CT physics 3

Question 1

kW of CT

Answer 1

100 kW and above

Question 2

mA of CT

Answer 2

800mA

Question 3

focal spot size of CT

Answer 3

0.5-1.2mm2

Question 4

scan sequences

Answer 4

30s

Question 5

large heat capacity and fast cooling

Answer 5

up to 8MU, 5MHU/min

Question 6

what is used to remove low energy photons that dont contribute to image but increase patient dose

Answer 6

flat filter

Question 7

purpose of beam shaping filter

Answer 7

• 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

Question 8

why is collimation important

Answer 8

• 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

Question 9

what is the acquired slice thickness determined by

Answer 9

z-axis collimation
- can range from 1 to 160 mm

Question 10

what does collimator width determine

Answer 10

the width of section sensitivity profile (SSP) which influences spatial resolution z-axis

Question 11

z-axis (longitudinal axis)

Answer 11

• primary axis od CT scanner
• longitudinal direction of patients body
• corresponds to direction of patient table movement through gantry

Question 12

what does z-axis determine

Answer 12

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

Question 13

y-axis (vertical axis)

Answer 13

corresponds to vertical direction as patient positioned on ct scanner table
- represents superior-inferior orientation within body

Question 14

combination of both axis

Answer 14

defines 3D coordinate system

Question 15

what does a larger z-axis collimator mean

Answer 15

higher photon flux reaching detectors = less noise and better contrast resolution –> partial volume artifacts

Question 16

what does the patient translation speed refer to

Answer 16

refers to the rate at which patient table moves through scanners gantry during scanning process

Question 17

definition of pitch (P)

Answer 17

patient translation speed and collimator width both define the quantity of P

Question 18

if P = 0

Answer 18

we have sequential CT slice

Question 19

if P = 1

Answer 19

patient moved one collimator width in one rotation of tube

Question 20

if P = 2

Answer 20

patient moved two collimator widths in one rotation

Question 21

CT detectors need to be …

Answer 21

• efficient (intrinsic and geometric)
• have rapid response time
• high stability + uniformity
• large dynamic range
• not too expensive
• easy to manufacture for multi-slice scanning

Question 22

idea of multi-slice scanning

Answer 22

multiple rows of detectors in patient translate direction (Z-axis)
- collect longer volume of data which can be reconstructed into many axial CT images

Question 23

advantages of multi-slice CT scanning

Answer 23

• 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

Question 24

disadvantages of helical multi-slice CT scanning

Answer 24

• cone beam artefacts
• increase in data storage as more images are produced

Question 25

bowtie filter

Answer 25

shaping filter

Question 26

what does the linear attenuation coefficient of any material depend on

Answer 26

• atomic number
• physical density
• x-ray photon energy

Question 27

what does ‘sampling aperture’ refer to

Answer 27

physical/ temporal window which a system samples/ captures data
- small sampling aperture required for high spatial resolution

Question 28

what is sampling aperture determined by

Answer 28

by focal spot size, detector size and scanner geometry

Question 29

how many samples in one projection

Answer 29

1000 samples
- attenuation profile for each view used to reconstruct into axial image slices