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MRT Year 1 Semester 2 > CT Theory > Flashcards

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
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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

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3
Q

What is a focal plane? Fulcrum?

A
  • Focal Plane: section thickness

- Fulcrum: center of the center (clear part)

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4
Q

Increasing the tomographic angle will _______ the section thickness?

A

Decrease

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5
Q

Advantages of tomography over radiography?

A
  • increase radiographic contrast
  • increase subject contrast
  • decrease superimposition
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6
Q

Disadvantages of tomography over radiography?

A

-increased patient dose

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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

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8
Q

Advantages of CT

A
  • Low contrast resolution
  • Data acquisition variability
  • Image reconstruction capabilities
  • 3D images
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9
Q

Disadvantages of CT?

A
  • increase dose
  • artifacts
  • decrease spatial resolution
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10
Q

Increasing the distance to the fulcrum will _______ blur

A

Increase

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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
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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
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13
Q

Basic CT equipment

A
  • Gantry
  • Couch/Table
  • Contrast Injector
  • Console
  • Workstation: post-processing
  • Accessory equipment: specialized headrest, sponges, immobilization straps, shielding
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14
Q

Types of contrast injectors and their purpose?

A

Power and mechanical

  • deliver precise flow rates
  • programmable
  • consistent
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15
Q

Characteristics of the table

A
  • concave
  • weight restrictions
  • moved vertically/horizontally
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16
Q

What is the scannable range of the table?

A

How much area can be scanned without having to move the patient

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17
Q

What is indexing?

A

How far you want the table to move for each slice

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18
Q

What is the scan point on the table?

A

A point used to determine the location of a pathology

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19
Q

What does the gantry house?

A
  • Tube
  • Detector Array
  • Generator
  • Filtration
  • Collimators
  • DAS
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20
Q

What is the aperture on the gantry? The Isocenter?

A

The hole in the middle

Isocenter: exact center point of the aperture

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21
Q

What other functions does the gantry have?

A
  • tilt (cephalad, caudad)
  • positioning lights
  • aperture
  • control panels
  • isocenter
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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

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23
Q

Why are high frequency generators used?

A
  • within gantry
  • compact
  • higher efficiency
  • stationary or rotating
  • high frequency inverter circuit
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24
Q

What is the x-ray tube designed for?

A

Heat dissipation

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25
Q

Which scanner use glass and which use metal envelopes? Why metal?

A

-Conventional: glass
-Modern: metal
Metal prevents arcing, increases tube current, and increased heat dissipation

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26
Q

Conventional scanner anode limitations

A
  • fixed
  • heavy
  • heat dissipation
  • tube life not as long
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27
Q

What is the anode made out of? Which material is most common?

A
  • All metal
  • Brazed graphite: most common
  • Chemical Vapor Deposition
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28
Q

Brazed graphite anode characteristics

A
  • 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
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29
Q

What is the purpose of filtration?

A

Remove long wavelengths

  • beam hardening
  • uniformity (with a more homogenous beam detectors can operate more consistently)
  • minimizes artifact
  • lowers patient dose
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30
Q

Types of filtration

A
  • Added and inherent

- Shape categorizations

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31
Q

Location and purpose of collimators

A
  • located in gantry
  • restrict the x-ray beam
  • protect patient, lower dose
  • increase image quality, decrease scatter
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32
Q

Collimation schemes

A
  • 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
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33
Q

Other names for Hounsfield Units?

A
  • CT numbers

- density values

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34
Q

Purpose of correction schemes (when assigning HU)

A
  • decrease artifacts

- decrease misdiagnosis

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35
Q

Attenuations principles

A
  • Increase atomic # (Z) = increased absorption
  • Increased Density = increased absorption
  • Increased energy = decreased absorption, increased scatter
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36
Q

HU for water, air, dense bone, and metal

A
  • Water: 0
  • Air: -1000
  • Dense bone: +1000
  • Metal: +2000 or higher
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37
Q

How are CT images acquired? (3 steps)

A
  • 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
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38
Q

What is data acquisition?

