CT Flashcards
What is partial volume effect?
What does it depend on?
What is the effect of partial voluming on low contrast objects?
A high contrast object that is smaller than the voxel may be seen larger
It depends on thickness of the slice,
Thinner slice provide better resolution
Partial volume effect may also reduce visibility of low contrast details.
Principle components of the scanners
- X-ray tube, collimator and detector are mounted in a rotating gantry.
- The rotation axis is the z axis.
- The x ray beam is collimated as a wide fan beam sufficient to cover the patient cross section at its widest
- It has a narrow width parallel to the z axis that for a single slice scanner defines the images slice thickness.
- With detector rays , the radius of the arc is equal to the focal distance. Total number of detectors 500-1000
What is the plane of the gantry rotation?
The plane of gantry rotation is perpendicular to long axis of tabletop.
It is possible to tilt the gantry by up to 30 degrees especially for head scans..
X-ray tube , position of the anodes cathode axis
Anode cathode axis parallel to the axis of rotation of scanner. This is to minimise the effect of the anode heel effect.
How many focal spot size? What is the smallest size?
What is the heat capacity?
Two focal spot size, the smallest being about 0.6mm
Heat capacities of 4MJ or more
What sort of filtration is used in CT?
Generally scanners have a total tube filtration of 6 mm aluminium
Earlier scanners used additional copper filters
Ideally a mono energetic beam would be used in CT.
CT matrix size
What is a CT number?
What is used for calibration of of CT number scale?
Variation in attenuation coefficient of each tissue relative to water is being dependent on what?
512x512 or 256x256 or 1024x1024
Average linear attenuation coefficient
Air and water
Variation in attenuation coefficient of each tissue relative to water being dependent on Kv and filtration of x ray beam.
Colimator
The beam is collimated to a fixed width ~50cm at the axis of rotation, sufficient to cover the full cross section of the patient
On z axis the size is variable and in the case of single slice scanner defines the image slice thickness
When would you use a post patient collimator?
Why would you use a post patient collimator?
For single slice scanners there might be a post patient collimator in front of the detectors.
Its purpose is to reduce scatter when the slice thickness is less than the width of detectors
Or to get accurate thickness selection for the smallest slice width.
Bow tie filters
Elliptical body shape
Low dose at the centre and unnecessarily high at the peripheries.
Thin at the centre and thicker at the periphery
Requirements for scanner detectors
- Small to allow good spatial resolution. For single slice scanners with 600-900 individual detectors are in detector bank, the width of each detector
What was the main detector type prior to development of the multi slice scanners ?
Ionisation chambers such as xenon or krypton because of high atomic number and k shell binding energy
Xe ( Z 54 and k shell 35 kev) at high atmospheric pressure 20atm
Detection efficiency 60%
Is ionisation chambers suitable for multi slice scanners?
What sort of detectors are used?
No
Solid state detectors
What is a solid state detectors?
They incorporate a scintillation with embedded silicon photodiode to detect the light output
They have negligible after glow ie signal lag and they have a stable response.
What is the scintillant is made up if ?
The scintillant is made up of
Cadmium tungstate
Bismuth germinate
Rare earth ceramics
What’s the detection efficiency of solid state detectors vs ionisation chambers?
Solid state detectors 98%. Although it is effectively less due to effectors have to be separated to prevent light cross over therefore the actual efficiency is closer to 80%
60% for ionisation chambers
The number of measurements taken in scanning a single section of the patient depend so what ?
Depends on :
Number of detectors
And number of measurements that are taken in the full rotation.
What is the most common method used for image reconstruction?
Filtered back projections
Is it possible to collect data over a reduced arc down to 180?
Give example of usage
Yes, this may be used for imaging rapidly changing scenes such as CT fluoroscopy and for cardiac imaging
What is a topogram/ scout view/ or scanogram?
This is a transmission image taken at a fixed projection angle.
The collimator is set to the narrowest slice width and the image is obtained as the patient table is moved through the gantry.
The resultant image has a poorer spatial resolution compared with the standard radiographs. The adv is minimal scatter.
Why is the image different on scout view to the conventional radiography?
Because there is no beam divergence along the z axis.
