CT Flashcards
what is the hounsfield value of water
0
data acquisition is done by
conventional slice by slice and volume data acquisitions
which structure is best imaged at window width 350 and window level +50
the mediastinum
how to reduce streaking artefact - abdomen
- ask patient to put arms above their head
- maybe use adaptive filtration
effects of increasing slice thickness on SNR and dose changes
- SNR will increase
- Dose will reduce patient dose
effects of decreasing the pitch on SNR and dose
- SNR increases
- dose increases
ideal properties of a CT detector
- high dynamic range - need it to respond to a range of x-ray intensities
- high efficiency
- robust and stable
- little afterglow
what happens as the x-ray beam passes through the body
- 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
difference between x-ray and CT in sending photons
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
How does a CT scan work
- 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
what is attenuation
Attenuation is the lowering/weakening of an x-ray beam
linear attenuation coefficients
quantifies how much the x-ray beam is attenuated by the material it is passing through
CT imaging is based on…..
the conversion of x-ray linear attenuation values to hounsfield units- which can be transformed to an extended grey scale of signal intensities
what is the pixel
the volume of tissue that has been scanned within the matrix
the hounsfield scale varies from..
-1000 to + 1000
what is the Hounsfield value for soft tissue
+200 to - 200
what is the Hounsfield value for dense bone
+1000 to +300
what is the Hounsfield value for lung tissue
-300 to -800
what is the Hounsfield value for air
-1000
hounsfield values above 1000 are referred to as..
CT numbers
why do the kidneys appear different from vertebrae in a CT image
- 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
CT windowing
is the process in which the CT image greyscale is manipulated via CT numbers
CT window width
is the range of CT numbers displayed as shades of grey
- controls image contrast
CT window level
- determines the CT number to be the centre of the window
- controls image density
window width and level for lungs
WW: 1500 WL: - 600
window width and level for the brain
W:80 L: 40
window width and level for the vertebrae
W:1800 L:400
window width and level for liver
W:150 L:30
window width and level for abdo soft tissue
W:400 L:50
CT artefacts types
patient-based artefacts
physics based artefacts
scanner-based artefacts
patient based artefacts
motion, body composition or implants
physics based artefacts
this is relating to x-ray attenuation and other processes
scanner-based artefacts
due to faults in scan function
- the appearances can include streaks, shadows, rings and distortion
causes of motion artefacts
- patient movement
- involuntary movement - cardiac, respiratory, bowel movement
motion artefacts result in
blurring, double images and long range streaks
motion artefacts solutions
- faster scans - increase pitch, increase slice thickness, reduce tube rotation time per slice
- increasing patient comfort
- immobilization or sedation of patient
- breath holds
Photon starvation- physics based artefact usually seen in
high attenuation areas, such as metal implants or if arms are placed down beside the patient
Photon starvation- physics based artefact cause
insufficient photons reach the detector surface
quantum noise is magnified and streaks in the image are seen along the direction of greatest attenuation
Photon starvation- physics based artefact can be reduced by
mA modulation - machine selects an appropriate exposure factor
Adaptive filtration
iterative reconstruction
ring artefact - scanner based
- a faulty detector, during its rotation around the patient
- can mimic pathology
ring artefact- scanner based solution
recalibration of the detector - put a phantom on the detector and recalibrate machine
