First Year Exam: Imaging Flashcards
1
Q
How are DRRs produced?
A
- 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
2
Q
Which is smaller and more concentrated, fan beam or cone beam?
A
Fan beams are smaller
3
Q
What is used for imaging in CT and Tomo, fan beam of cone beam?
A
Fan beams
4
Q
What is the major difference between a Cone beam vs a fan beam?
A
Cone beam does a larger scan area at once, and typically only requires one rotation since you’re scanning everything in one go
5
Q
What are the main uses of PET scans in radiation therapy? (three uses)
A
- 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
6
Q
Between PET and CT, which has the better spatial resolution?
A
CT
7
Q
What are the three limitations of PET spatial resolution? Which of the three is the main limiting constraint?
A
- 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
8
Q
What is the typically spatial resolution of a PET scan?
A
6 - 10 mm
9
Q
What does it mean to say that PET and CT are complementary of each other?
A
They cover each others inherent weaknesses
10
Q
In what 3 ways are PET and CT complementary?
A
- 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
11
Q
True or False
The majority of PET scanners are PET/CT scanners?
A
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
12
Q
How do you fuse a PET image to a CT sim image?
A
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
13
Q
What is the most common radiopharmaceutical for PET scans?
A
FDG
14
Q
What is FDG?
A
A F-18 isotope with a glucose molecule analog
15
Q
Why is FDG good for spotting malignant tumors?
A
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
16
Q
In what situations does using FDG fail? (2)
A
- When a tumor is masked by proximity to structures that normally have very high glucose uptake
- When a cancer does not readily uptake FDG
17
Q
In terms of physics, biology and practicality, why is FDG the most commonly used tracer? (4 reasons)
A
- 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)
18
Q
What is the half-life of F-18?
A
109.8 mins
19
Q
What is the approximate typical range in water of F-18 positron? What is the max range?
A
1-2 mm typically
Max range of 2.2 mm
20
Q
Why do annihilation photons not get emitted exactly 180 degrees from each other? Why is it 180 +- 0.25 degrees?
A
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.
21
Q
What is the HVL in tissue of an annihilation photon?
A
7 cm
22
Q
Why is it important to apply attenuation corrections to a PET scan?
A
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
23
Q
What is the time window for two photons to interact with the detector in order for a count to be considered?
A
20 ns
24
Q
What is the normally used computation algorithm for PET scans?
A
Filtered back-projection
25
What are the three types of detection events in PET?
1. True events
2. Scatter events
3. Random events
26
How does a true event occur?
Two annihilation photons undergo no scattering. Both make it tot he ring detector within the time window
27
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.
28
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
29
What artifacts can be present in a PET scan? (two types)
1. Motion artifacts (results in blurring of uptake value)
2. 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
30
What thickness of lead is typically needed to shield for PET?
2 cm
31
What is the typical patient dose per PET scan?
25 mSv
32
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
33
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
34
What is the most common application of ultrasound in radiation therapy?
Ultrasound guided prostate implants
35
Why is ultrasound not as clinically reliable as CT?
Because it's operator dependent. CT isn't.
36
What are the typical frequencies of ultrasound waves?
1 - 20 MHz
37
What is the signal that is registered in an ultrasound image?
Reflected sound
38
How do reflections occur?
Variations in acoustic impedance within tissue
39
# 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
40
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
41
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
42
Why is it very difficult to image with ultrasound beyond a bone surface?
Because bone heavily attenuates any beams
43
What types of tissues are very good transmitters of ultrasound energy?
Water, blood, fat, muscle
44
What is the most commonly used ultrasound mode in radiotherapy?
B mode
45
How does B mode ultrasound work? This is, in general, how pretty much all ultrasound works.
1. Send out a fan beam
2. Get reflections back to transducer at certain times
3. Distance is correlated directly with time
4. Intensity of reflected signal determines displayed brightness
46
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
47
Where is the piezoelectric crystal found?
In the probe/transducer
48
How does ultrasound resolution change from low frequency to high?
Resolution increases as frequency increases
49
How does ultrasound penetration change from low frequency to high?
Penetration decreases as frequency increases
50
What is the approximation attenuation coefficient of an ultrasound beam with respect to frequency?
0.5 F(Mhz)\*dB/cm
51
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)
52
Which is the near field name? Which is the far field?
Near field - Fresnel
Far field - Fraunhoffer
53
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
54
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
55
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
56
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)
57
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)
58
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
59
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
60
How is a dead zone reduced in ultrasound?
Using a backing block which reduces the ringing period
61
What is the speed of sound in tissue?
1540 m/s
62
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
63
What is the equation for impedance?
