First Year Exam: Imaging Flashcards
How are DRRs produced?
- Use a virtual source position
- Ray trace lines projected through CT onto a virtual imager plan (at same distance as a real imager)
- Sum attenuation coefficients in ray path through CT
Which is smaller and more concentrated, fan beam or cone beam?
Fan beams are smaller
What is used for imaging in CT and Tomo, fan beam of cone beam?
Fan beams
What is the major difference between a Cone beam vs a fan beam?
Cone beam does a larger scan area at once, and typically only requires one rotation since you’re scanning everything in one go
What are the main uses of PET scans in radiation therapy? (three uses)
- To provide functional images that can differentiate between malignant tumors and surrounding normal tissues AND benign tumors for contouring.
- Can also be used for clinical staging in certain sitauations.
- Follow changes in tumor over time
Note: Normal tissues and benign tumors appear relatively similar. Malignant tumors only appear different because of increased metabolic acitivity
Between PET and CT, which has the better spatial resolution?
CT
What are the three limitations of PET spatial resolution? Which of the three is the main limiting constraint?
- Main constraint is the energy of the emitted positron (higher energy means it travels more, so it’s harder to narrow down where the initial positron forming even occured)
- Deviation from 180 degree annihilation photon orientation (usually annihilation photons orient 180 +- 0.25 degrees)
- Detector thicknesses
What is the typically spatial resolution of a PET scan?
6 - 10 mm
What does it mean to say that PET and CT are complementary of each other?
They cover each others inherent weaknesses
In what 3 ways are PET and CT complementary?
- PET gives functional physiological information that CT lacks
- When fused onto a CT, a PET scan is given the correlative anatomy that it originally lacked
- Having a CT with a PET image allows you to apply attenuation correction factors on the PET image
True or False
The majority of PET scanners are PET/CT scanners?
True
Typically PET by itself is pretty useless. You need correlative anatomy to make sense of a PET scan, which is why you use a PET/CT scanner to make fusion of the PET to the CT much simpler
How do you fuse a PET image to a CT sim image?
This is a trick question
PET images are usually PET/CT. The PET is already fused onto its CT.
You then fuse the CT from the PET/CT to the CT sim, and that also brings along the PET with it
What is the most common radiopharmaceutical for PET scans?
FDG
What is FDG?
A F-18 isotope with a glucose molecule analog
Why is FDG good for spotting malignant tumors?
FDG, since it’s a glucose analog, will behave like glucose
Some tumors are highly metabolic to the point where they are able to distinguish themselves vs the surrounding tissue metabolically. Also tumors have a hard time breaking down glucose molecule analogs, so the FDG will be trapped in cancerous cells for longer
In what situations does using FDG fail? (2)
- When a tumor is masked by proximity to structures that normally have very high glucose uptake
- When a cancer does not readily uptake FDG
In terms of physics, biology and practicality, why is FDG the most commonly used tracer? (4 reasons)
- It has a short half-life
- It is easily biologically binded to glucose
- Cheap to produce F-18
- Has a low positron emission energy (allows for better spatial resolution)
What is the half-life of F-18?
109.8 mins
What is the approximate typical range in water of F-18 positron? What is the max range?
1-2 mm typically
Max range of 2.2 mm
Why do annihilation photons not get emitted exactly 180 degrees from each other? Why is it 180 +- 0.25 degrees?
Because when the positron annihilates with a medium electron, there is some inherent remaining kinetic energy right before the annihilation
Conservation laws dictate the photons can’t be perfectly 180 degrees, unless the kinetic energy before interaction is exactly 0, which is often isn’t.
What is the HVL in tissue of an annihilation photon?
7 cm
Why is it important to apply attenuation corrections to a PET scan?
Because the annihilation photons have a relatively small HVL in tissue
The annihilation photons from the periphery of a patient are much more likely to be counted than those from the center
Thus, if you don’t correct for attenuation, then the peripheral counts are gonna be skewed up relatively to the central counts
What is the time window for two photons to interact with the detector in order for a count to be considered?
20 ns
What is the normally used computation algorithm for PET scans?
Filtered back-projection
What are the three types of detection events in PET?
- True events
- Scatter events
- Random events
How does a true event occur?
Two annihilation photons undergo no scattering. Both make it tot he ring detector within the time window
How does a scatter event occur?
One or both annihilation photons undergo a scatter, resulting in a misplacement of the calculated origin point relative to the real origin point.
How does a random event occur?
Two simultaneous annihilations occur independent of each other. One photon from one annihilation happens to hit the detector at the same time as another photon from another annihilation
What artifacts can be present in a PET scan? (two types)
- Motion artifacts (results in blurring of uptake value)
- Artifacts in the CT scan that yield incorrect attenuation corrections in PET, which then yields artifacts in PET
Note: PET itself actually doesn’t produce any artifacts. it’s motion and the CT that make all the artifacts
What thickness of lead is typically needed to shield for PET?
2 cm
What is the typical patient dose per PET scan?
