CAMRT Review: CT Theory Flashcards
1
Q
Limitations of 2D imaging?
A
- Superimposition of structures
- Difficult to distinguish slight density differences
- Difficult to identify precise location of abnormalities
2
Q
What is a focal plane?
A
Tomography term
“Section thickness”
3
Q
What is the fulcrum?
A
Imaginary pivot point about which the x-ray tube and the IR move
“Dead center”
4
Q
Blur is increased as distance from the focal plane _________?
A
Increases
5
Q
Increased tomographic angle = _______ section thickness
A
Decreased
6
Q
Advantages of tomography over general radiography?
A
- Increased radiographic contrast
- Increased subject contrast
- Decreased superimposition
7
Q
Disadvantages of tomography over general radiography?
A
-Increased patient dose
8
Q
Basic principles of CT?: why do we use it over general radiography?
A
- minimize superimposition
- improve contrast
9
Q
Primary disadvantages of CT?
A
- Increased radiation dose
- Artifacts
- Decreased spatial resolution
10
Q
What is the scan point on the table?
A
-Can use to determine the location of a pathology
11
Q
What is the scannable range of the table?
A
-How much area can be scanned without having to move the patient
12
Q
What components are housed within the gantry?
A
- Tube
- Detector array
- Generator
- Filtration
- Collimators
- DAS
13
Q
What is the aperture of the gantry? What is the isocenter?
A
The hole in the gantry that the patient moves through. The isocenter is the center of that hole
14
Q
What is a CT x-ray tube designed for?
A
To dissipate heat
15
Q
Is a glass or metal envelope better? Why?
A
Metal
- prevents arcing
- increases current
- increases heat dissipation
16
Q
What is different about CT anodes? What are they made out of?
A
- Larger
- Thicker
- Smaller target angle
- High rotation speeds
- Rhenium, Tungsten, and Molybdenum
- Graphite base
17
Q
Types of filtration?
A
- Added
- Inherent
18
Q
What does filtration do?
A
- Hardens beam: removes longer wave-length x-rays
- Beam uniformity: more homogeneous
- Decreases patient dose
19
Q
Types of collimation schemes?
A
- Source: before patient, dose profile
- Post-patient: keeps beam a slice not a fan
20
Q
Hounsfield Units of water, air, bone, and metal?
A
Air: -1000
Water: 0
Bone: 1000
Metal: 2000
21
Q
Why does CT use high kVps?
A
- Decrease attenuation on coefficient : more penetration
- Decrease contrast of bone to soft tissue: more scatter
- Increase radiation flux at detector : more radiation to detector
22
Q
2 types of gantry geometries?
A
- Continuous: detectors and tube rotate
2. Stationary: detectors stationary, tube rotates
23
Q
What are scout images used for?
A
- Planning the scan
- not considered data acquisition
24
Q
2 methods of data acquisition?
A
- Axial
2. Helical
25
Another name for axial scans?
Conventional/serial
26
Advantages of axial scanning?
- Slices perpendicular to patient
- Acquisition can vary
- Highest image quality
27
3 ways data can be acquired during an axial scan?
1. Contiguous
2. Gapped
3. Overlapped
28
Disadvantages of axial scans?
- Increased exam time
- Scan delay: not good for scanning contrast filled vessels
- Increased likelihood of motion artifacts
29
What enables helical scans?
Slip ring technology
30
Advantages of helical scanning?
- Decreased scan time
- Scans a volume of tissue
- Reduces misregistration
- More reformatting and reconstruction
- Less contrast required
31
What type of data does image reconstruction use?
Raw data
32
What type of data does image reformatting use?
Image data
33
Disadvantage of helical scanning?
-Image quality is compromised: missing info: interpolation
34
4 types of data?
1. Scan data
2. Raw data
3. Filtered raw data
4. Image data
35
What is scan data?
Data that arise from the detectors
- require preprocessing corrections before the image reconstruction phase can occur
- prevents poor image quality
36
What is raw data?
