CT Theory Flashcards
1
Q
Limitations of general radiography?
A
- superimposition
- 2D
- difficult to distinguish between slight density changes
- difficult to determine precise location of abnormalities
2
Q
What is tomography?
A
Imaging modality that brings into focus only the anatomical structure lying in a plane of interest, while structures on either side of the plane are blurred
3
Q
What is a focal plane? Fulcrum?
A
- Focal Plane: section thickness
- Fulcrum: center of the center (clear part)
4
Q
Increasing the tomographic angle will _______ the section thickness?
A
Decrease
5
Q
Advantages of tomography over radiography?
A
- increase radiographic contrast
- increase subject contrast
- decrease superimposition
6
Q
Disadvantages of tomography over radiography?
A
-increased patient dose
7
Q
What is Computed Tomography? Why do we use it over radiography?
A
Creation of a cross-sectional tomographic section of the body
-Clear, axial slices
It minimizes superimposition and improves contrast
8
Q
Advantages of CT
A
- Low contrast resolution
- Data acquisition variability
- Image reconstruction capabilities
- 3D images
9
Q
Disadvantages of CT?
A
- increase dose
- artifacts
- decrease spatial resolution
10
Q
Increasing the distance to the fulcrum will _______ blur
A
Increase
11
Q
Basic steps in performing a CT scan?
A
- Turn machine on and perform Q/C test: maintains ALARA, CT producing best quality images
- Room prep: set scan parameters, clean room, equipment in working order
- PT care: explain procedure, obtain consent, remove artifacts, bladder empty
- PT positioning: anatomical landmarks
- Data acquisition: scan
- Image reconstruction: data processing: photons converted to electrical signal and then to digital
- PT Care: dismiss patient, clean room
- Post-processing
12
Q
The tech needs to understand the protocols to determine the following:
A
- Scan type: conventional vs. helical
- Positioning: of patient and centering
- Contrast: types and administration
- Scan parameters: select exam, modify for patient condition
13
Q
Basic CT equipment
A
- Gantry
- Couch/Table
- Contrast Injector
- Console
- Workstation: post-processing
- Accessory equipment: specialized headrest, sponges, immobilization straps, shielding
14
Q
Types of contrast injectors and their purpose?
A
Power and mechanical
- deliver precise flow rates
- programmable
- consistent
15
Q
Characteristics of the table
A
- concave
- weight restrictions
- moved vertically/horizontally
16
Q
What is the scannable range of the table?
A
How much area can be scanned without having to move the patient
17
Q
What is indexing?
A
How far you want the table to move for each slice
18
Q
What is the scan point on the table?
A
A point used to determine the location of a pathology
19
Q
What does the gantry house?
A
- Tube
- Detector Array
- Generator
- Filtration
- Collimators
- DAS
20
Q
What is the aperture on the gantry? The Isocenter?
A
The hole in the middle
Isocenter: exact center point of the aperture
21
Q
What other functions does the gantry have?
A
- tilt (cephalad, caudad)
- positioning lights
- aperture
- control panels
- isocenter
22
Q
What type of generators do conventional scanners and modern scanner use?
A
Conventional: 3 phase, not in gantry
Modern: high frequency, located in gantry
23
Q
Why are high frequency generators used?
A
- within gantry
- compact
- higher efficiency
- stationary or rotating
- high frequency inverter circuit
24
Q
What is the x-ray tube designed for?
A
Heat dissipation
25
Which scanner use glass and which use metal envelopes? Why metal?
-Conventional: glass
-Modern: metal
Metal prevents arcing, increases tube current, and increased heat dissipation
26
Conventional scanner anode limitations
- fixed
- heavy
- heat dissipation
- tube life not as long
27
What is the anode made out of? Which material is most common?
- All metal
- Brazed graphite: most common
- Chemical Vapor Deposition
28
Brazed graphite anode characteristics
- tungsten-rhenium focal track with graphite base
- larger and thicker
- smaller target angle (12 deg)
- high rotation speeds (3600-10000 rpm)
- 0.5-1.0mm focal spot size
- increased heat storage capacity because higher thermal capacity
- increased tube life
29
What is the purpose of filtration?
Remove long wavelengths
- beam hardening
- uniformity (with a more homogenous beam detectors can operate more consistently)
- minimizes artifact
- lowers patient dose
30
Types of filtration
- Added and inherent
| - Shape categorizations
31
Location and purpose of collimators
- located in gantry
- restrict the x-ray beam
- protect patient, lower dose
- increase image quality, decrease scatter
32
Collimation schemes
- Source: before patient, dose profile
- Post-patient: after patient, maintains beam width (keeps it a slice not a fan), prevents scatter form reaching detector array
33
Other names for Hounsfield Units?
- CT numbers
| - density values
34
Purpose of correction schemes (when assigning HU)
- decrease artifacts
| - decrease misdiagnosis
35
Attenuations principles
- Increase atomic # (Z) = increased absorption
- Increased Density = increased absorption
- Increased energy = decreased absorption, increased scatter
36
HU for water, air, dense bone, and metal
- Water: 0
- Air: -1000
- Dense bone: +1000
- Metal: +2000 or higher
37
How are CT images acquired? (3 steps)
- Data acquisition: how we collect data
- Image Reconstruction: only raw data, organizes the data
- Image display: see data, image data only, can be manipulated and sorted
38
What is data acquisition?
