Basic CT Principles Flashcards
1
Q
General radiography
A
Imaging modality that produces 2D images
Limitations:
-superimposition of all structures
-difficult to distinguish slight density changes of tissues
-difficult to identify precise location of abnormalities
2
Q
Tomography
A
-x-ray tube and film move simultaneously and in opposite directions
-imaging modality that brings into focus only the anatomical structure lying in a plane of interest, while structures on either side of that plane are blurred
-immediate goal us to eliminate structures above and below the focused section or focal plane. This is difficult to achieve
-limitations: persistent image blurring that cannot be completely removed, degradation of image contrast because of scatter, fail to demonstrate slight differences in subject contrast (tissue), contrast cannot be adjusted after it has been recorded on film
-focal plane (aka section thickness)
▪️blur ⬆️ as distance from the focal plane ⬆️
▪️fulcrum: imaginary pivot point about which the x-ray tube and the image recpetor move
-⬆️ tomographic angle = ⬇️ section thickness
3
Q
Focal plane
A
Section thickness
4
Q
Fulcrum
A
-imaginary pivot point about which the x-ray tube and the image receptor move
5
Q
Advantages of tomography vs general radiography
A
⬆️ radiographic contrast
⬆️ subject contrast
⬇️super imposition
6
Q
Disadvantages of tomography vs general radiography
A
⬆️ patient dose
7
Q
What is a shortcoming of radiography?
A
Superimposition of structures
8
Q
What is an example of a negative contrast agent?
A
Air, water
9
Q
How would a contrast agent with a high Z (atomic #) affect a radiographic image?
A
Increased image contrast, due to increased absorption
10
Q
If an object is slightly less dense than water, what would be the expected HU measurement?
A
-10
11
Q
The gantry houses all of the following except A) 3 phase generator B) beam collimators C) collimators D) DAS
A
A) 3 phase generator
12
Q
_____ interactions produce scatter, which degrades image quality and image _____. Anytime more of these types of interactions occur image quality is _____
A
COMPTON interactions produce scatter, which degrades image quality and image CONTRAST. Anytime more of these types of interactions occur image quality is DECREASED
13
Q
True or false: two essential components of a digital image processing system are the ADC and the DAC
A
TRUE
14
Q
What is another name for density values?
A
CT numbers
15
Q
Increasing _______ reduces patient dose, because the ____ energy photons that could be absorbed by the patient are removed from the x-ray beam
A
Filtration, low energy photons
16
Q
Data acquisition in CT refers to
A
Getting data from the patient through a systematic motion of the x-ray tube and detectors
17
Q
What is not a common anatomical landmark in CT?
A
Jugular notch
18
Q
Give an example of a low attenuation structure
A
Trachea (air filled)
19
Q
What does not produce a transverse axial sectional image?
A
Conventional radiography
20
Q
An image of a hyposthenic will have _____ than an image of an asthenic patient?
A
Decreased radiographic density, due to exponential attenuation
21
Q
What happens to the emission spectrum when a higher voltage waveform is utilized?
A
Increased amplitude; increased average energy
22
Q
True or false: area closest to the fulcrum has less blur
A
TRUE
Farther away from fulcrum = more blur
23
Q
True or false: fulcrum is dead centre on image
A
True
24
Q
True or false: big angle on the tube = smaller slice
A
True!
25
Tomography problems
- image blurring (poor image contrast outside of focal plane)
- patient dose
- GOOD for curved structures
26
True or false: The isocenter of a gantry is not an important consideration for patient positioning
False!
27
What is a universal limitation, and what is a common concern in x-ray imaging?
Heat production; motion
28
True or false: decreasing the tomographic angle increases slice thickness
True!
29
In CT, a high kilo-voltage technique is generally used for which reason?
Decrease the dependence of attenuation coefficients on photon energy
30
When a projectile electron interacts with the nuclear field of an atom and changes directions it loses _____ energy, in the form of _____ radiation
Kinetic energy, bremsstrahlung radiation
31
The x-ray generator used in modern CT scanners is a ____ generator
High frequency generator
32
What is not a synonym for the preliminary (or localizer) image taken at the start of a CT examination
Spiral
33
True or false: scouts are used to check for accurate patient positioning
False!
