1
Q
Gantry
A
- houses many of the components necessary to produce and detect x-rays
- components are mounted on a rotating scan frame
2
Q
Slip rings permit the gantry frame to rotate _____, making _____ scan modes possible
A
Continuously, helical
3
Q
Small focal spots in CT tubes produce _____ images (better spatial resolution)
A
Sharper images
-because they concentrate heat onto a smaller portion of the anode they cannot tolerate as much heat
4
Q
High ___ is used to increase the intensity of the beam
A
Kv
- increasing its penetrating ability and thereby reducing patient dose
- high kV settings also help to reduce the heat load on the x-ray tube by allowing a lower mA setting
5
Q
Filtering the x-ray beam helps to
A
- reduce radiation dose to the patient
- improves image quality (by reducing image artifacts that result from beam hardening)
6
Q
Collimators
A
- restrict x-ray beam to specific area, thereby reducing scatter radiation
- control the slice thickness by narrowing or widening the X-ray beam
7
Q
reducing scatter improves _____ and decreases patient dose
A
contrast resolution
8
Q
Source collimator aka prepatient collimation
A
- located near x-ray source and limits the amount of x-ray emerging to thin ribbons
- acts on x-ray beam before it passes through the patient
- affects patient dose and determines how the dose is distributed across the slice thickness
- resembles small shutters with an opening that adjusts, dependent on the operators selection of slice thickness
9
Q
Pre detector collimation aka postpatient collimation
A
- located below the patient and above the detector array
- shapes the beam after it has passed through the patient
- ensures the beam is the proper width as it enters the detector
- prevent scatter radiation from reaching the detector
10
Q
Scan field of view
A
-determines the size of the fan beam, which in turn, determines the number of detector elements that collect data
11
Q
Capture efficiency
A
Ability with which the detector obtains photons that have passed through the patient
12
Q
Absorption efficiency
A
The number of photons absorbed by the detector and is dependent on the physical properties of the detector face (ex thickness, material)
13
Q
Response time
A
The time required for the signal from the detector to return to zero after stimulation of the detector by x-radiation so that it is ready to detect another x-ray event
14
Q
Dynamic range
A
The ratio of the maximum signal measured to the minimum signal the detectors can measure
15
Q
What is the most common material a detector is made out of?
A
Solid-state crystal variety
16
Q
Solid state crystal detector aka scintillation detectors
A
- they used a crystal that fluoresces when struck by an x-ray photon
- a photodiode is attached to the crystal and transforms the light energy into electrical (analog) energy
- have high atomic numbers and density than gases, they have higher absorption characteristics
- absorb nearly 100% of the photons that reach them
17
Q
Detector spacing
A
- detectors are separated using spacing bars
- this allows detectors to be placed in an arc or a circle
- measured from the middle of one detector to the middle of the neighbouring detector and accounts for the spacing bar
- ideally all detectors should be placed together as close as possible, so all x-rays are converted to data
18
Q
A small detector is important for
A
- good spatial resolution
- scatter rejection
19
Q
Characteristics of solid state crystal detectors
A
- high photon absorption
- sensitive to temperature, moisture
- solid material
- can exhibit afterglow
- no front window loss
20
Q
Characteristics of pressurized xenon gas detectors (not used in newer models for CT)
A
- moderate photon absorption
- highly stable
- low-density material (gas)
- no afterglow
- losses attributable to front windows and the spaces taken up by the plates
21
Q
Stability
A
Refers to the steadiness of the detector response time
-if the system is not stable, frequent calibrations are required to render the signals useful
22
Q
Response time
A
Refers to the speed with which the detector can detect an x-ray event and recover to detect another event
-response times should be very short
23
Q
Dynamic range
A
“Ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated”
24
Q
Afterglow
A
The persistence of the image even after the radiation has been turned off
-CT detectors should have very low afterglow values
25
True/ false: MSCT scanners use gas ionization detectors
False!
| -because they have low quantum detection efficiency and low x-ray absorption
26
The detector elements of MSCT scanners use _______ materials
Solid-state materials
27
MSCT scanner should have what properties
- large dynamic range
- high quantum absorption efficiency
- high luminescence efficiency
- good geometric efficiency
- small after glow
- high precision machinability
- all detector elements must have a uniform response
28
What are the three steps for creating a CT image?
1. Data acquisition
2. Image reconstruction
3. Image display
- image post processing and image storage
29
Data acquisition
Refers to the method by which the patient is scanned to provide us with enough information to construct an image
30
Scanning
Defined by the beam geometry used (size, shape, and motion of the beam and its path during the scan)
- the beam is shaped by special filters as it leaves the tube
- the beam is collimated to pass only the slice of interest
- the beam is attenuated by the patient
31
Data acquisition-gantry geometries
Defined by the arrangement of the x-ray tube and detectors (for data collection)
Geometries:
-continuous: most common, rotating in the tube
-stationary: detectors built in a ring only thing rotating is the tube
32
What are the main components that assist with the data acquisition step?
