CT principles Flashcards
CT scanners need:
- exact source-detector geometrical relationship
- stable and fast detecotrs
- high resolution
- mono-energetic beam
- tube perpendicular to fan beam
- computing power
- strong radiolucent bed
why does the tube need to be perpendicular to the fan bea,m
to avoid the anode heel effect
what is CT measured according to
- hounsfield scale
- according to the distribution of ‘u’
- u value being scaled to that of WATER
what is the CT number for water, air and bone
water = 0
air = -1000
bone = 1000
(everything else is somewhere in-between)
what is the number of different CT numbers that can be recognised by the scanner
2000
what is the image display format for most CT monitors
8 bit
256 different grey levels
how many grey levels can the eye differentiate compared to CT
Eye can resolve about 30 grey levels between black and white
CT scanner can differentiate 256 grey levels
how many CT numbers does soft tissue excluding fat cover
80
what is image reconstruction in CT
a mathematical process that generates tomographic images from X-ray projection data acquired at many different angles around the patient
what equation is used in CT image reconstruction
I = I0 e^-micro(x)
why are 2 views required in planar radiology
to give an idea of depth in an object
in CT what component provides depth of an object
grey scale
what is back projection and how is this done
- used to convert the data from the detector (projection space) to the image space.
- standard method of reconstructing CT slices by ‘smearing back’ the projection across the image at the angle it was acquired 9this reconstructs the image)
- done using tube input and pixel value
how can you make a simple back projection less streaky + why is it streaky in the first place
- the more projections added the better the representation, but this gives an increasingly streaky image
- hence filter (image processing) is applied
(filtered back projection)
why is back projection similar to the real picture but blurry
- because you ‘smeared’ the bright pixels across the entire image instead of putting them exactly where they are
how is filtered back projection done
- alter the projection data before back projection is done
- uses high pass filter or sharpening filter
what is the use of a high pass or sharpening filter in filtering back prjections
this type of filter picks up sharp edges within the projection
what is reconstruction kernel
how ‘soft’ the filter used in filtering back projection is
why is reconstruction kernel important
- high pass/sharpening filter can reduce blur BUT this accentuates noise as noise looks like jagged edges
- so using high pass filters make images look too grainy and softer filters are required (reconstruction kernel)
how would you adjust between kernels for scans
different kernels chosen for different images needed
e.g if u need to spot fractures, small discrete features, use hard kernel to accentuate these features
or
for soft tissue e.g brainy want to look for larger features with mild differences so u use softer kernel to decrease noise
what is iterative image recontruction
- takes an initial guess and refines over several iterations
- starts with filtered back projection then stimulates making CT reconstruction of initial guess
- by looking at difference in reconstruction, you can make better guess
- process is repeated for defined number of cycles
why does iterative image reconstruction perform better in high-noise situations e.g low dose scans
simulation used by iterative reconstruction technique attempt to model real-world noise and attenuation process
what is the main negative of using iterative image reconstruction
much longer reconstruction time and image have different subjective quality than filtered back projection
what does CTDI stand for/what is it
computed tomography dose index
- distribution of radiation dose in a single CT scan
ideally, radiation dose would drop sharply at edges of scan but this does not happen, what does? what does this cause and why does it happen
how can the radiation dose of slices of tissue be affect because the radiation dose does not drop at edge of scans
- radiation dose profile process gradually at edges of scan with penumbra on either side
- these tails/penubra is due to both xray beam divergence and internal radiation scatter by body tissue
- so overall, each slice of tissue in CT not only receives radiation as it gets scanned but also gets some from when adjacent slices are scanned
what is the integration length of CT limited to
100mm
what is CTDI 100
A measure of the dose over a pencil chamber 100mm in length
what does NT define
number of detector rows x width of detector row (irradiated slice width)
CTDI is measured in air
how long is the pencil chamber used to measure CTDI
10CM
What is the size/diameter of the head and body phantom used for CTDI measurement
head = 16cm
body = 32 cm diameter
what is weighted CTDI
represent the average CTDI across an axial section of a cylindrical phantom.
doses in weighted CTDI are greater towards the periphery
what does DLP stand for and what is it and whats its units
dose length product
- a measure of CT tube radiation output/exposure (measured in mGy.cm).
