June 7, 2025 • 35 mins
Welcome back to the RadOnc Smart Review Physics Series for our concluding episode! In P16b, we explored the crucial planning modalities of CT and MRI. Today, in Episode P16c: PET & Ultrasound Physics for RadOnc, we round out our imaging tour by looking at two other important technologies: Positron Emission Tomography (PET), which gives us insights into metabolic function, and Ultrasound (US), often used for real-time guidance and visualizing certain structures.
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(00:00):
Welcome to Radon Smart Review Physics Edition.
This is actually it, our last session in the main physics
series. Hard to believe we've covered so
much ground. Definitely.
So today we're rounding out our imaging tour.
We're going to really dig into the physics of positron emission
tomography, that's PED and ultrasound or US, and crucially
how they fit into radiation oncology.
(00:21):
Right. We've done CT, you know,
essential for dose calculation with its electron density info,
and MRI which is just unbeatablefor soft tissue contrast when
you're contouring. Exactly.
And PET and ultrasound, they bring something different to the
table, things that are complementary.
PET shows us function right metabolic activity.
While ultrasound give you those real time anatomical views using
(00:45):
sound waves, totally different physics.
And often it's the combination that's so powerful.
I mean, think about how often weorder a pet CT.
Oh, constantly. It's standard practice for so
many sites. You get that metabolic hotspot
from the pet, overlayed directlyonto the detailed anatomy and
density from the CT. That fusion is well, it's what
makes pets so useful for us. Precisely.
(01:05):
It's not just seeing some anatomical abnormality, you're
seeing something that's metabolically active, which you
know, that's often the critical piece you need for staging or
figuring out exactly what to treat.
Or what not to treat sometimes. Good point.
And then ultrasound, that real time aspect is just
indispensable for procedures where you absolutely need to see
what you're doing while you're doing it.
(01:26):
OK, so let's dive into the physics.
Maybe start with PET? Sounds good.
So pet. Fundamentally, it's a nuclear
medicine technique. The goal is to see where a
specific radioactive tracer goesin the body, and that tells us
about metabolic processes. And it starts with giving the
patient that tracer, right? You inject them with a molecule
the body normally uses, like glucose, but it's been tagged
(01:48):
labeled with a specific type of radioactive isotope, one that
emits positrons. The workhorse for us in oncology
is 18 F Fluoro Deoxyglucose FDG.18 epic FDG exactly.
And the idea is FDG looks a lot like glucose to the body's
cells. Yep.
So cells that are really metabolically active and that
includes many types of aggressive cancer cells, they
(02:12):
gobble up glucose like crazy. They see FDG, think it's glucose
and take it right up. But there's a trick, right?
It gets stuck. That's the key.
Unlike regular glucose, which gets fully metabolized, FDG gets
phosphorylated that first step. But then it can't proceed
further down the metabolic pathway.
It just accumulates inside thosehighly active cells.
(02:32):
So the bright spots you see on aPET scan correspond to areas
where lots of FDG has built up because those cells are working
overtime metabolically. Correct, and the radioactive pag
that makes this whole thing visible is fluorine 1818 F
That's the atom that emits the positron.
OK. And why 18 F?
Is there something special aboutits property?
Well, it hits a sweet spot, especially for whole body
(02:53):
imaging. It's half life is about 110
minutes. Just under 2 hours.
Right. Which is, you know, long enough
to actually make the FTG in a cyclotron, do quality control,
ship it to the clinic if needed,and check the patient.
Let it distribute in the body, which usually takes about an
hour. Exactly.
And then get them into the scanner and acquire the images.
It all fits nicely within a few half lives, but it's also short
(03:15):
enough that the radioactivity decays away reasonably quickly
after the scan, minimizing the patient's dose.
Makes sense. OK, so the 18 F is in the cell.
What happens next? The core physics bit.
OK, so the 18 F nucleus is unstable.
It undergoes a type of radioactive decay called beta
plus decay. In this process, a proton inside
the nucleus transforms into a neutron.
(03:37):
And to conserve charge. It emits A positron that's the
antimatter particle of an electron, same as opposite
charge. We denoted beta plus the
fluorine 18 becomes oxygen 18 inthe process.
So this positron gets ejected from the nucleus.
What happens to it? Does it travel far?
Not very far, actually. It gets emitted with some
kinetic energy, and it travels avery short distance within the
(04:00):
tissue, bumping into atoms, scattering, losing energy along
the way. How short are we talking?
Depends on the initial energy inthe tissue density, but for 18 F
the average distance the positron range is typically less
than a millimeter in soft tissue, maybe up to a few
millimeters for the higher energy ones.
It's quite local. OK, so it travels a tiny
distance, slows down, and then what?
(04:21):
And then one of the lost most ofits kinetic energy, and it's
practically at rest. It inevitably encounters a
nearby electron, just a regular electron in the tissue.
Matter meets antimatter. Exactly.
And they annihilate each other. Poof.
They completely disappear and their total mass is converted
into pure energy, according to Einstein's Emcs.
And this energy comes out as photons.
(04:43):
Precisely two gamma photons to exact, and because of
conservation of energy and momentum, each photon carries
away exactly half of the total rest mass energy of the electron
positron pair. Which is 511 key.
I remember that number. That's the one 511K electron
volts. Each photon has exactly 511 PV
of energy, which corresponds to the rest mass energy of an
(05:05):
electron or a posit. OK, two photons, both 511 Kev.
Is there anything else special about them?
Yes, absolutely critical for imaging.
They fly off in almost exactly opposite directions, 180° apart.
Ah, OK. Like they're pushing off from
each other. You can think of it that way.
That back-to-back emission is the cornerstone of PET imaging.
(05:26):
So the key take away physics point PET relies on detecting
pairs of 511 Kev annihilation photons traveling 180° apart.
Right. And that's what the standard is
designed to look. For exactly the patient lies
inside this ring or multiple rings of detectors.
These detectors are typically scintillating crystals, things
like BGOLSOLASO materials that give off a tiny flash of light
(05:51):
when they absorb A gamma photon.And that light flash is
detected. Yeah, it's detected by a
photomultiplier tube, or maybe asolid-state photodiode which
converts that tiny light flash into an electrical pulse.
So the scanner sees individual photon hits, but you said it
looks for pairs. Right, the clever part is the
coincidence detection circuitry.The system isn't just counting
(06:13):
every 511 key photon that hits any detector.
It's specifically looking for two detectors on opposite sides
of the ring that register a 511 key photon hit at the same time.
How simultaneous does it have to?
Be very simultaneous within a very narrow time window, usually
just a few nanosecond. IF2 photons hit opposing
(06:34):
detectors within that tiny window, the system counts it as
a coincidence event. And what does that coincidence
event tell you? It tells you that the
annihilation event, the place where the positron met the
electron, must have occurred somewhere along the straight
line connecting those two detectors that fired in
coincidence. OK.
So you don't know exactly where along the line, but you know
(06:56):
it's on that line. Precisely that line is called
the line of response or LR. Got it.
So one annihilation gives you 1 LR.
Yep, and the scanner just sits there and collects millions,
even billions of these allores from all different angles as the
tracer decays throughout the patient's body.
And then you need to turn all those lines into an image.
Right, that's where the reconstruction algorithms come
in. It's conceptually similar to how
(07:18):
CT images are reconstructed fromprojections, but often more
complex. For PET, they use sophisticated
mathematical techniques, frequently iterative
reconstruction methods, to take all those intersecting Lors and
figure out the three-dimensionaldistribution of where the
annihilations were happening most frequently.
And since the annihilations onlyhappen where the tracer
accumulated. The final image gives you a 3D
(07:41):
map showing the concentration ofthe radio tracer and therefore
the metabolic activity throughout the body.
So unlike CT or MRI which show structure, PET shows function.
Exactly. It's a functional imaging
modality. Now, we often talk about SUV
values in PET reports. Standardized uptake value.
What's that about? Right SUV.
It's an attempt to make the pet image data more quantitative, or
(08:03):
at least semi quantitative. The raw image just shows
relative concentrations, but SUVtries to normalize that.
Normalize for what? Primarily for the amount of
radioactive tracer injected and the patient's body size, usually
their weight. The idea is to get a value that
reflects the tissues tracer uptake relative to the average
uptake if the tracer were distributed evenly throughout
(08:25):
the body. OK, how is it calculated
roughly? The common simplified formula is
SUV equals the measured radioactivity concentration in a
region of interest, say in unitslike kilo becquerels per
milliliter divided by the total injected dose, usually in
megabeck rails, and then multiplied by the patient's
weight in kilogram. Let.
