CT

Question 1

what is the hounsfield value of water

Answer 1

0

Question 2

data acquisition is done by

Answer 2

conventional slice by slice and volume data acquisitions

Question 3

which structure is best imaged at window width 350 and window level +50

Answer 3

the mediastinum

Question 4

how to reduce streaking artefact - abdomen

Answer 4

• ask patient to put arms above their head

- maybe use adaptive filtration

Question 5

effects of increasing slice thickness on SNR and dose changes

Answer 5

• SNR will increase

- Dose will reduce patient dose

Question 6

effects of decreasing the pitch on SNR and dose

Answer 6

• SNR increases

- dose increases

Question 7

ideal properties of a CT detector

Answer 7

• high dynamic range - need it to respond to a range of x-ray intensities
• high efficiency
• robust and stable
• little afterglow

Question 8

what happens as the x-ray beam passes through the body

Answer 8

• some of the beam are transmitted through the tissue it interacts with
• some are absorbed by the tissue
• some x-ray photons are deflected - causing scatter radiation

Question 9

difference between x-ray and CT in sending photons

Answer 9

x-ray tube sends out x-ray photons at a stationary position

• whereas CT x-ray tube revolves around the anatomical structure and sends out photons whilst moving 360 around the patient - produces a 3D image

Question 10

How does a CT scan work

Answer 10

• finely collimated x-ray beam is produced from the tube
• the beam is then attenuated by the body depending on atomic number and density
• the attenuated patterns received by the detector are processed by the computer with simultaneous equations to produce an axial image

Question 11

what is attenuation

Answer 11

Attenuation is the lowering/weakening of an x-ray beam

Question 12

linear attenuation coefficients

Answer 12

quantifies how much the x-ray beam is attenuated by the material it is passing through

Question 13

CT imaging is based on…..

Answer 13

the conversion of x-ray linear attenuation values to hounsfield units- which can be transformed to an extended grey scale of signal intensities

Question 14

what is the pixel

Answer 14

the volume of tissue that has been scanned within the matrix

Question 15

the hounsfield scale varies from..

Answer 15

-1000 to + 1000

Question 16

what is the Hounsfield value for soft tissue

Answer 16

+200 to - 200

Question 17

what is the Hounsfield value for dense bone

Answer 17

+1000 to +300

Question 18

what is the Hounsfield value for lung tissue

Answer 18

-300 to -800

Question 19

what is the Hounsfield value for air

Answer 19

-1000

Question 20

hounsfield values above 1000 are referred to as..

Answer 20

CT numbers

Question 21

why do the kidneys appear different from vertebrae in a CT image

Answer 21

• the kidneys have a much lower Hounsfield value compared to bone. so they appear darker than the vertebrae - between +200 and - 200. due to them having a different linear attenuation coeeficient - which quantifies how much the x-ray beam is attenuated by the material it is passing through
• the vertebrae has a hounsfield value of +1000

Question 22

CT windowing

Answer 22

is the process in which the CT image greyscale is manipulated via CT numbers

Question 23

CT window width

Answer 23

is the range of CT numbers displayed as shades of grey

• controls image contrast

Question 24

CT window level

Answer 24

• determines the CT number to be the centre of the window

- controls image density

Question 25

window width and level for lungs

Answer 25

WW: 1500 WL: - 600

Question 26

window width and level for the brain

Answer 26

W:80 L: 40

Question 27

window width and level for the vertebrae

Answer 27

W:1800 L:400

Question 28

window width and level for liver

Answer 28

W:150 L:30

Question 29

window width and level for abdo soft tissue

Answer 29

W:400 L:50

Question 30

CT artefacts types

Answer 30

patient-based artefacts physics based artefacts scanner-based artefacts

Question 31

patient based artefacts

Answer 31

motion, body composition or implants

Question 32

physics based artefacts

Answer 32

this is relating to x-ray attenuation and other processes

Question 33

scanner-based artefacts

Answer 33

due to faults in scan function - the appearances can include streaks, shadows, rings and distortion

Question 34

causes of motion artefacts

Answer 34

- patient movement | - involuntary movement - cardiac, respiratory, bowel movement

Question 35

motion artefacts result in

Answer 35

blurring, double images and long range streaks

Question 36

motion artefacts solutions

Answer 36

- faster scans - increase pitch, increase slice thickness, reduce tube rotation time per slice - increasing patient comfort - immobilization or sedation of patient - breath holds

Question 37

Photon starvation- physics based artefact usually seen in

Answer 37

high attenuation areas, such as metal implants or if arms are placed down beside the patient

Question 38

Photon starvation- physics based artefact cause

Answer 38

insufficient photons reach the detector surface quantum noise is magnified and streaks in the image are seen along the direction of greatest attenuation

Question 39

Photon starvation- physics based artefact can be reduced by

Answer 39

mA modulation - machine selects an appropriate exposure factor Adaptive filtration iterative reconstruction

Question 40

ring artefact - scanner based

Answer 40

- a faulty detector, during its rotation around the patient | - can mimic pathology

Question 41

ring artefact- scanner based solution

Answer 41

recalibration of the detector - put a phantom on the detector and recalibrate machine