A

When the patient is scanned to provide us with enough info to construct an image.
When we get raw data through scanning

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39
Q

What is scanning? What does it consist of?

A

The beam geometry used when exposing the patient to radiation

  • Size of beam
  • Shape of beam
  • Motion of beam
  • Path of scan
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40
Q

What are gantry geometries? Describe the two types

A

The way the x-ray tube and detectors are arranged for data collection

  • Continuous: detectors and tube rotate
  • Stationary: detectors in ring, tube rotates
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41
Q

Main components that assist with the data acquisition step?

A
  • gantry

- couch

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42
Q

2 methods of data acquisition?

A
  • Axial

- Helical

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43
Q

Other names for axial scans? How do they acquire data?

A
  • 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
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44
Q

Advantages and disadvantages of axial scans?

A

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

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45
Q

Other names for helical scans? How do they acquire data?

A
  • 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
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46
Q

Advantages of helical scanning?

A

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
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47
Q

Disadvantage of helical scanning?

A
  • lower image quality

- some missing info, needs interpolation

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48
Q

What is interpolation/extrapolation?

A

-a mathematical technique used to estimate the value of a function from know values on either side of the function, fills in missing information

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49
Q

What is the data collected as an x-ray photon considered as?

A

Analog data

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50
Q

Four forms of data

A
  • measurement data: scan data
  • raw data:
  • convolved data: filtered data
  • reconstructed data: image data
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51
Q

What is scan data?

A

Data that arise from the detectors

Require preprocessing corrections before the image reconstruction phase can occur

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52
Q

How does image data occur?

A

-image data occurs when we reconstruct the raw data using algorithms

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53
Q

4 reconstruction algorthims

A
  • 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
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54
Q

What is convolution?

A

-mathematical filter that is applied to raw data, removes blurring, improves image quality

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55
Q

Data processing in a nutshell

A
  • 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)
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56
Q

Advantages and disadvantages of image data over raw data

A
  • decreases storage capacity
  • decreases ability to manipulate
  • could cause misdiagnosis if tech plays with datas before sent through
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57
Q

Algorithms alter the way _______ data is reconstructed

A

Raw data

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58
Q

3 types of algorithms

A
  • Standard: balance noise and detail
  • Smoothing: soft tissue visualization, decreases spatial resolution
  • Edge enhancement: improves detail
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59
Q

What is needed for an image to be displayed?

A

DAC

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60
Q

Things to take into consideration when deciding slice thickness?

A

-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

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61
Q

What is slice thickness dependent on for SDCT?

A
  • source collimator width

- can only be as wide as the single detector

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62
Q

What is slice thickness dependent on in MDCT?

A
  • prepatient collimator width

- detector configuration

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63
Q

What is volume averaging influenced by?

A

Slice thickness

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64
Q

Increased volume averaging increases the likelihood of what? Is this good or bad?

A

The likelihood that structures will be superimposed

Bad because it hides pathologies because of less accurate pixel readings

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65
Q

What does retrospective slice incrementation do? How does this affect the partial volume effect and patient dose?

A

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
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66
Q

What are the limits of retrospective slice incrementation?

A
  • cannot change slice thickness in SDCT

- slices cannot be smaller than the slice thickness used during data acquisition because of image noise

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67
Q

Why do we use retrospective slice incrementation?

A

-can decrease volume averaging by changing the “starting point” of the slice

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68
Q

What is scan time controlled by?

A

-table movement (pitch)

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69
Q

Why can we adjust scan times?

A
  • patient condition
  • equipment limitations
  • to prevent misregistration
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70
Q

What is pitch? What are pitch calculations dependent on?

A

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)

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71
Q

Table speed and slice thickness have a ________ relationship

A

Direct

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72
Q

Formula for pitch for SDCT?