Z = density\*speed of sound
64
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
65
What are some artifacts that can occur in ultrasound (6 total)
1. Shadowing/enhancement
2. Refraction
3. Range distortion
4. Side lobes and/or grating lobes
5. Ringing
6. Reverb
66
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
67
What organ has the lowest attenuating property (so you're likely to see enhancement artifacts past it)
Bladder
68
What causes a shadowing/enhancement artifact?
Strong or weak attenuators
The effect is seen behind the attenuator
69
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
70
How do you minimize the refraction artifact in ultrasound?
Orient probe orthogonal to surface
71
What causes range distortion artifact in ultrasound?
Ultrasound assumes speed of sound of 1540 m/s no matter what
If it's not actually, then it will over or estimate distance of an object
72
What are side lobes/grating lobes? What causes them?
Since the ultrasound probe can't always send out a pencil beam, towards the edges of fields, if you run into a strong reflector of what should be outside the field, then it would be imaged anyway
Appears as a sort of phantom looking thing towards edges
73
What causes a ringing artifact?
Strong and multiple reflections back and forth between the transducer and a strong reflector (these occur right next to the transducer)
74
What causes a reverb artifact?
Strong and multiple reflections back and forth between two strong reflectors, NOT the transducer
75
How can you limit reverb artifact?
Slightly tilt the probe
76
How is contrast used to image vasculature in ultrasound?
Use of microbubbles (tiny air bubbles injected into the blood stream which cause interfaces of large impedance differences in the blood stream)
77
True or False
Snell's law is only applicable in the far field. In the near field, there's some slight variation from Snell's law.
False
It's the opposite. This is what's actually true...
Snell's law is only applicable in the near field. In the far field, there's some slight variation from Snell's law.
78
What is the difference in imaging plane preference between CT and MRI?
CT exclusively reconstructs and measures images on the transverse plane, so the optimal plane is always transverse/axial. And can use info on the transverse to create the other two planes. But they don't look as good
MRI can select which viewing plane to make optimal and measure on, and the other two will just be sub-optimal reconstructed from the optimal plane.
79
Between MRI and CT, which...
Has better contrast:
Have better soft tissue imaging:
Has better spatial resolution:
Images bones or calcifications better:
Has shorter treatment times:
Has better contrast: MRI
Have better soft tissue imaging: MRI
Has better spatial resolution: CT
Images bones or calcifications better: CT
Has shorter treatment times: CT
80
What is the gyromagnetic ratio for protons?
42.58 MHz/T
81
What is the equation for the Larmor frequency?
Larmor frequency = gyromagnetic ratio x magnetic field strength
82
Conceptually, what is a magnetic dipole moment?
A property that gives a measure of how quickly a magnetic will align itself to an external magnetic field
83
What happens when you induce an alternating RF field to a patient during an MRI?
Any nuclei that were processing at a Larmor Frequency = to the RF pulse frequency sent, will align themselves perpendicular to the magnetic field, then start relaxing back to parallel/antiparallel after the RF pulse ends
84
How is signal generated in an MRI?
1. Apply a magnetic field
2. Send RF pulse that excites some of the nuclei
3. As nuclei relax back to the magnetic field, they emit a signal to a receiving RF coil (tuned to the Larmor Frequency)
85
What does T2 (decay) tell you?
How many nuclei are still in the transverse position after the RF field is turned off
86
What does T1 (growth) tell you?
How many nuclei are returning to the original orientation of the magnetic field after RF field is turned off
87
Which nuclei can be used for signal in MRI?
Technically, any nucleus with a nonzero spin or angular momentum
But certain nuclei are better and give more signal than others
And usually the best candidate nuclei for MRI are those with unpaired protons
88
Why is hydrogen the best element for MRI imaging?
Lots of it in the body, and high intrinsic sensitivity (hydrogen has an unpaired proton)
89
What is the difference, between the number of parallel vs antiparallel protons when a magnetic field is applied?
3 ppm/T in favor of parallel
90
How many hydrogen atoms are there on average in a single voxel?
10^21
91
What affect does increasing magnetic field strength have on signal?
Proportionally increases signal
92
How is contrast selection done in MRI?
Vary the TR and TE accordingly to get max difference in decay and relaxations of certain material
93
How is slice thickness determined in a MRI?
Strength of magnetic field gradient and bandwidth
94
How is slice thickness related to...
Strength of magnetic field gradient:
Bandwidth:
Slice thickness is inversely proportional to magnetic field gradient strength (greater gradient = thinner slices)
Slice thickness is directly proportional to bandwidth (greater bandwidth, thicker slices (you sample more adjacent slices))
95
What are the steps to get a voxel by voxel image in MRI?
