iaea human health series publications - SEDIM

2.3.7.1. DICOM ‘for processing’ imageThe initial operations that take place in creating the ‘for processing’ image generally include a flat field orgain correction, where spatial non-uniformity in the detector sensitivity can be corrected by imaging a uniformlyattenuating object and creating a gain map that can be used to correct all subsequently acquired images. For flatpanel systems and scanning systems, this transformation also corrects for non-uniformities in the X ray field (e.g.the heel effect). For current CR systems, the correction is applied only to the laser readout system and not to theindividual phosphor plates or to the X ray field. Therefore, residual artefacts may occasionally be observed. Thepresence of the uncorrected heel effect in digital images can affect the results of image noise measurements in QC[29]. Other corrections that can occur at this stage include:— Partial correction for detector blurring through image processing by deconvolving the blurring function of thedetector. This procedure can be very effective, but if overdone it will also enhance image noise. It is thereforeimportant that the inherent detector resolution be adequate and that the image noise level be acceptable. Thelatter is accomplished in part through careful design of low noise detectors and also through appropriatedesign and use of AEC and/or automatic technique control.— Removal of ‘bad’ pixels. If a single detector element is defective, its signal can be replaced by some weightedcombination of signals from adjacent dels. This is acceptable if the defective dels are isolated and few innumber but is of greater concern if signals from entire patches or lines of the detector are absent or incorrect.Manufacturers specify the number and type of such defects that are acceptable.2.3.7.2. DICOM ‘for presentation’ imageAdditional processing is generally carried out to adapt the image for display and interpretation by theradiologist. Processing operations differ among manufacturers, but may include:— Peripheral compensation to flatten the signal level at the edge of the breast. This essentially suppresses theeffect of thickness reduction near the edge and reduces the dynamic range of image signal that the displaysystem must accommodate, allowing higher contrast settings to be used in image display. When such softwareis used, it is important that it does not unduly distort the contour of the breast.— Inversion of the greyscale (black represents high X ray transmission) and non-linear transformation(logarithm, square root, etc.) of the image.— Background suppression and masking.— Other image enhancements, for example, histogram equalization. These are also employed to attempt tooptimize contrast throughout the image of the breast and to best utilize the limited dynamic range of thedisplay system.Image enhancement techniques are proprietary to each vendor and frequently can be applied (or not) at theuser’s discretion. The best way to evaluate these algorithms is to observe the rendition of key structures(spiculations, microcalcifications, and margins of benign and malignant lesions), with and without the enhancementactivated, in a series of sample cases including images of both dense and fatty breasts. It is also important to knowwhether the results of image processing are only available locally at the viewing workstation or are preserved sothat the enhanced image can be viewed elsewhere.2.3.8. Display systemThe display system plays a major role in influencing overall performance of the digital mammography unit interms of both the ease of image interpretation and the image quality presented to the radiologist [30]. While someradiologists use hard copy systems (laser printed films) for interpretation, in the long term the benefits and costeffectiveness of digital mammography will only be fully realized if soft copy display is used.14