A

Pitch = table movement in 1 gantry rotation (d) / slice thickness or beam collimation (W)

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73
Q

Formula for pitch for MDCT?

A

Pitch = table movement per 1 gantry rotation (d) / (slice thickness x number of slices) (W)

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74
Q

If pitch doesn’t change slice thickness, what does it do?

A

It affect how much anatomy is in a single slice

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75
Q

Why do we need more interpolation/extrapolation when our pitch is larger?

A

Larger pitch = coils stretch = wider gap between slices (need to fill in missing information

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76
Q

Advantages of increased pitch (1.5 or less)

A

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
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77
Q

Consequences of too high pitch? Too low pitch?

A 
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)
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78
Q

Formula to calculate scan coverage for SDCT?

A

Amount of anatomy covered = pitch x total acquisition time x (1/rotation time) x slice thickness

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79
Q

Formula to calculate scan coverage for MDCT

A

Amount of anatomy covered = pitch x total acquisition time x (1/rotation time) x (slice thickness x slices per rotation)

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80
Q

Increased matrix size = ________ pixel size

A

Decreased

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81
Q

Is an increased or decreased pixel size better?

A

Decreased because improved spatial resolution

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82
Q

What do voxels represent?

A
  • a volume of tissue
  • a section of thickness
  • preferably isotropic (cube, same dimensions all around)
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83
Q

Each pixel has a ______ value that represents ________ level based on tissue attenuation characteristics

A

Discrete, brightness

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84
Q

What is pixel sampling?

A

The pixel detects radiation throughout the entire exam and then averages it to show a certain shade

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85
Q

What does bit depth define?What does bit depth give us?

A

Defines the grayscale capabilities of a pixel display (how many grays a pixel can show)
Gives us the contrast scale

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86
Q

What does bit depth affect?

A

Contrast resolution

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87
Q

What are Hounsfield unit calculations based on?

A

Based on the linear attenuation coefficients of tissue

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88
Q

Tissues more dense than water are negative or positive?

A

Positive

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89
Q

What do window width and level control?

A

WW: contrast
WL: brightness

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90
Q

Increased WW = _____ scale contrast = _______ contrast

A

Long scale, decreased

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91
Q

When is increased contrast good?

A

When looking at anatomy with similar tissue densities (ex. Brain). Need to increase contrast to differentiate between similar tissue densities and tell them apart

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92
Q

When is decreased contrast good?

A

When there are many different densities to be seen (ex. Abdomen). Need more shades of gray for all the densities

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93
Q

What should the window level be positioned at?

A

Should be centered near the average attenuation of the tissue of interest (ex. 0 for water)

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94
Q

When the WL is decreased the image will appear _________?

A

Brighter

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95
Q

When the WL is increased, the tissue will appear _______?

A

Darker

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96
Q

What does the SFOV determine?

A

The area within the gantry from which the raw data will be acquired
Determines the number of detector cells used for data collection

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97
Q

Can the SFOV be smaller than the gantry aperture?

A

Yes

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98
Q

If anatomy does not lie in the SFOV what can occur?

A

Artifacts (out-of-field)

  • Streaking
  • Shading
  • Incorrect HUs
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99
Q

What does DFOv determine? How does it affect pixel size and spatial resolution?

A

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
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100
Q

DFOV ______ (can/cannot) be larger than the SFOV?

A

Cannot, because if you didn’t scan it, it can’t be displayed

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101
Q

When does image archiving occur? What does it enable

A

When images have been reconstructed and image display is acceptable
Enables radiologist dictation/viewing

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102
Q

A pitch between ______ and _____ is most common in both SDCT and MDCT.

A

1 and 1.5

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103
Q

Basic CT equipment

A
  • table
  • operating console
  • power injector
  • gantry (DAS, x-ray tube, filtration, collimation, detector array, HF generator)
  • workstation
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104
Q

Characteristics of the couch?

