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RAD413 > CT physics > Flashcards

CT physics Flashcards

1
Q

principle behind creating CT images:

measurement of xray attenuation at multiple angles through the object

A
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2
Q

what are the 3 components of a CT scanner

A
  • donut shaped gantry
  • patient table
  • computer and console
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3
Q

DAS is used to send data from scanner to computer and DAC is used to send processed date to the monitor, what do they stand for

A

data acquisition system
digital to analogue converter

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4
Q

what do you have within the gantry of the scanner

A
  • patient table
  • xray tube that spins 360 around table
  • xray detector opposite of tube
  • xray fan beam coming out of tube
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5
Q

not that xray tube in ct scanner has filtration and collimation for xray beam coming out

A
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6
Q

what is the voltage used in CT Xray tube

A

80-140 kV

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7
Q

find a diagram of a CT Xray tube and label the diagram

A
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8
Q

what is the size of the movable collimator in found outside the tube housing (outside the window of the tube)

A

1 - 160mm

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9
Q

what is the focal spot size of an xray tube

A

0.5 - 1.2 mm

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10
Q

modern ct scanners need powerful generators, up to what power is required

A

100kw

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11
Q

due to the fast rotation of the xray tube, focal spot stability and heating can be issues, what adaptation are made to accommodate for these things

A
  • dual shaft support anode
  • spiral groove bearings (facilitate rapid heat transfer)
  • segmental anode
  • direct oil cooled anode
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12
Q

ideally CT needs mono energetic xrays, filtering in tube and housing can absorb low energy xrays (affecting patient dose not image), what material and what size material is used as filter

A

anything equivilant to a minimum of 2.5mm

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13
Q

how does filtration affect beam hardening artefacts

A

it reduces beam hardening artefacts

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14
Q

despite filtration, beam hardening can still happen due to patient shape, how is this so

A

patients are thicker towards their midline in general

  • longer path through center = greater attenuation and more beam hardening
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15
Q

how can the affect of beam hardening due to patient shape be prevented

A

using a beam shaping (bow tie) filter to ensure more uniform dose across patient

  • so beam is more intense exiting at centre
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16
Q

what is the width of the fan beam in the axial plane

A

500mm (at isocentre)

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17
Q

what is the width of the fan beam in the z-axis

A

1-160 mm

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18
Q

what are the 2 types of CT detectors

A

CT detectors can have ionisation (gas) or scintillation (solid) detectors

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19
Q

what is detector efficiency

A

ability to capture, absorb and convert xray photons to electrical signals

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20
Q

what is geometric efficiency of detector

A

active detector area / irradiated detector area

  • Characterises the extent of the radiation beam that is used for image creation
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21
Q

what is quantum efficiency of the detecotr

A

number of photons absorbed by detector

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22
Q

what 3 things is quantum efficiency dependant on

A
  • atomic number
  • density
  • thickness
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23
Q

what is conversion efficiency of detector

A

conversion of captures photons to electrical signal

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24
Q

how is geometric efficiency determined

A

via ‘dead’ spaces between elements

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25
Q

compare the quantum efficiency between gas and scintillator detectors

A

gas = 60% QE

scintillator = 100% QE

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26
Q

what is response time of detector

A

refers to speed with which the detector can detect an xray event and recover to detect another event

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27
Q

what are the 3 things a detector does during the response time

A
  • transfer energy of photn
  • record electrical signal
  • discard evidence of the photon in readiness for next event
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28
Q

scintillators have a slower decay phase during the response time (after glow)

A
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29
Q

define stability in CT

A

output is constant over a period of time

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30
Q

define uniformity in CT

A

for given incident intensity, all detectors should give same output

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31
Q

how do you ensure stability and uniformity in CT machine

A

regular callibration

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32
Q

detectors need to respond to both very high and low photon intensities

ratio of largest signal measure to smallest signal is discriminated

A
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33
Q

how do CT detectors operate

A
  • operate in ‘current’ mode
  • does not measure individual pulses and energy
  • pulses are integrated to produce steady current flow
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34
Q

what is the size of current flow of detector determined by

A

product of the average event rate and charge produced per event

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35
Q

what gas molecule is used in ionisation detectors

A

Xe gas

single chamber containing pressurised Xe gas

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36
Q

how does an ionisation detector work

A
  • tungsten septae creating regions in detector
  • pressure increased in detector to increase probability of xray hitting gas (30 atoms)
  • xrays ionise the Xe (remove an electron)
  • electrons pulled towards anode via electric field
  • read as electrical signal
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37
Q

what are some pros and cons with ionising detectors

A

pros
- simple
- high precision and stable
- uniform response
- no afterglow
- fast response time

cons
- poor quantum efficiency
- difficult to manufacture multirow

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38
Q

how does a scintillation detector work

A
  • xray converted to light photon via scintillation crystals
  • visible photon/light converted to electrical signal via photomultiplier or photodiode
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39
Q

how is the signal amplified in a photomultiplier tube in a scintillation detector