Me make sure I heard that right.SUV equals tissue activity.
(08:46):
Injected both patient weight. That's the basic form.
Sometimes lean body mass or bodysurface area is used instead of
total weight, which can be a bitmore accurate, especially
comparing patients of very different builds.
And what does the number mean? Like an SUV of eight versus 2.
Generally, a higher SUV value suggests higher tracer
concentration in that tissue, which implies higher metabolic
(09:06):
activity. So an SUV of eight in a lymph
node is much more concerning formalignancy than an SUV of two.
But you said semi quantitative. What are the caveats?
Oh, there are many. SUV is really useful, but you
have to be careful. It's not a perfect measure of
metabolic rate. It can be affected by lots of
things. Like what?
Well, the time between injectionand scanning is critical.
(09:29):
Uptake changes over time. The patient's blood glucose
level at the time of injection matters hugely if their blood
sugar is high. The sugar competes with FDG for
uptake into cells, lowering the tumor SUV.
Hence the fasting requirement before a PET scan.
Exactly. Also, patient motion during the
scan can blur the image and affect SUV.
(09:51):
Even the specific reconstructionparameters used by the scanner
software and the type of scanneritself can influence the final
SUV numbers. So comparing SUV values
absolutely requires consistent protocols.
You can't just compare an SUV from one hospital with another
taken under different conditions.
You really can't. It's best used for tracking
changes within the same patient scanned on the same scanner
using the same protocol, like comparing pretreatment and post
(10:13):
treatment. OK.
That's a really important clinical Pearl.
So how do we actually use PET? Usually PET CT in radiation
oncology? Well, several key ways.
First diagnosis and staging. PET is incredibly sensitive for
detecting metabolically active disease.
Finding primary tumors, or maybemore often finding metastases
(10:33):
that weren't obvious on anatomical imaging alone.
Exactly. Especially lymph node Mets or
distant metastasis. Think about lymphoma, lung
cancer, head and neck cancer, esophageal colorectal Melanoma.
PET. CT is often standard for initial
staging. It can find tiny spots of
disease in nodes or organs that look perfectly normal on CT.
(10:54):
And finding those can completelychange the patient's stage,
right, which changes prognosis and the whole treatment plan.
Absolutely. It might shift someone from
potentially curative local treatment to needing systemic
therapy or maybe palliative radiation instead of aggressive
treatment. It's hugely impactful.
OK, staging is 1 big use. What else?
Target volume delineation. This is huge for RT planning.
(11:16):
For tumors that avidly take up FDG, the PET scan helps us
define the gross tumor volume, the GTV.
How does it help refine the GTV compared to just using the CT or
MRI? It can show the metabolically
active part of the mass. Sometimes ACT shows a large
complex mass, maybe with centralnecrosis or surrounding
inflammation. The PET can highlight just a
(11:37):
viable active tumor within that,which is what you really need to
target with the highest dose. So it helps separate tumor from
non tumor essentially. Yes, or differentiate tumor from
treatment related changes like fibrosis or inflammation which
can be really tricky on CTMRI alone, especially after prior
treatment. And we contour this GTV pet
directly on the fused pet CT images.
(11:59):
Typically yes. Use the CT for the underlying
anatomy and Bony landmarks, and the PET overlay helps you define
the metabolically active extent of the tumor and involve nodes.
It's standard practice now for planning many head and neck,
lung, lymphoma and esophageal cases, among others.
OK, staging target definition? Any other major uses?
Treatment response assessment, especially for FDG avid tumors
(12:22):
treated with chemotherapy or chemo radiation.
How does that work? Because.
PET measures metabolic activity.You can sometimes see a decrease
in FDG uptake, a drop in SUV indicating A metabolic response
to treatment before the tumor actually starts to shrink
noticeably on CT or MRI. So it can be an earlier
indicator of whether the treatment is working.
Potentially yes. A significant drop in SUV post
(12:45):
treatment is generally a good prognostic sign.
Conversely, persistent high uptake might suggest resistant
disease. This is used routinely in
lymphoma response assessment Deville criteria and explored in
other cancers too. Sounds powerful, but are there
downsides or limitations to pet?Definitely.
We mentioned the spatial resolution isn't as good as CT
(13:05):
or MRI, typically around 4 to 6mm.
Best case, you won't see super fine anatomical details on the
PET image itself. That's why the CT fusion is so
vital. And the big one, clinically, FDG
uptake isn't tumor specific. Lots of things besides cancer
can cause hotspots. Infection and inflammation are
major culprits. An Abscess or active
(13:26):
inflammatory process can be intensely FDG avid.
Also some normal tissues have high uptake.
The brain always does. The heart muscle often does
active muscle. So you need to correlate
clinically and with the CT findings.
A hotspot in a weird place mightbe inflammation, not Mets.
Absolutely. Clinical context is key, and
(13:47):
patient preparation, like fasting and resting before the
scan, helps minimize physiological uptake in muscles.
Any other limitations? Well, there's a radiation dose.
The injected tracer gives the patient a dose, typically around
5 to 10 millisieverts for an FDGscan.
It's not huge, but it's additiveto other imaging doses and it
requires specialized equipment, cyclotrons for many tracers, and
it's relatively expensive. OK, so that's a good overview of
(14:09):
PET, positron emission annihilation, 511 QV photons,
coincidence detection, LORS reconstruction, SUV and its uses
in staging, targeting and response, with caveats about
resolution and specificity. Nicely summarized, ready to
switch gears to sound waves? Let's do it.
Ultrasound. Completely different beast.
Totally different physics. Ultrasound uses high frequency
(14:32):
sound waves. Remember, sound is a mechanical
wave. It needs a medium like tissue to
travel through. It can't travel in a vacuum like
light or X-rays. And these waves have properties
like frequency, wavelength and velocity.
Exactly, and they're related by that fundamental equation
velocity V equals frequency F times wavelength Lambda V.
(14:53):
But ultrasound just means the frequency is really high, right?
Right higher than humans can hear.
Human hearing typically tops outaround 20,000 Hertz, or 20 kHz
kHz. Medical ultrasound uses
frequencies way above that, typically in the range of 1 to
15 megahertz megahertz, millionsof cycles per second.
Wow, and how do we generate and detect these high frequency
(15:14):
sound waves? That's the job of the
transducer, the part you hold against the patient.
Inside the transducer are special materials called
piezoelectric crystal. Piezoelectric.
What does that mean? It means they have this amazing
property. They can convert electrical
energy into mechanical energy, sound waves, and they can
convert mechanical energy returning sound waves back into
(15:34):
electrical energy. It works both ways.
When the ultrasound machine applies A voltage pulse across
these crystals, they physically deform or vibrate very rapidly
at a specific frequency. That visibration pushes on the
surrounding tissue, creating thepulse of ultrasound waves that
travels into the body. OK, so voltage in, sound wave
(15:55):
out. Yep.
Then the transducer waits and listens.
When echo sound waves reflected back from tissues inside the
body hit the transducer, they make those same piezoelectric
crystals vibrate again. And that vibration creates.
A small electrical voltage or signal.
The ultrasound machine detects this silk roll, amplifies it,
and processes it to form the image.
So the transducer is both the speaker and a microphone
(16:17):
essentially. OK, so it sends a pulse.
It listens for echoes. What determines whether an echo
comes back? It all comes down to a property
of the tissue called acoustic impedance, usually denoted by
the letter Z. Acoustic impedance, Yeah.
What? What is that?
It's basically a measure of how much resistance a tissue
presents to the passage of soundwaves.
Think of it like how hard or easy it is for sound to travel
(16:38):
through it. It's calculated as the product
of the tissues density row and the speed of sound within that
tissue V, so ZE qv. And different tissues have
different densities and sound speeds.
Yes, the speed of sound varies quite a bit.
It's relatively slow in air, around 330 meters per second.
It's around 15140 meters in mostsoft tissues, which is the
(17:00):
average value of the machine Assumes it's faster and denser
materials like bone. Maybe around 4000 meters.
So different tissues have different Z values.
How does that create an echo? An echo is generated whenever
the ultrasound pulse encounters A boundary and interface between
two tissues that have different acoustic impedances.