A
  • moves horizontally and vertically
  • made of carbon fibre due to strength, low absorption, and vibrational properties
  • has weight limits
  • concave
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105
Q

Characteristics and uses of the operating console?

A
  • 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)
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106
Q

Why do we use contrast injectors?

A
  • to inject a bolus of contrast into the patient IVs
  • injection consistency
  • programable
  • precise flow rates and volume
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107
Q

Limitations of contrast media injectors?

A
  • kinked tubing
  • high viscosity
  • incompatible equipment
108
Q

Purpose of the gantry and its characteristics

A

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
Q

Why is the isocenter so important?

A
  • 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
Q

Major functions of the DAS

A
  • Measures the electric signal that comes from the detectors and converts it to digital
  • Sends the signal to the computer for recording
111
Q

How is the x-ray tube designed to decrease heat loads?

A
  • high anode rotation speed
  • larger and thicker anode
  • metal envelopes
  • dual focal spots
  • power supplied by high frequency generator
112
Q

What does beam filtration do?

A
  • create a more homogenous beam (uniform) by filtering out low energy photons
  • good for patient
  • detectors operate more consistently with a homogenous beam
113
Q

2 types of collimators

A
  • Source: pre-patient

- Post-patient

114
Q

What do collimators do?

A
  • decrease patient dose
  • decrease amount of scatter reaching the detectors
  • controls slice thickness by shaping the x-ray beam
  • controls the voxel length
115
Q

What do detectors do?

A

Measure x-ray photon energy and convert it to an electrical signal

116
Q

Are short and wide or long and skinny detectors better? Why?

A

Long and skinny because they don’t pick up as much scatter

117
Q

Are smaller or larger detectors better for spatial resolution

A

Smaller

118
Q

Characteristics we look for in detectors?

A
  • high DQE
  • good calibration frequency
  • fast response time
  • wide dynamic range
119
Q

What is DQE? What makes a detector have better DQE?

A

How well a detector can capture, absorb, and convert

  • increased surface area
  • decreased spacing
  • high ‘Z’
  • high density
  • high thickness
120
Q

What is stability with reference to detectors?

A

How much radiation it can take before it needs to be recalibrated (calibration frequency)

121
Q

2 types of detectors

A
  • Ionization chamber

- Solid state/ scintillation

122
Q

How does an ionization chamber detector work? Advantages/disadvantages?

A
  • 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
Q

How do solid state scintillation detectors work? Advantages/disadvantages?

A
  • 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
Q

Multi-slice detector configurations

A
  • matrix array: isotropic

- adaptive array: anisotropic

125
Q

Mulit-slice detector advantages?

A
  • faster scans (increased anatomical coverage, less chance of motion)
  • thinner slices minimizes partial volume averaging
  • retrospective slice thickness
126
Q

What does data management consist of?

A

-collection
-classification
-storage
-retrieval
-distribution
of recorded information

127
Q

Motion of beam for each generation scanner?

A

1st: translate-rotate 180deg
2nd: translate-rotate 180 deg
3rd: rotate-rotate 360 deg
4th: rotate-fixed 360 deg

128
Q

Shape of beam geometry for each generation scanner?

A

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
Q

Detectors arrays for different generation scanner?

A

1st: no array
2nd: linear detector array
3rd: curved detector array
4th: circular single row detector array

130
Q

Scan times for each generation scanner?

A

1st: 4.5 -5.5 mins
2nd: 20s-3.5min
3rd: a few seconds
4th: very short

131
Q

What is not part of HIS/CIS?

A

Storing reports

132
Q

What is fundamental for the success of a PACS?

A

The network

133
Q

What does the network do?

A

Regulates the movement of data and directly affects all users

134
Q

Which generation of scanner is known as the Dual source CT scanner? Why? What would it be useful for?