A

light photon hits photocathode, producing electron

electron hits dynode stimulating electron avalanche

avalanche eventually reaches anode forming electrical signal

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40
Q

how does a photodiode generate an electric signal

A
  • has a p-n junction
  • p side is packed with holes, n side is packed with electrons
  • When light energy is detected by the device (usually above a certain set level called the bandgap) (at the lens) this causes new holes and electrons to be created, generating an electrical current in the p-n junction.

In photodiodes that are reverse-biased, the holes move towards the anode and the electrons to the cathode, creating a current in the depletion region. As the brightness of light increases, so does the current in the device.

(light incident on semiconductor creates electron-hole pairs)

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41
Q

why might a photodiode be better than photomultiplier tube

A
  • higher QE
  • more compact size
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42
Q

what crystal is typically used in scintillators and why

A

caesium iodide

  • high absorption efficiency
  • wavelength matches response of PM tube
  • slow decay
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43
Q

why might a gemstone detector be better

A
  • recovery time is x4 faster than comparable detectors
  • primary speed is 100 times faster than competitors
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44
Q

pros and cons of scintillation detectors

A

pro
- good QE
- easy to manufacture multi-row

cons
- low dynamic range
- slow response time (afterglow)
- issues with stability
- can suffer from non-uniform response (ring artefacts)
- require regular re-calibration

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45
Q

what is DAS, its use

A

data acquisition system

  • electronics between detector and computer
  • measures transmitted radiation (analogue)
  • encodes to digital data and transmoits to computer
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46
Q

log amplifier converts transmission data into attenuation and thickness

A
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47
Q

detector —> log amplifier —> ADC —-> digital transmission —-> computer

A
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48
Q

what is DAC and how does it work

A

analogue to digital converter
- divides signal into multiple parts

  • parts are measured in bits so e.g 1 bit ADC = 2 digital values (2^1) 2 bit ADC = 4 digital values (2^2)
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49
Q

what size bit ADC does a modern CT scanner use

A

16 bit (2^16)

determines grey scale resolution

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50
Q

data transmission to computer by light emitting diodes (optical transmission)

A
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51
Q

what 3 things does the attenuation of xrays between tube and detector depend on

A
  • linear attenuation coefficient
  • μ
  • thickness (t)
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52
Q

what is μ and what are the units of μ

A
  • average linear attenuation coefficient
  • cm-1
53
Q

what 3 things does the μ of a material depend on

A
  • atomic numer (Z)
  • density
  • xray energy (E)
54
Q

total attenuation ‘summed’ along the path between tube and detectors

A
55
Q

how do the number of projections and spatial resolution relate

A

increased projection = improved spatial resolution

56
Q

3rd generation CT scanner features?

A
  • wide fan beam
  • 500-100 detectors
  • rotation only
  • less than 1sec per rotatin
57
Q

4th generation CT scanner features?

A
  • fan beam
  • static 360 degree detector ring
  • only tube rotates
  • poorer scatter discrimination
58
Q

how has the 4th gen improved from the 3rd gen CT scanner

A
  • avoided ring artefact problem of 3rd
59
Q

advantages of 3rd gen scanner

A
  • fan beam allows patient coverage without translation stage
  • fast acquisition than 1 and 2 gen
  • rotating detectors reduce number of detector elements needed
  • detector collimation reduces scatter
  • more easily adaptable to multi row
60
Q

define algorithm in ct

A

set of defined mathematical rules for solving a problem

61
Q

define convolution filter/ kernel (reconstruction filter)