Let's say tissue one has a vetina Z and tissue 2 has Z OK.
(17:22):
When the sound wave hit that boundary, part of its energy is
reflected back towards the transducer as an echo, and the
rest is transmitted deeper into tissue 2.
And does the strength of the echo depend on how different Z&Z
are? Exactly.
The bigger the difference in acoustic impedance between the
two tissues, the stronger the reflection and the brighter that
interface will appear on the ultrasound image.
(17:43):
Ah okay, so if 2 tissues have very similar Z values.
You get a very weak echo or maybe none at all.
Boundary will be hard to see but.
If the difference is huge, like between soft tissue and bone, or
soft tissue and air. Then the impedance mismatch is
massive. Air has extremely low impedance,
Bone has very high impedance compared to soft tissue.
(18:04):
So with those interfaces you getalmost total reflection of the
sound wave. Which means a very bright echo.
A very bright echo, yes, but also critically, very little
sound energy gets transmitted past that interface.
That's why you can't really image things behind bone or air.
Precisely. The sound just bounces right
back from the surface of the bone or the air pocket.
You get acoustic shadowing behind bone and you just can't
(18:26):
penetrate air filled structures like the lungs or bowel gas
effectively. That's a fundamental limitation.
OK, so impedance differences cause reflections.
Does anything else happen to thesound wave as it travels?
Yes, it also gets weaker due to attenuation.
Attenuation is the gradual loss of sound energy as it propagates
through tissue. Why does it lose energy?
(18:47):
2 main reasons. Absorption, where the sound
energy is converted into heat within the tissue, and
scattering, where the found waves bounce off tiny structures
within the tissue in various directions so less energy
continues straight ahead. And does Tenuation depend on the
sound wave itself? Absolutely, and this is probably
the most important practical concept in ultrasound physics.
(19:07):
Attenuation increases significantly with increasing
ultrasound frequency. OK.
Higher frequency means higher attenuation.
What does that mean for imaging?It leads to a fundamental trade
off, a direct conflict between image resolution and how deep
you can see into the body the penetration depth.
Or break that down for me. Let's say I use a high frequency
transducer like the 10 or 12 megahertz.
(19:28):
OK, high frequency. That means the wavelength is
short. Short wavelength is good for
resolution. Specifically, it gives you
better axial resolution. Axial resolution.
What's that? That's your ability to
distinguish between two small objects.
They're located close together along the direction the sound
beam is traveling, one behind the other.
Shorter wavelength means you canresolve finer details Along that
(19:50):
axis. You get a sharper, clearer
image. OK, high frequency, high
resolution sounds great. What's the catch?
The catch is that high attenuation because attenuation
is so much higher at 12 millihertz compared to say, 3
millihertz. Those high frequency sound waves
get absorbed and scattered very quickly.
They lose their energy rapidly. So they can't travel very far
(20:11):
into the tissue. Exactly.
They don't penetrate deep enoughto create usable echoes from
structures far below the surface, so high frequency
transducers have limited penetration depth.
OK, so high frequency gives great detail, but only for
things close to the surface. Perfect.
That's why you use high frequency pros, maybe 7 to 15
megahertz for imaging superficial structures like the
(20:31):
thyroid, breath lumps tests, superficial nodes, good skin
lesions. You need that detail and
penetration isn't an issue. All right, now, what if I need
to see something deep like the liver or the kidneys in the
abdomen or the uterus? Then you have to use a lower
frequency transducer, maybe in the 2 to 5 megahertz range.
OK, low frequency, what happens now?
Low frequency means a longer wavelength.
(20:52):
Longer wavelength means worse axial resolution.
Your image won't be as sharp, the details won't be as fine.
That's the. Downside What's the upside?
The upside is much lower attenuation.
Because attenuation is less at lower frequencies, the sound
waves can travel much deeper into the body before they become
too weak. O low frequency transducers give
(21:13):
you greater enetration death. So you sacrifice some resolution
to gain the ability to see deeper structures.
That's the trade off exactly. You have to use lower
frequencies for abdominal imaging, pelvic imaging,
obstetrics. You need that penetration.
OK, so let's try to summarize that trade off.
It's crucial. High frequency equals high
resolution but low penetration. And low frequency equals low
(21:34):
resolution but high penetration.Got it.
That's the mantra you have to remember when choosing an
ultrasound probe or evaluating an image.
Did they use the right frequencyfor the job?
OK, that makes sense. So we send a pulse, it reflects
off interfaces based on impedance differences, and we
choose the frequency based on depth versus resolution needed.
(21:56):
How does the machine actually build the picture?
It uses the pulse echo principle.
Send out a short pulse, then listen for returning echoes.
And how does it know how deep the echo came from?
Based on timing, it measures thetime it takes for the echo to
return to the transducer after the pulse was sent out.
Since sound travels at a relatively constant speed and
(22:16):
soft tissue that assumed 15140 meters per second, a longer
return time means the echo came from a deeper structure.
Simple geometry. Basically, time makes distance
speed. Yep, the machine calculates the
depth, distance, speed, time too, because it's a round trip
for every echo it receives. And then it displays this.
How does it make the grayscale image we usually see?
That's called B mode or brightness mode.
(22:38):
It's the standard anatomical image.
For each echo received, the machine calculates its depth
based on time time and determines its strength or
amplitude. And the altitude determines the
brightness. Exactly.
The stronger the echo, meaning abigger impedance difference at
the interface, the brighter the pixel corresponding to that
depth will be on the screen. So really strong reflections
(22:58):
like from bone or maybe gas lookbright white.
Right, those are called hyperechoic tissues that reflect
moderately like solid organs appear as various Shades of
Grey. And what about things that don't
reflect at all, like fluid? If there's no impedance
difference, or if the sound justpasses through without
reflecting, no echo returns fromthat region.
(23:19):
The machine displays those areasas black.
We call those anechoic, like thefluid inside a simple cyst, or
in the bladder, or inside blood vessels.
OK, B mode gives us the anatomical map in grayscale.
Are there other modes? Yes, though B mode is the main
one for anatomical imaging. There's A mode, amplitude mode,
which is mostly historical now. It just showed a graph of echo
(23:39):
amplitude versus depth along a single line.
Then there's M mode or motion mode.
It repeatedly sends pulses alongone line and displays how the
positions of reflecting structures along that line
change over time. It looks like a WAVY line graph.
Where would you use M mode? It's essential in
echocardiography for looking at heart valve motion or measuring
chamber dimensions accurately over the cardiac site.
(24:01):
Got it. Any others?
The other big one is Doppler ultrasound.
This uses the Doppler effect to detect motion, primarily blood
flow. The Doppler effect, like the
pitch change from an ambulance siren.
Exactly the same principle. If the sound waves reflect off
something moving towards the transducer, like red blood
cells, the returning echo has a slightly higher frequency.
(24:23):
If they reflect off something moving away, the echo has a
slightly lower frequency. And the machine can detect this
tiny frequency shift. Yes, and the magnitude of the
frequency shift tells you how fast the blood is moving, and
the sign of the shift higher or lower tells you the direction
relative to the transducer. And this is shown with colors.
Often yes. Color Doppler overlays a color
(24:44):
map onto the B mode image. Typically red means flow towards
the transducer, blue means flow away, and the brightness of the
color indicates the relative velocity.
Gives you a quick map of where blood is flowing.
Are there other Doppler types? Yes, pulsed wave PW and
continuous wave CW Doppler give you more quantitative
information. They produce a spectral
(25:07):
waveform, a graph showing the range of blood velocities
present over time within a specific sample volume, PW or
along the entire beam. CW cardiologists and vascular
techs use this extensively. Sometimes useful in oncology to
assess tumor vascularity, thoughnot a primary RT tool.
OK, quite a range of informationfrom sound waves.
(25:27):
Now let's bring it back home. How do we actually use
ultrasound and radiation therapy?
You mentioned it's not a primaryplanning tool like CT.
Right. Its role in external beam
planning is pretty limited, mainly because of those issues
we discussed, operator dependence, poor penetration
through bone and air, and difficult to getting
quantitative data for dose calculation.
So where is it essential? It's main niche is real time
(25:49):
guidance during specific procedures, leveraging that
ability to see anatomy instantly.
And the classic example is. Prostate bracket therapy.
It's absolutely fundamental there.
We use transrectal ultrasound orTRUS.