1. Apply a magnetic field gradient along one of the 3 axis using your gradient RF coils
2. Apply RF signal to whatever slice has that resonance frequency window, then next slice, then next, then next, etc.
3. Then further phase encode (this utilizes a different axis gradient RF coils)
4. Then frequency encode (turn on another axis gradient RF coil to select voxel by voxel)
96
What are the encoding steps to select a voxel?
slice select (z gradient coils) --\> phase encode (y gradient coils) --\> frequency encode (x gradient coils)
97
What are the 8 steps to a basic spin-echo sequence?
(Don't focus too hard on knowing all this. Just review it)
1. Apply slice select gradient and transmit RF pulse
2. Apply phase encoding gradient
3. apply frequency encoding gradient (you now have the voxel figured out)
4. Fourier transform the received signal
5. Repeat steps 1-4 and average
6. Repeat steps 1-5 with new phase encoding gradients (sweeping your entire y direction in the slice)
7. Take inverse fourier-transform of data from steps 1-6
8. Repeat steps 1-7 with new slice select gradient
So essentially....
pick slice with z gradient --\> make y gradient --\> go voxel by voxel using x gradient --\> transform raw signal into data --\> go to next slice --\> repeat until whole body is scanned
98
How does fat, CSF and water appear in a T1 image?
Fat is bright
CSF and water are dark
99
How do fat, white matter, water, CSF and bone appear in a T2 image?
CSF and water are bright
Fat, white matter and bone are dark
100
How do bone, CSF and fat appear in a PD image?
Anything with lots of hydrogen concentration is bright... so...
CSF and fat are bright
Bone is dark
101
What is T1 used for in RT?
Separating tumor from surrounding edema (since water is suppressed)
This is the go-to for drawing the GTV in RT
102
What is T2 used for in RT?
Contouring edema surrounding brain tumors
This is the go-to for drawing the CTV in RT
103
What are FLAIR sequences good for?
Suppressing all fluids
Good for drawing GTV when a lot of edema is present
104
What are diffusion weighted sequences good for?
Imaging stroke
105
What are fMRI good for?
Locating what parts of the brain are responsible for certain tasks
106
What is the most commonly used contrast for MRI? How does it work?
Gadolinium
Shortens T1. if a tumor takes up Gadolinium, it shows as bright on a T1 weighted image.
107
What are the two most important to know artifacts in MRI?
Metallic objects and chemical shift
108
How does a chemical shift artifact work in MRI?
Protons in two different tissues may have different enough frequencies that they could be placed in incorrect voxels despite being right next to each other
This is the case of the border between fat and water
This shift is exaggerated for greater magnetic field strength
The artifact is only seen at interfaces of tissues
109
What is the difference between an axial/helical vs a slice by slice CT scan?
Axial/helical the patient is moving longitudinally through the bore while data is being acquired
Slice by slice the patient moves, stops, is scanned, moves, stops, is scanned, etc.
110
Which modality has a thicker set of detectors and less focused beam?
Axial/helical
Because you need to scan slices multiple times. Typically you need to get the same slice on 3 different rotations in order to compensate for the less data per rotation per slice compared to slice by slice
111
How are most modern CTs designed?
Helical scanning
Slip-ring technology
Multidetector array (4-320 channels of spatial data)
No wobble artifact
Bowtie filter to equalize image noise
Wide fan beam
Solid-state scintillator detector array
112
What is the concept of "One HU"
One HU is a change of 0.1% in the attenuation coefficient of water
113
What does a HU curve plot?
HU number vs compton electron density
114
Why is an HU curve non-linear?
In the lower HU's it is linear, because at your scanning energy Compton dominates. So the CT number (attenuation) is proportional to electron density
In higher HU's, there in non-linearity because at your scanning energies, photoelectric begins to dominate again (cause Z gets higher)
115
What phantom is used to get an HU curve?
Gammex tissue Characterization Phantom
116
What two things effect the HU curve measured using the Gammex phantom?
Location of plugs and energy being used
Note: if you switch out plugs and move them around, their measure HU values can actually change by a significant amount (several percent)
117
Why does location in scan play a role in measured HU value?
The effective energy changes radially due to scattering
118
What are the differences between HU curves for changing energy?
In general, from lung - tissue, the plots will be pretty close regardless of energy
But in the denser regions, energy differences play a much larger role due to photoelectric dominance
119
In addition to energy and plug location, what else is an HU curve dependent to?