34
This factor controls beam quality
kVp
35
KVp affects what 3 aspects of the emissions spectrum
Quantity, average energy, peak energy
36
What contributes no useful information to the radiographic image
Compton scatter
37
The total number of photons remaining in an x-ray beam after penetrating through a given thickness also stated as the degree to which an x-ray beam is reduced by an object is referred to as
Attenuation
38
A complete set of ray sums is
A view (aka projection)
39
A patient who appears thin, but healthy can be described by this term
Hyposthenic
40
This term describes an average sized patient
Sthenic
41
Only the amplitude of the emission spectrum is affected by this controlling factor
Quantity (aka mAs)
42
The detector (DAS) captures each arriving ray and senses how much of the beam was attenuated is called
Ray sum (aka data sample)
43
This type of interaction is defined by a projectile electron interaction with the nuclear field of an atom
Bremsstrahlung
44
The path that the x-ray beam takes from the tube to the detector is a
Ray
45
This factor controls beam intensity
MAs
46
The attenuation measurements of the transmitted radiation, that reconstruct an image of the scanned object is a
Projection profile
47
What are the main advantages of CT over conventional radiography?
- elimination of superimposed structures
- ability to differentiate small differences in density of anatomic structures and abnormalities
- superior quality of images
48
The preliminary image each scanner produces may be referred to as
- scout
- topogram
- scanogram
49
Spatial resolution
-the ability of a system to define small objects distinctly
50
Low-contrast resolution
The ability of a system to differentiate, on the image, objects with similar densities
51
Temporal resolution
- refers to the speed that data can be acquired
- this speed is particularly important to reduce or eliminate artifacts that result from object motion ex when imaging the heart
52
The images created by CT are
Cross sectional
53
Each CT slice represents a specific plane in the patients body. The thickness of the plane is referred to as the ___ axis
Z axis
| -z axis determines the thickness of the slices
54
True or false: selecting a slice thickness limits the x-ray beam so that it passes only through this volume
True!
55
In CT the most common matrix size is
512
| -this translates to 512 rows of pixels down and 512 pixels across
56
True or false: a larger matrix size will contain smaller individual pixels?
True!
57
X-ray photons that pass through objects unimpeded are represented by a ____ area on the image. These areas on the image are commonly referred to as having ___ attenuation
Black, low
58
X-ray beam that is completely absorbed by an object cannot be detected; the place on the image is ____. An object that has the ability to absorb much of the x-ray beam is referred to as having ____ attenuation
White, high attenuation
59
Areas of intermediate attenuations are represented by various shades of ____
Grey
60
The number of photons that interact depends on what 3 things?
1. Thickness
2. Density (objects that are more dense provide more opportunities for photon interaction compared to less dense elements)
3. Atomic number of object
61
Two examples of positive contrast agents
- barium sulfate
| - iodine
62
To differentiate adjacent objects on a CT image, there must be a ____ difference between the two objects
Density difference
63
____ units quantify the degree that a structure attenuates an x-ray beam
Hounsfield units (HU) may also be referred to as CT numbers
64
Housnfield assigned distilled water the number ___. He assigned the number ____ to dense bone and - ____ to air
Water the number 0, 1000 to dense bone, and -1000 to air
65
Objects with a beam attenuation less than that of pure water have an associated ____ number. And substances with an attenuation greater than that of water have a ____ hounsfield value
Negative number, positive number
66
Polychromatic x-ray beams
The x-ray beam comprises photons with varying energies
67
Beam hardening artifacts
Results from preferential absorption of low energy photons, which leaves higher intensity photons to strike the detector array
-appear as dark streaks or vague areas of decreased density, sometimes called cupping artifacts
68
The smaller the object being scanner the ___ the CT slice required
Thinner
69
Thicker CT slices increase the likelihood of missing very ____ objects
Small
70
The process in CT by which different tissue attenuation values are averaged to produce one less accurate pixel reading is called
Volume averaging
| -using a small pixel size reduces the likelihood of volume averaging by limiting the amount of data to be averaged
71
Raw data vs image data
Raw data:
-all the bits of data acquired by the system with each scan aka scan data
-the computer data waiting to be processed to create an image
-has not yet been sectioned to create pixels; hence no hounsfield unit values have been assigned
Image data:
-once raw data has been processed so that each pixel is assigned a hounsfield unit value, an image can be created, the data included in the image is referred to as image data
72
Transverse planes
- parallel to the floor
| - horizontal
73
Longitudinal planes
- perpendicular to the floor
| - vertical
74
Coronal
Divides body into anterior and posterior
75
Axial plane
Divides the body into upper and lower sections
76
Gantry
Ring-shaped part of the CT scanner that houses many of the components necessary to produce and detect x-rays
77
True or false: x-rays are produced when a substance is bombarded by fast-moving electrons
True
78
Intensity of x-ray beam controlled by
KVp setting
79
Data acquisition system (DAS)
Each detector cell is sampled and converted into a digital format by the DAS
-each complete sample is called a view
80
Central processing unit (CPU)
The digital data from the DAS are then transmitted to the central processing unit (CPU).