When the patient is scanned to provide us with enough info to construct an image.
When we get raw data through scanning
39
What is scanning? What does it consist of?
The beam geometry used when exposing the patient to radiation
- Size of beam
- Shape of beam
- Motion of beam
- Path of scan
40
What are gantry geometries? Describe the two types
The way the x-ray tube and detectors are arranged for data collection
- Continuous: detectors and tube rotate
- Stationary: detectors in ring, tube rotates
41
Main components that assist with the data acquisition step?
- gantry
| - couch
42
2 methods of data acquisition?
- Axial
| - Helical
43
Other names for axial scans? How do they acquire data?
- Conventional/serial scan
- tube rotates around patient to get first slice, then stops
- table moves into position
- tube rotates back in opposite direction to get another slice
- "step and shoot" method
44
Advantages and disadvantages of axial scans?
Advantages
-Highest image quality
-slices perpendicular to patient
-data can be contiguous, gapped, or overlapped
Disadvantages
-increased exam time
-limits reformatting
-decreases ability to scan contrast filled vessels
-increases likelihood of motion artifacts
45
Other names for helical scans? How do they acquire data?
- Spiral or helical beam geometry
- beam rotates around patient as multiple projections are taken in a 360 deg scan, table moves continuously as well
- scans a volume of tissue and puts it into slices
- slip ring technology
- continuous movement
- volume scanning
46
Advantages of helical scanning?
Advantages
- Reduces misregistration: when a patient takes different breaths info is missed
- more reformatting and reconstruction software
- decreases scan times: good for peds, trauma, physical conditions
- less contrast required: cost effective, safer for patient
47
Disadvantage of helical scanning?
- lower image quality
| - some missing info, needs interpolation
48
What is interpolation/extrapolation?
-a mathematical technique used to estimate the value of a function from know values on either side of the function, fills in missing information
49
What is the data collected as an x-ray photon considered as?
Analog data
50
Four forms of data
- measurement data: scan data
- raw data:
- convolved data: filtered data
- reconstructed data: image data
51
What is scan data?
Data that arise from the detectors
| Require preprocessing corrections before the image reconstruction phase can occur
52
How does image data occur?
-image data occurs when we reconstruct the raw data using algorithms
53
4 reconstruction algorthims
- Back projection: data gets smeared, projection data is dragged or smeared to get the shape of the anatomy
- Filtered Back Projection: removes blurring that results from smearing, aka convolved data
- Fourier Transform: used to reconstruct MRI images, based on measuring frequencies
- Iterative Reconstruction: think automatic rescaling
54
What is convolution?
-mathematical filter that is applied to raw data, removes blurring, improves image quality
55
Data processing in a nutshell
- raw data undergoes some form of preprocessing and is reconstructed
- raw data is converted into a digital image characterized by CT numbers
- Image data (data that has been averaged for post-processing)
56
Advantages and disadvantages of image data over raw data
- decreases storage capacity
- decreases ability to manipulate
- could cause misdiagnosis if tech plays with datas before sent through
57
Algorithms alter the way _______ data is reconstructed
Raw data
58
3 types of algorithms
- Standard: balance noise and detail
- Smoothing: soft tissue visualization, decreases spatial resolution
- Edge enhancement: improves detail
59
What is needed for an image to be displayed?
DAC
60
Things to take into consideration when deciding slice thickness?
-spatial resolution: size of pathology
-Size of area to be scanned: pt dose, tube heat limits
-Reformatting: improves with thinner slices, stair step artifacts
Different for SDCT and MDCT
61
What is slice thickness dependent on for SDCT?
- source collimator width
| - can only be as wide as the single detector
62
What is slice thickness dependent on in MDCT?
- prepatient collimator width
| - detector configuration
63
What is volume averaging influenced by?
Slice thickness
64
Increased volume averaging increases the likelihood of what? Is this good or bad?
The likelihood that structures will be superimposed
| Bad because it hides pathologies because of less accurate pixel readings
65
What does retrospective slice incrementation do? How does this affect the partial volume effect and patient dose?
Enables the operator to change the slice center of an image and create overlapping slices after scan acquisition.
- decreases partial volume effect
- no increase in patient dose
66
What are the limits of retrospective slice incrementation?
- cannot change slice thickness in SDCT
| - slices cannot be smaller than the slice thickness used during data acquisition because of image noise
67
Why do we use retrospective slice incrementation?
-can decrease volume averaging by changing the "starting point" of the slice
68
What is scan time controlled by?
-table movement (pitch)
69
Why can we adjust scan times?
- patient condition
- equipment limitations
- to prevent misregistration
70
What is pitch? What are pitch calculations dependent on?
The table movement throughout the helical scan acquisition
The ratio of the distance the table travels per tube rotation to the collimated x-ray beam width
-dependent on detector configurations (SDCT, MDCT)
71
Table speed and slice thickness have a ________ relationship
Direct
72
Formula for pitch for SDCT?
Pitch = table movement in 1 gantry rotation (d) / slice thickness or beam collimation (W)
73
Formula for pitch for MDCT?
Pitch = table movement per 1 gantry rotation (d) / (slice thickness x number of slices) (W)
74
If pitch doesn't change slice thickness, what does it do?