```
Preprocessed measurement (scan data) data
-can be stored and retrieved as needed
```
37
When does raw data turn into image data?
When it is reconstructed using algorithms
38
Reconstruction algorithms? (4)
1. Back projection: data gets smeared
2. Filtered back projection: removes blurring from smearing “convolved data”
3. Fourier transform: used to reconstruct MRI
4. Iterative Reconstruction: think automatic rescaling “makes image look better electronically”
39
Image reconstruction algorithms? (Changes the way raw data is manipulated to create image data)
1. Standard: balance noise and detail
2. Smoothing: soft tissue visualization, but decreases spatial res.
3. Edge Enhancement: improved detail, but decreases contrast resolution
40
How does a CT image appear on a computer? How does it go from raw data to be displayed?
-Analog data is first converted to digital to assign HU, then it is converted back so that we can see it as tissue not numbers
41
Reformatting improved with _______ (thinner/thicker) slices?
Thinner
42
When do stair step artifacts occur?
When thick slices are used for reformatting
43
How is slice thickness controlled in SDCT?
It is determined by pre-patient collimators, can only be as wide as the detector, but can be narrower
44
How is slice thickness determined for MDCT?
- pre patient collimator width
| - detector configuration
45
What is volume averaging determined by?
Slice thickness
Thicker slice = more averaging
-increases likelihood that structured will be superimposed
46
What is retrospective slice incrementation? (Post-processing). Why is it useful?
Enables the operator to chance the slice center of an image
- no increase in patient dose
- can decrease partial volume effect
- **cannot change slice thickness in SDCT**
47
Retrospective slice incrementation: can you make the slice smaller than the acquisition slice? Why/why not?
No.
| -noise
48
What is pitch?
Ratio of the table movement per 1 gantry rotation to the slice thickness
49
Does pitch change slice thickness?
No, it just changes how much anatomy is in 1 slice
50
Advantages of increased pitch? (Less than 1.5)
- decreased scan time: pt motion, holding breath
- improved imaging of contrast filled vessels
- decreased patient dose
- decreased heat load
- minimal loss of image sharpness
51
Pitch formula: SDCT
Pitch = table movement in 1 gantry rotation / slice thickness
52
Pitch formula: MDCT
Pitch = table movement per 1 gantry rotation / slice thickness x number of slices
53
Anatomy coverage formula: SCDT?
Pitch x total acquisition time x (1/rotation time) x slice thickness
54
Anatomy coverage formula: MDCT?
Pitch x total acquisition time x (1/rotation time) x slice thickness x number of slices
55
Increased matrix = _________ pixel size
Decreased pixel size
56
Decreased pixel size = __________ spatial resolution?
Increased
57
What is a voxel?
Volume of tissue
| -isotropic = same dimensions all around, what we want
58
What is sampling when referring to pixels?
The pixel detects radiation throughout the entire scan and then averages it at the end to display a shade of grey
59
What is bit depth?
The amount of greys that a pixel can show
60
Where should the window level be set at for a scan?
Close to what the HU is of what you want to see
61
Increased WW = __________ contrast
Decreased, long scale contrast
62
When is long scale contrast good?
When there are many different densities to be seen
63
When is short scale contrast good?
When looking at similar densities
64
Decreased WL = __________ brightness
Increased brightness
65
What is the SFOV?
Scan field of view: determines the area within the gantry that the raw data will be acquired from
66
What is the DFOV?
Display field of view: determines how much of the acquired raw data will be used to create an image
67
Does DFOV change pixel size?
Yes
68
Decreased DFOV = _________ spatial resolution?
Increased
69
Is DFOV of magnifying better for spatial resolution?
Decreasing DFOV
70
What is the difference between analog and digital data?
Analog is continuous data
| Digital is one value
71
Why is the table made out of carbon fibre?
- strength
- low absorption
- vibrational properties
72
Advantages of the power injector?
- volume, injection rate, and delay can be preprogrammed
- protocols can be user defined and stored for injection consistency
- injection pressure is monitored enabling administration precision
73
Limitations of the power injector?