Your specific machine
Different CTs will give different HU curves even if the setup and kVp are exactly the same
120
Why for CT based heterogeneity corrections do we use electrons per cc instead of electrons per gram?
Because bone has a greater physical density than tissue
121
What is the most commonly used algorithm in CT?
Filtered back projection
122
What is a benefit of iterative reconstruction vs filtered back projection?
Eliminates streaking artifacts
123
What is a major limitation to iterative reconstruction vs filtered backprojection?
Time consuming, guesses at a solution with minimal info, and computationally expensive
124
How does filtered back projection work?
1. Create a sinogram data file that has a series of projections (bunch of rays), at each source angle
2. Take ray projections and back project (trace back) result along ray line
3. Assume the attenuation is even along the entire path that gives you that final ray
4. Do that for a bunch of different angles
5. This gives you a blurry artifact
6. Filtered back pojection gets rid of the blurriness
125
How does iterative reconstruction work?
Computer takes a guess at an approximate solution to the linear attenuationc coefficients of each voxel
Keeps adjusting using an algorithm to more closely match measured data
Keep doing this multiple times until it maximizd a match
It's computationally demanding but most modern computers can do it
It's actually kind of synonymous to statistical optimization from treatment planning
126
Which gives better image quality, FBJ or IR?
Iterative reconstruction
127
# Fill in the blank...
A narrower window width results in ____ contrast
A **narrower** window width results in **greater** contrast
128
What is a "level" parameter?
The CT number at the center of the window
129
What is a pitch?
Pitch = table increment per rotation / beam collimation
130
What is the typical range of pitches in CT?
0.5 - 2.0
131
What does a pitch = 1 mean?
We gather similar information to a contiguous axial scan
132
What does a pitch \< 1 mean?
Implies overlapping slices (higher dose and increased image quality)
133
What does a pitch \> 1 mean?
Implies gaps between slices (reduced dose and decreased image quality)
134
How long do CBCTs take?
1 minute for a full scan
135
Why are CBCTs worse image quality than CT?
Large fields generating much more scatter (you're measuring a lot of slices at once)
136
How many slices do you usually measure at once for CT?
3-5 at any given rotation
137
What are the 5 common CT artifacts? What causes them?
**1. Ring artifact** - bad detector
**2. Beam hardening** - Effective energy changing with attenuation, leads to over-estimation of CT numbers in regions that it occurs
**3. Streaking** - Photon starvation
**4. Motion** - breathing, cardiac, or patient motion
**5. Partial volume averaging** - more than one tissue type in a CT slice becomes averaged into a single voxel, yielding an incorrect CT number
138
How much elad shielding is typically needed for a CT?
1.5 - 2 mm
Doors and windows are all leaded
139
How much dose is delivered during an adult CT? Per scan
50 - 75 mGy
140
CT Dose is proportional to...
kVp2 , mAs, beam quality,
141
Does dose increase or decrease with proper collimation?
Increases
(More collimation means you lose signal)
142
Does dose in CT increase or decrease with greater patient size?
Increases
You need to pump more mAs
143
What are the CTDIW values for head scans and body scans?
Head: 80 mGy
Body: 30 mGy
144
What are the CTDI and DLP equations?
145
What are the contrasts typically used with CT?
Iodine
Barium (most often used in GI studies)
146
What are the benefits to Multi-Energy CT?
Better characterization of amss or electron density and atomic number
Improved contrast detection
Correct beam hardening artifacts
Extrapolating photon attenuation to other energies
Separate materials that have same CT numbers at a single energy
147
What is the purpose of a port film (besides the obvious setup purpose)
To verify treatment volume under actual treatment conditions
148
When we say "port film", what does this actually mean?
In the clinic, a port is done with EPID
But the actual terminology of a port is with film or radiographic imaging which has poor image quality, delayed viewing time, and it's not very practical before each treatment
149
True or False
It's actually required as legal record to have a port fi;m showing field boundaries
True
EPID may be used as the port film
150
What are the pros of using EPID? (4)
Can view image instantly
Images can be stored digitally
Comercially available on almost all LINACS
BEst for imaging target, MLC positions and relative location of OARs
151
What are the cons of EPID, particularly compared to a planar kV? (2)
Relatively low image quality (particularly contrast)
Higher dose than a planar kV
152
What are the pros/uses of kV CBCT? (4 or 5)
Good contrast
Sub mm spatial resolution
Acquire iamges in therapy room coordinates
Re-alignment (IGRT)
Use 2D radiographic modes to verify portal accuracy and manage motion
153
What are the cons to using kV CBCT? (3)
Large discrepancies and poor imaging in non-homogenous media
Highly suceptible to high Z artifacts
Add imaging dose that is difficult to completely account for
154
What are the pros/uses of MV CBCT? (7)
Good image quality for bony anatomy (and sometimes soft tissue)
Can be used for IGRT and position verification
Less susceptible to high Z artifacts
Do not need to extrapolate attenuation coefficients
Handles heterogeneities better than kV CBCT
Easy to account for imaging dose in TPS
In some cases, (Tomo) it is actually easily implemented in the system
155
What are the cons of MV CBCT compared to kV CBCT? (2)
Worse contrast, especially soft tissue, compared to kV CBCT
Dose from an equivalent image quality MV CBCT is s lot higher than kV CBCT (~2-10x higher)
156
What are 4 features of CBCT that are in favor of using CBCT over planar imaging?