A
  • 6th generation
  • has 2 tubes, detectors, and DAS
  • useful for cardiac imaging
135
Q

What is temporal resolution?

A

Being able to image a moving object as if it wasn’t moving

136
Q

Other technical applications of CT?

A
  • 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
Q

What is image quality? What is it also known as?

A
  • describes how well the image represents the object scanned

- aka image fidelity or image accuracy

138
Q

2 categories of image quality?

A
  • Contrast resolution: 2 objects with similar densities will look different
  • Spatial resolution: how small of an object can we image?
139
Q

Subcategories of image quality?

A
  • uncontrolled: body habitus

- controlled: mA, scan time, kVp, slice thickness, FOV, reconstruction algorithms, *pitch (only in helical scans)

140
Q

How is spatial resolution measured?

A

lp/mm or lp/cm

141
Q

What is spatial frequency?

A

How many objects can fit in a given space
High spatial frequency = small objects
Low spatial frequency = big objects

142
Q

What is the modulation transfer function?

A

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
Q

MTF increases as object size ________?

A

Increases

-the bigger the object the better for image accuracy

144
Q

Is the relationship of spatial frequency and image accuracy linear?

A

No

145
Q

Increased spatial resolution = _______ contrast resolution?

A

Decreased

146
Q

What is the uncoupling effect?

A

Overexposures can look great thanks to automatic rescaling

-dose creep

147
Q

What is automatic tube current modulation?

A

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
Q

Can automatic tube current modulation fix quantum mottle?

A

No, can only manipulate what it has been given

149
Q

Usual kVp range for adults vs. peds

A

Adults: 120-140kVp
Peds: 80kVp

150
Q

Does kVp affect contrast resolution, spatial resolution, or both?

A

Only contrast resolution

151
Q

Does tube current (small or large filament selection) affect contrast resolution, spatial resolution, or both?

A

Only spatial resolution

-smaller filament = less penumbra = increased spatial resolution

152
Q

How do quantum mottle and overexposure affect the image?

A

They decrease contrast

153
Q

Increased pitch = _______ image quality

A

Decreased

154
Q

Does pitch change slice thickness?

A

No

155
Q

Increased slice thickness = _______ image quality? Why?

A

Decreased because there is increased volume averaging because more info crammed into the slices

156
Q

Increased slice thickness = ________ SNR

A

Increased because the thicker slices have more photons in them

157
Q

Increased matrix = _____ pixel size = _______ spatial resolution

A

Decreased, increased

Small pixels are better for spatial resolution

158
Q

Decreased DFOV = __________ spatial resolution ________ contrast resolution

A

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
Q

A pixel should be ____ the size of the object being scanned?

A

1/2

160
Q

Bony algorithms = ______spatial resolution and _______contrast resolution?

A

Increased spatial, decreased contrast

161
Q

Soft tissue algorithms = _______spatial resolution and _______ contrast resolution?

A

Decreased spatial, increased contrast

162
Q

What do reconstruction algorithms do?

A

Change how the raw data is manipulated to reconstruct the image

163
Q

Narrow window width is good for ?

A

Similar densities

164
Q

Wide window width is good for?

A

Very different densities

165
Q

General definition of artifacts? CT definition?

A

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
Q

3 general appearances of artifacts on image?

A
  1. Streak
  2. Ring/band
  3. Shading
167
Q

Types of artifacts?

A
  • 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
Q

Beam hardening artifacts: what are they caused by? How do they appear? How do we fix them?

A

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
Q

Partial Volume Artifact: what are they caused by? How do they appear? How do we fix them?

A

Caused by: more than 1 type of tissue contained within a voxel
Appears as: shading
Fix: use smaller pixel sizes (thinner slices)

170
Q

Motion artifacts: what are they caused by? How do they appear? How do we fix them?

A

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
Q

Metallic artifacts: what are they caused by? How do they appear? How do we fix them?