- kinked tubing
- high viscosity
- incompatible equipment
74
Miscentering of 3-6cm can result in an increase in dose of _____%?
18-41%
75
Miscentering in elevation by 20-60mm can result in a dose of up to ____%
140%
76
Major function of the DAS?
Data acquisition system: measures the # of photons that strike the detector (measures the electric signal that comes from the detectors and coverts it to digital)
- ADC
- sends signal to computer for recording
77
Detectors operate more consistently with a _________ beam?
Homogenous
78
What do collimators do?
- decrease patient dose
- decrease scatter reaching IR
- controls slice thickness by shaping the x-ray beam
- controls voxel length
79
What do the detectors do?
Measure the x-ray photon energy and convert it into an electric signal
80
Is a long and skinny detector or a short and wide detector better? Why?
Long and skinny because wider ones pick up more scatter
81
Smaller detector = ________ spatial resolution?
Increased
82
Detector characteristics? (4)
1. DQE
2. Stability
3. Fast response time
4. Wide dynamic range
83
What is DQE?
Detective quantum efficiency: how well a detector can capture, absorb and convert x-rays
84
What increases DQE?
- Wider detector : increases surface area
- Decreased spacing
- Composition: high atomic #, high density, high thickness
85
What is detector stability?
How much can you take before you snap
| -how much radiation can it take before it has to be recalculated
86
What is detector response time?
How quickly can a detector detect another photon?
| -ready to detect another photon quickly after receiving one already
87
What is detector dynamic range?
- we want wide
- variety of signals able to be detected
- able to capture a wide range of photon energies and display them in a variety of electric signals
88
Types of detectors?
1. Ionization chamber xenon gas
| 2. Solid-state scintillation
89
How does a xenon gas detector work?
- Tungsten plate in between the chambers attract ions
- The movement of the electrons causes an electric current proportional to the number of photons absorbed by the system
- *Photons come in and hit the xenon gas and lose an electron that is attracted to the negative plate and the now positive ion is attracted to the positive plate
90
Advantages and disadvantages of a xenon gas detector?
```
Advantages?
-less expensive
-highly stable
-no afterglow
Disadvantages?
-needs to be under pressure
-lots of space
-loss of photons at aluminum casing (decreased DQE)
```
91
How does a solid state/scintillation detector work?
-converts x-rays to light to electrical
92
What do scintillation detectors require?
Amplifiers to beef up the signal to get it proportional to the x-ray energy
93
MDCT detector configurations?
1. Matrix array: same size, isotropic
| 2. Adaptive array: anisotropic, different sizes, larger further from center
94
Do thin slices or thick slices take up more storage space?
Thin
95
CT generations?
1st Gen: narrow pencil beam, no detector array, translate-rotate 180 degrees
2nd Gen: narrow fan beam, linear detector array, translate-rotate 180 degrees
3rd Gen: wide fan beam, rotate-rotate 360 degrees, curved detector array
4th Gen: wide fan beam, rotate-fixed 360 degrees, circular single row detector array
5th Gen: fan beam, stationary-stationary, used for cardiac in 1980s, decreased spatial res
6th Gen: dual source, fan beam, rotate-rotate, improved spatial resolution, cardiac imaging
96
Which generation is most susceptible to scatter?
4th because of the circular detector array
97
Other technical applications of CT?
1. Cardiac imaging
2. CT fluoroscopy
3. CT Angiography
4. CT screening
5. CT simulation
98
What is CT simulation?
- used to determine the exact size, location, and shape of a tumor for treatment
- patients skin marked for treatment
99
How do CT simulators differ from conventional CT scanners?
- larger gantry aperture
- flat table top
- software capable of creating a targeted treatment path
100
Can CT simulation machine do conventional CT? Can conventional CT scanners do CT simulation?
CT simulation can do conventional CT, but not the other way around
101
What are the 2 parts that make up radiopharmaceuticals?
1. The drug portion: goes to the source
| 2. The radioactive isotope: so we can see it
102
What is the most common tracer used in nuclear medicine?