It affect how much anatomy is in a single slice
75
Why do we need more interpolation/extrapolation when our pitch is larger?
Larger pitch = coils stretch = wider gap between slices (need to fill in missing information
76
Advantages of increased pitch (1.5 or less)
Advantages:
- less patient motion, less change of misregistration
- improved imaging of contrast filled vessels
- decreased patient dose
- decreased heat load
- minimal loss of image sharpness
77
Consequences of too high pitch? Too low pitch?
```
High pitch
-more extrapolation
-faster scan
-decreases resolution due to volume averaging
Low pitch:
-increased patient dose
-overlapping
-longer scan
-more accurate extrapolation for data reconstruction purposes (decreases partial volume averaging)
```
78
Formula to calculate scan coverage for SDCT?
Amount of anatomy covered = pitch x total acquisition time x (1/rotation time) x slice thickness
79
Formula to calculate scan coverage for MDCT
Amount of anatomy covered = pitch x total acquisition time x (1/rotation time) x (slice thickness x slices per rotation)
80
Increased matrix size = ________ pixel size
Decreased
81
Is an increased or decreased pixel size better?
Decreased because improved spatial resolution
82
What do voxels represent?
- a volume of tissue
- a section of thickness
- preferably isotropic (cube, same dimensions all around)
83
Each pixel has a ______ value that represents ________ level based on tissue attenuation characteristics
Discrete, brightness
84
What is pixel sampling?
The pixel detects radiation throughout the entire exam and then averages it to show a certain shade
85
What does bit depth define?What does bit depth give us?
Defines the grayscale capabilities of a pixel display (how many grays a pixel can show)
Gives us the contrast scale
86
What does bit depth affect?
Contrast resolution
87
What are Hounsfield unit calculations based on?
Based on the linear attenuation coefficients of tissue
88
Tissues more dense than water are negative or positive?
Positive
89
What do window width and level control?
WW: contrast
WL: brightness
90
Increased WW = _____ scale contrast = _______ contrast
Long scale, decreased
91
When is increased contrast good?
When looking at anatomy with similar tissue densities (ex. Brain). Need to increase contrast to differentiate between similar tissue densities and tell them apart
92
When is decreased contrast good?
When there are many different densities to be seen (ex. Abdomen). Need more shades of gray for all the densities
93
What should the window level be positioned at?
Should be centered near the average attenuation of the tissue of interest (ex. 0 for water)
94
When the WL is decreased the image will appear _________?
Brighter
95
When the WL is increased, the tissue will appear _______?
Darker
96
What does the SFOV determine?
The area within the gantry from which the raw data will be acquired
Determines the number of detector cells used for data collection
97
Can the SFOV be smaller than the gantry aperture?
Yes
98
If anatomy does not lie in the SFOV what can occur?
Artifacts (out-of-field)
- Streaking
- Shading
- Incorrect HUs
99
What does DFOv determine? How does it affect pixel size and spatial resolution?
Determines how much of the collected raw data will be used to create an image for display
- changes pixel size
- increased spatial resolution because less partial volume averaging
100
DFOV ______ (can/cannot) be larger than the SFOV?
Cannot, because if you didn't scan it, it can't be displayed
101
When does image archiving occur? What does it enable
When images have been reconstructed and image display is acceptable
Enables radiologist dictation/viewing
102
A pitch between ______ and _____ is most common in both SDCT and MDCT.
1 and 1.5
103
Basic CT equipment
- table
- operating console
- power injector
- gantry (DAS, x-ray tube, filtration, collimation, detector array, HF generator)
- workstation
104
Characteristics of the couch?
- moves horizontally and vertically
- made of carbon fibre due to strength, low absorption, and vibrational properties
- has weight limits
- concave
105
Characteristics and uses of the operating console?
- consists of computer, keyboard, and multiple monitors
- used to input all factors related to the CT scan (patient demographics, scan protocol, slice thickness, pitch, technical factors including kVp and mA)
106
Why do we use contrast injectors?
- to inject a bolus of contrast into the patient IVs
- injection consistency
- programable
- precise flow rates and volume
107
Limitations of contrast media injectors?
- kinked tubing
- high viscosity
- incompatible equipment
108
Purpose of the gantry and its characteristics
Houses the imaging component of the scanner including x-ray tube, detectors, slip rings, and DAS
- had positioning lights
- can angle 30 deg caudad and cephalad
- aperture usually 70-90cm in diameter
109
Why is the isocenter so important?
- miscentering of 3-6cm can result in an increase in patient dose by 18-41%
- miscentering in elevation by 20-60mm with a mean position of 23mm below isocenter can result in a dose of up to 140%
110
Major functions of the DAS
- Measures the electric signal that comes from the detectors and converts it to digital
- Sends the signal to the computer for recording
111
How is the x-ray tube designed to decrease heat loads?
- high anode rotation speed
- larger and thicker anode
- metal envelopes
- dual focal spots
- power supplied by high frequency generator
112
What does beam filtration do?
- create a more homogenous beam (uniform) by filtering out low energy photons
- good for patient
- detectors operate more consistently with a homogenous beam
113
2 types of collimators
- Source: pre-patient
| - Post-patient
114
What do collimators do?
