CT physics 3 Flashcards
(29 cards)
kW of CT
100 kW and above
mA of CT
800mA
focal spot size of CT
0.5-1.2mm2
scan sequences
30s
large heat capacity and fast cooling
up to 8MU, 5MHU/min
what is used to remove low energy photons that dont contribute to image but increase patient dose
flat filter
purpose of beam shaping filter
- dose modification; can attenuate/ enhance frequency of x-ray beam
- scatter reduction; selectively attenuates lower-energy x-rays
- energy selection
- shape modification; match anatomy/ target area
- contrast enhancement
- dose optimization
why is collimation important
- limit patient dose and maximise image quality
- need to collimate slice acquisition in z-axis
- detectors arranged to obtain constant beam width at detector which helps remove scatter
what is the acquired slice thickness determined by
z-axis collimation
- can range from 1 to 160 mm
what does collimator width determine
the width of section sensitivity profile (SSP) which influences spatial resolution z-axis
z-axis (longitudinal axis)
- primary axis od CT scanner
- longitudinal direction of patients body
- corresponds to direction of patient table movement through gantry
what does z-axis determine
the thickness of slices/ sections of body that are imaged
- when patient moved through gantry, creates a series of cross-sectional images that represent different slices of body
- patient translate direction
y-axis (vertical axis)
corresponds to vertical direction as patient positioned on ct scanner table
- represents superior-inferior orientation within body
combination of both axis
defines 3D coordinate system
what does a larger z-axis collimator mean
higher photon flux reaching detectors = less noise and better contrast resolution –> partial volume artifacts
what does the patient translation speed refer to
refers to the rate at which patient table moves through scanners gantry during scanning process
definition of pitch (P)
patient translation speed and collimator width both define the quantity of P
if P = 0
we have sequential CT slice
if P = 1
patient moved one collimator width in one rotation of tube
if P = 2
patient moved two collimator widths in one rotation
CT detectors need to be …
- efficient (intrinsic and geometric)
- have rapid response time
- high stability + uniformity
- large dynamic range
- not too expensive
- easy to manufacture for multi-slice scanning
idea of multi-slice scanning
multiple rows of detectors in patient translate direction (Z-axis)
- collect longer volume of data which can be reconstructed into many axial CT images
advantages of multi-slice CT scanning
- reduced motion artefacts
- longer scan runs in single breath
- improved contrast media enhancement
- finer slice acquisition
- same amount of heat load on x-ray tube for each rotation - greater volume of patient data acquired during multi-slice CT scanning
disadvantages of helical multi-slice CT scanning
- cone beam artefacts
- increase in data storage as more images are produced