A

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
Q

Out-of-field artifacts: what are they caused by? How do they appear? How do we fix them?

A

Caused by: anatomy that extends outside the SFOV
Appears as: streaks and shading
Fix: move arms out of way, increase SFOV size

173
Q

Cone beam artifacts: what are they caused by? How do they appear? How do we fix them?

A

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
Q

Rings and bands artifacts: what are they caused by? How do they appear? How do we fix them?

A

Caused by: malfunctioning or miscalibrated detector elements
Appear as: rings or bands
Fix: recalibrate scanner or call service engineer

175
Q

3 types of noise artifacts and how they’re caused, appearance, how they’re fixed

A
  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
Q

3 main categories of post-processing?

A
  1. Basic functions
  2. Retrospective reconstruction (raw data)
  3. Reformatting (image data)
177
Q

List the basic post-processing functions

A
  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
Q

What is retrospective reconstruction? What things can you change?

A

Using raw data to create new images

  • DFOV
  • image center
  • reconstruction algorithms
179
Q

Why would we use overlapping reconstructions?

A

To create thicker slices to send to PACS, less info

MDCT*

180
Q

What is reformatting? What things are required to reformat an image?

A

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
Q

Types of reformatting?

A 
2D:
-MPR
3D:
-MIP
-MinIP
-SR
-VR
182
Q

What is MPR?

A

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
Q

What is MIP?

A

Maximum intensity projection

  • pixels show the highest values detected
  • eliminate other values
184
Q

What is MinIP?

A

Minimum intensity projection

  • pixels show lowest value displayed
  • eliminate other values
185
Q

What structured are best visualized with MIP and MinIP?

A

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
Q

Advantages of MIP and MinIP?

A

-highlight structured of interest and minimize superimposition

187
Q

What is surface rendering? What is it good for?

A

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
Q

What happens if the threshold is too high or too low when surface rendering?

A

Too high: materials like fluid will also be displayed as tissues of interest
Too low: structures can be excluded

189
Q

What is volume rendering?

A

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
Q

What is segmentation? What is another term for it?

A

Selectively removing or isolating structures from an image

Aka ROI editing

191
Q

Most common artifacts that degrade reformats?

A
  • motion
  • metal
  • asymmetrical voxels
192
Q

What causes the “stair-step” appearance when reformatting? How do you fix it?

A

Different DFOVs, image centers, etc.

-fix it by using the image overlap to make more symmetrical voxels

193
Q

Why is contrast used in CT?

A

Objects that share similar densities are difficult to visualize

  • easier viewing
  • localization
  • differentiation
194
Q

What is contrast media?

A

A material that temporarily enhances the radiographic visualization of structured with similar densities

195
Q

Forms of contrast media?

A
  • liquid (most common)
  • gas
  • powder/paste
196
Q

How is it decided whether contrast media is a positive or negative agent?

A

Lower density than structure that required enhancement = negative

Higher density = positive

197
Q

What do you take into consideration when choosing a contrast material?

A
  • radiographic appearance
  • protocol
  • equipment (transit time)
  • anatomy
  • patient (risk/complications)
  • administration
198
Q

Types of contrast we use?

A
  • Air
  • C02
  • Water
  • Barium sulfate solutions
  • Iodinated water-soluble solutions
199
Q

Air and gas contrast: characteristics, purpose, pros/cons

A 
Characteristics:
-non-toxic
-readily absorbed by body
-radiolucent
Purpose:
-distention
-increase contrast
Pros:
-distention
Cons
-spastic colon
200
Q

Why is C02 preferred over air?

A
  • absorption

- patient tolerance

201
Q

Air and gas can be used for what kind of exams? How is it administered?

A
  • virtual colonoscopies
  • GI studies
  • arthrograms
  • myelograms
  • injection
  • orally
  • rectally
202
Q

Water (contrast): characteristics, purpose, pros/cons

A 
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
Q

What studies is water used in? How can it be administered?