- decrease patient dose
- decrease amount of scatter reaching the detectors
- controls slice thickness by shaping the x-ray beam
- controls the voxel length
115
What do detectors do?
Measure x-ray photon energy and convert it to an electrical signal
116
Are short and wide or long and skinny detectors better? Why?
Long and skinny because they don't pick up as much scatter
117
Are smaller or larger detectors better for spatial resolution
Smaller
118
Characteristics we look for in detectors?
- high DQE
- good calibration frequency
- fast response time
- wide dynamic range
119
What is DQE? What makes a detector have better DQE?
How well a detector can capture, absorb, and convert
- increased surface area
- decreased spacing
- high 'Z'
- high density
- high thickness
120
What is stability with reference to detectors?
How much radiation it can take before it needs to be recalibrated (calibration frequency)
121
2 types of detectors
- Ionization chamber
| - Solid state/ scintillation
122
How does an ionization chamber detector work? Advantages/disadvantages?
- photons come in and hit the xenon gas molecules and lose an electron. The new positive ions are attracted to the tungsten plate
- the movement of electrons causes an electric current
- less expensive
- highly stable
- no afterglow (fast response time)
- need to be under pressure
- lots of space
- loss of photons at aluminum casing (decrease conversion efficiency)
123
How do solid state scintillation detectors work? Advantages/disadvantages?
- x-rays to light to electrical
- use a photodiode or photomultiplier to covert x-rays to light
- need light to be proportional to x-rays it is hit with so we use amplifiers to beef up signal
- high x-ray stopping power
- good spectral matching
- low afterglow
- temp and moisture sensitive
- afterglow (disadvantage)
- spectral matching (disadvantage)
124
Multi-slice detector configurations
- matrix array: isotropic
| - adaptive array: anisotropic
125
Mulit-slice detector advantages?
- faster scans (increased anatomical coverage, less chance of motion)
- thinner slices minimizes partial volume averaging
- retrospective slice thickness
126
What does data management consist of?
-collection
-classification
-storage
-retrieval
-distribution
of recorded information
127
Motion of beam for each generation scanner?
1st: translate-rotate 180deg
2nd: translate-rotate 180 deg
3rd: rotate-rotate 360 deg
4th: rotate-fixed 360 deg
128
Shape of beam geometry for each generation scanner?
1st: narrow pencil beam, parallel
2nd: multiple pencil beam, narrow fan
3rd: continuously rotating fan beam, wide fan
4th: continuously rotating fan beam, wide fan
129
Detectors arrays for different generation scanner?
1st: no array
2nd: linear detector array
3rd: curved detector array
4th: circular single row detector array
130
Scan times for each generation scanner?
1st: 4.5 -5.5 mins
2nd: 20s-3.5min
3rd: a few seconds
4th: very short
131
What is not part of HIS/CIS?
Storing reports
132
What is fundamental for the success of a PACS?
The network
133
What does the network do?
Regulates the movement of data and directly affects all users
134
Which generation of scanner is known as the Dual source CT scanner? Why? What would it be useful for?
- 6th generation
- has 2 tubes, detectors, and DAS
- useful for cardiac imaging
135
What is temporal resolution?
Being able to image a moving object as if it wasn't moving
136
Other technical applications of CT?
- Cardiac imaging: all about heart rate, bet blockers can be used to lower heart rate, patient cant consume caffein prior to exam
- CT Fluoroscopy: guides radiologist for biopsies and drainages, patient sedation not required, high patient dose
137
What is image quality? What is it also known as?
- describes how well the image represents the object scanned
| - aka image fidelity or image accuracy
138
2 categories of image quality?
- Contrast resolution: 2 objects with similar densities will look different
- Spatial resolution: how small of an object can we image?
139
Subcategories of image quality?
- uncontrolled: body habitus
| - controlled: mA, scan time, kVp, slice thickness, FOV, reconstruction algorithms, *pitch (only in helical scans)
140
How is spatial resolution measured?
lp/mm or lp/cm
141
What is spatial frequency?
How many objects can fit in a given space
High spatial frequency = small objects
Low spatial frequency = big objects
142
What is the modulation transfer function?
Most commonly used method of describing a system spatial resolution
- defined as the ratio of the accuracy of the image compared with the actual object scanned
- measured on a scale from 0-1
143
MTF increases as object size ________?
Increases
| -the bigger the object the better for image accuracy
144
Is the relationship of spatial frequency and image accuracy linear?
No
145
Increased spatial resolution = _______ contrast resolution?
Decreased
146
What is the uncoupling effect?
Overexposures can look great thanks to automatic rescaling
| -dose creep
147
What is automatic tube current modulation?
Balanced dose and image quality based on the size and weight (density) of the object you are scanning
-ex. Would adjust mA when scanning from a chest (less mA) to an abdomen (more mA)
148
Can automatic tube current modulation fix quantum mottle?
No, can only manipulate what it has been given
149
Usual kVp range for adults vs. peds
Adults: 120-140kVp
Peds: 80kVp
150
Does kVp affect contrast resolution, spatial resolution, or both?
Only contrast resolution
151
Does tube current (small or large filament selection) affect contrast resolution, spatial resolution, or both?
Only spatial resolution
| -smaller filament = less penumbra = increased spatial resolution
152
How do quantum mottle and overexposure affect the image?