1. Better contrast and spatial resolution
2. Better at imaging and aligning soft tissue
3. Allows you to match using sim CT as reference
4. Allows you to image body contour much better than planar
157
What are 6 features of planar imaging that are advantages over CBCT?
1. Planar avoids most artifacts that CBCT is susceptible to (especially high Z and motion)
2. Planar is best for field alignment
3. Planar is best for imaging and alignment to bone
4. Planar is lower dose
5. Planar is faster
6. Planar allows you to match using DRR as reference
158
What is typical 4D CT dose?
100 - 400 mGy
159
What is typical CT sim dose?
30 - 50 mGy
160
Relative to CT pitch, what is the 4DCT pitch?
1/8th the pitch of a CT
Meaning you expect a 4DCT should have roughly 8x more dose than a CT. Which is true.
161
When do you use DAP? When do you use DLP?
DAP is an area integration for entrance skin dose, specifically for planar imaging
DLP is a volume integration for dose, specifically for CT
162
What is approximate HU for titanium?
Around 5000
163
What is the minimum possible HU value?
-1000
This occurs if your attenuation coefficient and compton electron density = 0. Then the equation goes as...
1000\* ((0-uwater) /uwater)
And remember, there is no such thing as a negative attenuation coefficient. So HU can never go below -1000
164
What are all the CTDI equations to go from CTDI measured to DLP?
CTDIw = 1/3CTDIcenter + 2/3CTDIperipheral
CTDIvol = CTDIw/pitch
DLP = CTDIvol\*Irradiated Length
165
What is the maximum corresponding mass density for an unextended vs an extended CT scan?
Unexctended curve: 3 g/cc max
Extended curve: 19.33 g/cc max
166
Up to what HU values do we have an established mass density to material type table?
2.2 g/cc
Beyond this we need to manually assign
167
What is the maximum HU value of a CT? (extended)
~ 30000
168
How long does a PET scan take from very start to very end?
Takes about 30 - 60 minutes for injected tracer to absorb in body
Takes an additional 30 minutes to complete the PET-CT scan (or 45 mins for a PET-MRI)
Total: 1 - 2 hours
169
Which of the following has the least geometric distortion? Which has the most?
CT, Ultrasound, MRI
CT has the least geometric distortion and is often called the **gold standard** for minimal geometric distortion
Ultrasound has the most
170
What causes geometric distortion in MRI?
Magnetic field non-uniformity
Typically the magnetic field is very uniform near isocenter, but as you move away from isocenter the field loses uniformity, so you can start to experience geometric distortions. For very large patients this can be a big planning challenge.
171
Select the following items below that are true regarding collimators in a CT scanner (may be more than one)
A) Located near x-ray tube
B) Reduce scatter hitting detectors
C) Located near the detectors
D) Determine one dimension of pixels
Select the following items below that are true regarding collimators in a CT scanner (may be more than one)
**A) Located near x-ray tube**
**B) Reduce scatter hitting detectors**
C) Located near the detectors
D) Determine one dimension of pixels
172
To improve image quality in MRI, what is something that can be placed on the patient?
A receiver coil specific to the site being imaged
(Below is an example of a receiver coil for brain scans)
173
What are typical doses for...
kV planar:
MV planar:
kV CBCT:
MV CBCT:
What are typical doses for...
kV planar: 0.5 - 1 **mGy**
MV planar: 1-3 **cGy**
kV CBCT: 1 **cGy**
MV CBCT: 15-20 **cGy**
174
What gen of CTs are most CTs that w euse in radiology and radonc?
3rd gen
175
What is the usual width of a fan beam in CT?
1-2 cm
176
What is the typical width of a cone beam?
10 cm
177
What is the benefit to 2D-3D match?
You can simulate x-ray source in 3D. So on the fly, the program can create a new DRR everything you shift
Essentially you can match a 3D image to 2D