OK, explain that workflow. Well, first you do a tree recess
volume study, usually before theimplant day.
The physician uses the probe in the rectum to acquire a series
(26:10):
of transverse images through theprostate.
The planning system reconstructsthese into a 3D model of the
prostate, urethra and rectum. This is used for treatment
planning, figuring out where theradioactive seeds need to go.
And then during the actual seed implant procedure?
During the implant, the Tree US probe stays in place, providing
continuous real time imaging. The physician uses a template
(26:34):
grid attached to the probe. As they insert each needle
carrying the seeds toward the prostate, they watch its path
live on the ultrasound screen. So they can see exactly where
the needle tip is relative to the prostate boundary and the
planned position. Precisely, they guide the needle
to the correct location based onthe plan, deposit the seeds and
withdraw. Repeat dozens of times.
(26:54):
TUS makes that level of accuracypossible in real time.
It's a perfect marriage of imaging and intervention.
That's a great example. Anywhere else US might pop up.
It can sometimes be useful for localizing superficial things,
maybe helping to mark out a small skin lesion before
electron planning or guiding a biopsy of a superficial node,
but these are less common roles compared to TRUS for bracket
(27:17):
therapy. It's not typically used for
daily setup guidance and external beam like CBCT or MV
imaging. Right, because of those
limitations, operator skill needed, can't see through bone
or air well, doesn't give density info.
Exactly its strengths lie in that real time visualization for
procedures, especially in the pelvis or for superficial
structures. OK, That clarifies its role
(27:38):
nicely. Shall we try to crystallize some
key clinical pearls for both petand ultrasound?
Things you absolutely need to remember?
Good idea. High yield points.
Number one for PET. Remember it uses positron
emitters like 18 FFDG. It detects coincident 511 key
annihilation photons flying off 180° apart.
(27:59):
That's the signal. And it images metabolic activity
semi quantified by SUV, almost always fused with CT for
anatomical localization and planning.
Maybe a mnemonic like peat 511 a180° for FDG links the energy
angle and common tracer. Nice OK Pearl #2 for ultrasound.
It uses high frequency sound, way above hearing 20 kHz.
(28:22):
The transducer with its piezoelectric crystals sends
pulses and receives the echoes. And those echoes are generated
at interfaces between tissues with different acoustic
impedance Z, which is density times sound velocity Z equals
this F. Pearl 3 The big ultrasound trade
off. High frequency gives you high
resolution but low penetration. And low frequency gives you low
resolution but high penetration.You must choose the right
(28:45):
frequency for the depth you needto image.
Pearl 4 Ultrasound B mode is thestandard grayscale image.
Brightness depends on echo strength, Depth depends on echo
return time, assuming that constant 1540 meters speed and
tissue. And Pearl Five summing up the RT
rolls. PET CT fusion is a workhorse for
staging and target definition inmany cancers.
Ultrasound's primary role is real time guidance, especially
(29:08):
TRUS for prostate brachytherapy.Less use for routine EBRT
planning or delivery. Got it.
Those are excellent summaries. Maybe let's try a quick clinical
scenario, sort of like a mini case to apply this.
OK, imagine you have a patient who finished chemeradiation for
say, locally advanced head and neck cancer a few months ago.
On their follow up CT scan, there's some fullness or maybe
(29:30):
subtle enhancement in one area of the treated neck, but it's
hard to tell if it's just post treatment inflammation, scar
tissue, or if it's residual or recurrent tumor.
What imaging study would be mosthelpful, and what's the physics
principle you're relying on? Yeah, that's a super common and
often difficult clinical question.
The go to study here would almost certainly be a PET CT
scan. And why Pet CT.
(29:51):
Because you need that functionalinformation Pet provides.
Both scar tissue and active tumor might look abnormal on CT,
but active tumor cells are typically highly metabolic and
will avidly take up FDG, showingup as a hotspot with a high SUV
on the PET scan. Scar tissue or mature fibrosis
should be metabolically quiet with low or no FDG uptake.
(30:14):
So you're fundamentally relying on the difference in glucose
metabolism between viable tumor and inactive post treatment
tissue, which PET visualizes viaFDG uptake.
Exactly. The PET tells you if that
questionable area on CT is metabolically active or not.
The fused CT part, of course, tells you precisely where that
activity, or lack thereof is located anatomically.
(30:35):
It directly addresses the clinical question, is there
active disease here? Perfect illustration.
Understanding the physics, what each modality actually shows
you, lets you choose the right test.
OK, ready for a board blitz? Let's do it.
Fire away. Question one.
The two photons detected in coincidence by a PET scanner
originate from which physical process?
A Compton scattering of primary gamma rays from the tracer.
(30:56):
B Pair production caused by highenergy beta particles.
C Annihilation of a positron emitted by the tracer with an
electron. D Bremstrawing radiation
produced as positron slow down. OK, we covered this positron
emitted travels short distance meets electron, they annihilate,
producing the two photons. So that's answer C.
(31:17):
Correct. C annihilation of a positron
emitted by the tracer with an electron.
That annihilation is the source of the coincident 511 key V
photons, PET. Detects OK next.
Question 2A Physician wants to use ultrasound to get a high
resolution image of a superficial thyroid nodule.
Which transducer frequency wouldbe most appropriate?
A 3 megahertz B5 megahertz, C7 megahertz, D12 megahertz.
(31:40):
Superficial thyroid nodule keywords superficial high
resolution that points directly to high frequency.
Of the choices, 12 megahertz is the highest, so D.
Correct D12 megahertz. For superficial structures where
penetration isn't the main concern, you use the highest
available frequency to maximize spatial resolution.
Makes sense. Keep them coming.
Question 3 The standardized uptake value or SUV calculated
(32:05):
from an FDG PET scan is primarily intended to provide a
semi quantitative measure of a tissue electron density.
BT2 relaxation time. C regional blood flow.
D glucose metabolic activity. SUV from FDG pet.
FDG mimics glucose, so SUV reflects glucose uptake and
metabolism. That's the answer D.
(32:26):
Correct D Glucose metabolic activity SUV attempts to
normalize tracer concentration, which for FDG reflects glucose
metabolism often elevated in tumors.
Right, last one. Last one, question 4.
Ultrasound waves are strongly reflected at an interface
between soft tissue and air, primarily because of the large
difference in a temperature B acoustic impedance, C electrical
(32:46):
conductivity, D shear modulus. Soft tissue to That's a huge
mismatch. We've said echoes happen due to
differences in acoustic impedance.
Z air has very low Z tissue has much higher Z.
So the large difference in Z causes the strong reflection
answer B. Correct again.
B Acoustic impedance. Strong reflections occur at
(33:06):
boundaries with large differences in acoustic
impedance. Z equals density, velocity.
Nicely done. So just to wrap things up then
we've covered PET. It's physics based on positron
annihilation, coincidence, detection of those back-to-back
five on 11 KV photons, how SUV gives us a metabolic snapshot
and it's huge role in staging and target definition using PET
(33:28):
CT. And then ultrasound based on
high frequency sound waves, piezoelectric transducer sending
and receiving echoes generated by acoustic impedance
differences and that crucial trade off between frequency
resolution and penetration. And we've seen how they fit into
our world. PET CT is really integral to
modern planning for many sites, while ultrasound shines in real
time procedural guidance, especially for prostate bracket
(33:49):
therapy, even if it has limitations for routine external
beam work. Absolutely.
And really understanding the whybehind these images, the physic
principles, it's just so fundamental.
It helps you interpret them correctly, understand their
strengths, know their limitations, and ultimately use
them in the safest and most effective way for patient care.
Couldn't agree more. And with that, well, that's a
(34:10):
wrap on our main Radonk Smart Review physics series.
We've gone from basic interactions all the way through
machines planning Brackie safetyand now finishing up imaging.
It's been quite a journey. It really has.
Hopefully it provides a solid foundation or a good review for
everyone listening though. You know the joke, right?
Which one? They say the hardest part about
(34:30):
radiation physics isn't learningit for the boards, it's
remembering any of it six monthsafter the boards.
That is painfully true. You'll be sitting in clinic
trying to remember was that annihilation or pair production?
Was it 180° or 90? Did SUV stand for standard
uptake value or seriously unsurevalue?
(34:51):
Exactly. Hopefully revisiting these
concepts helps make them stick abit better.
Repetition and clinical application are key.