There is minimal scatter
These are used for planning the ct sequences including the selecting the start and end points and displaying scan slice position.
What is a CT fluoroscopy?
This is a display of a CT image in real time.
This is achieved by continuous rotation of the gantry without table movement.
Using fast reconstruction techniques from 180 data sets frame rates of 5 frames per sec or greater might be achieved.
Give example where CT fluoroscopy is used.
Biopsy needle placement
Although effective dose may be less than standard diagnostic scan, one should be aware of the high patient skin doses.
What is the other name of axial scanning?
Step and shoot
ie patient being moved along the z axis of the scanners between each succeeding slice
What does a slip ring technology entail?
It comprises a metal ring mounted on the gantry. This is connected to the signal output from the detectors. Adjacent to the gantry there is a connector that is able to retain good contact with the ring as the gantry rotates.
This connector takes the signal and passes it to the computer system for image reconstruction.
Slip ring technology
The detector signals have to be sent sequentially at high frequency in order to accommodate the total number of individual detectors on the gantry and the sampling frequency
Additional rings are used to connect the X-ray generator to the external voltage supply
What is the advantage of continuous rotation?
To allow faster rotation times.
To allow continuous acquisition of data while the table moves the patient through the scan plane.
Data for the complete volume can be collected in a single exposure.
What is the continuous acquisition of data called?
Volume , spiral or helical CT
What is a pitch?
A pitch is a table movement per rotation divided by slice thickness
T/ F?
The slice width can be less than the detector width.
False
The slice width cannot be less than the detectors width.
Advantages of helical scanning.
Scan speed- possible to do in single breath hold.
This reduces the risk of slice misregistration. Teh other adv are patient throughput and the reduced use of contrast medium.
What are the advantages of increasing pitch more than 1?
Reducing exposure time and reducing patient dose.
However the resolution is reduced because of the need of greater interpolation.
Pitch greater than 2 gives unacceptable image quality.
T or F
True 3D imaging requires isotropy that is the voxel size must be equal in all three dimensions.
True
what is the voxel size in transaxial plane is determined by?
by
matrix size
FOV
usually in the region of 1mm
why would detectors have a minimum gap between them with a separator?
to prevent light cross over between detectors and to reduce the effect of scatter produced in the detectors.
what is the typical length of the individual detectors in the central rows?
0.5-1mm
slice width is determined by …
detector size and not the collimation.
what is a pitch in multislice CT?
tabletop movement per rotation divided by collimator length.
what is the maximum spatial resolution achieved on a CT scan?
20 lp/cm
lp/mm is generally used for fluoro and radioghraphy
what is the pixel size dependent on?
matrix size
FOV
*the spatial resolution is limited by pixel size.
what affects the projection path of x ray beam as it passes through the patient?
- focal spot size
- geometry of the scanner- source to isocentre and source to detector distance.
- physical size of the sensitive area of the detector itself
- sampling frequency or no of projections sampled on each rotation.
effect of sampling frequency on spatial resolution
increases the resolution.
T/F
The spatial resolution in the z direction (parallel to the rotation axis) depend on slice thickness.
True
what are the sources of noise in CT images?
- quantum noise caused by random variation in no. of photons detected.
- electronic noise produced in the measuring system.least significant.
- structural noise affected by reconstruction algorithm
what affects the quantum noise?
mA
Scan time
slice width
what does increasing the slice width and reducing noise do to SR and partial volume effect?
it reduces SR and increases partial volume effect and this would have an adverse effect on image quality.
what is the effect of kV on noise?
at increased kV penetration is greater and therefore greater no of photons are detected. the mA is lowered to reduce the dose and therefore the noise will bereduced.
what is the effect of increased kV on contrast
reduces contrast!
what is the effect of FOV and matrix size on noise?
smaller FOV or larger matrix , there would be a smaller detector area defining each pixel, therefore there would be less photons per pixel and noise is therefore increased.
T/F
for single slice scanners pitch affects noise.
false
for single slice scanners pitch does not affectfor single slice scanners pitch affects noise.for single slice scanners pitch affects noise. noise.
what happens to noise as window width is reduced?
noise becomes more apparant.
The greater the pitch….(dose/image quality and slice misregistration)
less the patient dose
unacceptable image quality
reduction of slice misregistration.
