iaea human health series publications - SEDIM

GLOSSARYair kerma. The energy deposited per unit mass in air. The unit used to measure air kerma is the gray (Gy). ForX rays with energies less than 300 kilo-electron volts (keV), the magnitude of air kerma and absorbed dose inair are equivalent.amorphous selenium (a-Se, or a-Se). Amorphous selenium layers have the same structure as single crystals overshort distances, but are less ordered over larger distances. As a result, amorphous selenium layers provideuniform X ray detection over the large areas needed by flat panel X ray detectors. When X rays are absorbedin a-Se, electron–hole charge pairs are created and these can be collected under an electric field appliedbetween the two faces of the detector to form the signal. The a-Se can be deposited onto amorphous siliconTFT arrays to provide readout of the signal.amorphous silicon (a-Si, or a-Se). A type of silicon which has the same structure as single silicon crystals overshort distances but is less ordered over larger distances The ability to produce relatively large areas ofamorphous silicon at a cost that is very much lower than that for single crystal silicon has enabled fabricationof flat panel TFT arrays large enough to be used as the readout structure for all flat panel X ray detectors.artefact. Any structure or pattern visible in the image that is not part of the object being imaged.automatic exposure control (AEC) systems.Fully automatic AEC. A device designed to determine the spectrum (target material, filtration material andkV) and/or the exposure (mAs) needed to produce an adequately penetrated X ray image. This istypically done by sampling the X ray intensity after it passes through the patient and image receptor.Automatic exposure time (photo timing). The beam quality parameters are selected manually and the exposuretime is controlled automatically.bit depth. The number of bits used to digitize the detector signal, giving rise to the number of digital signal levels(greyscale). For a bit depth of n, the number of possible grey levels is equal to 2 n (i.e. 12 bits = 4096 shades ofgrey). Bit depth cannot be changed after equipment is purchased and is a vendor specific system characteristic.cassette. A light-tight case usually made of thin, low X ray absorption plastic, for holding intensifying screens andfilm or the CR plate. The screen stores the latent image and requires further processing to produce a visibleimage.compression device. A plastic paddle used to flatten and immobilize the breast. Compression helps reduce motionblurring in the breast, separates structures within the breast, and decreases the thickness of breast tissue. Thisminimizes the amount of radiation required and the amount of scattered radiation reaching the image receptor.Ideally, the compression device is made of rigid, thin plastic and has a flat bottom surface that is parallel to theplane of the image receptor, with edges perpendicular to the plane of the image receptor to assist in movingbreast tissue away from the chest wall and into the field of view.contrast resolution. The smallest relative exposure change that can be usefully imaged by a system. Ultimately,contrast resolution is limited by the dynamic range and the quantization (number of bits per pixel) of thedetector. Increased contrast resolution is considered one of the major advantages of digital receptors and tendsto counteract the lower spatial resolution of many digital systems.control chart. A graphical means of displaying data in which the variable of interest is plotted on the vertical axisas a function of time on the horizontal axis. The control chart allows for easy and rapid review of the data todetermine whether the process is within the desired control limits (‘in control’).173