Image Quality

4.1 Introduction

It is widely recognized that high-quality images are imperative for the reliable detection and accurate characterization of subtle lesions in the breast with mammography. The quality of the images depends critically on the design and performance of the x-ray unit and image receptor, and on how that equipment is used to acquire the mammogram. In addition, the type of display and the conditions under which the image is viewed have an important effect on the ability of the radiologist to extract the information recorded in the mammogram.

4.1.1 Mammographic Image Quality

In general, the phrase "mammographic image quality" can be considered to indicate the clarity with which radiologically-significant details can be perceived in an image. In turn, high mam-mographic image quality should contribute to high performance in detecting and diagnosing breast cancer. There is, however, no well-defined standard for specifying mammographic image quality. The relationship between physical properties of the radiographic image (such as contrast, resolution and noise) and the ability of the observer to properly detect and interpret relevant image features is not well understood (Haus and Yaffe, 2000; NCRP, 1986). Currently, probably the most effective tool for inferring this is receiver operating curve (ROC) methodology (Metz, 1979), applied retrospectively to clinical images where the true disease state is known. ROC testing can be used to assess the overall performance of a radiologist in combination with a particular imaging system at detecting or diagnosing breast cancer in terms of sensitivity at varying levels of specificity. This creates a measure that is based on perception of information in the mammogram and is essentially independent of the level of conservatism of the radiologist in calling abnormal findings. Unfortunately, ROC studies are complex, time-consuming experiments, requiring large image databases with known truth data regarding disease. They require many image readings by many observers, and are, therefore, often not practical for routine measurement of image quality.

Logically, mammographic image quality should be related to certain image attributes that can be described technically, such as spatial resolution, contrast, image noise and SNR and the absence of artifacts (Haus and Yaffe, 2000; Vyborny and Schmidt, 1994). It is accepted that these are important parameters that will affect the ability to detect or characterize microcalcifications, to visualize fine fibrillar structures radiating from a mass, or to identify the presence of architectural distortion. It is still not known, however, how to define what constitutes "optimal" or "necessary" quality for diagnostic accuracy in terms of technical parameters. At present, about the best that can be done is to attempt to correlate differences in ROC performance with differences in the technical aspects of image acquisition and display. This issue is the subject of continuing research (Bencomo et al., 1982; Bunch, 1999; Chan et al., 1987b). Nevertheless, there is a strong correlation between radiologist's rejection of mammograms as having inadequate quality and low measured values of resolution, contrast and SNR, or the excessive prevalence of artifacts.

In many cases, the optimum values of these parameters are not all simultaneously achievable, at least not at reasonable radiation dose and, therefore, trade-offs must generally be considered. The most acceptable compromise between technical parameters in forming the image to achieve high mammographic quality is likely to be task dependent. For example, the image characteristics required to allow detection of a large lesion in a fatty breast can be very different from those needed to visualize microcalcifications in a rather dense breast.

4.1.2 Screen-Film and Digital Mammography

By the early 1980s, screen-film mammography had largely replaced direct-film mammography and xeroradiography as the main technique for producing mammograms. Digital mammogra-phy systems have now emerged. There are key differences in the technologies of screen-film and digital mammography. These are discussed in Sections 3.2 and 3.3. These affect both the image quality and the approach to optimization of technique, and therefore, specific reference will be made to digital mammography in this Section where these differences exist.

In the following, the discussion is restricted to those factors that can be quantified objectively. The term, "technical image quality" is used to include those factors that are measurable in the imaging process rather than variables entering into interpretation of the image.

4.2 Factors Affecting Image Quality and Radiation Dose

In this Section, the descriptors of technical image quality and their dependence on the many variables of image acquisition and display in screen-film and digital mammography are discussed. The major parameters describing image quality are: radiographic contrast, spatial resolution (blur), noise (mottle), and the presence of artifacts. These must be considered in relation to the dose to the breast required to produce the mammogram. The technical factors that influence these parameters are listed in Table 4.1 (Haus and Jaskulski, 1997; NCRP, 1986). Image quality, as used here, refers to the aggregate affect of these elements on the appearance of the mammographic image. Inevitably, there are trade-offs among the many components involved (NCRP, 1986). Many of the factors in image acquisition and display can be controlled and optimized so that mammograms having good image quality can be obtained at appropriate radiation dose to the patient.

Using the outline in Table 4.1, the factors affecting technical image quality are reviewed for screen-film and digital mammogra-phy image receptors with emphasis on those that can be controlled by the user.

4.2.1 Contrast

Radiographic contrast refers to the magnitude of the signal difference between the structure of interest and its surroundings in the displayed image. Radiographic contrast is influenced by two factors: subject contrast and receptor contrast. Contrast is typically considered for larger areas (1 cm2 or greater) in the image where spatial resolution of the detector is not a limiting factor. Subject contrast is measured in terms of the relative difference in x-ray exposure to the image receptor, transmitted through one part of the breast and through an adjacent part, while overall radiographic contrast is quantified as the optical density difference between two areas on the processed film, or as the relative brightness difference between the corresponding areas in an image displayed on a monitor. Subject Contrast. Subject contrast is especially important in mammography because of the subtle differences in the soft-tissue density of normal and pathologic structures of the breast, and because of the importance of detecting minute details such as

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