Evaluating the Performance of Digital Mammography Systems: Physical and Psychophysical Characterization

This chapter illustrates some of the methods used for evaluating the physical and psychophysical performances of digital detector for imaging in mammography. Objective and subjective measurements by means of human observers and automatic methods can be achieved to better characterize the performance of digital systems. Some results from commercial units are also presented. To assess the image quality of the various systems, spatial resolution and noise properties can be evaluated using metrics such as the modulation transfer function (MTF), noise power spectra (NPS), noise-equivalent quanta (NEQ), and detective quantum efficiency (DQE). While the image quality of different systems can be characterized by using these objective measures, medical diagnosis also involves perception of the observer. Since an image is, by definition, a means for visually representing the clinical information captured by the equipment, at some stage quality must be based on the judgment of a human observer. A more direct image-based method of evaluating overall system performance is by using contrast detail (CD) phantoms. These phantoms contain test objects of different size and contrast and are traditionally used to determine the boundary between visible and invisible objects represented by the imaging system. An issue with this type of experiment is that subjective observation can often result in relatively high inter- and intra-observer variability. A high-quality image is of major importance to assure an accurate diagnosis, and this is generally determined by three primary physical image-quality parameters: contrast, spatial resolution, and noise. These quality parameters can be evaluated by objective image-quality measurements such as signal-to-noise ratio (SNR), MTF, and NPS. Together they form a basis for the description of image quality that encompasses the three primary physical image-quality parameters. These factors contribute to the measurement of DQE, which has been well established as the most suitable parameter for describing the imaging performance of an x-ray digital imaging device. DQE is the measurement of the combined effects of the noise and contrast performance of an imaging system, expressed as a function of object detail. Parameters such as NEQ and DQE have the potential for providing objective assessments of imaging performance for an image viewed by an ideal observer. Measurements of this type allow objective comparisons to be made between different systems. More research is required to establish mathematical relationships between these measures of performance and the requirements for clinical examinations. Some studies have investigated the relationship between physical performance characteristics (MTF, NPS, and DQE), and psychophysical performance (such as those derived from CD analysis). The relationship between the results of physical measurements, phantom evaluations, and clinical performance is not fully understood. Object detectability could be directly linked to DQE, although the response of the human eye, system noise, and variation of the DQE as a function of the exposure need to be taken into account. Image quality can be evaluated by combining the physical characteristics of the imaging system, overall system performance, and observer performance studies.