CT physics Flashcards
principle behind creating CT images:
measurement of xray attenuation at multiple angles through the object
what are the 3 components of a CT scanner
- donut shaped gantry
- patient table
- computer and console
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
data acquisition system
digital to analogue converter
what do you have within the gantry of the scanner
- patient table
- xray tube that spins 360 around table
- xray detector opposite of tube
- xray fan beam coming out of tube
not that xray tube in ct scanner has filtration and collimation for xray beam coming out
what is the voltage used in CT Xray tube
80-140 kV
find a diagram of a CT Xray tube and label the diagram
what is the size of the movable collimator in found outside the tube housing (outside the window of the tube)
1 - 160mm
what is the focal spot size of an xray tube
0.5 - 1.2 mm
modern ct scanners need powerful generators, up to what power is required
100kw
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
- dual shaft support anode
- spiral groove bearings (facilitate rapid heat transfer)
- segmental anode
- direct oil cooled anode
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
anything equivilant to a minimum of 2.5mm
how does filtration affect beam hardening artefacts
it reduces beam hardening artefacts
despite filtration, beam hardening can still happen due to patient shape, how is this so
patients are thicker towards their midline in general
- longer path through center = greater attenuation and more beam hardening
how can the affect of beam hardening due to patient shape be prevented
using a beam shaping (bow tie) filter to ensure more uniform dose across patient
- so beam is more intense exiting at centre
what is the width of the fan beam in the axial plane
500mm (at isocentre)
what is the width of the fan beam in the z-axis
1-160 mm
what are the 2 types of CT detectors
CT detectors can have ionisation (gas) or scintillation (solid) detectors
what is detector efficiency
ability to capture, absorb and convert xray photons to electrical signals
what is geometric efficiency of detector
active detector area / irradiated detector area
- Characterises the extent of the radiation beam that is used for image creation
what is quantum efficiency of the detecotr
number of photons absorbed by detector
what 3 things is quantum efficiency dependant on
- atomic number
- density
- thickness
what is conversion efficiency of detector
conversion of captures photons to electrical signal
how is geometric efficiency determined
via ‘dead’ spaces between elements
compare the quantum efficiency between gas and scintillator detectors
gas = 60% QE
scintillator = 100% QE
what is response time of detector
refers to speed with which the detector can detect an xray event and recover to detect another event
what are the 3 things a detector does during the response time
- transfer energy of photn
- record electrical signal
- discard evidence of the photon in readiness for next event
scintillators have a slower decay phase during the response time (after glow)
define stability in CT
output is constant over a period of time
define uniformity in CT
for given incident intensity, all detectors should give same output
how do you ensure stability and uniformity in CT machine
regular callibration
detectors need to respond to both very high and low photon intensities
ratio of largest signal measure to smallest signal is discriminated
how do CT detectors operate
- operate in ‘current’ mode
- does not measure individual pulses and energy
- pulses are integrated to produce steady current flow
what is the size of current flow of detector determined by
product of the average event rate and charge produced per event
what gas molecule is used in ionisation detectors
Xe gas
single chamber containing pressurised Xe gas
how does an ionisation detector work
- 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
what are some pros and cons with ionising detectors
pros
- simple
- high precision and stable
- uniform response
- no afterglow
- fast response time
cons
- poor quantum efficiency
- difficult to manufacture multirow
how does a scintillation detector work
- xray converted to light photon via scintillation crystals
- visible photon/light converted to electrical signal via photomultiplier or photodiode
how is the signal amplified in a photomultiplier tube in a scintillation detector
light photon hits photocathode, producing electron
electron hits dynode stimulating electron avalanche
avalanche eventually reaches anode forming electrical signal
how does a photodiode generate an electric signal
- 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)
why might a photodiode be better than photomultiplier tube
- higher QE
- more compact size
what crystal is typically used in scintillators and why
caesium iodide
- high absorption efficiency
- wavelength matches response of PM tube
- slow decay
why might a gemstone detector be better
- recovery time is x4 faster than comparable detectors
- primary speed is 100 times faster than competitors
pros and cons of scintillation detectors
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
what is DAS, its use
data acquisition system
- electronics between detector and computer
- measures transmitted radiation (analogue)
- encodes to digital data and transmoits to computer
log amplifier converts transmission data into attenuation and thickness
detector —> log amplifier —> ADC —-> digital transmission —-> computer
what is DAC and how does it work
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)
what size bit ADC does a modern CT scanner use
16 bit (2^16)
determines grey scale resolution
data transmission to computer by light emitting diodes (optical transmission)
what 3 things does the attenuation of xrays between tube and detector depend on
- linear attenuation coefficient
- μ
- thickness (t)
what is μ and what are the units of μ
- average linear attenuation coefficient
- cm-1
what 3 things does the μ of a material depend on
- atomic numer (Z)
- density
- xray energy (E)
total attenuation ‘summed’ along the path between tube and detectors
how do the number of projections and spatial resolution relate
increased projection = improved spatial resolution
3rd generation CT scanner features?
- wide fan beam
- 500-100 detectors
- rotation only
- less than 1sec per rotatin
4th generation CT scanner features?