-often referred to as the brain of the CT scanner
81
The CT process can be broken down into 3 general segments
- data acquisition (get data)
- image reconstruction (use data)
- image display (display data)
82
Localizer images
- all routine CT studies require at least one localizer scan
- not cross sectional, very similar to conventional radiography
- are digital image acquisitions that are created while the tube is stationary and the table moves through the scan field
- position of tube determines orientation of image ex if tube is positioned above the patient, resulting scan will be AP view, whereas positioning the tube to the side of the patient will result in a lateral view
- can be called scout
- the optimal scout includes all areas to be scanned and therefore ensures that the anatomy to be imaged has been placed within the range of the system so you know if ex lying too close to edge of table or if patient isn't centered appropriately in the gantry in both x and y directions
- miscentering in x or y can result in out of field artifacts
- proper centering particularly important when AEC options are used
- allows tech to prescribe the location of cross sectional slices and find anatomical landmarks
- help tech to select optimal display field of view (DFOV) and correct image center
- AP localizer can only be used to adjust the image center in the right left direction
- a lateral localizer can be used to adjust the subsequent image center in the top-bottom (or anterior-posterior) direction
83
True or false: incorrectly inputting directional instructions into the CT scanner can result in images that have been mislabeled and can result in misdiagnosis and serious medical errors. Important to input correct directional instructions before data acquisition is initiated
True!
84
Step and shoot scanning aka axial scanning or conventional scanning
- CT table moves to the desired location and remains stationary while the x-ray tube rotates within the gantry, collecting data
- there is a slight pause in scanning between data acquisitions, referred to as interscan delay, as the table moves to the next location
- result in highest image quality (superior to helical methods)
- axial slices can be programmed so that the data acquired are contiguous (in which one slice abuts the next. Non contiguous = some areas of the patient are skipped between slices or slice data overlaps), gapped, or overlapping
- all current scanners offer the option of axial scanning
- disadvantage: takes a long time, data is more limited in how they can be reconstructed
85
When does misregistration occur?
When a patient breathes differently with each data acquisition
-valuable information may be missed
86
Helical scanning aka spiral
- dramatic improvement in scanning speed by eliminating interscan delay
- 3 basic ingredients that define a helical scan process
1. Continually rotating x-ray tube
2. Constant x-ray output
3. Uninterrupted table movement
- advantages:
- improved image resolution
- decreased cardiac and respiratory motion artifact
- superior multiplanar and 3D reformation capabilities
- optimize iodinated contrast agent administration
- the end result is a block of data, not separate slices although the information is acquired in ribbons
87
Improvements of helical scanning over axial
- x-ray gantries with a slip ring design
- more-efficient tube cooling
- higher x-ray output (increase mA capability)
- smoother table movement
- software that adjusts for table motion
- improved raw data management
- more efficient detectors
88
Slip rings
- before helical, CT gantries moved first in one direction, then stopped as the table moved to the next position. The gantry then reversed direction for the next acquisition
- slip rings allow the tube to move continually in the same direction
- much faster
- slip rings are in helical scanning
89
Helical scanning and more efficient tube cooling
- constant x-ray output with helical scan acquisition
- more heat built up in the system
- place tremendous stress on the x-ray tube
- improvements in tube heat capacity and heat dissipation have helped to rid some of the heat generated
- tube heat can still be a significant issue, often limiting the length of helical acquisition and mAs used to scan
90
True or false: the overall mAs needed to produce an adequate image is approximately equal, whether the slice is produced in an axial or a helical mode
True!