Technetium 99M
103
Contraindications for a nuclear medicine scan?
- no barium for 48hrs
- no previous NM tests within the last week
- appropriate prep: fasting, stop meds
- removed all attenuating artifacts (metal)
104
During a NM scan, gamma radiation is emitted from the _______ and captured by the detector?
Patient
105
During a skeletal NM scan, which parts of the body usually have increased uptake?
- bladder
- sternum
- SI joints
106
What is SPECT?
Single Proton Emission Computed Tomography
107
What is PET?
Positron Emission Tomography
108
Advantages of SPECT over PET?
- requires less space and consumables
- no cyclotron required
- operating costs
- no positron emission
109
What is spatial resolution? How is it measured?
The ability of a scanner to display 2 different objects immediately adjacent to one another without blurring them together
-lp/mm
110
Increased spatial frequency = ________ object size?
Decrease
111
Decreased spatial frequency = __________ object size?
Increased
112
What is the modulation transfer function?
The ratio between image accuracy in comparison to the actual object scanned
-scale of 0-1
113
Increased MTF = __________ object size
Increased
114
What is contrast resolution?
2 objects with very similar densities will look different rather than the same
115
Is the relationship of spatial frequency and image size linear?
No
116
What is the uncoupling effect?
Automatic rescaling: overexposures can still look great on digital
117
What is automatic tube current modulation?
-adjusts mAs as you scan the patient
118
Can automatic tube current fix quantum mottle?
No, it can only manipulate what its been given
119
Typical kVp for adults and children?
Adults: 120-140kVp
Children: 80kVp
120
Small filament = _________ penumbra = ______ detail
Small filament = decreased penumbra = increased detail
121
Focal spot size only affect ____________?
Spatial resolution
122
Quantum mottle = _________ contrast?
Decreased
123
Increased SNR = _________ contrast
Increased contrast
124
Increased slice thickness = ________ contrast
Decreased because of more averaging
125
Increased slice thickness = ___________ SNR
Increased because thicker slices have more photons in them
126
Decreased pixel size = ___________ contrast resolution
Decreased because it catches less photons
127
Pixel size formula?
Pixel size = DFOV/matrix size
128
The pixel should be ____ the size of the object being scanned?
1/2
129
3 general appearance of artifacts on CT image?
1. Streaks
2. Ring/band
3. Shading
130
Types of artifacts?
1. Beam hardening
2. Partial Volume
3. Motion
4. Metallic
5. Out-of-field
6. Cone beam
7. Rings and Bands
8. Noise
131
Beam hardening artifact?
- Caused by natural filtration of the x-ray beam by the object
- Shading/streaks/
- select appropriate SFOV or use beam hardening reducing software to avoid
132
Partial volume artifact?
- Caused by more than 1 type of tissue contained in a pixel
- Shading
- Use smaller pixels and thinner slices to avoid
133
Motion artifacts?
- Caused by: voluntary or involuntary motion
- Shading, streaking, or blurring
- Short scan times, sedation, immobilization, communication, software correction and cardiac gating to avoid
134
Metallic artifacts?
- Caused by the density of the metal
- streaks
- Improved HU range, proper changing instructions, high kVp, thinner slices
135
Out of field artifacts?
- Caused by: anatomy that extends out of the SFOV
- streaks/shading
- move arms out of way, increased size of SFOV to avoid
136
Cone beam artifacts?
- Cause by interpolation of data
- streaks/shading near areas with large density differences
- **ONLY IN MDCT***
- use lower pitch, cone beam reconstruction algorithms
137
Ring and Band artifacts?
- appear from malfunctioning or miscalibrated detector elements
- rings/bands
- **unique to 3rd gen CT**
- recalibrate scanner, call service engineer
138
3 types of noise artifacts?
1. Quantum noise: scanner efficiency, patient size
2. Inherent Physical Limitations: electronic noise in the DAS
3. Reconstruction Parameters: high res reconstruction algorithms increase noise
139
How to reduce noise artifacts?
