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CT Theory > Notes From 3rd Weeks Readings > Flashcards

Notes From 3rd Weeks Readings Flashcards

1
Q

Gantry

A
  • houses many of the components necessary to produce and detect x-rays
  • components are mounted on a rotating scan frame
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2
Q

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

A

Continuously, helical

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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

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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
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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)

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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
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7
Q

reducing scatter improves _____ and decreases patient dose

A

contrast resolution

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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
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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
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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

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11
Q

Capture efficiency

A

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

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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)

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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

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14
Q

Dynamic range

A

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

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15
Q

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

A

Solid-state crystal variety

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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
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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
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18
Q

A small detector is important for

A
  • good spatial resolution

- scatter rejection

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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
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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
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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

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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

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23
Q

Dynamic range

A

“Ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated”

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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

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25
Q

True/ false: MSCT scanners use gas ionization detectors

A

False!

-because they have low quantum detection efficiency and low x-ray absorption

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26
Q

The detector elements of MSCT scanners use _______ materials

A

Solid-state materials

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27
Q

MSCT scanner should have what properties

A
  • 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
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28
Q

What are the three steps for creating a CT image?

A
  1. Data acquisition
  2. Image reconstruction
  3. Image display
    - image post processing and image storage
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29
Q

Data acquisition

A

Refers to the method by which the patient is scanned to provide us with enough information to construct an image

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30
Q

Scanning

A

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
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31
Q

Data acquisition-gantry geometries

A

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
Q

What are the main components that assist with the data acquisition step?

A

Gantry

Table

33
Q

What are the two methods of data acquisition?

A
  1. Axial (slice by slice)

2. Helical (spiral path)

34
Q

Data acquisition-axial scans

A
  • 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
Q

Advantages of axial scanning

A
  • 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
Q

Disadvantages of axial scanning

A

-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
Q

Data acquisition- helical scans

A

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
Q

Advantages of helical scanning

A
  • 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
Q

Disadvantages of helical scanning

A
  • 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
Q

Data acquisition

A
  • 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
Q

Interpolation

A

-a process of filling information in helical scanning

42
Q

What are the four forms of data?

A
  1. Measurement data
  2. Raw data
  3. Convolved data
  4. Reconstructed data
43
Q

Scan data (aka measurement/ raw data)

A
  • Data measured by the detectors
  • preprocessed data
    • storage capacity
    • image quality
    • artifacts (minimizes)
  • prevents poor image quality
44
Q

Image data (aka reconstructed raw data)

A
  • 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
Q

Data processing in a nutshell

A 
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
Q

Image reconstruction (algorithms)

A

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
Q

Stair step artifacts occur when ____slices are used for reformatting

A

Wide

48
Q

what determines slick thickness in SDCT systems

A

Source collimator width

49
Q

What determines slice thickness in MDCT systems

A
  • pre-patient collimator width

- detector configuration

50
Q

Volume averaging

A

-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
Q

Scan parameters

A 
Scan time
-acquisition speed
-table increments
-pitch
Pre-programmed scan software 
Considerations:
-pt. condition
-equipment limitations
52
Q

Pitch

A
  • 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
Q

Volume averaging can be decreased by changing _____ of the slice

A

The starting point

-there’s no change in slice thickness

54
Q

when the table speed and the beam collimation are identical pitch = ____

A

1

55
Q

When are thick slices used

A
  • when there could be pt motion
  • dont need great detail
  • repeat images
56
Q

⬆️ in pitch =

A

A scan that covers more anatomy lengthwise for a given total acquisition

57
Q

Manipulating pitch can be useful in controlling

A

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
Q

Image thickness vs slice thickness

A

Image thickness: Reconstructed

Slice thickness: acquisition thickness, raw data

59
Q 
⬆️pitch= \_\_\_ acqusition time
⬆️pitch = \_\_\_ pt motion
⬆️pitch = \_\_\_ contrast
⬆️pitch = \_\_\_ pt dose
A

⬆️ pitch = ⬇️acquisition time
⬆️ pitch = ⬇️patient motion
⬆️ pitch = ⬆️contrast
⬆️pitch = ⬇️ pt dose

60
Q

If pitch is equal to or less than 1.5:

A
  • Decreased heat load

- minimal loss of image sharpness

61
Q

How to calculate pitch for MDCT calculations

A

Pitch (P)= table movement in one gantry rotation (d) divided by number of slices multiplied by slice thickness (w)

62
Q

How to calculate pitch for SDCT calculations

A

Pitch (P) = table movement in one gantry rotation (d) divided by slice thickness or beam collimation (w)

63
Q

P= 2

A
  • extrapolation
  • acquisition speed
  • resolution
    • partial volume averaging
64
Q

P= 0.5

A
  • overlap

- pt. dose

65
Q 
Pitch and scan coverage 
Pitch adjustments:
-required anatomy
-equipment limitations
SDCT calculations:
A

Amount of anatomy covered = P x total acquisition time x 1/rotation time x slice thickness

66
Q 
Pitch and scan coverage 
Pitch adjustments:
-required anatomy
-equipment limitations
MDCT calculations:
A

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

67
Q

Matrix (image display)

A
  • 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
Q

Windowing

A

-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
Q

WW

A

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
Q

WL

A

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
Q

SFOV

A

-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
Q

DFOV

A

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
Q

Anything above WW =

A

White

74
Q

Anything below WW =

A

Black

75
Q

A pitch between what is most common in SD and MD CT

A

1-1.5

CT Theory (11 decks)

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