91
What are the improvemts of helical over axial scanning?
- slip rings
- more efficient tube cooling
- higher x-ray output
- more efficient detectors
- smoother table movement and special software (can compensate for only table motion)
- improved raw data management (more data storage)
92
An axial image is taken so that each slice is ____ to every slice. In contrast helical CT creates slices that are at a slight ____
Parallel, tilt
93
What is the most conspicuous difference between conventional radiographic imaging and CT imaging?
-CT shows cross sectional or transaxial anatomy, which can be subject to digital post processing operations to produce 3D images
94
The 3 physical principles of CT
1. Data acquisition: systematic collection of information from the patient to produce the CT image
2. Data processing
3. Image display, storage and communication
95
Data acquisition
Systematic collection of information from the patient to produce the CT image
-two methods: slice by slice and volume data
Slice by slice data acquisition: the X-ray tube rotates around the patient and collects data from the first slice. The tube stops, and the patient moves into position to scan the next slice. This process continues until all slices have been individually scanned
Volume data acquisition: spiral or helical geometry is used to scan a volume of tissue rather than one slice at a time
96
Spiral or helical CT
- x-ray tube rotates around the patient and traces a spiral/ helical path to scan an entire volume of tissue while the patient holds a single breath
- generates a single slice per one revolution of the x-ray tube
- aka single-slice spiral/ helical CT
- to improve the volume coverage speed performance of SSCT, multislice spiral/ helical CT MSCT has become available for faster imaging of patients
- MSCT scanners generate multiple slices per one revolution of the x-ray tube
97
Attenuation in CT depends on what 3 things
Atomic number (Z), density, photon energy
98
In a homogenous beam the ____ of the beam, or beam energy does not change
Quality
| -however just quantity is reduced
99
During attenuation of a heterogenous beam both the ___ and the ___ of the photons change
Quantity and quality (energy)
100
Photoelectric effect
- occurs mainly in tissues with a high atomic number, Z (such as bone and contrast medium)
- occurs minimally in some soft tissues and substances with a lower Z
- depends on beam energy
101
Compton effect
- occurs in soft tissues, and differences in density result in differences in compton interaction
- less likely to dominate as the beam energy increases
- "energy dependence" not nearly as dramatic as with PE effect
102
Two data acquisition geometries
1. Continuous rotation- x-ray tube and detectors are couples and rotated 360 degrees around the patient to collect transmission measurements by using a fan beam of radiation
- most common
2. Stationary detectors- x-ray tube rotates 360 degrees around the patient and is positioned inside a stationary ring of detectors
103
Data processing
Basically a two step process
- first RAW DATA (data received from the detectors) undergo some form of pre processing, in which corrections are made and some reformatting of the data occurs
- IMAGE RECONSTRUCTION, the scan data which represent attenuation readings, are converted into a digital image characterized by CT numbers of HU
- conversion of the attenuation readings into a CT image is accomplished by reconstruction algorithms
104
The range of CT numbers is referred to as ____, and the center of the range is the ____ or window center
window width (WW), and window level (WL)
- both located on control console
- WW controls the image contrast
- WL controls image brightness
- changing the CT image grayscale is called windowing
105
Field of view (FOV)
The tech must the FOV or reconstruction circle, which is a circular region from which the transmission measurements are recorded during scanning
-this region is specially referred to as the scan field of view
106
Ray
The part of the beam that falls on one detector
107
Each transmission measurement is referred to as a ____
Data sample
108
Straton x-ray tube
- the electron beam from the cathode is deflected to strike the anode at two precisely located focal spots that vary in size
- anode in direct contact with cooling oil
- never accumulates heat during exposure
- vectron is improvement on this: electron beam is now accurately and rapidly deflected, creating two focal spots alternating at 4480 times/s. this significantly increases in-plane resolution
109
Filtration
- serves a dual purpose
- removes long wavelength x-rays because they do not play a role in CT image formation and contribute to patient dose
- as a result of filtration, the mean energy of the beam increases and the beam becomes "harder", which may cause beam-hardening artifacts
- it shapes the energy distribution across the radiation beam to produce uniform beam hardening when X-rays pass through the filter and the object
- placed between the x-ray tube and the patient