A

mathematical function for smoothing or sharpening data

62
Q

define convolution

A

digital imaging processing technique to modify images with filter/kernel

63
Q

define interpolation

A

used in reconstruction process of helical data

64
Q

Fourier series:
objects can be described by spatial frequencies

amplitude and frequency of sine waves determine the apperance of the image

A
65
Q

what are the 3 preprocessing adjustments for better image quality

A
  • beam hardening correction
  • detector non-uniformity
  • scatter correctiosn
66
Q

what is the typical matrix size for CT image displau

A

512 x 512

67
Q

how do you calculate pixel size

A

FOV / matrix size

68
Q

smaller fields of view allow viewing of smaller detail

A
69
Q

pixel size can be limiting factor in resolution as you cannot resolve details smaller than 1 pixel

A
70
Q

what is RFOV and DOFV

A

reconstruction field of view : The reconstruction field of view is the size of the scan field of view (SFOV) that is reconstructed to form a CT image.

display field of view : The display field of view is a selectable scan factor measured from the center of the patient to the most distant located edge of the patient.

71
Q

each pixel value represent s a small volume in the patient

A

pixel size is less than 1mm

72
Q

what is the slice width

A

0.5 - 10mm

73
Q

what is isotropic resolution

A

the spatial resolution in the transaxial plane (X-Y plane) and that in the longitudinal direction (Z direction) are equivalent.

74
Q

image data represents distribution of µ values

values are normalised to give the CT number, how do you calculate the CT number if you know the voxel value (tissue value)

A

CT number =
((tissue x water) / water) x1000 (air CT no)

75
Q

what is the dimensions/range of typical grey scale workstation in CT

A

256 grey levels (8 bits)

76
Q

what is the dimension of true colour workstation in CT

A

24 OR 32 BIT (with only 8 bit variation grey)

77
Q

optimise apperance by varying the window

A
78
Q

what is windowing in ct

A

selected range of CT numbers can be displayed over grey scale by defining window width and window length (WL, WW)

79
Q

what is window width and window length and how does this adjust ct image

A
  • width - range of CT number display over grey scale
  • length = CT number of mid grey

width and length determine the contrast and brightness of the image

80
Q

what are multi-planar reformats

A

the process of converting data from an imaging modality acquired in a certain plane, usually axial, into another plane

  • allows user to view data in arbitrary direction, combing data from multiple slices
81
Q

normal axial image shows single trans-axial slices only

A
82
Q

how can MPR be used for curved planes

A
  • curved planar reformatting can ‘straighten; an arbitrary curve through the patient
  • useful for following blood vessels etc
83
Q

what is MIP, why is this useful

A

maximum intensity projection

  • brightness of pixel given by maximum CT number along path through patient
  • useful in CT angiography where small size vessels would be averaged out in an average projection
84
Q

what is surface rendering

A

surfaces are created that connect pixels with selected attenuation values

  • can be used in things like virtual endoscopy do see interior of structures
85
Q

what is volume rendering

A
  • assigns opacities and colours to different tissue type according to attentuation
86
Q

how does rotation in a conventional CT scanner work

A
  • data transmission and power cables in old scanners meant to start-stop rotation
  • change of direction at end of rotation to unwind cables
  • series of images acquired by rotating clock and anti clockwise for subsequent scans
87
Q

what is the rotation time for a conventional CT scanner

A

1 sec + 1 sec for interscan delay

88
Q

what is slip ring technology

A

allows continuous rotation

  • power and signals transmitted to rotating gantry using ‘brushes; on static rings
89
Q

helical/spiral CT uses slip ring tech, how long are their rotation times and why is it so short

A

less than 0.3 seconds

  • no need to start and stop rotation
90
Q

mains —> high voltage generator —> high voltage slip ring —-> xray tube

A
91
Q

mains —> low voltage slip ring —-> high voltage generator —> xray tube

used on modern helical scanners

A
92
Q

why is slip ring tech so beneficial for imaging

A

the continuous rotation allows :
- continuous imaging

  • helical scanning (continuous movement of patient through gantry while imaging

(removes the step and shoot imaging)

93
Q

how do you calculate helical pitch

A

table travel per rotation / xray beam width

94
Q

speed of table movement through the gantry relative to beam rotation, determine the spacing of helices

A
95
Q

how does pitch affect coverage and data storage

A

high pitch = faster coverage and less data samples

96
Q

for the same xray beam width:

the greater the table travel per rotation, the higher the pitch

the higher the pitch the greater the spacing between the helical spins

A
97
Q

what is the difference between data points collected from an axial rotation (in step and shoot) and a helical rotation

A

in axial, both the first and second half of the rotation collects the same data points as it is rotating around the same slice

in helical, the second half of the rotation does not cover the same data as the first half of the rotation (you have a v pattern of data points. (helical structure)

