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Fig. 4.14. The relationship of the phantom failure rates and failure rates for fibers, specks and masses to the mean glandular dose.

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aggravated by inadequate viewbox intensity to allow proper viewing of these very dark films.

For any radiographic imaging system, including digital mam-mography, quantum noise is a fundamental factor that can never be eliminated, only minimized. This is accomplished by attempting to maximize quantum efficiency and by using an adequate radiation dose. In screen-film mammography for a specified screen and film, the amount of radiation used is largely determined by the need to achieve a given optical density. In digital mammography, the detector and electronics should be designed to have adequate dynamic range and number of bits of digitization to precisely record the entire range of x-ray intensities transmitted through the breast. If this is the case, the electronic image can be amplified as much as desired so that there is really no constraint on image brightness. If an inadequate number of quanta is used, however, the SNR will be inadequate. Therefore, it is really the desired SNR that should determine the radiation dose used for a given examination (Haus and Yaffe, 2000).

In digital mammography, film granularity is eliminated. However, there may be variations in sensitivity of the receptor which would cause the image to have structure that is unrelated to the tissues in the breast. As long as the system design insures that these variations are temporally stable, this "fixed pattern noise" can be eliminated by imaging a uniform field of x rays and using the recorded image as a correction mask to make the image uniform. This procedure is known as "flat fielding" (Critten et al., 1996).

If flat fielding is not performed properly (e.g., if the mask image itself is noisy), residual noise can result in the digital image. In addition, there can be noise associated with the electronic circuitry that amplifies and digitizes the detector signal. Finally, there will be some level of granularity associated with either the soft-copy display device or with the film used to print the hard-copy digital images. For a digital system to perform well, it must be designed to minimize these nonquantum noise sources such that the SNR is determined by the level of radiation used.

4.2.4 Artifacts

Artifacts are unwanted contrasts that appear in the image and are unrelated to anatomical structures within the breast. Artifacts have two detrimental effects on mammographic quality: (1) they can mask the detection or impair the characterization of lesions by adding "clutter" or noise to the image and (2) they can simulate lesions that do not exist. In screen-film mammography, artifacts can be caused by the x-ray source, beam filter, compression device, breast support table, grid, screen, film, processor, and darkroom. These have been well documented in the literature (Haus and Jaskulski, 1997) and their evaluation should be part of any QC program for mammography (ACR, 1999).

In digital mammography, all of the artifacts caused by components before the image receptor are still possible. In addition, there can be artifacts caused by nonuniformities in the detector response over the image area. These may be a result of improper flat fielding, errors in scanning (Type 2 and 4 units), or mismatches in "stitching together" sub-images from detectors that contain multiple modules (Types 1 and 4). Artifacts can also be caused by nonuniformity or miscalibration associated with the hard- or soft-copy display systems (Roehrig et al., 1995). With good design, and proper maintenance and system calibration, it should be possible to control or eliminate these artifacts.

4.3 Summary

It is important to remember that the goal in making a mammo-gram is to obtain as much diagnostic information as possible at the lowest dose compatible with obtaining that information. As noted earlier, this necessitates compromises (i.e., an optimization of factors that affect image quality). These include: beam quality, compression, imaging geometry, grids, receptor characteristics, processing of the film or digital image, and display and viewing conditions. If this is done correctly, a high-quality mammogram can be obtained at a reasonably low dose to the patient. The goal is not simply to use as low a dose as possible because, if this is done, there is a risk of degrading the performance of mammography in detecting or accurately characterizing small, node-negative cancers.

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