110
Total filtration is equal to the sum of ___ filtration and ___ filtration
Added and inherent filtration
111
Bowtie filter
- shaped filter
- features bilateral symmetry with a thickness that increases with the distance from the center
- compensate for the difference in beam path length through the axial plane of the object such that a more uniform fluence can be delivered to the detector
112
There are ___ types of beam shaping filters in CT
2
| -these filters attenuate the beam so that a more uniform (monochromatic) beam falls onto the detectors
113
Collimator
- help define the slice thickness
| - restrict the x-ray beam to the cross section of interest
114
Two important features of the Gantry are
The gantry aperature (the donut hole) and gantry tilting range
-also includes a set of laser beams to aid in patient positioning
115
A complete set of ray sums is a
View
116
Ray sum
Detector senses each arriving ray and measures how much of the beam has been attenuated
117
Attenuation profile
The system accounts for the attenuation properties of each ray sum and correlates it to the position of the ray
- created for each view in the scan
- the processes of converting the data from the attenuation profile to a matrix is known as back projection
118
Back projection
Compiles the information from all of the attenuation profiles to create an image
119
Convolution
The process of applying a filter function to an attenuation profile
120
True or false: filter functions can only be applied to raw data (not image data)
True!
121
True or false: There is a significant drawback to back projecting data onto a matrix: it produces streak artifacts in a star pattern on the image
True!
| -to minimize these artifacts, a process called filtering is applied to the scan data before back projection occurs
122
Which of the following is a shortcoming of radiography?
A) image blur due to tube motion
B) contrast degradation since low kVp techniques are used
C) superimposition of structures
D) the presence of ghost images due to tube movement
C) superimposition of structures
123
CT overcomes the limitations of radiography and tomography by:
A) reducing the problem of superimposition
B) improving the contrast of the image
C) using quantitive detectors
D) all are correct
D) all are correct
124
Data acquisition in CT refers to:
A) acquiring information from the CT detectors
B) acquiring information from the computer
C) acquiring data from the patient through a systematic motion of the x-ray tube and detectors
D) all are correct
C) acquiring data from the patient through a systematic motion of the x-ray tube and detectors
125
```
Attenuation of a beam of radiation depends on
A) atomic number
B) density and electrons/ gm of tissue
C) the energy of the radiation
D) all are correct
```
D) all are correct
126
```
The range of CT numbers is called the:
A) widow width
B) window level
C) field of view
D) dynamic range
```
A) window width
127
```
Convolved data are:
A) reformatted data
B) data that have been filtered
C) data from the preprocessor
D) data from the back projector
```
B) data that have been filtered
128
```
Data from the CT detectors are first sent to the:
A) array processor
B) host computer
C) back projector
D) preprocessor
```
D) pre processor
129
Beam attenuation can be defined as:
A) the ability of an x-ray beam to penetrate tissue (beam quality)
B) the degree to which an x-ray beam is reduced by an object
C) the x-ray photons that remain after the beam exits through an object (exit radiation/transmitted radiation)
D) the scale of contrast that is sued to assess the nature of tissue
B) the degree to which an x-ray beam is reduced by an object
130
Attenuation depends on an objects:
A) thickness and density only
B) thickness, weight and density
C) thickness, weight and atomic number (z)
D) thickness, density, and atomic number (z)
D) thickness, density, and atomic number (z)
131
Photons that pass through objects unimpressed would be represented as:
A) white on the image
B) grey on the image
C) black on the image
C) black on the image (high radiographic density)
132
Areas of intermediate attenuation would be represented as:
A) white on the image
B) grey on the image
C) black on the image
B) grey on the image
133
The energy of a photon affects beam attenuation (true/false)
True
| Higher energy photons are more likely to be directly transmitted
134
```
Bony anatomy produces images that contain ____ and appear ____
A) high attenuation; white
B) high attenuation; black
C)low attenuation; white
D) high attenuation; white
```
A) high attenuation; white
| If photons are absorbed image will appear white
135
Images that contain tissue with similar atomic numbers (z) and are uniform in thickness would have:
A) high contrast
B) long scale contrast (lots of greys on the image that have similar shades
C) short scale contrast
D) low spatial resolution
B) long scale contrast (lots of greys on the image that have similar shades
136
```
What is the purpose of using a positive contrast agent?