98
Q

helical image reconstruction:

only a single projection is acquired exactly in the image plane

  • need to get rest of data from other projection planes
A
99
Q

what is helical interpolation

A
  • estimation of a value at certain position using known data from nearby points
100
Q

what is linear interpolation

A

assume a straight line between points

101
Q

what is 360 linear interpolation

A

uses attenuation data from points 360 degrees apart on the helix to interpolate data at slice location

102
Q

what are the 2 main adv of helical scanning

A
  • speed
  • flexibility of reconstruction (any position any interval)
103
Q

3 mind disadvantages for helical scanning

A
  • broadening of slice profile
  • helical interpolation artefacts
  • overscan
104
Q

what is a multi slice CT

A

enables acquisition of multiple slices in a single gantry rotation

thicker the slice thickness, the greater the number of slices

105
Q

compare the detectors from single slice to multi-slice CT

A

single slice = single slice reaching single row of detector

multi-slice = multiple slices reaching a parallel banks of detectors

106
Q

what are the components of dual-slice ct/ how does it work

A
  • simple extension to single slice ct
  • 2 parallel detector banks, 2 data sets per rotation
  • slice width chose by varying beam width
107
Q

what is the range of number detectors needed in multi slice ct

A

8-34 rows of detector needed

108
Q

four slice scanners need more than 4 detector rows

A
109
Q

what are the 2 types of detector arrays

A

fixed matrix

adaptive array

110
Q

what is a fixed matrix detector

A
  • matrix of detectors with fixed width (e.g 16 detectors of 1.25mm each)
111
Q

what is a adaptive array detector

A
  • matrix of detectors with varying width
    (e.g 8 detectors with widths of 1, 1.25 and 3mm etc)
112
Q

in multislcie detector switching,

  • data from more than one detector group can be summed to reconstruct wider slices
A
113
Q

how would an image at a required z position be produced from a helical multi slice scanner

A
  • projection data formed by interpolating projections from same scan angle at similar z-position (left or right to the specified z position)
114
Q

helical multislice slice width:

  • filter width approach allows flexibility in reconstructed slice width
  • range of slices equal or greater than detector acquisition width can be reconstructed
A
115
Q

what is a cone beam artefact

A
  • as number of slices increases, beam is less planar
  • becomes cone shaped and opposing projection follows different path thru patient
116
Q

what 2 ways is cone beam reconstruction done

A
  • tilted reconstruction produce non axial images, then filtered to make standard axial image
  • feldkamp reconstruction uses 3D back-projection
117
Q

what is feldkamp reconstruction

A

a widely used filtered-backprojection algorithm for three-dimensional image reconstruction from cone-beam (CB) projections measured with a circular orbit of the x-ray source.5

118
Q

what are the 3 main advantages of multislcie scanners

A
  • faster (reduced motion artefacts, improved IV contrast use)
  • further (long scan runs in trauma)
  • finer (routine availability of isotropic resolution scans)
119
Q

what is temporal resolution

A

discrete resolution of a measurement with respect to time

120
Q

how would you avoid mis-registration artefacts in ct scan

A
  • cover heart scan in minimum number of heart beats
  • ideally in a single heart beat
121
Q

in coronary CT angiography CCTA, why do you need fast rotation times and how fast

A
  • fast rotation times to freeze motion of heart
  • around 300ms for a full 360 rotation
122
Q

What was an electron beam computed tomography used for

A

developed for cardiac scanning to overcome poor temporal resolution of conventional scanners

123
Q

how does electron beam CT work

A
  • beam is accelerated, focused and deflected at angle to hit tungsten ring
  • beam is steered along rings
  • when electron beam hits tungsten rings, xray is produced
  • temporal resolution up to 50 ms an be achieved
124
Q

how many rows of detectors are in EBCT and how many slices are there depending on the number of target rings used

A
  • 2 rows of detectors
  • 2 slices if one target rings
  • 8 slices if 4 target rings etc
125
Q

what type of detector used in EBCT

A

scintillator

126
Q

Dual energy scanning:
scanning xray at 2 energies allowing better differentiation between materials

A
127
Q

for functional imaging in CT, you need to trace the flow of contrast in organ over time

  • increase coverage of detectors OR
  • perform axial or helical shuttle scans
A
128
Q
A

RAD413 (24 decks)

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