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66 terms

Image Acquisition and Evaluation CH4

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density decreases when grids are used, unless...
mAs in increased to compensate for the loss of scatter fog
density decreases by beam restriction, unless...
mAs is increased to compensate
subject contrast: atomic number
high atmoic number increases contrast owing to photoelectric effect
subject contrast: tissue density
increase in tissue density decreases constrast owing to Compton effect
subject contrast: tissue thickness
as tissue thickness increases, contrast decreases owing to Compton interaction
contrast primary function is...
to make detail visible
spatial resolution or recorded detail
`sharpness which anatomical structures are displayed on an image receptor
increased OID...
causes magnification, loose recorded detail
short SID...
causes magnification of the image, loose recorded detail
focal-spot size...
large focal-spot decreases sharpness
distortion
geometric misrepresentation of an anatomical part or image receptor; size and shape
AEC
uses fixed kVp while machine controls mAs; changing kVp doesn't change density, but change contrast
grid is used when part thickness is...
10cm or greater or using 60 kVp or higher
grid construction...
lead strips separated by aluminum interspacers
grid ratio
height of lead strips divided by the distance between the lead strips; H/D
grid frequency
number of lead strips per inch(cm)
grid radius(focal distance or focal range
distance at which focused grid may be used
focal range is wide for...
low-ratio grids
focal range is narrow for...
high-ratio grids
grid characteristics
contrast improvement factor, grid selectivity, grid conversion factor
contrast improvement factor
measure the ability of a grid to increase contrast; ratio of contrast w/ a grid to contrast w/o a grid
grid selectivity
ratio of primary radiation transmitted through the grid to secondary transmitted through the grid
high grid selectivity indicates...
high efficiency of scatter cleanup
grid conversion factor(GCF) or Bucky Factor
exposure increase is necessary to compensate for x-ray and scatter absorption
grid conversion factor or bucky factor pt2...
increase in mAs when converting from nongrid to grid...multiply mAs by GCF
grid conversion factor or bucky factor pt3...
decrease in mAs when converting from grid to nongrid status...divide mAs by GCF
stationary grids
do not move during the exposure; grid lines may be seen
moving grids
moves back and forth during exposure; eliminate grid lines; move just before exposure and continue until just after the exposure
upside-down grids
normal density in the middle of the radiograph w/ decreased density on the sides
off-level grid
decreased density visible over the entire radiograph; absorb entire image-forming rays
lateral decentering
cutoff visible, more to one side of the radiograph
grid-focus decentering
normal density in the middle of the radiograph w/cutoff visible on the sides
air gap technique
uses increased OID; increased OID allows scatter to exit the patient and miss the image receptor
3 types of technique charts:
fixed kVp-variable mAs; variable kVp-fixed mAs; varible kVp-varible mAs
primary purpose of technique charts is...
radiation protection through decreased repeat exposure; more consistent image quality
fixed kVp-varible mAs assumes...
thicker parts absorb more rays and more rays must be placed in the primary beam
variable kVp-fixed mAs assumes...
thicker parts require a beam w/ shorter wavelength rays that are more penetrating
variable kVp and variable mAs assumes...
variations of pathological conditions, age, casts, contrast media
CR(computed radiography)
use imaging plate
imaging plate (IP)
made of a photostimulable phosphor (PSP); remnant x-rays strike electrons in the phosphors, placing electrons in a higher state
flat panel DR system use either...
direct or indirect detectors
direct detectors convert exit radiation...
directly into electrical charges, speeding image construction
indirect detectors convert exit radiation first to...
visible light then to electrical charges
digital image is composed of rows and columns called ...
a matrix
smallest component of the matrix is the ...
pixel (picture element)
each pixel corresponds to a shade of gray representing an area in the patient called...
a voxel (volume element)
a histogram (graphic display)...
is constructed to show the radiographer the distribution of pixel values(indicating low, proper, or high exposure)
information can be manipulated through various postprocessing steps...
subtraction, edge enhancement, contrast enhancement, and black/white reversal
subtraction
removal of superimposed or unwanted structures from the image
contrast enhancement
altering of image to display varying brightnesses
edge enhancement
improves visibility of small high-contrast areas
black/white reversal
reversal of the gray scale in the image
postprocessing can compensate for overexposures or underexposures of considerable degree...
-100% to +500%
resolution is finer with DR than with CR because...
DR involves less conversion of the information
changing window level to adjust image brightness throughout the range of densities is a what type of relationship...
direct relationship; change image brightness
changing window width to adjust contrast is what type of relationship...
inverse relationship; changes contrast
spatial frequency resolution
level of detail or sharpness on the CR image
look-up table (LUT)
histogram of brightness level values from image acquisition that can be used to correct, or enhance, luminance values
quantum mottle
grainy or noisy image; caused by too few x-ray photons hitting the image receptor
PACS
Pictures archiving and communications system
DICOM
digital imaging and communications in medicine; it's used for blending PACS and various imaging modalities
contrast
visible differences between any two selected areas of density levels w/in the radiographic image
long scale or low contrast
increase in density
short scale or high contrast
decrease in density
processing algorithm(math formula)...
used by the computer to reconstruct the image specific to the examination that was performed
density
the amount of blackness on a given area