A) increase spatial resolution
B) decrease spatial resolution
C) increase contrast resolution
D) decrease contrast resolution
```
C) increase contrast resolution (makes the contrast filled area easier to see, more white)
137
An x-ray beam is homogenous (true/false)
False (no, beam is heterogenous)
138
```
Filtration is designed to make an x-ray beam:
A) more uniform
B) heterogenous
C) less penetrating
D) more attenuating
```
A) more uniform
139
```
Which imaging plane is perpendicular to the body?
A) sagittal
B) coronal
C) oblique
D) axial (aka transverse)
```
D) axial (aka transverse)
140
Pixel size is ____ when the matrix size is ____
A) decreased; increased
B) unchanged; increased
C) increased; increased
A) decreased; increased
141
A matrix size of 512 has greater resolution than a matrix size of 1024 (true/false)
False
142
Computed tomography (main advantages)
- low contrast resolution
- data acquisition variability
- image reconstruction capabilities
- 3D imaging
143
Primary disadvantages of computed tomography
- radiation doses
- artifacts
- decreased spatial resolution
144
Gantry
Houses many of the CT system components
- tube
- detector array
- generator (enables high kV settings)
- filtration
- collimators
- DAS
145
Scan data (aka measurement/raw data)
- Data measured by the detectors
| - preprocessed data
146
Image data (aka reconstructed raw data)
- convolution
| - algorithms ("directions" applied to raw data)
147
Back projection algorithm
- data gets "smeared"
| - process of converting the data from the attenuation profile to the matrix
148
Filtered back projection
Removes blurring that results from "smearing"
149
Fourier transform algorithm
Used to reconstruct MRI images based on measuring frequencies
150
Iterative reconstruction algorithm
Makes the image look nicer and more like it should same idea as automatic rescaling
-must terminate after a finite number of steps
151
When mAs will affect contrast resolution
- ⬇️ SNR = ⬆️ noise quantum mottle
- extreme overexposure
mAs DOES NOT change spatial resolution
152
Signal =
The number of photons in the image = decreased noise
153
1st generation
▪️Motion of system: translate-rotate (180 degree rotation) (translate means move in a straight line)
▪️Shape or geometry of the beam: narrow pencil beam
▪️Path of tube travel: 180 degree rotation
▪️Detectors: no array (1-2 max)
▪️Scan time: 4.5-5 min
154
2nd generation
▪️motion of system: translate-rotate
▪️shape or geometry of the beam: narrow fan beam (multiple pencil beam scanning)
▪️path of tube travel: 180 degree rotation
▪️detectors: linear detector array (max 30)
▪️scan time: 20s-3.5 min
155
3rd generation (most common)
▪️motion of the system: rotate-rotate (continuosly rotating fan beam)
▪️shape or geometry of the beam: wide fan beam
▪️path of tube travel: 360 dgeree rotation (cables eliminated and now using slip rings)
▪️detectors: curved detector array (30-40 degree arc)
▪️scan time: a few seconds
156
4th generation
▪️motion of system: rotate-fixed (continuously rotating fan beam)
▪️shape or geomtery of the beam: wide fan beam
▪️path of tube travel: 360 degree rotation (tube rotates but detectors are fixed)
▪️detectors: circular single row detector array, 4800 detectors, ⬆️ scatter
▪️scan time: very short