- fan beam
- static 360 degree detector ring
- only tube rotates
- poorer scatter discrimination
how has the 4th gen improved from the 3rd gen CT scanner
- avoided ring artefact problem of 3rd
advantages of 3rd gen scanner
- 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
define algorithm in ct
set of defined mathematical rules for solving a problem
define convolution filter/ kernel (reconstruction filter)
mathematical function for smoothing or sharpening data
define convolution
digital imaging processing technique to modify images with filter/kernel
define interpolation
used in reconstruction process of helical data
Fourier series:
objects can be described by spatial frequencies
amplitude and frequency of sine waves determine the apperance of the image
what are the 3 preprocessing adjustments for better image quality
- beam hardening correction
- detector non-uniformity
- scatter correctiosn
what is the typical matrix size for CT image displau
512 x 512
how do you calculate pixel size
FOV / matrix size
smaller fields of view allow viewing of smaller detail
pixel size can be limiting factor in resolution as you cannot resolve details smaller than 1 pixel
what is RFOV and DOFV
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.
each pixel value represent s a small volume in the patient
pixel size is less than 1mm
what is the slice width
0.5 - 10mm
what is isotropic resolution
the spatial resolution in the transaxial plane (X-Y plane) and that in the longitudinal direction (Z direction) are equivalent.
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)
CT number =
((tissue x water) / water) x1000 (air CT no)
what is the dimensions/range of typical grey scale workstation in CT
256 grey levels (8 bits)
what is the dimension of true colour workstation in CT
24 OR 32 BIT (with only 8 bit variation grey)
optimise apperance by varying the window
what is windowing in ct
selected range of CT numbers can be displayed over grey scale by defining window width and window length (WL, WW)
what is window width and window length and how does this adjust ct image
- 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
what are multi-planar reformats
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
normal axial image shows single trans-axial slices only
how can MPR be used for curved planes
- curved planar reformatting can ‘straighten; an arbitrary curve through the patient
- useful for following blood vessels etc
what is MIP, why is this useful
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
what is surface rendering
surfaces are created that connect pixels with selected attenuation values
- can be used in things like virtual endoscopy do see interior of structures
what is volume rendering
- assigns opacities and colours to different tissue type according to attentuation
how does rotation in a conventional CT scanner work
- 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
what is the rotation time for a conventional CT scanner
1 sec + 1 sec for interscan delay
what is slip ring technology
allows continuous rotation
- power and signals transmitted to rotating gantry using ‘brushes; on static rings
helical/spiral CT uses slip ring tech, how long are their rotation times and why is it so short
less than 0.3 seconds
- no need to start and stop rotation
mains —> high voltage generator —> high voltage slip ring —-> xray tube
mains —> low voltage slip ring —-> high voltage generator —> xray tube
used on modern helical scanners
why is slip ring tech so beneficial for imaging
the continuous rotation allows :
- continuous imaging
- helical scanning (continuous movement of patient through gantry while imaging
(removes the step and shoot imaging)
how do you calculate helical pitch
table travel per rotation / xray beam width
speed of table movement through the gantry relative to beam rotation, determine the spacing of helices
how does pitch affect coverage and data storage
high pitch = faster coverage and less data samples
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
what is the difference between data points collected from an axial rotation (in step and shoot) and a helical rotation
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)
helical image reconstruction:
only a single projection is acquired exactly in the image plane
- need to get rest of data from other projection planes
what is helical interpolation
- estimation of a value at certain position using known data from nearby points
what is linear interpolation
assume a straight line between points
what is 360 linear interpolation
uses attenuation data from points 360 degrees apart on the helix to interpolate data at slice location
what are the 2 main adv of helical scanning
- speed
- flexibility of reconstruction (any position any interval)
3 mind disadvantages for helical scanning
- broadening of slice profile
- helical interpolation artefacts
- overscan
what is a multi slice CT
enables acquisition of multiple slices in a single gantry rotation
thicker the slice thickness, the greater the number of slices
compare the detectors from single slice to multi-slice CT
single slice = single slice reaching single row of detector
multi-slice = multiple slices reaching a parallel banks of detectors
what are the components of dual-slice ct/ how does it work
- simple extension to single slice ct
- 2 parallel detector banks, 2 data sets per rotation
- slice width chose by varying beam width
what is the range of number detectors needed in multi slice ct
8-34 rows of detector needed
four slice scanners need more than 4 detector rows
what are the 2 types of detector arrays
fixed matrix
adaptive array
what is a fixed matrix detector
- matrix of detectors with fixed width (e.g 16 detectors of 1.25mm each)
what is a adaptive array detector
- matrix of detectors with varying width
(e.g 8 detectors with widths of 1, 1.25 and 3mm etc)
in multislcie detector switching,
- data from more than one detector group can be summed to reconstruct wider slices
how would an image at a required z position be produced from a helical multi slice scanner
- projection data formed by interpolating projections from same scan angle at similar z-position (left or right to the specified z position)
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
what is a cone beam artefact
- as number of slices increases, beam is less planar
- becomes cone shaped and opposing projection follows different path thru patient
what 2 ways is cone beam reconstruction done
- tilted reconstruction produce non axial images, then filtered to make standard axial image
- feldkamp reconstruction uses 3D back-projection
what is feldkamp reconstruction
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
what are the 3 main advantages of multislcie scanners
- faster (reduced motion artefacts, improved IV contrast use)
- further (long scan runs in trauma)
- finer (routine availability of isotropic resolution scans)
what is temporal resolution
discrete resolution of a measurement with respect to time
how would you avoid mis-registration artefacts in ct scan
- cover heart scan in minimum number of heart beats
- ideally in a single heart beat
in coronary CT angiography CCTA, why do you need fast rotation times and how fast
- fast rotation times to freeze motion of heart
- around 300ms for a full 360 rotation
What was an electron beam computed tomography used for
developed for cardiac scanning to overcome poor temporal resolution of conventional scanners
how does electron beam CT work
- 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
how many rows of detectors are in EBCT and how many slices are there depending on the number of target rings used
- 2 rows of detectors
- 2 slices if one target rings
- 8 slices if 4 target rings etc
what type of detector used in EBCT
scintillator
Dual energy scanning:
scanning xray at 2 energies allowing better differentiation between materials
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