They decrease contrast
153
Increased pitch = _______ image quality
Decreased
154
Does pitch change slice thickness?
No
155
Increased slice thickness = _______ image quality? Why?
Decreased because there is increased volume averaging because more info crammed into the slices
156
Increased slice thickness = ________ SNR
Increased because the thicker slices have more photons in them
157
Increased matrix = _____ pixel size = _______ spatial resolution
Decreased, increased
| Small pixels are better for spatial resolution
158
Decreased DFOV = __________ spatial resolution ________ contrast resolution
Increased because less partial volume averaging makes better images
Decreased because the smaller the pixel, the less likely a photon will interact with it
159
A pixel should be ____ the size of the object being scanned?
1/2
160
Bony algorithms = ______spatial resolution and _______contrast resolution?
Increased spatial, decreased contrast
161
Soft tissue algorithms = _______spatial resolution and _______ contrast resolution?
Decreased spatial, increased contrast
162
What do reconstruction algorithms do?
Change how the raw data is manipulated to reconstruct the image
163
Narrow window width is good for ?
Similar densities
164
Wide window width is good for?
Very different densities
165
General definition of artifacts? CT definition?
A distortion or error in an image that is unrelated to the subject being imaged
Any discrepancy between the CT numbers in the image and the true attenuation coefficients of the object
166
3 general appearances of artifacts on image?
1. Streak
2. Ring/band
3. Shading
167
Types of artifacts?
- Beam hardening
- Partial volume artifact
- motion
- Metallic artifacts
- out-of-field artifacts
- cone beam artifacts
- rings and bands
- noise artifacts (3): quantum noise, inherent physical limitations, reconstruction parameters
168
Beam hardening artifacts: what are they caused by? How do they appear? How do we fix them?
Caused by: natural filtration of the x-ray beam by the scanned object (attenuation causes beam to harden)
Appears as: dark bands (aka shading) or streaks
Fix: select the appropriate SFOV, or beam hardening reducing software
169
Partial Volume Artifact: what are they caused by? How do they appear? How do we fix them?
Caused by: more than 1 type of tissue contained within a voxel
Appears as: shading
Fix: use smaller pixel sizes (thinner slices)
170
Motion artifacts: what are they caused by? How do they appear? How do we fix them?
Caused by: voluntary or involuntary patient motion
Appears as: shading, streaking, or blurring
Fix: built in features such as software correction and cardiac gating, communication, immobilization, sedation, short scan times
171
Metallic artifacts: what are they caused by? How do they appear? How do we fix them?
Caused by: the density of the metal is beyond the range of HU values that the system can display
Appear as: streaks
Fix: newer software improved HU range, proper changing instructions, high kVp techniques, thinner slices
172
Out-of-field artifacts: what are they caused by? How do they appear? How do we fix them?
Caused by: anatomy that extends outside the SFOV
Appears as: streaks and shading
Fix: move arms out of way, increase SFOV size
173
Cone beam artifacts: what are they caused by? How do they appear? How do we fix them?
Caused by: interpolation of data, more difficult to calculate missing info, can misrepresent CT number in an image, can be misrepresented as a disease
ONLY IN MDCT HELICAL SCANS
Appears as: streaks or bright and dark shading near areas of large density differences, more pronounced in outer detector rows (less straight beams)
Fix: lower pitch when possible, cone beam reconstruction algorithms
174
Rings and bands artifacts: what are they caused by? How do they appear? How do we fix them?
Caused by: malfunctioning or miscalibrated detector elements
Appear as: rings or bands
Fix: recalibrate scanner or call service engineer
175
3 types of noise artifacts and how they're caused, appearance, how they're fixed
1. Quantum noise
-scanner efficiency, patient size
-increase mAs
2. Inherent physical limitations
-electronic noise in the DAS
-use low-noise DAS systems
3. Reconstruction parameters
-high resolution reconstruction algorithms produce noise
-use smoothing algorithms
ALL LOOK LIKE SALT AND PEPPER
176
3 main categories of post-processing?
1. Basic functions
2. Retrospective reconstruction (raw data)
3. Reformatting (image data)
177
List the basic post-processing functions
1. Windowing
2. Distance measurements: reports size of abnormalities
3. Image annotation: words, phrases, markers on image
4. Multiple image display: view more than one image at once
5. Reference image: helps identify which viewed slice corresponds to specific anatomic landmarks
6. Image magnification: helps with measurement accuracy
7. Histogram display: graph showing how often a range of HUs occur in a specific ROI
178
What is retrospective reconstruction? What things can you change?
Using raw data to create new images
- DFOV
- image center
- reconstruction algorithms
179
Why would we use overlapping reconstructions?
To create thicker slices to send to PACS, less info
| MDCT*
180
What is reformatting? What things are required to reformat an image?
Manipulates image data for viewing
- DFOV the same
- image centers the same
- gantry tilt the same
- contiguous (each slice right next to each other, no missing info)
181
Types of reformatting?
```
2D:
-MPR
3D:
-MIP
-MinIP
-SR
-VR
```
182
What is MPR?
Multiplanar reconstruction
- done to show anatomy in various planes
- transverse, coronal, sagittal, oblique, curved
- 2D
- represent original HU
- automatically generate sagittal and coronal
- curved and obliques manually
183
What is MIP?
