CT

Question 1

CT basic principle

Answer 1

images taken all around the patient

data reconstructed
tomogram: slice through patient

Question 2

CT scanner

Answer 2

gantry:
tube
aperture
detectors

Question 3

x-ray source

Answer 3

high voltages for high energies needed to scan full body

120-140kV

heavily filtered to get rid of low energies

use effective energy

compton scatter dominates attenuation

measures electron density

Question 4

detectors

Answer 4

fast, no lag
high absorption efficiency
small and compact to fit in gantry
high stability to take more images with the same detector response
large dynamic range

Question 5

xenon ionisation chamber

Answer 5

x rays ionise gas

electric field attracts ions

charge collected proportional to xray intensity

Question 6

solving low quantum efficiency of gas

Answer 6

use high Z gas (xenon)
gas at high pressures
make detector long

solid state detectors
scintillator and photodiode
higher absorption efficiency

Question 7

first generation CT

Answer 7

pencil beam
single detector
translate and rotate around object

configuration adv:
easy calibration
low cost
high scatter rejection due to beam collimation
parallel-beam reconstruction, geometry allows easier reconstruction

Question 8

second generation CT

Answer 8

multiple detectors
fan beam - fewer rotations
translate and rotate
reduces acquisition time

Question 9

third generation CT

Answer 9

more detectors
only rotate
fan beam

vulnerable to ring artefacts because the same physical detector channel measures all the rays that form a ring

Question 10

fourth generation CT

Answer 10

detectors all around gantry
only source rotates
avoids ring artefacts

inflexible system, can’t change source-detector distance
fixed

Question 11

helical/spiral scanning

Answer 11

continuously rotating xray tube

patient is moved through a rotating x-ray beam and detector set. From the perspective of the patient, the x-ray beam from the CT traces a helical path. The helical path results in a three-dimensional data set, which can then be reconstructed into sequential images for an image stack.

balance speed with loss of data
higher pitch (speed) lowers radiation dose but at expense of partial volume effect (loss of detail)

quick, done in one breath
less movement artefacts

Question 12

slice interpolation

Answer 12

at every bed position, only 1 projection

360 degree linear interpolation
uses attenuation data from points 360 apart on the helix for interpolation

makes effective image width broader

180 degree linear interpolation
distance shorter
slice profile narrower (good thing)
but noisier

Question 13

multislice CT

Answer 13

allow acquisition of multiple slices in a single rotation

cone beam artefacts

4-slice, 16-slice, 64-slice

faster

allows heart imaging

Question 14

multislice CT slice thickness

Answer 14

thick slices have lower noise

thin slices reduce partial volume effects and allow off axis image creation with isotropic resolution

Question 15

CT numbers

Answer 15

mu for each pixel converted to CT number
allows a greater range of gray levels to be displayed

in Hounsfield units

polychromatic beam
so scales values to reduce variation between difference scanners (different energies)

CT number for water is 0

lower CT number if less dense than water

Question 16

why a high range of CT values for same material

Answer 16

consistency not the same
intra-tissue variation
inter-patient variation

Question 17

windows and levels

Answer 17

window = width of CT numbers
level = centre of window

distributes grayscale across values

if CT number:
below window = black
above = white
within = gray shade

Question 18

image quality

Answer 18

determined by
image contrast
spatial resolution: line pair test object, acrylic bars
noise: tube mA, scan time, slice thickness
artefacts

Question 19

QA of CT systems

Answer 19

use phantoms to measure
noise
size dependence
spatial resolution
contrast scale
sensitivity
alignment
absorbed dose
linearity
slice thickness

Question 20

artefacts

Answer 20

ring
motion
spectral
streak
partial volume
cone beam
noise

Question 21

ring artefacts

Answer 21

3rd gen
non uniform detector response
concentric rings in image
removed by averaging or appropriate filters
identified and removed by software algorithms

Question 22

spectral artefacts

Answer 22

beam not monochromatic
beam hardened across patient (the low-energy photons are absorbed more than the high-energy photons)
average energy higher in centre
CT numbers higher at edge
cupping effect

correct:
use bow tie filter
to modulate spectrum
software solutions

Question 23

streaking
artefact

Answer 23

high density objects
cause streaks
eg bullet, surgical pin

Question 24

partial volume effects

Answer 24

loss of apparent activity in small objects or regions because of the limited resolution of the imaging system

one 3D voxel occupied by more than one tissue type

CT number is intermediate

Question 25

cone beam artefacts

Answer 25

partial volume
problem for multislice CT

Question 26

noise

Answer 26

statistical noise dominated
measured with phantom
to reduce:
more photons
so increase mA (tube current), rotation time, slice thickness

Question 27

dose in CT

Answer 27

higher dose than diagnostic radiography

needs image reconstruction from large number of projections

dose scales with number of projections

Question 28

dose index

Answer 28

dose to a depth in a scanned volume for a complete series of slices

measure with a long ionisation chamber inserted into a phantom (14 slices)

large contribution from scatter

Question 29

dose reduction

Answer 29

expose less (reduce mA, or shorten scan time):
lowers CNR contrast to noise ratio

keep CNR constant:
expose less and increase slice thickness
may affect spatial resolution

adaptively change exposure as patient anatomy changes

automatic exposure control

angular dose modulation

longitudinal dose modulation

Question 30

table feed

Answer 30

table travel per rotation