CT physics 5 Flashcards
what do you need to consider to determine pixel size in image display?
reconstruction field of view (RFOV)
how to calculate pixel size
RFOV/ matrix size
what does smaller RFOV allow
viewing of smaller objects in patient
main limiting factor in spatial resolution
pixel size
- you cannot resolve details smaller than 1 pixel
isotropic resolution
when all x, y and z planes are the same size
pros of isotropic resolution
allows for accurate measurements in any direction within image
what is K in the hounfield unit in the formula
K is a number contrast factor and we will take this being 1000
Hounsfield unit defined using following formula
HU = attenuation of X-rays in tissue-attenuation of X-rays in water/ attenuation of X-rays in water
how many grey levels are CT images displayed using
256 grey levels
what is window width
the number of HU values chosen to be displayed
what is window level
is the mid-point of HU values chosen in the window width
- should be centered near average attenuation of tissues of interest
how can contrast be changed
altering window width
- if ww wide contrast is poor
window width of 10
image will appear very black and white
large window width
400-2000 HU
- transition from low HU value (black) to high HU value (white) is large
- should encompass tissues of differing attenuation within image
- makes it possible to visualise different types of tissue at once
small window width
50 to 400 HU
- transition from low HU values (black) to high HU values (white) is small
- only a few HU values assigned some grey level value
