HU unit
whats the attenuation coefficient of water?
0.19 or 0.21
whats the ct number of water/air/bone?
0/-1000/+1000
what are the Advantages of image Manipulation &Reformatting?
1- Enable visualization of specific structures relation to others 2- Determine extent of lesions and #s 3- help to localize lesions or F.B
what is region of interest (ROI)?
A display function available on all scanners is that of defining an area on the image
whats Reference Image?
A reference image is an image function that displays the slice lines in corresponding locations on the scout image.
The brightness value of the gray-scale image correspond to ?
CT numbers of the digital data they represent.
The gray level assigned to each pixel corresponds to the ?
Hounsfield units for that pixel .
Image Magnification ?
Image magnification only uses image data, by simply enlarging the existing image without improving resolution. Magnification is not decreasing DFOV
whats The Window level (W L): ?
Is the midpoint (center) of the whole range of CT numbers it is used to adjust the center of the window.
whats The Window Width (W W): ?
It adjusts the range of CT numbers that will be displayed with the contrast and therefore it controls contrast in the displayed image.
Effect of WL on CT image?
It changes the overall image brightness
Effect of WW on CT image?
It defines image contrast. Wider WW decreases image contrast and vise versa
Types of manipulation techniques?
Linear techniques: include image smoothing, and enhancement. Nonlinear techniques: include gray scale manipulation, in which the gray scale of the image can be modified (windowing).
what are the disadvantages of image Manipulation &Reformatting?
Loss of image details
whats A voxel(volume element)?
Slice thickness gives the pixel and added dimension.
whats linear attenuation coefficient?
The amount of the attenuation per unit thickness
whats relation of tissue attenuation/density and water with ct number?
Tissue with density (or attenuation) greater than water have +ve CT numbers. Those less dense than water will have -ve CT numbers.
whats Distance Measurements ?
allow distance measurements. This feature is helpful in reporting the size of the abnormality. It is also essential for the placement of a biopsy needle or drainage apparatus.
windowing is used to ?
alter the contrast of the displayed image by adjusting the window width and window level
after calculating ct numbers the computer then does?
converts it into a gray-scale image
whats relationship between linear attenuation coefficients (μ) of the tissues and CT number?
ct number= μtissue-μH2o/μH2ox1000
attenuation depends upon?
density and atomic number (Z) of materials and energy of x-ray photons.
how to calculate lowest and highest grey scale value ?
divide the WW by half ex:1000/2=+500/-500 then add the WL to both of them ex:500+100=600,-500+100=400
whats ct number?
is a digital value assigned to each pixel.
whats the matrix?
is made up a series of picture element, arranged in rows and columns, called pixels.
whats windowing ?
manipulation of CT image gray scale using CT numbers of that images.
The window level should be set at a point that is ?
roughly the same value as the average attenuation number of the primary tissue of interest.
whats Hounsfield measurement ?
several valuable tools that aid in image interpretation.
The amount of CT number variance within the region of interest is indicated by ?
standard deviation
define attenuation?
the reduction of the intensity of a beam of radiation as it passes through an object
why water is used is ct equation?
used because it is in abundance in the body and has a uniform density.
what does reformatting means?
using ct image to produce images in different planes
Window level selects ?
which Hounsfield numbers are displayed on
when using window width All values higher than the selected range appear ?
whiter, and vise versa
The number of Hounsfield units assigned to each level of gray is determined by?
window width.