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Flashcards in Notes From 3rd Weeks Readings Deck (75):
1

Gantry

-houses many of the components necessary to produce and detect x-rays
-components are mounted on a rotating scan frame

2

Slip rings permit the gantry frame to rotate _____, making _____ scan modes possible

Continuously, helical

3

Small focal spots in CT tubes produce _____ images (better spatial resolution)

Sharper images
-because they concentrate heat onto a smaller portion of the anode they cannot tolerate as much heat

4

High ___ is used to increase the intensity of the beam

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

Filtering the x-ray beam helps to

-reduce radiation dose to the patient
-improves image quality (by reducing image artifacts that result from beam hardening)

6

Collimators

-restrict x-ray beam to specific area, thereby reducing scatter radiation
-control the slice thickness by narrowing or widening the X-ray beam

7

reducing scatter improves _____ and decreases patient dose

contrast resolution

8

Source collimator aka prepatient collimation

-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

Pre detector collimation aka postpatient collimation

-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

Scan field of view

-determines the size of the fan beam, which in turn, determines the number of detector elements that collect data

11

Capture efficiency

Ability with which the detector obtains photons that have passed through the patient

12

Absorption efficiency

The number of photons absorbed by the detector and is dependent on the physical properties of the detector face (ex thickness, material)

13

Response time

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

Dynamic range

The ratio of the maximum signal measured to the minimum signal the detectors can measure

15

What is the most common material a detector is made out of?

Solid-state crystal variety

16

Solid state crystal detector aka scintillation detectors

-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

Detector spacing

-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

A small detector is important for

-good spatial resolution
-scatter rejection

19

Characteristics of solid state crystal detectors

-high photon absorption
-sensitive to temperature, moisture
-solid material
-can exhibit afterglow
-no front window loss

20

Characteristics of pressurized xenon gas detectors (not used in newer models for CT)

-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

Stability

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

Response time

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

Dynamic range

"Ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated"

24

Afterglow

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