CT Flashcards
What is the usual HU for water/urine/CSF?
Around 0
What is the usual HU for Bone?
500-1000
What is the usual HU for fat?
-20 to -100
What is the usual HU for metal?
> 2000
What is the typical tube voltage of CT in adults?
120 kVp, mean energy of 60 keV
What is the typical tube voltage of CT in paediatrics?
80 kVp, mean energy of 40 keV
What are the focal spot sizes in CT and X-ray? (Same)
0.6mm, 1.2mm
What is the predominant mode of interaction in CT?
Compton scatter
What determines the slice thickness in single-slice CT?
Collimator width
What determines the slice thickness in multi-slice CT?
Detector size
What level of filtration and of what material are used in CT?
Up to 6mm Al/ Cu
What is the least significant type of noise in CT?
Electronic noise
What happens to image contrast with increased window width?
Reduced contrast
CT: What happens to spatial resolution with increased window width?
Window width does not affect spatial resolution
How to reduce partial volume effect? (3)
- decrease slice thickness
- decrease field of view
- decrease pixel size/increase matrix size
The CT number represents the average…
Average LAC of tissues in the voxel
What effect does thick slices have in noise?
Less noise
CT detector arrays use what type of scintillators in their detectors?
Ceramic scintillation detectors (gadolinium oxysulphide)
How do CT detectors work?
Scintillation layer detects attenuated X-rays and then emits light, converted to electricity by photodiodes
How do we eliminate multi slice CT overbeaming?
We need to increase the width of the Xray beam to allow the penumbras to fall to the side of the active detectors
CT pitch = what?
Table feed per rotation/width of acquired slices
It’s a Measure of overlaps
CT pitch - what does it mean when it’s <1, 1 and >1?
Pitch < 1 = overlapping slices
Pitch 1 = complete sampling no overlap
Pitch > 1 = gaps in data acquired
What is CT overranging ?
Getting additional data on either end of the scanned area to be able to reconstruct the image
CT filtered back projection: choosing filters is a trade off between…?
Noise and resolution, which are proportional
Drawbacks of filtered back projection?
Streak artefact
Susceptible to Poisson noise
Quantum noise in CT is reduced by…? (3)
Use thicker slices
Increase kVs
Increase mAs
How do we decrease structural noise? (in CT)
increase number of projections used to create image
When does beam hardening artefact commonly occur?
At bone:soft tissue junctions
CT Number is dependent on which 2 factors?
CT Number is the average LAC of the tissues.
It depends on kVP and filtration
CT Spatial resolution depends on what…? (5)
- … focal spot
- … number of smaller detectors
- … scan time
- … pixels
- Lack of …
- Small focal spot
- Increased number of smaller detectors
- Increasing scan time
- Small pixels, for finer sampling
- No motion/artefacts
CT Noise is generally increased by…? (2)
- less x-rays hitting detector
- increased matrix size
CTDI measurements units?
mGy
CTdI= Area under axial dose profile/slice width
What is CTDI?
Average dose delivered to a standard plastic phantom
what is the relationship between CTDI and current (mA)?
Proportional to each other.
What effect does pitch have on CTDI?
High pitch = faster scan = Low CTDI
CT imaging dose is proportional to…? (1)
- Tube current
Dose increased by anything that causes more x-rays to be detected
Typical CT spatial resolution is…? Mins Pairs per cm?
10mins pairs per cm.
(1lp/mm therfor less SR than plain film)
CT gantry speeds can travel up to…
5 revs per second
When is CT overbeaming more of a problem?
When the X-ray beam is narrow, so the penumbras make up relatively more of the dose the patient is receiving
Iterative reconstruction advantages?
Very accurate, fewer artefacts
How does Hybrid iterative reconstruction work?
Initial image generated with FBP
Iterative methods are then used to optimise image characteristics eg noise
Advantages of Hybrid Ir? (4)
Less patient dose
Less noise
Better than FBP alone
faster than IR alone
What is DLP in terms of Ct
What is the equation?
Scan length x average CTDI volume
A measure of the total dose in a CT study
Will thin Ct slices increase or decrease contrast?
Increase
How to decrease Noise in CT? (5)
- Bigger pixels
- Thicker slices
- Thinner patients
- More X-rays (kvP, mA, filtration) but will affect dose
- Soft reconstruction filters
How to reduce beam hardening artefact in Ct?
Use filtration or correction software
CT: What does Cone beam artefact do
Can cause streaking in images, especially in outer detector rows