what is CTDI vol
represents the dose through a slice of an appropriate phantom
calculated by CTDIw/pitch
how is dose length product calculated
length of scan x CTDI vol
what is a CT NDRL
CT national dose reference levels
what 5 factors affect dose and image quality in CT
- slice thickness
- pitch
- kV/mAs
- reconstruction filter
- contrast
what is slice thickness
how wide the image is in the z-direction
compare the pros and cons of narrow and wide slice thickness
narrow =
- more noise
- better z-axis resolution
wide =
- less noise
- worse z axis resolution
- fewer slices to view
- faster AOI coverage
what is pitch in CT
the table distance traveled in one 360-gantry rotation divided by the beam collimation
compare pros and cons of having pitch at 1, <1 and >1.
pitch = 1 (similar to axial scan at isocenter)
pitch = <1
- slices overlap
- higher dose
- better image quality
- slower scan
pitch = >1
- gaps in slices
- lower dose
- more interpolation
- faster scan
what is axial scanning in CT
the patient is moved forward along the longitudinal axis of the CT scanner, pausing at intervals to allow a trans-axial image to be captured at each position along the axis.
what is helical scanning
- With helical CT, the patient is moved through a rotating x-ray beam and detector set.
- From the perspective of the patient, the x-ray beam from the CT traces a helical path.
- The helical path results in a three-dimensional data set, which can then be reconstructed into sequential images for an image stack.
what are the main pros and cons of helical scanning
- faster
- reduced dose
but - reduced image quality
how is the data reconstructed in helical scanning
via interpolation
- data from same projection angle at different table position
what is the relation between radiation dose and tube potential (kVp)
non linear
radiation dose proportional to kVp^2
how does kVp, noise and contrast resolution relate in CT
increased kVp = less noise BUT reduces contrast resolution
what is the typical kVp used in ct
120
what relation does dose and tube current have
linear relationship (if everything is constant)
photon fluency is proportional to tube current for fixed rotation time
how does noise and current mA relate
increased mA = less noise
most CT contrast agents take advantage of a k-edge to increase photon attenuation. what is k-edge
- binding energy of inner k-shell electron orbital of an atom
there is a sudden increase in attenuation coefficient for photons of energies just above the k-edge of an atom
what is the most commonly used CT contrast agent for vascular studies
iodine (intravenously)
define artefact
info contained within image thats not present in object
what is a partial volume artefact
occurs when a dense object protrudes into an adjacent slice where there is only soft tissue
- portions of several objects are averaged on singular slice
what does a partial volume artefact look like on an image
characteristic ‘ white blob’
- reduced spacial images
- when imaging fine bony structures, may get blurred edges
what is the beam hardening artefact
xray beam contains spectrum of different energies,
- as it travels thru patient, lower energy photons absorbed and average energy of beam increases “harder’
beam hardening causes cupping, what is this
- beam has higher average energy than incident beam
- as beam passes thru object, effective attenuation coefficient of any material diminishes making short ray paths more attenuating than long ray paths
- overall causes edges of object to appear brighter than centre
what is the sighting of beam hardening/cupping
darker centre image with brighter edges
what is photon starvation artefact
When too few photons reach detector elements, strong streaks appear through paths of high X-ray attenuation and an image becomes completely useless
what type of patients does photon starvation occur in and why is there streak artefacts seen
- occurs in larger patients/thick regions
- noise is magnified during reconstruction giving streak artefacts
what is the ring artefact
occurs due to the miscalibration or failure of one or more detector elements in a CT scanner
- non uniformity amongst detectors
what is the visualised characteristic of ring artefact
appears as rings
becomes a circle with enough projections
what is a metal artefact in CT
combination of beam hardening, scatter, noise and edge enhancement
- causes bright and dark streaks and photon starvation
what is used by the artefact reduction software to combat metal artefact
IR
What is an out of field artefact
- causes streaks and errors in reconstructed FOV due to presence of photon-interacting medium outside FOV
how might you try to fix the out of field artefact
use wider form and/or place patient arms up
what are the characteristics of out of field artefact
streaking and increase or decreased density of structures
(The lack of data from these out of field tissue/objects interferes with the ability of the software to generate a correct image leading to streaking, and areas of unusual increased or decreased density. )
what type of patients does the out of field artefact apply to
obese patients who just about fit into the bore
