Multi Slice CT Flashcards
-SSCT Slice thickness is defined by :
-MSCT slice thickness is defined by :
- collimation
- detector array
Pitch - SSCT
defined as
patient couch movement per rotation divided by the slice thickness.
SSCT the x-ray beam width=
slice thickness
A pitch < 1 leads
oversampling (double irradiation of some tissues)
- this means patient is moving faster than you are rotating around
EXAMPLE- TABLE MOTION = 2mm and SLICE THICKNESS =5mm
Pitch = 1
each slice is continuous
Pitch >1 implies
-gaps between the x-ray beams from adjacent rotations
-gaps get bigger between individual slices.
-tf lower image quality but lower dose
In MSCT “Beam Pitch” is defined
patient couch movement per rotation divided by the acquired length (ACQUISITION BEAM WIDTH)
For SSCT image noise depends on
-KV
-slice thickness
-mA
-Rotation time
Kv
↑kV = ↑ penetrating photons = more hitting the detector and less noise.
Slice thickness
-thicker slices=Detectors collect more photons(less noise)….produces more partial volume effect.
mA
↑mA= ↑higher photon flux tf less noise
Rotation time
faster rotation=shorter time acquiring photons tf more noise
Image noise is generally independent of
pitch
linear CT
CT rotates around 360º w patient moving.
the patients moves in a few mm for the next rotation
Heat loading
continous helical scanning increases the heat loading on the tube by 100.
causing a limiting factor
tf we need to produce tubes w better heat capacity
Linear CT : Dead time
it is relavitely small
it is the am of time it takes for the cT detector to recover between being hit and being able to have another scan
Linear CT: Quality of images
low due to photon starvation and high noise
tf need a increase in the am of photons hitting the detector… done by increasing the tube current
signal to noise ratio
if greater than 1 . signal is bigger than noise
noise
is a random effect and over time it will average to 0
signal
is a systematic effect
if you keep averaging measuring and sampling , the signal will get stronger.
this will decrease noise and increase image quality
Multislice detectors
wider than linear
20 mm wide
Multislice : if extra side scatter affects. the adjacent banks of the detector
deepen the sepital thickness of colimator
what does over sampling lead to
more signal and no noise and no photon starvation.
This increases image quality but allows double irradication
tube rating
max Kv x mA = total power output
In MSCT , there is different slice thickness bc
we are acquiring in different directions
for MSCT quality depends on
-noise is dependent on pitch
-Photon flux and tf energy flux is spread out by ↑pitch
- leading to dose ↓ and noise ↑
-large slices are good for
interpretation
-small slices are good
isotropic imaging