Maximum intensity projection
- pixels show the highest values detected
- eliminate other values
184
What is MinIP?
Minimum intensity projection
- pixels show lowest value displayed
- eliminate other values
185
What structured are best visualized with MIP and MinIP?
MIP: high attenuation structures (bone and contrast filled vessels)-high contrast structured
MinIP: low attenuation structured (bronchioles, negative contrast filled vessels)-low contrast structures
186
Advantages of MIP and MinIP?
-highlight structured of interest and minimize superimposition
187
What is surface rendering? What is it good for?
Images are created by having the pixels only show a predetermined threshold CT value
- we see the shell of an object
- good for examining tubular structures
188
What happens if the threshold is too high or too low when surface rendering?
Too high: materials like fluid will also be displayed as tissues of interest
Too low: structures can be excluded
189
What is volume rendering?
A 3D semi-transparent representation of the imaged structure
- all voxels contribute to image
- enables display of multiple tissues and their relationship to one another
190
What is segmentation? What is another term for it?
Selectively removing or isolating structures from an image
| Aka ROI editing
191
Most common artifacts that degrade reformats?
- motion
- metal
- asymmetrical voxels
192
What causes the "stair-step" appearance when reformatting? How do you fix it?
Different DFOVs, image centers, etc.
| -fix it by using the image overlap to make more symmetrical voxels
193
Why is contrast used in CT?
Objects that share similar densities are difficult to visualize
- easier viewing
- localization
- differentiation
194
What is contrast media?
A material that temporarily enhances the radiographic visualization of structured with similar densities
195
Forms of contrast media?
- liquid (most common)
- gas
- powder/paste
196
How is it decided whether contrast media is a positive or negative agent?
Lower density than structure that required enhancement = negative
Higher density = positive
197
What do you take into consideration when choosing a contrast material?
- radiographic appearance
- protocol
- equipment (transit time)
- anatomy
- patient (risk/complications)
- administration
198
Types of contrast we use?
- Air
- C02
- Water
- Barium sulfate solutions
- Iodinated water-soluble solutions
199
Air and gas contrast: characteristics, purpose, pros/cons
```
Characteristics:
-non-toxic
-readily absorbed by body
-radiolucent
Purpose:
-distention
-increase contrast
Pros:
-distention
Cons
-spastic colon
```
200
Why is C02 preferred over air?
- absorption
| - patient tolerance
201
Air and gas can be used for what kind of exams? How is it administered?
- virtual colonoscopies
- GI studies
- arthrograms
- myelograms
- injection
- orally
- rectally
202
Water (contrast): characteristics, purpose, pros/cons
```
Characteristics:
-non-toxic
-readily absorbed by body
-low density
Purpose:
-decreases superimposition
-increases contrast
Pros:
-alternative to contrast agents
-not obscure
-3D reformatting
-accessibility
-no allergic reactions
Cons:
- fast transit time
-poor bowel distentions
```
203
What studies is water used in? How can it be administered?
- evaluation of pancreas
- GI studies
- gastric neoplasms
-orally
204
Barium sulfate: characteristics, purpose, considerations
```
Characteristics:
-specifically formulated for CT (1-3% concentration)
Purpose:
-increase contrast
-fill atomic structures (radiopaque)
Considerations:
-concentration
-routes of adminstration
```
205
2 forms of barium sulfate and their characteristics
Liquid: low concentration, low viscosity
Paste: low concentration, high viscosity
206
Pros and cons of barium sulfate
```
Pros:
-slower transit time
-clings to bowel walls
-additives
-distention
Cons:
-allergies
-fills bowel
-complications
-contraindications
-artifacts
-time
```
207
What is the usual HU value and concentration of VoLumen?
15-30 HU
| -0.1% barium sulfate solution
208
Pros of VoLumen compared to normal barium sulfate?
- increased bowel distention
- improved transit time
- increased visualization of bowel wall and mucosa
209
What studies is VoLumen used for? How can it be administered?
-GI studies
- orally
- rectally
210
Iodinated water soluble: characteristics, purpose, and considerations
```
Characteristics:
-liquid
-osmolality: HOCM, IOCM, LOCM
-Non-ionic
-radiopaque
-viscosity
Purpose:
-increase contrast
-fills anatomical structures
Considerations:
-variable routes of administration
-most commonly used in CT
-filters out via kidneys
```
211
Pros and cons of iodinated media?
```
Pros:
-not metabolized
-water soluble
-easy to administer
-safe
-increases attenuation differences
Cons:
-allergic-like reactions
-contraindications
-differential enhancement (good thing except with pathologies)
-diarrhea
-poor mucosal coating
```
212
What studies do we use iodinated media for? How can it be administered?
- GI studies
- Arthrograms
- postmyelograms
- vascular studies
- arterial studies
- solid organs
- tumors
- orally
- rectally
- injection
213
What methods of administration of contrast medias do we use in CT?
- IV
- Oral
- Rectal
- Injection
214
What can be administered rectally? What exams require this? Considerations?
- Air, C02, Barium Sulfate, Iodinated water soluble
- CT colonoscopy
- only for large bowel
- only for patients with specific histories
215
What can we inject intrathecally? What exams do we do this for? Considerations?