Definitely. Well, for more practice,
especially gearing up for orals,you can find
completepracticeoralboards@radonsmartlearn.com.And keep an eye out for future
Radon Smart review sessions covering different areas of
oncology. Thanks so much for joining us
throughout this physics series.
Welcome to Radon Smart Review Physics Edition.
This is actually it, our last session in the main physics
series. Hard to believe we've covered so
much ground. Definitely.
So today we're rounding out our imaging tour.
We're going to really dig into the physics of positron emission
tomography, that's PED and ultrasound or US, and crucially
how they fit into radiation oncology.
(00:21):
Right. We've done CT, you know,
essential for dose calculation with its electron density info,
and MRI which is just unbeatablefor soft tissue contrast when
you're contouring. Exactly.
And PET and ultrasound, they bring something different to the
table, things that are complementary.
PET shows us function right metabolic activity.
While ultrasound give you those real time anatomical views using
(00:45):
sound waves, totally different physics.
And often it's the combination that's so powerful.
I mean, think about how often weorder a pet CT.
Oh, constantly. It's standard practice for so
many sites. You get that metabolic hotspot
from the pet, overlayed directlyonto the detailed anatomy and
density from the CT. That fusion is well, it's what
makes pets so useful for us. Precisely.
(01:05):
It's not just seeing some anatomical abnormality, you're
seeing something that's metabolically active, which you
know, that's often the critical piece you need for staging or
figuring out exactly what to treat.
Or what not to treat sometimes. Good point.
And then ultrasound, that real time aspect is just
indispensable for procedures where you absolutely need to see
what you're doing while you're doing it.
(01:26):
OK, so let's dive into the physics.
Maybe start with PET? Sounds good.
So pet. Fundamentally, it's a nuclear
medicine technique. The goal is to see where a
specific radioactive tracer goesin the body, and that tells us
about metabolic processes. And it starts with giving the
patient that tracer, right? You inject them with a molecule
the body normally uses, like glucose, but it's been tagged
(01:48):
labeled with a specific type of radioactive isotope, one that
emits positrons. The workhorse for us in oncology
is 18 F Fluoro Deoxyglucose FDG.18 epic FDG exactly.
And the idea is FDG looks a lot like glucose to the body's
cells. Yep.
So cells that are really metabolically active and that
includes many types of aggressive cancer cells, they
(02:12):
gobble up glucose like crazy. They see FDG, think it's glucose
and take it right up. But there's a trick, right?
It gets stuck. That's the key.
Unlike regular glucose, which gets fully metabolized, FDG gets
phosphorylated that first step. But then it can't proceed
further down the metabolic pathway.
It just accumulates inside thosehighly active cells.
(02:32):
So the bright spots you see on aPET scan correspond to areas
where lots of FDG has built up because those cells are working
overtime metabolically. Correct, and the radioactive pag
that makes this whole thing visible is fluorine 1818 F
That's the atom that emits the positron.
OK. And why 18 F?
Is there something special aboutits property?
Well, it hits a sweet spot, especially for whole body
(02:53):
imaging. It's half life is about 110
minutes. Just under 2 hours.
Right. Which is, you know, long enough
to actually make the FTG in a cyclotron, do quality control,
ship it to the clinic if needed,and check the patient.
Let it distribute in the body, which usually takes about an
hour. Exactly.
And then get them into the scanner and acquire the images.
It all fits nicely within a few half lives, but it's also short
(03:15):
enough that the radioactivity decays away reasonably quickly
after the scan, minimizing the patient's dose.
Makes sense. OK, so the 18 F is in the cell.
What happens next? The core physics bit.
OK, so the 18 F nucleus is unstable.
It undergoes a type of radioactive decay called beta
plus decay. In this process, a proton inside
the nucleus transforms into a neutron.
(03:37):
And to conserve charge. It emits A positron that's the
antimatter particle of an electron, same as opposite
charge. We denoted beta plus the
fluorine 18 becomes oxygen 18 inthe process.
So this positron gets ejected from the nucleus.
What happens to it? Does it travel far?
Not very far, actually. It gets emitted with some
kinetic energy, and it travels avery short distance within the
(04:00):
tissue, bumping into atoms, scattering, losing energy along
the way. How short are we talking?
Depends on the initial energy inthe tissue density, but for 18 F
the average distance the positron range is typically less
than a millimeter in soft tissue, maybe up to a few
millimeters for the higher energy ones.
It's quite local. OK, so it travels a tiny
distance, slows down, and then what?
(04:21):
And then one of the lost most ofits kinetic energy, and it's
practically at rest. It inevitably encounters a
nearby electron, just a regular electron in the tissue.
Matter meets antimatter. Exactly.
And they annihilate each other. Poof.
They completely disappear and their total mass is converted
into pure energy, according to Einstein's Emcs.
And this energy comes out as photons.
(04:43):
Precisely two gamma photons to exact, and because of
conservation of energy and momentum, each photon carries
away exactly half of the total rest mass energy of the electron
positron pair. Which is 511 key.
I remember that number. That's the one 511K electron
volts. Each photon has exactly 511 PV
of energy, which corresponds to the rest mass energy of an
(05:05):
electron or a posit. OK, two photons, both 511 Kev.
Is there anything else special about them?
Yes, absolutely critical for imaging.
They fly off in almost exactly opposite directions, 180° apart.
Ah, OK. Like they're pushing off from
each other. You can think of it that way.
That back-to-back emission is the cornerstone of PET imaging.
(05:26):
So the key take away physics point PET relies on detecting
pairs of 511 Kev annihilation photons traveling 180° apart.
Right. And that's what the standard is
designed to look. For exactly the patient lies
inside this ring or multiple rings of detectors.
These detectors are typically scintillating crystals, things
like BGOLSOLASO materials that give off a tiny flash of light
(05:51):
when they absorb A gamma photon.And that light flash is
detected. Yeah, it's detected by a
photomultiplier tube, or maybe asolid-state photodiode which
converts that tiny light flash into an electrical pulse.
So the scanner sees individual photon hits, but you said it
looks for pairs. Right, the clever part is the
coincidence detection circuitry.The system isn't just counting
(06:13):
every 511 key photon that hits any detector.
It's specifically looking for two detectors on opposite sides
of the ring that register a 511 key photon hit at the same time.
How simultaneous does it have to?
Be very simultaneous within a very narrow time window, usually
just a few nanosecond. IF2 photons hit opposing
(06:34):
detectors within that tiny window, the system counts it as
a coincidence event. And what does that coincidence
event tell you? It tells you that the
annihilation event, the place where the positron met the
electron, must have occurred somewhere along the straight
line connecting those two detectors that fired in
coincidence. OK.
So you don't know exactly where along the line, but you know
(06:56):
it's on that line. Precisely that line is called
the line of response or LR. Got it.
So one annihilation gives you 1 LR.
Yep, and the scanner just sits there and collects millions,
even billions of these allores from all different angles as the
tracer decays throughout the patient's body.
And then you need to turn all those lines into an image.
Right, that's where the reconstruction algorithms come
in. It's conceptually similar to how
(07:18):
CT images are reconstructed fromprojections, but often more
complex. For PET, they use sophisticated
mathematical techniques, frequently iterative
reconstruction methods, to take all those intersecting Lors and
figure out the three-dimensionaldistribution of where the
annihilations were happening most frequently.
And since the annihilations onlyhappen where the tracer
accumulated. The final image gives you a 3D
(07:41):
map showing the concentration ofthe radio tracer and therefore
the metabolic activity throughout the body.
So unlike CT or MRI which show structure, PET shows function.
Exactly. It's a functional imaging
modality. Now, we often talk about SUV
values in PET reports. Standardized uptake value.
What's that about? Right SUV.
It's an attempt to make the pet image data more quantitative, or
(08:03):
at least semi quantitative. The raw image just shows
relative concentrations, but SUVtries to normalize that.
Normalize for what? Primarily for the amount of
radioactive tracer injected and the patient's body size, usually
their weight. The idea is to get a value that
reflects the tissues tracer uptake relative to the average
uptake if the tracer were distributed evenly throughout
(08:25):
the body. OK, how is it calculated
roughly? The common simplified formula is
SUV equals the measured radioactivity concentration in a
region of interest, say in unitslike kilo becquerels per
milliliter divided by the total injected dose, usually in
megabeck rails, and then multiplied by the patient's
weight in kilogram. Let.