Gantry
| Table
33
What are the two methods of data acquisition?
1. Axial (slice by slice)
| 2. Helical (spiral path)
34
Data acquisition-axial scans
- x-ray tube rotates around the patient and collects data from the first slice
- tube stops, the patient moves and the next slice is scanned
- only acquisition method in older generation scanners before the invention of the slip ring
35
Advantages of axial scanning
- image quality
- data can have acquisition variability
- contiguous (no gap of info, no missing info where one slice ends, another begins. All slices are touching)
- gapped (can tell scanner where one bad slice was then it can go back and just reconstruct that one image)
- overlapped (not common, doesn't give more info just gives more dose)
36
Disadvantages of axial scanning
-examination time
-scan delay (there is a pause between slices)
-reconstruction capabilities
⬆️ likelihood of motion artifacts
⬇️ ability to scan contrast filled vessels
-limits reformatting
37
Data acquisition- helical scans
Aka spiral or helical beam geometry
- beam rotates around the patient as multiple projections are taken in a 360 degree scan
- scans a volume of tissue rather than one slice
- slip rings (eliminates cables, faster scan times, continuos acquisition protocols)
- continuous movements
- volume scanning (scans whole thing at once then puts it into slices for us)
- there is a gap in information
38
Advantages of helical scanning
- misregistration (seeing different amount of anatomy because the patients breathing wasn't exactly the same)
- data manipulation capabilities
- scan times (good for uncooperative or trauma pts, or pts unable to lay for long periods of time or kids)
- volume of contrast (less required)
39
Disadvantages of helical scanning
- image quality
- 360 degree of data is not obtained for each helix (image quality is compromised, could miss information)
- extrapolation (means the same thing as interpolation)
- reconstruction
40
Data acquisition
- Radiation beam that transmits through the patient is recorded, then manipulated
- exit data (x-ray energy) is converted to an electrical signal, digitized and assigned a HU number and a specific shade so it can be processed by the computer to create an image
- x-ray photon = analog data
- analog data -> digital data and sent to the computer (converted by the ADC (analog to digital data) using the DAS
41
Interpolation
-a process of filling information in helical scanning
42
What are the four forms of data?
1. Measurement data
2. Raw data
3. Convolved data
4. Reconstructed data
43
Scan data (aka measurement/ raw data)
- Data measured by the detectors
- preprocessed data
- storage capacity
- image quality
- artifacts (minimizes)
- prevents poor image quality
44
Image data (aka reconstructed raw data)
- convolution
- storage capacity
- image quality
- algorithms (directions applied to raw data)
- back projection: data gets "smeared"
- filtered back projection: removes blurring that results from "smearing"
- fourier transform: used to reconstruct MRI images, based on measuring frequencies
- iterative reconstruction: think about automatic rescaling, makes data look more like it should and a nicer. Must terminate after a finite number of steps
45
Data processing in a nutshell
```
Raw data undergoes some form of preprocessing
-raw data is reconstructed
-corrections are made
Image reconstruction
-raw data is converted into a digital image characterized by CT numbers
Image data
-averaged for post processing
-can be reformatted
```
46
Image reconstruction (algorithms)
Algorithms alter the way raw data is reconstructed
- designed to suppress noise and improve detail
- types of algorithms
- standard (balance noise and detail)
- smoothing (soft tissue)
- decreases contrast resolution
- edge enhancement (improve detail, image noise)
47
Stair step artifacts occur when ____slices are used for reformatting
Wide
48
what determines slick thickness in SDCT systems
Source collimator width
49
What determines slice thickness in MDCT systems
- pre-patient collimator width
| - detector configuration
50
Volume averaging
-Partial volume artifact occurs when tissues of widely different absorption are encompassed on the same CT voxel producing a beam attenuation proportional to the average value of these tissues.