- increased mAs
- low noise DAS
- smoothing algorithms
140
3 main categories of post-processing for CT?
1. Basic functions
2. Retrospective reconstruction
3. Reformatting
141
Type of 2D reformatting?
-MPR: formatting the image data into different planes that the one it was taken in
142
Types of 3D reformats?
- MIP :max intensity projection
- MinIP: minimum intensity
- SR: surface rendering
- VR: volume rendering
143
What is MIP?
Maximum Intensity Projection
- pixels show the highest value detected and eliminate other values
- high attenuation structures visualized
144
What is MinIP?
Minimum Intensity Projection
- pixels show the minimum values detected and eliminate other values
- low attenuating structures
145
What is SR? How does it work?
- predetermined threshold
- if the CT number is within the threshold, it is shown, if not, it is eliminated
- we see the shell of an object
- if threshold is too low: structures can be excluded
- if threshold is too high (large): other materials that we don’t want to see might be included
- only 10% of voxels contribute to image
146
What is VR? How does it work?
- all voxels contribute to image
| - 3D semi-transparent representation of an object
147
What is segmentation?
- ROI editing
- removed or isolates structures on an image for better demonstration of an area of interest
- automatic or manual
148
Forms of contrast media?
- Liquid
- Powder/paste
- Gas
149
Types of contrast we use?
- Air
- C02
- Water
- Barium sulfate
- Iodinated water soluble
150
Purpose of air and gas?
- distention
| - increase contrast
151
Is spastic colon more likely to happen with air or C02?
Air
152
How can air/gas be administered?
- Injection
- Orally
- Rectally
153
Cons of using water as a contrast media?
- fast transit time/absorption
| - poor bowel distention
154
How can water be administered?
-orally
155
Barium sulfate concentration for CT?
1-3%
156
2 forms of barium sulfate?
Liquid: low concentration and viscosity
Paste: high concentration and viscosity
157
Routes of administration of barium sulfate?
- orally
| - rectally
158
What is VoLumen? Why is it used?
- 0.1% barium sulfate solution
- HU: 15-30 (less than mucosal lining)
- resembles water on image, but increases bowel distention, transit time, and visualization of mucosa and bowel wall
159
Iodinated water soluble contrast media routes of administration?
- orally
- rectally
- injection: IV
160
What can we inject intrathecally?
- air
- gas
- iodinated water soluble
161
3 methods of IV administration from least used to most commonly used in CT?
1. Drip infusion
2. Hand bolus
3. Mechanical Injection
162
What vein is preferred when establishing a line in the CT department?
AC vein
163
What is the minimum amount of saline flushed to verify the patency of the line?
10ml
164
Mechanical injector components?
- Warming Device
- Syringes
- Pressure mechanism
- Control panel
165
3 phases in order from first to enhance to last to enhance?
- Arterial (bolus)
- Non-equilibrium (venous)
- Equilibrium (delayed)
166
Peak enhancement of arterial organs occurs ______seconds after the injection?
15-45
167
Which organ requires a scan delay of 4 mins or longer. Why?
The brain because of the blood brain barrier
168
When does each phase occur?
1. Arterial (bolus): immediately after injection, 15-22secs
2. Non-Equilibrium (venous): 1 min after bolus phase
3. Equilibrium (delayed): 2 mins after bolus phase
169
What do we see during the delayed phase?
Organ parenchyma
170
What is the worst phase for liver scans?
Delayed
171
Portal venous scans of the liver should occur ____ seconds after the bolus injection to include the arterial and venous enhancement?
60 seconds
172
Peak enhancement for the kidneys is ______ seconds after the bolus injection because they also excrete contrast?
80-120 seconds
173
The timing of the 3 phases is controlled by what 3 things?
1. Pharmacokinetics
2. Patient factors
3. Equipment
174
Increased volume = ________ magnitude of peak enhancement?
Increased
175
Increased volume = _______ time to peak?
Increased
176
Increased volume = ________ time a given level of enhancement is maintained?