- Air, Gas, Iodinated water soluble
- CT intrathecal imaging
- Types of contrast media used is imperative
- patient head must stay elevated
- imaging takes place 1-3hrs post injection (log roll)
216
What exams required intra-articular contrast administration? Considerations?
- CT arthrogram imaging
- simultaneous viewing of soft tissue and bone
- only imaging completed in CT
217
What can we administer orally? What exams require oral administration?
- CT GI tract imaging
| - Air, Gas, Barium Sulfate, Iodinated water soluble, Water
218
What is the only contrast media to be used for intravascular injection? Most commonly used brands?
Non-ionic contrast
- Optiray
- Omnipaque
219
How many times higher are the chances of having an adverse reaction with HOCM vs. LOCM?
4-5 times
220
Give an example of an IOCM contrast media
-Visipaque
221
What is the "gold standard" test to confirm if the kidneys can handle contrast?
GFR
222
What makes creatinine levels higher?
- men vs. women (gender)
- younger vs. older (age)
- blacks vs. whites (race)
- consumption of cooked meat (protein) = increase
- malnutrition = decreased
223
What systems do contrast overdoses affect?
- pulmonary
| - cardiovascular
224
What is the upper limit of grams of iodine?
64gI
225
Categories of iodinated contrast reactions
1. Subjective/normal
2. Chemotoxic
3. Idiosyncratic
226
Normal side effects of IV contrast?
- feeling of heat/warmth
- mild flushing
- metallic taste
- nausea and/or vomiting
- anxiety may increase probability of reactions
227
What is a chemotoxic response to IV contrast? Examples?
Result from physiochemical properties of the contrast media, the dose, and the speed of injection
- hemodynamic disturbances and injuries to organs/vessels
- pain at injection site
- contrast induced nephropathy (CIN)
228
What is the 3rd leading cause of ARF?
CIN
229
What is CIN? Signs? Treatment?
Acute impairment of renal function resulting from the administration of IV contrast
- elevated SeCR levels within 24hrs of contrast admin.
- may cause need for temporary or chronic dialysis
- increased risk of death from non-renal causes
230
CIN risk factors?
1. Diabetes mellitus: low risk (0.6%), high risk if coupled with preexisting renal impairment (19.7%)
2. Volume of contrast material: increased volume = increased risk, allow 48hrs between procedures requiring contrast
3. Dehydration
4. Age, sex (increased risk in men), atherosclerotic disease
231
How to prevent CIN?
- Use of LOCM or IOCM
- Hydration: patients should be well hydrated
- Dose: use smallest dose possible, 48hrs between procedures
- Temporarily discontinue medications
232
What is an idiosyncratic response to IV contrast?
Include all other forms of reactions
- unpredictable, can occur within 1hr of administration
- most adverse reactions occur within minutes
- majority are non-life-threatening
- unrelated to dose
- allergic-like
- children have lower incidence
233
How to prevent idiosyncratic reactions?
- avoid using HOCM
| - premedication
234
How are idiosyncratic responses to IV contrast classified?
- mild
- moderate
- severe
235
Signs/symptoms of mild/moderate idiosyncratic responses? What can you do?
* *they are acute
- itchy skin, hives, nasal congestion, sneezing, watery eyes, coughing, laryngeal swelling, peripheral tingling, tachycardia, bradycardia, hypotension, feeling of fullness/tightness in mouth and throat, anxiety, nervousness
- stop injection
- calm and reassure patient
- apply cool compress to itchy area
- observe patient for changes
- document details of reaction
- obtain medical assistance
236
Signs/symptoms of severe idiosyncratic reactions? What can you do?
* *Potentially or immediately life threatening
- abrupt onset, bradycardia (<50bpm), hypotension, sever dyspnea, cardiac arrhythmias, laryngeal swelling, convulsions, seizures, loss of consciousness, respiratory arrest, cardiac arrest
- call a code
- ensure integrity of IV site
- calm and reassure patient
- prepare oxygen, suction, and crash cart
- have patients history ready and available
- be ready to assist physicians
237
What should you document about a contrast reaction?
- Amount and type of contrast
- Signs and symptoms of the reaction
- Interventions or medications given during reaction
- Final outcome (sent home? Admitted to hospital?)
- Update patient info in computer system
238
Risk factors for adverse reactions?
- Asthma: 3x more likely
- Allergies to food, drugs, and other substances: 2x higher risk (beta blockers can impair response to treatment of reactions, but do not increase likelihood of reaction)
- Previous contrast reaction: 11x more likely
239
Contraindications to IV contrast?
- Kidney disease or renal failure: must take precautions
- Dialysis: cannot have, could lead to CRF if administered contrast
- Diabetes and taking metformin: must take precautions
- Heart disease or hypertension: bolus can cause vasodilation (pt can already have weakened blood vessels, can cause vasovagal reaction
- Hyperthyroidism: contrast media can increase thyroid hormone levels (thyroid storm = fatal)
- CNS disorders: disruptive BBB (increased risk to develop seizures)
240
Can patients with end stage renal failure undergo contrast media exams?
Yes because it cannot increase the extent of preexisting kidney damage
241
What can happen if renal dysfunction occurs and the patient is taking metformin?
Metformin can accumulate and cause lactic acidosis
| Patient can only resume taking meds 48 hrs after exam
242
What is important to get if you know a patient suffers from high BP?