Me make sure I heard that right.SUV equals tissue activity.
(08:46):
Injected both patient weight. That's the basic form.
Sometimes lean body mass or bodysurface area is used instead of
total weight, which can be a bitmore accurate, especially
comparing patients of very different builds.
And what does the number mean? Like an SUV of eight versus 2.
Generally, a higher SUV value suggests higher tracer
concentration in that tissue, which implies higher metabolic
(09:06):
activity. So an SUV of eight in a lymph
node is much more concerning formalignancy than an SUV of two.
But you said semi quantitative. What are the caveats?
Oh, there are many. SUV is really useful, but you
have to be careful. It's not a perfect measure of
metabolic rate. It can be affected by lots of
things. Like what?
Well, the time between injectionand scanning is critical.
(09:29):
Uptake changes over time. The patient's blood glucose
level at the time of injection matters hugely if their blood
sugar is high. The sugar competes with FDG for
uptake into cells, lowering the tumor SUV.
Hence the fasting requirement before a PET scan.
Exactly. Also, patient motion during the
scan can blur the image and affect SUV.
(09:51):
Even the specific reconstructionparameters used by the scanner
software and the type of scanneritself can influence the final
SUV numbers. So comparing SUV values
absolutely requires consistent protocols.
You can't just compare an SUV from one hospital with another
taken under different conditions.
You really can't. It's best used for tracking
changes within the same patient scanned on the same scanner
using the same protocol, like comparing pretreatment and post
(10:13):
treatment. OK.
That's a really important clinical Pearl.
So how do we actually use PET? Usually PET CT in radiation
oncology? Well, several key ways.
First diagnosis and staging. PET is incredibly sensitive for
detecting metabolically active disease.
Finding primary tumors, or maybemore often finding metastases
(10:33):
that weren't obvious on anatomical imaging alone.
Exactly. Especially lymph node Mets or
distant metastasis. Think about lymphoma, lung
cancer, head and neck cancer, esophageal colorectal Melanoma.
PET. CT is often standard for initial
staging. It can find tiny spots of
disease in nodes or organs that look perfectly normal on CT.
(10:54):
And finding those can completelychange the patient's stage,
right, which changes prognosis and the whole treatment plan.
Absolutely. It might shift someone from
potentially curative local treatment to needing systemic
therapy or maybe palliative radiation instead of aggressive
treatment. It's hugely impactful.
OK, staging is 1 big use. What else?
Target volume delineation. This is huge for RT planning.
(11:16):
For tumors that avidly take up FDG, the PET scan helps us
define the gross tumor volume, the GTV.
How does it help refine the GTV compared to just using the CT or
MRI? It can show the metabolically
active part of the mass. Sometimes ACT shows a large
complex mass, maybe with centralnecrosis or surrounding
inflammation. The PET can highlight just a
(11:37):
viable active tumor within that,which is what you really need to
target with the highest dose. So it helps separate tumor from
non tumor essentially. Yes, or differentiate tumor from
treatment related changes like fibrosis or inflammation which
can be really tricky on CTMRI alone, especially after prior
treatment. And we contour this GTV pet
directly on the fused pet CT images.
(11:59):
Typically yes. Use the CT for the underlying
anatomy and Bony landmarks, and the PET overlay helps you define
the metabolically active extent of the tumor and involve nodes.
It's standard practice now for planning many head and neck,
lung, lymphoma and esophageal cases, among others.
OK, staging target definition? Any other major uses?
Treatment response assessment, especially for FDG avid tumors
(12:22):
treated with chemotherapy or chemo radiation.
How does that work? Because.
PET measures metabolic activity.You can sometimes see a decrease
in FDG uptake, a drop in SUV indicating A metabolic response
to treatment before the tumor actually starts to shrink
noticeably on CT or MRI. So it can be an earlier
indicator of whether the treatment is working.
Potentially yes. A significant drop in SUV post
(12:45):
treatment is generally a good prognostic sign.
Conversely, persistent high uptake might suggest resistant
disease. This is used routinely in
lymphoma response assessment Deville criteria and explored in
other cancers too. Sounds powerful, but are there
downsides or limitations to pet?Definitely.
We mentioned the spatial resolution isn't as good as CT
(13:05):
or MRI, typically around 4 to 6mm.
Best case, you won't see super fine anatomical details on the
PET image itself. That's why the CT fusion is so
vital. And the big one, clinically, FDG
uptake isn't tumor specific. Lots of things besides cancer
can cause hotspots. Infection and inflammation are
major culprits. An Abscess or active
(13:26):
inflammatory process can be intensely FDG avid.
Also some normal tissues have high uptake.
The brain always does. The heart muscle often does
active muscle. So you need to correlate
clinically and with the CT findings.
A hotspot in a weird place mightbe inflammation, not Mets.
Absolutely. Clinical context is key, and
(13:47):
patient preparation, like fasting and resting before the
scan, helps minimize physiological uptake in muscles.
Any other limitations? Well, there's a radiation dose.
The injected tracer gives the patient a dose, typically around
5 to 10 millisieverts for an FDGscan.
It's not huge, but it's additiveto other imaging doses and it
requires specialized equipment, cyclotrons for many tracers, and
it's relatively expensive. OK, so that's a good overview of
(14:09):
PET, positron emission annihilation, 511 QV photons,
coincidence detection, LORS reconstruction, SUV and its uses
in staging, targeting and response, with caveats about
resolution and specificity. Nicely summarized, ready to
switch gears to sound waves? Let's do it.
Ultrasound. Completely different beast.
Totally different physics. Ultrasound uses high frequency
(14:32):
sound waves. Remember, sound is a mechanical
wave. It needs a medium like tissue to
travel through. It can't travel in a vacuum like
light or X-rays. And these waves have properties
like frequency, wavelength and velocity.
Exactly, and they're related by that fundamental equation
velocity V equals frequency F times wavelength Lambda V.
(14:53):
But ultrasound just means the frequency is really high, right?
Right higher than humans can hear.
Human hearing typically tops outaround 20,000 Hertz, or 20 kHz
kHz. Medical ultrasound uses
frequencies way above that, typically in the range of 1 to
15 megahertz megahertz, millionsof cycles per second.
Wow, and how do we generate and detect these high frequency
(15:14):
sound waves? That's the job of the
transducer, the part you hold against the patient.
Inside the transducer are special materials called
piezoelectric crystal. Piezoelectric.
What does that mean? It means they have this amazing
property. They can convert electrical
energy into mechanical energy, sound waves, and they can
convert mechanical energy returning sound waves back into
(15:34):
electrical energy. It works both ways.
When the ultrasound machine applies A voltage pulse across
these crystals, they physically deform or vibrate very rapidly
at a specific frequency. That visibration pushes on the
surrounding tissue, creating thepulse of ultrasound waves that
travels into the body. OK, so voltage in, sound wave
(15:55):
out. Yep.
Then the transducer waits and listens.
When echo sound waves reflected back from tissues inside the
body hit the transducer, they make those same piezoelectric
crystals vibrate again. And that vibration creates.
A small electrical voltage or signal.
The ultrasound machine detects this silk roll, amplifies it,
and processes it to form the image.
So the transducer is both the speaker and a microphone
(16:17):
essentially. OK, so it sends a pulse.
It listens for echoes. What determines whether an echo
comes back? It all comes down to a property
of the tissue called acoustic impedance, usually denoted by
the letter Z. Acoustic impedance, Yeah.
What? What is that?
It's basically a measure of how much resistance a tissue
presents to the passage of soundwaves.
Think of it like how hard or easy it is for sound to travel
(16:38):
through it. It's calculated as the product
of the tissues density row and the speed of sound within that
tissue V, so ZE qv. And different tissues have
different densities and sound speeds.
Yes, the speed of sound varies quite a bit.
It's relatively slow in air, around 330 meters per second.
It's around 15140 meters in mostsoft tissues, which is the
(17:00):
average value of the machine Assumes it's faster and denser
materials like bone. Maybe around 4000 meters.
So different tissues have different Z values.
How does that create an echo? An echo is generated whenever
the ultrasound pulse encounters A boundary and interface between
two tissues that have different acoustic impedances.
Let's say tissue one has a vetina Z and tissue 2 has Z OK.
(17:22):
When the sound wave hit that boundary, part of its energy is
reflected back towards the transducer as an echo, and the
rest is transmitted deeper into tissue 2.