A
  • evaluation of pancreas
  • GI studies
  • gastric neoplasms

-orally

204
Q

Barium sulfate: characteristics, purpose, considerations

A 
Characteristics:
-specifically formulated for CT (1-3% concentration)
Purpose:
-increase contrast
-fill atomic structures (radiopaque)
Considerations:
-concentration
-routes of adminstration
205
Q

2 forms of barium sulfate and their characteristics

A

Liquid: low concentration, low viscosity
Paste: low concentration, high viscosity

206
Q

Pros and cons of barium sulfate

A 
Pros: 
-slower transit time
-clings to bowel walls
-additives
-distention
Cons:
-allergies
-fills bowel
-complications
-contraindications
-artifacts
-time
207
Q

What is the usual HU value and concentration of VoLumen?

A

15-30 HU

-0.1% barium sulfate solution

208
Q

Pros of VoLumen compared to normal barium sulfate?

A
  • increased bowel distention
  • improved transit time
  • increased visualization of bowel wall and mucosa
209
Q

What studies is VoLumen used for? How can it be administered?

A

-GI studies

  • orally
  • rectally
210
Q

Iodinated water soluble: characteristics, purpose, and considerations

A 
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
Q

Pros and cons of iodinated media?

A 
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
Q

What studies do we use iodinated media for? How can it be administered?

A
  • GI studies
  • Arthrograms
  • postmyelograms
  • vascular studies
  • arterial studies
  • solid organs
  • tumors
  • orally
  • rectally
  • injection
213
Q

What methods of administration of contrast medias do we use in CT?

A
  • IV
  • Oral
  • Rectal
  • Injection
214
Q

What can be administered rectally? What exams require this? Considerations?

A
  • Air, C02, Barium Sulfate, Iodinated water soluble
  • CT colonoscopy
  • only for large bowel
  • only for patients with specific histories
215
Q

What can we inject intrathecally? What exams do we do this for? Considerations?

A
  • 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
Q

What exams required intra-articular contrast administration? Considerations?

A
  • CT arthrogram imaging
  • simultaneous viewing of soft tissue and bone
  • only imaging completed in CT
217
Q

What can we administer orally? What exams require oral administration?

A
  • CT GI tract imaging

- Air, Gas, Barium Sulfate, Iodinated water soluble, Water

218
Q

What is the only contrast media to be used for intravascular injection? Most commonly used brands?

A

Non-ionic contrast

  • Optiray
  • Omnipaque
219
Q

How many times higher are the chances of having an adverse reaction with HOCM vs. LOCM?

A

4-5 times

220
Q

Give an example of an IOCM contrast media

A

-Visipaque

221
Q

What is the “gold standard” test to confirm if the kidneys can handle contrast?

A

GFR

222
Q

What makes creatinine levels higher?

A
  • men vs. women (gender)
  • younger vs. older (age)
  • blacks vs. whites (race)
  • consumption of cooked meat (protein) = increase
  • malnutrition = decreased
223
Q

What systems do contrast overdoses affect?

A
  • pulmonary

- cardiovascular

224
Q

What is the upper limit of grams of iodine?

A

64gI

225
Q

Categories of iodinated contrast reactions

A
  1. Subjective/normal
  2. Chemotoxic
  3. Idiosyncratic
226
Q

Normal side effects of IV contrast?

A
  • feeling of heat/warmth
  • mild flushing
  • metallic taste
  • nausea and/or vomiting
  • anxiety may increase probability of reactions
227
Q

What is a chemotoxic response to IV contrast? Examples?

A

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
Q

What is the 3rd leading cause of ARF?

A

CIN

229
Q

What is CIN? Signs? Treatment?

A

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
Q

CIN risk factors?

A
  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
Q

How to prevent CIN?

