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45 terms

Criteria for image evaluation

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Resolution
describes how closely fine details may be associated and still be recognized as separate details before seeming to blend into each other and appear "as one."
The degree of resolution transferred to the IR is a function of:
the resolving power of each of the system components and can be expressed in line pairs per millimeter (lp/mm), line-spread function (LSP), or modulation transfer function (MTF).
Line pairs per millimeter can be measured using:
a resolution test pattern; a number of resolution test tools are available
LSP is measured using a
10-m x-ray beam
MTF measures
the amount of information lost between the object and the IR
line-focus principle
The effective focal spot is the foreshortened size of the actual focal spot as it is projected down toward the IR, that is, as it would be seen looking up into the emerging x-ray beam
Small crescent-shaped artifacts on processed x-ray film are usually the result of
acute bending of the film before or after exposure.
A radiograph made with a parallel grid demonstrates decreased density on its lateral edges. This is most likely due to
decreased SID.
If there was a centering or tube angle problem
there would more likely be a noticeable density loss on one side or the other.
focused grid had been placed upside down
only the central portion of the image (along the long axis of the image receptor) would have been imaged
crescent-shaped kink, or crinkle, marks
caused by acutely bending the x-ray film.
an artifact will usually appear as a plus-density (dark) artifact if it is produced:
before exposure
an artifact will usually appear as a minus-density (light) artifact if it is produced:
after exposure.
X-ray film emulsion is very sensitive to mishandling, particularly after
exposure
Which of the following terms is used to describe unsharp edges of tiny radiographic details?
Blur
geometric blur
the area of blurriness that may be associated with small image details
blurriness can be produced by
using a large focal spot, or by diffused fluorescent light from intensifying screens
umbra
The image proper (ie, without blur)
Mottle
a grainy appearance caused by fast imaging systems
If the grid failed to move during the exposure
there would be grid lines throughout
If the central ray was off-center
there would be uniform loss of density.
When a slow screen-film system is used with a fast screen-film automatic exposure control system, the resulting images
are too light
When an AEC (phototimer or ionization chamber) is used
the system is programmed for the use of a particular screen-film speed (eg, 400 speed).
If a slower-speed screen image receptor is placed in the bucky tray:
the AEC has no way of recognizing it as different, and will time the exposure for the system that it is programmed for
if the AEC is programmed for a 400 screen-film combination, and if a 200-speed screen image receptor is placed in the bucky tray, the resulting radiograph:
will have half the required radiographic density
Foreshortening can be caused by
the radiographic object being placed at an angle to the IR
to reduce shape distortion
Aligning the x-ray tube, anatomic part, and image recorder so that they are parallel
elongation
Tube angulation causes elongation of the part
Size distortion (magnification)
is inversely proportional to SID and directly proportional to OID
Decreasing the SID and increasing the OID serve to
increase size distortion
What are the effects of scattered radiation on the radiographic image?
It produces fog
Scattered radiation
produced as x-ray photons travel through matter, interact with atoms, and are scattered (change direction).
If these scattered rays are energetic enough to exit the body:
they will strike the IR from all different angles
scattered rays
do not carry useful information and merely produce a flat, gray (low-contrast) fog over the image.
Grid cutoff
increases contrast and is caused by an improper relationship between the x-ray tube and the grid, resulting in absorption of some of the useful/primary beam
Treelike, branching black marks on a radiograph are usually the result of
static electrical discharge.
Problems with static electricity are especially prevalent during
cold, dry weather and can be produced by simply removing a sweater in the darkroom.
Grid cutoff due to off-centering would result in
overall loss of density
Over- or underexposure under the anode is usually the result of
exceeding the focusing distance limits in addition to being off-center
Foreshortening of an anatomic structure means that
it is projected on the IR smaller than its actual size
What effect will a stained intensifying screen have on the finished radiograph?
Decreased density
If an intensifying screen becomes stained, either partly or wholly, the stained area:
will not react to x-ray photons as completely and will emit less light.
the film emulsion adjacent to the stained area(s) will exhibit:
decreased density on the finished radiograph.
how would changing exposure factors affect film
The original mAs (regulating radiographic density) was 200. The original kVp (regulating radiographic contrast) was 90. The mAs was cut in half, to 100, causing a decrease in density. The kVp was increased (by 15%) to compensate for the density loss and thereby increase the scale of contrast.
developer temperature
1. The development process slows at the lower temperature, and a much lighter radiograph results.
2. at too high temperature, the development process is accelerated and the radiograph is too dark