25 mSv
Between CT and Ultrasound, which of the following…
Has better image quality:
Is more clinically reliable:
Does not involve ionizing radiation:
Is cheaper:
Has better image quality: CT
Is more clinically reliable: CT
Does not involve ionizing radiation: Ultrasound
Is cheaper: Ultrasound
When do we use ultrasound in radiation therapy?
Localization of malignancy-prone structures in the lower pelvis, retroperitoneum, upper abdomen, breast, and chest wall
Used in localization of prostate
What is the most common application of ultrasound in radiation therapy?
Ultrasound guided prostate implants
Why is ultrasound not as clinically reliable as CT?
Because it’s operator dependent. CT isn’t.
What are the typical frequencies of ultrasound waves?
1 - 20 MHz
What is the signal that is registered in an ultrasound image?
Reflected sound
How do reflections occur?
Variations in acoustic impedance within tissue
Fill in the blank…
The ________ the difference in impedance, the greater the magnitude in reflected signal
The ___larger___ the difference in impedance, the greater the magnitude in reflected signal
How does total reflection occur. What is the downside of this?
Occurs when the impedance differences are drastically different. Ex. Air-tissue, tisuse-bone, chest wall-lung
Downside: you can’t image anything beyond a total reflection surface
How do you get an image in uniform soft tissue?
The impedance has relatively small differences, but it’s just enough to still give enough reflections
Why is it very difficult to image with ultrasound beyond a bone surface?
Because bone heavily attenuates any beams
What types of tissues are very good transmitters of ultrasound energy?
Water, blood, fat, muscle
What is the most commonly used ultrasound mode in radiotherapy?
B mode
How does B mode ultrasound work? This is, in general, how pretty much all ultrasound works.
- Send out a fan beam
- Get reflections back to transducer at certain times
- Distance is correlated directly with time
- Intensity of reflected signal determines displayed brightness
What is the role of a piezoelectric crystal? Aka, what does the piezoelectric effect do?
It’s an effect in which electrical energy is converted to ultrasound energy, and vice versa
Where is the piezoelectric crystal found?
In the probe/transducer
How does ultrasound resolution change from low frequency to high?
Resolution increases as frequency increases
How does ultrasound penetration change from low frequency to high?
Penetration decreases as frequency increases
What is the approximation attenuation coefficient of an ultrasound beam with respect to frequency?
0.5 F(Mhz)*dB/cm
At what point in the ultrasound beam is resolution at its best?
At the focal point (between the near fresnel zone and the far zone)
Which is the near field name? Which is the far field?
Near field - Fresnel
Far field - Fraunhoffer
How does the location of the focal spot in ultrasound move as you increase frequency?
The focal spot gets closer to the probe as frequency increases
In general, how does resolution change going from lower frequency to higher frequency when you’re near vs further from the probe?
Near probe: higher frequency is better
Further from probe: lower frequency is better
How do you choose frequency in ultrasound imaging?
Based off of where the thing you want to image is
If it’s closer to the probe, you want a higher frequency that way the focal spot gets near the object
If it’s further from the probe, you want a lower frequency for better penetration and to move your focal spot towards the further object
In the near field, how does image quality change as you move to greater depths?
Image quality improves moving to greater depths (cause you’re getting closer to the focal spot)
In the far field, how does image quality change as you move to greater depths?
Image quality worsens moving to greater depths
(cause you’re getting further from the focal spot)
What is a dead zone in ultrasound?
It’s a region in which no reflections are measured what so ever. It usually occurs closest to the transducer
What causes a dead zone in ultrasound?
when the piezoelectric crystal rings, there is a brief moment of time for it to settle down
This settling period is an inherent deatime, in which any reflected pulses arriving to the crystal will not be detected
Because of this, the closest reflections will not be measured, since they arrive during the dead time
How is a dead zone reduced in ultrasound?
Using a backing block which reduces the ringing period
What is the speed of sound in tissue?
1540 m/s
How does the speed of sound change depending on medium?
Speed of sound is proportional to density
So denser mediums have faster speed of sound
What is the equation for impedance?
Z = density*speed of sound
How does doppler mode work?
Displays moving objects in red (if they’re moving towards transducer) and blue (if moving away from transducer). This is opposite to stars
Uses the doppler effect
What are some artifacts that can occur in ultrasound (6 total)
- Shadowing/enhancement
- Refraction
- Range distortion
- Side lobes and/or grating lobes
- Ringing
- Reverb
What is a “strong attenuation”
It’s either a dense medium (like bone), or any interface where reflection is so high (such as tissue-lung), that most of the beam is reflected instead of transmitted
What organ has the lowest attenuating property (so you’re likely to see enhancement artifacts past it)
Bladder
What causes a shadowing/enhancement artifact?
Strong or weak attenuators
The effect is seen behind the attenuator
What are refraction artifacts in ultrasound?
They are mispositioned reflection events due to refraction of waves in the medium, originating from different angles than what was assumed
Typically the artifact is pretty subtle
How do you minimize the refraction artifact in ultrasound?
Orient probe orthogonal to surface