A
  • Use of LOCM or IOCM
  • Hydration: patients should be well hydrated
  • Dose: use smallest dose possible, 48hrs between procedures
  • Temporarily discontinue medications
232
Q

What is an idiosyncratic response to IV contrast?

A

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
Q

How to prevent idiosyncratic reactions?

A
  • avoid using HOCM

- premedication

234
Q

How are idiosyncratic responses to IV contrast classified?

A
  • mild
  • moderate
  • severe
235
Q

Signs/symptoms of mild/moderate idiosyncratic responses? What can you do?

A
  • *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
Q

Signs/symptoms of severe idiosyncratic reactions? What can you do?

A
  • *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
Q

What should you document about a contrast reaction?

A
  • 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
Q

Risk factors for adverse reactions?

A
  • 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
Q

Contraindications to IV contrast?

A
  • 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
Q

Can patients with end stage renal failure undergo contrast media exams?

A

Yes because it cannot increase the extent of preexisting kidney damage

241
Q

What can happen if renal dysfunction occurs and the patient is taking metformin?

A

Metformin can accumulate and cause lactic acidosis

Patient can only resume taking meds 48 hrs after exam

242
Q

What is important to get if you know a patient suffers from high BP?

A

**baseline blood pressure

243
Q

3 methods of IV administration?

A
  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
Q

What do you do if an IV needs to be established in the Ct department?

A
  • 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
Q

Questions to ask yourself if patient arrives with pre-established IV

A
  • 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
Q

What is a CVAD? What is it for?

A

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
Q

What is a PICC?

A

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
Q

Can all PICCs handle mechanical injection?

A

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
Q

Techs should adhere to what kind of practices when using CVADs? What does this include?

A

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
Q

When is documentation for contrast required? What should be documented?

A

Anytime contrast is administered

  • volume/rate
  • injection site
  • injection time
  • GFR, SeCR levels
251
Q

What can techs adjust on mechanical injectors?

A
  • volume and concentration
  • flow rate(s)
  • pressure limits
  • timing between injection and start of scan acquisition
252
Q

What are mechanical injectors used for?

A

Deliver our pre-set bolus of contrast in a short period of time

253
Q

What is a concern with power injectors? How can we alleviate this concern?

A

Extravasation

-a test bolus of saline using the power injector is common practice

254
Q

What must be done to prevent fatal emboli when using a power injector?

A

Air bubbles must be removed

255
Q

Components of mechanical injectors?

A
  • 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
Q

How to load a pressure injector according to AHS

A
  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
Q

3 phases of enhancement (from first to last to enhance)

A
  1. Arterial (bolus)
  2. Non-equilibrium (venous)
  3. Equilibrium (delayed)
258
Q

How long is the peak enhancement of arterial organs after injection?

A

15-45 seconds

259
Q

Which organs breaks the enhancement rules? Why?

A

The brain

  • the BBB
  • scan delay can be 4 mins or longer
260
Q

The exact timing of the start and end of each of the 3 phases are affected by?

A
  • Pharmacokinetics: all about contrast
  • Patient factors: age, cardiac output, body habitus
  • Equipment: system speed, increased scanner speed = you need to increase delay requirement
261
Q

How does volume and dose affect peak enhancement?

A
  • 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
Q

Formula for total volume of contrast injected is?

A

V=(ml/sec)(sec)

263
Q

Effects of flow rate on enhancement?

A
  • 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
Q

Effects of body habitus on enhancement?

A
  • Increased weight = decreased magnitude of peak enhancement
  • Increased weight = no effect on time to peak
  • Increased flow rate (or iodine concentration) = increased enhancement
265
Q

Effects of cardiac output on enhancement?

A
  • Increased heart rate = no effect magnitude of peak

- Increased heart rate = decreased time to peak enhancement

266
Q

What helps us determine the right time to start the scan after an injection?

A

Computer software

267
Q

2 automated injection triggering methods?

A
  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

MRT Year 1 Semester 2 (11 decks)

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