digital imaging CT

ADC

converts analog info to digital, making the analog info useful to the computer

used for image processing and reconstruction in CT

steps of ADC

scanning

sampling

quantization

scanning

For the picture to be digitized it is separated into small regions on a grid (matrix)​

hounsfield unit

pixel

sampling

brightness of each pixel is measured

quantization

brightness value of the pixel is assigned

scan field of view

area to be imaged within the gantry

wherein raw data is collected

smaller

better: larger of smaller SFOV?

SFOV

determines the number of detectors collecting data per scan

reconstruction or display field of view

determines how much of the collected raw data is used to create an image

DFOV

equal to or less that the size of the SFOV

matrix

the computerized grid of data onto which the slice is placed for the purpose of reconstructing the density info

512×512

1024×1024

common CT matrix sizes

over the SFOV

where is the CT matrix placed

decreased

smaller matrix: increase/ decrease noise

smaller matrix

larger pixels

decreased spatial resolution

decrease noise

larger matrix

smaller pixels

increased spatial resolution

increase noise

pixel size (d)

= FOV / matrix

voxels - volume element

tissue volume

determined by multiplying the pixel size by the thickness of the CT image slice

voxel

(pixel size)² x slice thickness =

hounsfield units

the levels of brightness displayed on the monitor for each pixel

0

water CT number

1000+

dense bone CT number

-1000

air CT number

CT number

µt - µw X 1000µw =​

µw

attenuation coefficient of water

µt

attenuation coefficient of tissue

higher CT numbers

light shades of gray/ white

lower CT numbers

darker shades of gray/ black

attenuation

reduction i the intensity of the beam a sit passes through an object

windowing

adjusting the gray scale

256

monitor can detect how many shades of gray

window level (wl)

HU value of the area of interest

center of range

window width (ww)

range of grays (CT numbers)

determines the number of HUs represented on an image

white

numbers above range will be?

black

numbers below range will be?

lower contrast

larger window width

high contrast

smaller window width

calculate CT numbers HU

divide widow width in half

subtract the quotient form the window level for lower limit

add the quotient to the window level to determine upper limit

slice thickness

the width of the volume of tissue being examined (voxel)

anatomical coverage

noise in image

patient dose

what does slice thickness affect?

slice thickness

thickness of the x-ray beam / slices per rotation =

magnification

permit viewer to enlarge part of the reconstructed image on the monitor for viewing or filming

mag

allows for easier examination of image

enlarges the individual pixels

post processing done to the image not to raw data

“zoom in”