What does fan angle define?
Angle the xray beam extends from anode in order to cover entire width of detector array
What is cone angle and what makes it?
Depth of beam in 3rd gen CTs and is made by collimator.
What is beam width and what creates it?
Data covering the Z axis. Created by cone angle and collimators as patient moves through machine.
In 3rd gen CT machines, does beam width equal slice thickness? Why?
No, because unlike 1st gen, there are multiple rows of detectors.
Does beam width narrow or widen the closer to the detector?
Wider
What is slice thickness determined by?
Width of detectors
In very thin slices, is signal to noise ration higher or lower?
Just one detector width being used in a thin slice so signal to noise ratio is low. Lots of noise for not a lot of photons hitting.
What does combining multiple detectors do to signal to noise ratio?
Increases it.
Is the beam width slightly wider or slightly more narrow than the detector in Z axis?
Wider to account for penumbra where regions on peripheries have fewer photons. This is because the focal spot has width.
What effect does having a beam width slightly wider or narrower than the detector have on dose and image quality?
If wider, increases patient dose and reduces image quality because area of scatter made where photons bounce off patient.
What is the isocentre?
The axis of rotation for a 3rd gen CT.
What is the maximum field of view?
If you expanded a circle around the isocentre to the edge of the beam. It is the area where attenuation data can be calculated. So, transaxial area to be reconstructed.
True or false, patient needs to be directly in the isocentre?
True
Does increasing fan angle increase or decrease maximum field of view?
Increase
Does increasing source to isocentre distance increase or decrease maximum field of view?
Increase
Equation for magnification factor at isocentre (M)?
Source to detector / Source to isocentre
If source to detector distance is increased, does magnification increase or decrease?
Increase
Describe axial/helical acquisition
Patient in machine and source rotates 360 around them. Machine turned off whilst patient then moved same distance as beam width at isocentre. Repeat.
Why is axial/sequential acquisition no longer used?
Takes ages and get movement artefact eg) peristalsis, aorta moving, patient moving
Describe helical/spiral acquisition
Patient moves through rotating source that’s always on. No longer parallel to source or perpendicular to patient, slanted.
Equation for pitch?
Pitch = table speed x rotation time over isocentre beam width (slice thickness).
If the rotation time is longer what does this do to pitch?
Higher pitch
If the table speed is lower, what does this do to pitch?
Lower pitch
If beam is narrower, what does this do to pitch?
Higher pitch