Increased
177
Total volume of contrast formula?
V- (ml/sec)(sec)
178
Optimal imaging occurs at ____HU?
200
179
Increased flow rate = ________ magnitude of peak enhancement?
Increased
180
Increased flow rate = _______ time to peak enhancement?
Decreased
181
Increased flow rate = _________ duration of contrast injection?
Decreased
182
Increased weight = _________ magnitude of peak enhancement?
Decreased
183
Increased weight = __________ time to peak
No effect
184
Increased heart rate = __________ magnitude of peak?
No effect
185
2 automated triggering methods?
1. Test bolus: 10-15 images taken at same anatomic location, gives you the time at which you should start the real scan
2. Bolus triggering: when HU threshold is reached, injection starts
186
The level of risk associated with a CT exam is considered acceptable if?
- the patient is aware of the risk
- the patient receives some type of benefit
- ALARA is maintained to reduce risk
187
What is absorbed dose? What is it measured in?
The amount of energy absorbed per unit mass of material
| -grays (Gy)
188
What is effective dose?
Attempts to account for the effects specific to the patient’s tissue that absorbed the radiation dose
189
2 main classifications of radiation effects?
- Stochastic: no threshold, late effects, cancer, hereditary effects
- Deterministic: threshold, early effects
190
Max dose limit for canadian radiation workers?
20mSv whole body
191
Early- non-stochastic effects? (3)
- skin erythema
- epilation
- pericarditis
192
In CT, which dose is greater: the entrance skin dose, or dose at the center of the patient?
Entrance dose
193
What is the partial shielding effect?
The dose at the periphery of the patient is higher than that of the middle of the patient (organs absorb/block) the radiation as it goes through the patient
194
Are organ doses higher for thinner or thicker patients?
Thinner, because less partial shielding
195
What is over-ranging?
Exposed scan length is greater than planned scan length
| -occurs in helical scans
196
Decreased SFOV = ____________dose?
Increased because smaller patients have a more uniform dose distribution which means a higher total dose (entrance + exit radiation)
197
Decreased slice thickness = _________ patient dose?
Increased because you must increase technique or else there is too much noise
198
3 cardinal rules of radiation protection?
1. Time
2. Distance
3. Shielding
199
CT shielding?
1. In-plane (bismuth): don’t use during scout, in the image
2. Out of plane (lead): must be wrapped 360 deg around patient, reduces scatter
3. Secondary: room walls due to scatter
200
A 1yr old is _____ times more likely than a 50yr old to get cancer when the same dose is used?
6 times
201
What is a DRL? What is their purpose?
Diagnostic Reference Level
- promote better control of patient exposures to x-rays
- identifies acceptable limits for an exam
202
What does dosimetry allow?
-techs to compare their doses with a national average
203
What dosimeter is the easiest method for recording exposure in CT?
Ionization chamber
204
What do dosimetry phantoms measure?
CTDI (computed tomography dose index)
205
3 CT dose measurements that we should be aware of and their units of measurement
1. CTDI: grays (Gy): dose of 1 slice
2. DLP: dose length product: mGy/cm: dose of entire scan
3. ED: effective dose: sieverts (Sv)
206
What dose measurement allows dose to be compared between scanners?
CTDI
207
Does CTDI work for helical scans?
No, we must estimate for helical scans as this only works with contiguous slices
208
Does CTDI include scatter in dose?
Yes
209
Types of CTDI measurements? (4)
1. CTDI(fda): index that relates to # of slices and slice width used
2. CTDI(100): modification to accommodate smaller slice widths
3. CTDI(w): dose in the x-y axis to accommodate dose uniformity
4. CTDI(volume): dose in the z-axis (slice thickness)
210
CTDI volume formula?
CTDI(volume) = CTDI(w)/pitch
211
DLP formula?
DLP = CTDI(volume) x scan length
212
______ (dose measurement) is directly proportional to scan length?
DLP
213
ED formula?
ED = DLP x K (factor always given)