And does the strength of the echo depend on how different Z&Z
are? Exactly.
The bigger the difference in acoustic impedance between the
two tissues, the stronger the reflection and the brighter that
interface will appear on the ultrasound image.
(17:43):
Ah okay, so if 2 tissues have very similar Z values.
You get a very weak echo or maybe none at all.
Boundary will be hard to see but.
If the difference is huge, like between soft tissue and bone, or
soft tissue and air. Then the impedance mismatch is
massive. Air has extremely low impedance,
Bone has very high impedance compared to soft tissue.
(18:04):
So with those interfaces you getalmost total reflection of the
sound wave. Which means a very bright echo.
A very bright echo, yes, but also critically, very little
sound energy gets transmitted past that interface.
That's why you can't really image things behind bone or air.
Precisely. The sound just bounces right
back from the surface of the bone or the air pocket.
You get acoustic shadowing behind bone and you just can't
(18:26):
penetrate air filled structures like the lungs or bowel gas
effectively. That's a fundamental limitation.
OK, so impedance differences cause reflections.
Does anything else happen to thesound wave as it travels?
Yes, it also gets weaker due to attenuation.
Attenuation is the gradual loss of sound energy as it propagates
through tissue. Why does it lose energy?
(18:47):
2 main reasons. Absorption, where the sound
energy is converted into heat within the tissue, and
scattering, where the found waves bounce off tiny structures
within the tissue in various directions so less energy
continues straight ahead. And does Tenuation depend on the
sound wave itself? Absolutely, and this is probably
the most important practical concept in ultrasound physics.
(19:07):
Attenuation increases significantly with increasing
ultrasound frequency. OK.
Higher frequency means higher attenuation.
What does that mean for imaging?It leads to a fundamental trade
off, a direct conflict between image resolution and how deep
you can see into the body the penetration depth.
Or break that down for me. Let's say I use a high frequency
transducer like the 10 or 12 megahertz.
(19:28):
OK, high frequency. That means the wavelength is
short. Short wavelength is good for
resolution. Specifically, it gives you
better axial resolution. Axial resolution.
What's that? That's your ability to
distinguish between two small objects.
They're located close together along the direction the sound
beam is traveling, one behind the other.
Shorter wavelength means you canresolve finer details Along that
(19:50):
axis. You get a sharper, clearer
image. OK, high frequency, high
resolution sounds great. What's the catch?
The catch is that high attenuation because attenuation
is so much higher at 12 millihertz compared to say, 3
millihertz. Those high frequency sound waves
get absorbed and scattered very quickly.
They lose their energy rapidly. So they can't travel very far
(20:11):
into the tissue. Exactly.
They don't penetrate deep enoughto create usable echoes from
structures far below the surface, so high frequency
transducers have limited penetration depth.
OK, so high frequency gives great detail, but only for
things close to the surface. Perfect.
That's why you use high frequency pros, maybe 7 to 15
megahertz for imaging superficial structures like the
(20:31):
thyroid, breath lumps tests, superficial nodes, good skin
lesions. You need that detail and
penetration isn't an issue. All right, now, what if I need
to see something deep like the liver or the kidneys in the
abdomen or the uterus? Then you have to use a lower
frequency transducer, maybe in the 2 to 5 megahertz range.
OK, low frequency, what happens now?
Low frequency means a longer wavelength.
(20:52):
Longer wavelength means worse axial resolution.
Your image won't be as sharp, the details won't be as fine.
That's the. Downside What's the upside?
The upside is much lower attenuation.
Because attenuation is less at lower frequencies, the sound
waves can travel much deeper into the body before they become
too weak. O low frequency transducers give
(21:13):
you greater enetration death. So you sacrifice some resolution
to gain the ability to see deeper structures.
That's the trade off exactly. You have to use lower
frequencies for abdominal imaging, pelvic imaging,
obstetrics. You need that penetration.
OK, so let's try to summarize that trade off.
It's crucial. High frequency equals high
resolution but low penetration. And low frequency equals low
(21:34):
resolution but high penetration.Got it.
That's the mantra you have to remember when choosing an
ultrasound probe or evaluating an image.
Did they use the right frequencyfor the job?
OK, that makes sense. So we send a pulse, it reflects
off interfaces based on impedance differences, and we
choose the frequency based on depth versus resolution needed.
(21:56):
How does the machine actually build the picture?
It uses the pulse echo principle.
Send out a short pulse, then listen for returning echoes.
And how does it know how deep the echo came from?
Based on timing, it measures thetime it takes for the echo to
return to the transducer after the pulse was sent out.
Since sound travels at a relatively constant speed and
(22:16):
soft tissue that assumed 15140 meters per second, a longer
return time means the echo came from a deeper structure.
Simple geometry. Basically, time makes distance
speed. Yep, the machine calculates the
depth, distance, speed, time too, because it's a round trip
for every echo it receives. And then it displays this.
How does it make the grayscale image we usually see?
That's called B mode or brightness mode.
(22:38):
It's the standard anatomical image.
For each echo received, the machine calculates its depth
based on time time and determines its strength or
amplitude. And the altitude determines the
brightness. Exactly.
The stronger the echo, meaning abigger impedance difference at
the interface, the brighter the pixel corresponding to that
depth will be on the screen. So really strong reflections
(22:58):
like from bone or maybe gas lookbright white.
Right, those are called hyperechoic tissues that reflect
moderately like solid organs appear as various Shades of
Grey. And what about things that don't
reflect at all, like fluid? If there's no impedance
difference, or if the sound justpasses through without
reflecting, no echo returns fromthat region.
(23:19):
The machine displays those areasas black.
We call those anechoic, like thefluid inside a simple cyst, or
in the bladder, or inside blood vessels.
OK, B mode gives us the anatomical map in grayscale.
Are there other modes? Yes, though B mode is the main
one for anatomical imaging. There's A mode, amplitude mode,
which is mostly historical now. It just showed a graph of echo
(23:39):
amplitude versus depth along a single line.
Then there's M mode or motion mode.
It repeatedly sends pulses alongone line and displays how the
positions of reflecting structures along that line
change over time. It looks like a WAVY line graph.
Where would you use M mode? It's essential in
echocardiography for looking at heart valve motion or measuring
chamber dimensions accurately over the cardiac site.
(24:01):
Got it. Any others?
The other big one is Doppler ultrasound.
This uses the Doppler effect to detect motion, primarily blood
flow. The Doppler effect, like the
pitch change from an ambulance siren.
Exactly the same principle. If the sound waves reflect off
something moving towards the transducer, like red blood
cells, the returning echo has a slightly higher frequency.
(24:23):
If they reflect off something moving away, the echo has a
slightly lower frequency. And the machine can detect this
tiny frequency shift. Yes, and the magnitude of the
frequency shift tells you how fast the blood is moving, and
the sign of the shift higher or lower tells you the direction
relative to the transducer. And this is shown with colors.
Often yes. Color Doppler overlays a color
(24:44):
map onto the B mode image. Typically red means flow towards
the transducer, blue means flow away, and the brightness of the
color indicates the relative velocity.
Gives you a quick map of where blood is flowing.
Are there other Doppler types? Yes, pulsed wave PW and
continuous wave CW Doppler give you more quantitative
information. They produce a spectral
(25:07):
waveform, a graph showing the range of blood velocities
present over time within a specific sample volume, PW or
along the entire beam. CW cardiologists and vascular
techs use this extensively. Sometimes useful in oncology to
assess tumor vascularity, thoughnot a primary RT tool.
OK, quite a range of informationfrom sound waves.
(25:27):
Now let's bring it back home. How do we actually use
ultrasound and radiation therapy?
You mentioned it's not a primaryplanning tool like CT.
Right. Its role in external beam
planning is pretty limited, mainly because of those issues
we discussed, operator dependence, poor penetration
through bone and air, and difficult to getting
quantitative data for dose calculation.
So where is it essential? It's main niche is real time
(25:49):
guidance during specific procedures, leveraging that
ability to see anatomy instantly.
And the classic example is. Prostate bracket therapy.
It's absolutely fundamental there.
We use transrectal ultrasound orTRUS.
OK, explain that workflow. Well, first you do a tree recess
volume study, usually before theimplant day.
The physician uses the probe in the rectum to acquire a series
(26:10):
of transverse images through theprostate.