What artefact does metal object/bone/contrast make?
streak artefact
What artefact does hip implant produce
photon starvation which is a variant of streak artefact
what does beam hardening mean?
As the beam passes through the patient, the low energy photons are filtered out and the beam becomes harder.
what does beam hardening do to CT number?
It reduces the CT number along the beam path.
Compare the CT number at the centre and periphery of the patient?
What does this effect called?
The CT no is lower in the centre and higher and in the periphery.
Cupping
What sort of artefact does cupping produce?
How can it be corrected?
dark streak and it can be corrected by beam hardening algorithm.
It can also be reduced with bow tie filter that provides increased filtration of outer rays of the fan beam.
What artefact does a faulty detector produce?
Ring artefact.
can be seen on both single and multi slice scanners.
What would happen if there were inadequate correction for cone beam artefact
If there is inadequate correction for this artefact, a blurring is seen at the boundaries between high contrast details - eg the boundaries of bone and soft tissue.
What % of total population dose that is attributed to medical exposures is from CT?
What % of total ionizing radiation is from CT?
40%
4%
what is a CTDI?
It is a measure of dose from a single rotation of the gantry.
This is a measure of dose from a single rotation of the gantry.
CTDI
How do we measure CTDI?
Pencil ionisation chamber
What are the dimensions of pencil ionisation chamber?
diameter 8mm
length 100mm
This is greater than the max slice width that is to be measured.
T/F?
the peak dose is dependent on slice width
False
the peak dose is independent on slice width
It depends on:
scanner design-
Filteration, KV, mA
Cylindrical perspex phantom are used to measure CTDI.
What type/measurements are used for head and body?
16 cm- head
32 cm-body
Several measurements are done in several locations to derive a weighted value- CTDIw
What is the range of CTDIw?
10-40 mGy
What is a CTDIvol?
CTDIw/pitch
this is the average absorbed dose within the scanned volume
What is the max skin dose in scan of head and body?
max skin dose is CTDIw of head
and about 20% higher for body.
T/F?
Skin doses are higher in CT than for radiographic examination.
True
but not as high as doses from prolonged fluoroscopy
What parameter is used to describe dose efficiency between scan models?
CTDI
This is less useful for comparing doses to individual patients.
What is dose length product? DLP
DLP= CTDIw x L
L total scan length.
In DLP, how does the length of scan calculated?
number of rotations x collimated length and pitch
what does the conversion coefficient depend on?
it depends on body region and scanner design.
What is the tube potential in kv?
120kv
Increasing the kv by 20 , increases the radiation output by 40%
but attenuation in patient is lowered.
therefore, higher kv increases dose, but for same level of noise the mA is lowered to give an overall dose reduction.
T/F?
in a single slice scanner patient dose is independent of slice width.
True
T/F
CTDI varies with slice width.
False
CTDI does not vary with slice width.
this is because the integration of dose profile is normalised to slice width.
Therefore the DLP should also remain constant with changes in slice width. ( there are two caveats)
What are teh caveats with regards to slice width and dose
- for some single slice scanners the narrowest beam may be defined by post pt collimation and the beam size passing through the pt might be sig greater. for such equipments use of the narrowest slice width could produce a sig greater dose. This only applies to some single slice scanners.
- slice thickness does not increase dose (this is only if the mA is not changed). Reducing reconstructed slice thickness increases noise.
use of narrower reconstructed slices is asso with increased dose, this is not because of slice width per se but it is due to increased in MA
What is the relationship bwn dose and pitch
pt dose is inversely proportional to pitch.
Therefore, use of a pitch of 1.5 rather than 1 leads to 33% dose reduction.
what is the effect of detector rows numbers on dose
no effect
T/F?
Doses in multislice scanners are greater than with single slice scanners.
True
Need for over scanning at the start and end of each scan.
what defines the slice width in :
single slice and
multi slice scanners?
for single slice scanners the collimator length defines slice width. 8mm slice- collimator is set at 8mm
for multi-slice scanners the reconstructed slice width is defined by the length of the detectors in the detector row.
what is the effect of reconstruction on thinner slices on:
noise
it increases noise and to compensate mAs, the dose may be increased.