**baseline blood pressure
243
3 methods of IV administration?
1. Mechanical injection: regularly used, consistent and safe
2. Hand bolus: reproducibility issues, tech dose
3. Drip infusion: not used for med administration, not used in CT due to speed
244
What do you do if an IV needs to be established in the Ct department?
- use AC or a large forearm vein
- ensure IV is not over a site that will cause the cannula patency to be compromised
- injection speed must be compatible with the size of the vein and needle gauge
- test IV site for resistance and blood aspiration prior to injection
245
Questions to ask yourself if patient arrives with pre-established IV
- Where is it located
- When was it established
- Is it being used for medication administration
- Is there discolouration or swelling of the surrounding skin
- Is the catheter being used at the IV site compatible with the power injector
246
What is a CVAD? What is it for?
Central Venous Access Device
Designed to deliver meds and fluids directly into the SVC, IVC, or RA
-used for days, weeks, months, or years
-durable
-may contain 1-3 lumens to prevent medication mixing
-catheters have open or closed ends
247
What is a PICC?
Peripherally Inserted Central Catheter
- open ended
- must be clamped when not in use
- should be flushed with heparin to maintain patency
- closed ended catheters contain a valve that controls fluid flow and prevents reflux of blood, only require saline flush
248
Can all PICCs handle mechanical injection?
No, only specially designed ones can.
Make sure you verify it can be used and if not, start a separate IV or decreased injection rate and perform a hand injection
249
Techs should adhere to what kind of practices when using CVADs? What does this include?
Sterile practices
- disinfect the injection caps and ensure they are dry prior to use
- follow same protocol as newly established IVs (injection speed must be compatible with size of catheter, test catheter prior to injection)
250
When is documentation for contrast required? What should be documented?
Anytime contrast is administered
- volume/rate
- injection site
- injection time
- GFR, SeCR levels
251
What can techs adjust on mechanical injectors?
- volume and concentration
- flow rate(s)
- pressure limits
- timing between injection and start of scan acquisition
252
What are mechanical injectors used for?
Deliver our pre-set bolus of contrast in a short period of time
253
What is a concern with power injectors? How can we alleviate this concern?
Extravasation
| -a test bolus of saline using the power injector is common practice
254
What must be done to prevent fatal emboli when using a power injector?
Air bubbles must be removed
255
Components of mechanical injectors?
- Warming device: maintains temp (37deg), does not warm contrast
- Syringe(s): 2 usually, sterilized, removable, disposable, 200ml common, contain pistons, multi-use
- Pressure mechanism: loading assembly, ceiling or floor mounted, moveable, has a motor to control delivery of contrast, safety devices built in (PSI limits)
- Control panel: manual parameter selection, flow rate, volume, delay, injection pressure, programmable protocol presets for certain exams
256
How to load a pressure injector according to AHS
1. Wash hands and open sterile package of syringes
2. Insert syringes into loading assembly and allow injector to purge the air from both syringes
3. Attach a transfer spike to each set of syringes
4. Attach a multi-patient Y connector tube to the spike connectors (short end to contrast)
5. Attach saline transfer spike to saline and contrast transfer spike to contrast bottle
6. Hang the contrast and saline containers from hooks on the ceiling mounted injector arm
7. Attach a single use patient line to the Y connector
8. Manually purge system of air bubbles (contrast 1st up to Y, saline 2nd through single use line)
9. Turn injector head down indicating injector is ready for use
10. Post injection: turn injector head up and remove the patient line and place a new patient line on immediately
257
3 phases of enhancement (from first to last to enhance)
1. Arterial (bolus)
2. Non-equilibrium (venous)
3. Equilibrium (delayed)
258
How long is the peak enhancement of arterial organs after injection?
15-45 seconds
259
Which organs breaks the enhancement rules? Why?
The brain
- the BBB
- scan delay can be 4 mins or longer
260
The exact timing of the start and end of each of the 3 phases are affected by?
- Pharmacokinetics: all about contrast
- Patient factors: age, cardiac output, body habitus
- Equipment: system speed, increased scanner speed = you need to increase delay requirement
261
How does volume and dose affect peak enhancement?
- Increased volume = increased magnitude of peak enhancement
- Increased volume = increased time to peak
- Increased volume = increased time a given level of enhancement is maintained
262
Formula for total volume of contrast injected is?
V=(ml/sec)(sec)
263
Effects of flow rate on enhancement?
- Increased flow rate = increased magnitude of peak enhancement
- Increased flow rate = decreased time to peak enhancement
- Increased flow rate = decreased duration of contrast injection
264
Effects of body habitus on enhancement?
- Increased weight = decreased magnitude of peak enhancement
- Increased weight = no effect on time to peak
- Increased flow rate (or iodine concentration) = increased enhancement
265
Effects of cardiac output on enhancement?
- Increased heart rate = no effect magnitude of peak
| - Increased heart rate = decreased time to peak enhancement
266
What helps us determine the right time to start the scan after an injection?
Computer software
267
2 automated injection triggering methods?
1. Test bolus: start 10 secs after injection, uses 10-20ml IV bolus of contrast, 10-15 images taken, 1 every 2 secs, all same anatomical location
2. Bolus triggering: when HU threshold is reached, injection starts