The planning system reconstructsthese into a 3D model of the
prostate, urethra and rectum. This is used for treatment
planning, figuring out where theradioactive seeds need to go.
And then during the actual seed implant procedure?
During the implant, the Tree US probe stays in place, providing
continuous real time imaging. The physician uses a template
(26:34):
grid attached to the probe. As they insert each needle
carrying the seeds toward the prostate, they watch its path
live on the ultrasound screen. So they can see exactly where
the needle tip is relative to the prostate boundary and the
planned position. Precisely, they guide the needle
to the correct location based onthe plan, deposit the seeds and
withdraw. Repeat dozens of times.
(26:54):
TUS makes that level of accuracypossible in real time.
It's a perfect marriage of imaging and intervention.
That's a great example. Anywhere else US might pop up.
It can sometimes be useful for localizing superficial things,
maybe helping to mark out a small skin lesion before
electron planning or guiding a biopsy of a superficial node,
but these are less common roles compared to TRUS for bracket
(27:17):
therapy. It's not typically used for
daily setup guidance and external beam like CBCT or MV
imaging. Right, because of those
limitations, operator skill needed, can't see through bone
or air well, doesn't give density info.
Exactly its strengths lie in that real time visualization for
procedures, especially in the pelvis or for superficial
structures. OK, That clarifies its role
(27:38):
nicely. Shall we try to crystallize some
key clinical pearls for both petand ultrasound?
Things you absolutely need to remember?
Good idea. High yield points.
Number one for PET. Remember it uses positron
emitters like 18 FFDG. It detects coincident 511 key
annihilation photons flying off 180° apart.
(27:59):
That's the signal. And it images metabolic activity
semi quantified by SUV, almost always fused with CT for
anatomical localization and planning.
Maybe a mnemonic like peat 511 a180° for FDG links the energy
angle and common tracer. Nice OK Pearl #2 for ultrasound.
It uses high frequency sound, way above hearing 20 kHz.
(28:22):
The transducer with its piezoelectric crystals sends
pulses and receives the echoes. And those echoes are generated
at interfaces between tissues with different acoustic
impedance Z, which is density times sound velocity Z equals
this F. Pearl 3 The big ultrasound trade
off. High frequency gives you high
resolution but low penetration. And low frequency gives you low
resolution but high penetration.You must choose the right
(28:45):
frequency for the depth you needto image.
Pearl 4 Ultrasound B mode is thestandard grayscale image.
Brightness depends on echo strength, Depth depends on echo
return time, assuming that constant 1540 meters speed and
tissue. And Pearl Five summing up the RT
rolls. PET CT fusion is a workhorse for
staging and target definition inmany cancers.
Ultrasound's primary role is real time guidance, especially
(29:08):
TRUS for prostate brachytherapy.Less use for routine EBRT
planning or delivery. Got it.
Those are excellent summaries. Maybe let's try a quick clinical
scenario, sort of like a mini case to apply this.
OK, imagine you have a patient who finished chemeradiation for
say, locally advanced head and neck cancer a few months ago.
On their follow up CT scan, there's some fullness or maybe
(29:30):
subtle enhancement in one area of the treated neck, but it's
hard to tell if it's just post treatment inflammation, scar
tissue, or if it's residual or recurrent tumor.
What imaging study would be mosthelpful, and what's the physics
principle you're relying on? Yeah, that's a super common and
often difficult clinical question.
The go to study here would almost certainly be a PET CT
scan. And why Pet CT.
(29:51):
Because you need that functionalinformation Pet provides.
Both scar tissue and active tumor might look abnormal on CT,
but active tumor cells are typically highly metabolic and
will avidly take up FDG, showingup as a hotspot with a high SUV
on the PET scan. Scar tissue or mature fibrosis
should be metabolically quiet with low or no FDG uptake.
(30:14):
So you're fundamentally relying on the difference in glucose
metabolism between viable tumor and inactive post treatment
tissue, which PET visualizes viaFDG uptake.
Exactly. The PET tells you if that
questionable area on CT is metabolically active or not.
The fused CT part, of course, tells you precisely where that
activity, or lack thereof is located anatomically.
(30:35):
It directly addresses the clinical question, is there
active disease here? Perfect illustration.
Understanding the physics, what each modality actually shows
you, lets you choose the right test.
OK, ready for a board blitz? Let's do it.
Fire away. Question one.
The two photons detected in coincidence by a PET scanner
originate from which physical process?
A Compton scattering of primary gamma rays from the tracer.
(30:56):
B Pair production caused by highenergy beta particles.
C Annihilation of a positron emitted by the tracer with an
electron. D Bremstrawing radiation
produced as positron slow down. OK, we covered this positron
emitted travels short distance meets electron, they annihilate,
producing the two photons. So that's answer C.
(31:17):
Correct. C annihilation of a positron
emitted by the tracer with an electron.
That annihilation is the source of the coincident 511 key V
photons, PET. Detects OK next.
Question 2A Physician wants to use ultrasound to get a high
resolution image of a superficial thyroid nodule.
Which transducer frequency wouldbe most appropriate?
A 3 megahertz B5 megahertz, C7 megahertz, D12 megahertz.
(31:40):
Superficial thyroid nodule keywords superficial high
resolution that points directly to high frequency.
Of the choices, 12 megahertz is the highest, so D.
Correct D12 megahertz. For superficial structures where
penetration isn't the main concern, you use the highest
available frequency to maximize spatial resolution.
Makes sense. Keep them coming.
Question 3 The standardized uptake value or SUV calculated
(32:05):
from an FDG PET scan is primarily intended to provide a
semi quantitative measure of a tissue electron density.
BT2 relaxation time. C regional blood flow.
D glucose metabolic activity. SUV from FDG pet.
FDG mimics glucose, so SUV reflects glucose uptake and
metabolism. That's the answer D.
(32:26):
Correct D Glucose metabolic activity SUV attempts to
normalize tracer concentration, which for FDG reflects glucose
metabolism often elevated in tumors.
Right, last one. Last one, question 4.
Ultrasound waves are strongly reflected at an interface
between soft tissue and air, primarily because of the large
difference in a temperature B acoustic impedance, C electrical
(32:46):
conductivity, D shear modulus. Soft tissue to That's a huge
mismatch. We've said echoes happen due to
differences in acoustic impedance.
Z air has very low Z tissue has much higher Z.
So the large difference in Z causes the strong reflection
answer B. Correct again.
B Acoustic impedance. Strong reflections occur at
(33:06):
boundaries with large differences in acoustic
impedance. Z equals density, velocity.
Nicely done. So just to wrap things up then
we've covered PET. It's physics based on positron
annihilation, coincidence, detection of those back-to-back
five on 11 KV photons, how SUV gives us a metabolic snapshot
and it's huge role in staging and target definition using PET
(33:28):
CT. And then ultrasound based on
high frequency sound waves, piezoelectric transducer sending
and receiving echoes generated by acoustic impedance
differences and that crucial trade off between frequency
resolution and penetration. And we've seen how they fit into
our world. PET CT is really integral to
modern planning for many sites, while ultrasound shines in real
time procedural guidance, especially for prostate bracket
(33:49):
therapy, even if it has limitations for routine external
beam work. Absolutely.
And really understanding the whybehind these images, the physic
principles, it's just so fundamental.
It helps you interpret them correctly, understand their
strengths, know their limitations, and ultimately use
them in the safest and most effective way for patient care.
Couldn't agree more. And with that, well, that's a
(34:10):
wrap on our main Radonk Smart Review physics series.
We've gone from basic interactions all the way through
machines planning Brackie safetyand now finishing up imaging.
It's been quite a journey. It really has.
Hopefully it provides a solid foundation or a good review for
everyone listening though. You know the joke, right?
Which one? They say the hardest part about
(34:30):
radiation physics isn't learningit for the boards, it's
remembering any of it six monthsafter the boards.
That is painfully true. You'll be sitting in clinic
trying to remember was that annihilation or pair production?
Was it 180° or 90? Did SUV stand for standard
uptake value or seriously unsurevalue?
(34:51):
Exactly. Hopefully revisiting these
concepts helps make them stick abit better.
Repetition and clinical application are key.
Definitely. Well, for more practice,
especially gearing up for orals,you can find
completepracticeoralboards@radonsmartlearn.com.And keep an eye out for future
Radon Smart review sessions covering different areas of
oncology. Thanks so much for joining us
throughout this physics series.
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