CT Flashcards
Doses
- CT Head - 2mSv
- CT Chest - 7mSv
- CT Abdo - 8mSv
- CTPA - 10mSv
- CT Abdo/Pelvis - 15mSv
Partial Volume
Makes high contrast objects appear larger
Makes low contrast objecto appear smaller
Beam Hardening
Causes a reduction in houndsfield units towards the centre of the image
Anode-cathode axis
Parallel with centre of scanner
In Z axis
Fan Beam
Is perpendicular to the anode-cathode axis
What is resolution in CT?
Measured in line pair/cm
7lp/cm in the transaxial plane
When directly compared with radiography:
1lp/mm vs 3lp/mm for radiography
Is contrast resolution better in CT than plain film?
Yes contrast resolution better in CT
(Spatial resolution better in plain film)
Does a narrow window increase or decrease noise?
Increases noise
Smaller FOV = less signal and more noise
What is anode cathode axis in CT?
Parallel to the axis of the scanner to minimise heel effect
What filtration is used in CT?
6mm of aluminium
or
0.5mm Copper which is equilivent
In newer scanners:
- 3mm aluminium
- 0.1mm copper
Total filtration 6mm equivilent
What collimation distance is used in CT?
50cm usually sufficient for most patients
Are detectors separated in CT?
Yes, this is to reduce light crossover
Reduces detection efficiency however (to 80%)
What is cone beam effect due to?
Due to divergence of beam in Z axis
Usually problematic when there are more than 4 slices
Increased number of slices = worse cone beam effect
Affects outer detectors rows more
For same mA and kV, does slice thickness affect dose?
No
Dose is independent of slice thickness
In an obese patient, is kV or mA increased? or both?
Just mA is increased
Which part of patient receives higher dose?
Periphery
Which scanner produces more scatter? 1st or 3rd gen
Third generation
Are detector crystal embedded in a matrix in CT?
NO
What is pixel size formula?
Pixel size = FOV /matrix size
In single slice scanners, does pitch affect noise?
NO
What is CTDIw
CTDIw = 1/3CTDI centre + 2/3CTDI periphery
Units = mGy
What is CTDI vol?
CTDIw / Pitch
Units = mGy
What is DLP?
CTDIvol x Total Scan length
Units mGy/cm
What is effective dose?
DLP x fiddle factor (dependent on body part)
Units are mSv
How to calculate noise in CT?
1/ square root of number of photon
Inversely proportional to the square root of number of photons
What effect will doubling of mA have on the noise?
Will decrease noise by a factor of:
square root of 2
Halving rotation time also decreases noise by the square root of 2
What is filter interpolation and 2 point interpolation used in?
Filter interpolation = Multislice scanners
Two point interpolation = single slice scanners
What is normal CT matrix size?
512 x 512
For high resolution = 1024 x 1024
Fluoro CT uses 256 x 256 for reconstruction
How to calculate CT number?
What measures can be taken to improve cupping artefact?
Cupping artefact is otherwise known as beam hardening
Made better by:
- Bow tie filter
- Algorithms
Which has more afterglow?
Sodium iodide
vs
Bismuth germinate
Sodium iodidie has more afterglow
Bismuth germinante is more preferable
Which CTDI most closely corresponds with absorbed CT dose?
CTDIvol
CT dose = mA/pitch
Is effective dose measured on every CT scan?
No
What is pixel size in CT?
0.5mm usually
In CT does magnification increase spatial resolution?
Yes
Can CT gantry tilt in the vertical axis?
Yes
It can tilt up to 30 degrees in the vertical axis
What is heat capacity in CT?
4MJ
What is CT fluoroscopy used for?
Needle guidance in IR procedures
- 180 degree rotation
- low dose technique
- higher skin dose than normal CT
- lower effective dose
When is post patient collimation used?
Used in single slice scanners
Used to reduce scatter
Results in increased dose
Do 4th gen scanners result in a higher dose?
Yes
The patient to detector distance is greater
They have 6x the amount of detectors than other scanners
Calibration can be adjusted through each scanning cycle
