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

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