iaea human health series publications - SEDIM

to X rays. In the case of flat panel detectors, this is the area occupied by the switches and the readout lines, resultingin a reduced ‘fill factor’ and directly influencing the efficiency of use of the incoming X rays. A smaller aperturecauses the image to be sharper but can cause a reduction in detector sensitivity and can result in information beingmissed. When the aperture is smaller than the pitch, undersampling occurs and a phenomenon called ‘signalaliasing’ is more likely to be observed. Aliasing causes information to be incorrectly rendered in the image, bothsuppressing some spatial frequencies and giving the impression of signal information that does not actually exist.Noise aliasing is a similar process that causes an increase in the apparent image noise. In most current digitalmammography systems, signal aliasing is not clinically apparent, while noise aliasing may be measurable.The effective aperture can also be larger than the pitch. This can be due to blurring by spread of light in a CsIphosphor, or in the case of CR systems, scattering of the readout laser light in the screen. In this case the image maybe less sharp; however, one possible benefit of this blurring is the reduction of aliasing. This phenomenon may beimportant in considering differences in performance between direct conversion and phosphor based systems. In theformer the effective del aperture is more likely to be close to the pitch, giving rise to an inherently sharper imagewith more aliasing, whereas in the latter the larger effective aperture caused by slight blurring may result in lessaliasing, so there may be a trade-off between sharpness and noise.2.3.4. Data representation and exposure index valuesMost captive detector digital mammography units provide data in a linear format. The ‘for processing’ imagesfor these devices are normally flat field corrected to remove stationary effects such as detector gainnon-uniformities (individual dels or lines of dels) and beam non-uniformities (filter, heel artefact). The linear valuesmay or may not have a zero offset. Some manufacturers add a constant offset of either 50 or 200 to the data values,which are normally 12 bits (4096), 13 bits (8129) or 14 bits (16 384).Typically, CR systems compress the signal during image acquisition by applying a logarithmic (or logarithmiclike) transformation. Each vendor has a system for defining the relationship between detector exposure and signalin their ‘for processing’ images. These are variously known as S# value and L# value, exposure index (EI), scanaverage level (SAL) or log median exposure level (lgM). Annex IV discusses these parameters and other DICOMtags used in digital mammography for the purposes of exposure and dose estimation.The ‘for presentation’ images normally cannot be used to accurately estimate local exposure to the detectorbecause most vendors apply a non-reversible, non-stationary transformation of the data, which may include locallyadaptive frequency enhancement and peripheral equalization (see Section 2.3.7).2.3.5. Automatic exposure control on digital mammography systemsThe design of the automatic exposure control (AEC) in digital mammography units differs from that inanalogue mammography X ray units. First, the dose to the detector no longer must be constrained to the relativelynarrow range suitable for screen film. Thus, doses can be lower or higher, and can be widely varied as requiredaccording to the breast thickness. This extra freedom also extends to the choice of technique factors such as kV, andtarget and filter material. Generally, digital systems will select X ray spectra that are more penetrating than wouldbe the case with screen film systems. This is possible because the loss of subject contrast may be compensated forby enhancement of displayed contrast during image viewing as well as by additional computer image processing.Moreover, greater detector dose can be used, if desired, leading to better image noise characteristics.It is important to understand that once the dose is increased beyond a noise limited image, the image providesvery little subjective indication that the dose is excessive, and ‘dose creep’ may well result in long term increasesbeyond optimal levels.Most DX systems use a measurement of the compressed breast thickness (produced by a sensor in thecompression mechanism) to choose some of the technique factors (e.g. kV, target, filter) to be employed in theexposure. Furthermore, some DX units use a trial exposure to determine the transmission through the breast. Theimage from this trial exposure may or may not be incorporated into the resultant image formation and/or includedin the specified post-exposure mAs. In a further refinement of this approach to determining the exposure factors,some DX systems utilize sophisticated AECs that identify the area of greatest attenuation within a defined area ofthe detector during this trial exposure. This is then used to select an appropriate kV and filtration, and sufficientexposure to achieve a predetermined pixel value, contrast or detector dose set by the manufacturer. As a result, the12