Affected by slice thickness
-can hide pathology because of less accurate pixel readings
⬆️ slice thickness = ⬆️ partial volume effect
-inaccurate pixel readings
Retrospective slice incrementation (this causes no increase in pt dose)
-post processing
-overlapping slices
-can help ⬇️partial volume effect
-SDCT (fixed slice thickness)
-MDCT (variable slice thickness) the slices however cannot be smaller than the slice thickness used during data acquisition
51
Scan parameters
```
Scan time
-acquisition speed
-table increments
-pitch
Pre-programmed scan software
Considerations:
-pt. condition
-equipment limitations
```
52
Pitch
- Used to describe CT table movement throughout a helical scan acquisition
- travel distance of the couch per 360 degree rotation of the x-ray tube, divided by the x-ray beam collimation width
- during helical scanning the tube is on for the entire acquisition (as the table moves through the gantry)
- when the table speed and the beam collimation are identical pitch = 1
- only affects how fast the pt moves through the gantry no change in slice thickness
- table will move twice the distance of the slice thickness for each rotation of the tube
53
Volume averaging can be decreased by changing _____ of the slice
The starting point
| -there's no change in slice thickness
54
when the table speed and the beam collimation are identical pitch = ____
1
55
When are thick slices used
- when there could be pt motion
- dont need great detail
- repeat images
56
⬆️ in pitch =
A scan that covers more anatomy lengthwise for a given total acquisition
57
Manipulating pitch can be useful in controlling
Coverage and scan time
-pitch can also be expressed as a ratio
Ex 5mm section with a table speed of 10mm/s = pitch ratio of 2:1
-extrapolation
20 mm slice with a table speed of 10mm/s = a pitch of 0.5
-overlap (bad)
-more accurate extrapolation and data for reconstructive purposes
- increased pt dose
-slower
58
Image thickness vs slice thickness
Image thickness: Reconstructed
| Slice thickness: acquisition thickness, raw data
59
```
⬆️pitch= ___ acqusition time
⬆️pitch = ___ pt motion
⬆️pitch = ___ contrast
⬆️pitch = ___ pt dose
```
⬆️ pitch = ⬇️acquisition time
⬆️ pitch = ⬇️patient motion
⬆️ pitch = ⬆️contrast
⬆️pitch = ⬇️ pt dose
60
If pitch is equal to or less than 1.5:
- Decreased heat load
| - minimal loss of image sharpness
61
How to calculate pitch for MDCT calculations
Pitch (P)= table movement in one gantry rotation (d) divided by number of slices multiplied by slice thickness (w)
62
How to calculate pitch for SDCT calculations
Pitch (P) = table movement in one gantry rotation (d) divided by slice thickness or beam collimation (w)
63
P= 2
- extrapolation
- acquisition speed
- resolution
- partial volume averaging
64
P= 0.5
- overlap
| - pt. dose
65
```
Pitch and scan coverage
Pitch adjustments:
-required anatomy
-equipment limitations
SDCT calculations:
```
Amount of anatomy covered = P x total acquisition time x 1/rotation time x slice thickness
66
```
Pitch and scan coverage
Pitch adjustments:
-required anatomy
-equipment limitations
MDCT calculations:
```
Amount of anatomy covered = P x total acquisition time x 1/rotation time x (slice thickness x slices per rotation)
67
Matrix (image display)
- a 2D array of numbers that make up a digital image
- made up of columns (M) and rows (N)
- define small square regions called picture elements (aka pixels)
- shape
- the larger the matrix size, the smaller the pixel size for the same FOV
- this will result in better spatial resolution
68
Windowing
-post processing feature
-manipulates only image data
-controls image display contrast and brightness
WINDOW WIDTH:
-contrast
-HU range
-controls the range of CT numbers displayed in an image
-represents the max number of shades of grey that can be displayed on the image
-⬆️ WW = good when viewing very different densities on an image
-narrow WW = good when viewing brain because densities are similar ex greys and whites
WINDOW LEVEL
-controls brightness
-increase WL = decrease brightness
-centre point
-center over the anatomy of interest
-acts as a reference point
-determines the central value range of CT numbers
-selects which CT numbers are displayed as shades of grey
-does not affect grey scale
69
WW
WINDOW WIDTH:
- contrast
- HU range
- controls the range of CT numbers displayed in an image
- represents the max number of shades of grey that can be displayed on the image
- ⬆️ WW = good when viewing very different densities on an image
- narrow WW = good when viewing brain because densities are similar ex greys and whites
70
WL
WINDOW LEVEL
- controls brightness
- increase WL = decrease brightness
- centre point
- center over the anatomy of interest
- acts as a reference point
- determines the central value range of CT numbers
- selects which CT numbers are displayed as shades of grey
- does not affect grey scale
71
SFOV
-everything that is going to be imaged
-select which area we want to scan
Aka scan field of view
-determines amount of space used within the aperature during data acqusition
-scan diameter
-isocenter
-number of detector cells
- half field and full field
-out of field artifacts
-anything outside the SFOV is not imaged (no data collected)
72
DFOV
Cannot be larger than SFOV but can be equal
-a smaller DFOV produces a zoomed image
Ex vertebra within the SFOV of the abdominal slice
Aka display field of view
-data used for image display
-affects pixel size
-spatial resolution
(Displays region of interest in greater detail)
-determines how much of the collected raw data will be used to create an image for display
73
Anything above WW =
White
74
Anything below WW =
Black
75
A pitch between what is most common in SD and MD CT
1-1.5
CT Theory
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