Other expenses to consider are the need for high-frequency (e.g., 170 lines per inch) antiscatter grids for stationary (non-Bucky) applications such as portable bedside imaging and fixed grid cassette holders, and for readjusting phototimer
sensitivity to account for the lower detection efficiency of the conventional PSP imaging plate compared to 400 speed screen-film detectors.
Essentially a phototimer
for mammography, it measures both x-ray intensity and quality.
This expected exposure level is the basis of construction of any radiographic technique guide, or phototimer
He or she can set the exposure time manually or use the phototimer. Patient dose is directly related to exposure time, so this test is very important.
Phototimers are designed to provide a constant image density regardless of the thickness of the body part.
The backup timer for the phototimer
should stop the exposure at:
* The cassette should have a low absorption characteristic to maintain low levels of patient radiation exposure and accurate phototimer
One of the most tedious, time-consuming tasks involved in the evaluation of an x-ray imaging system is checking the performance of the automatic exposure control, commonly known as the phototimer. On most modern x-ray systems, the phototimer consists of an ionization chamber located in front of the film-screen cassette.
The Mammography Quality Standards Act of 1992, a federal law, requires that the phototimer be tested extensively during annual inspections of mammographic units.
A well-calibrated phototimer tracks both thickness and kVp.
Generally, the image will still be acceptable if the S value is between 200-400; however, with an S value above 400 there will be more noise and the image should probably be repeated." Ross further stated that "the Xpress CR is optimized when your equipment's phototimers
are calibrated to a 200 speed equivalent.
Table 2 Comparison of Increased mAs, Optical Density and Patient Exposure Output For Center and Side Cell Phototimers
Using 40" FFD and a 48" to 72" Grid(*)