In this issue, Cole et al report on the performance of a photon-counting digital mammography system (PCM) compared to a more established conventional full-field digital mammography (FFDM) system under clinically relevant image interpretation conditions involving 16 radiologists. The study was aimed at demonstrating noninferiority of PCM compared to the FFDM reference standard.
In this study, the performance of two distinctly different digital mammography systems was evaluated using an enriched population that included 133 women, 67 of whom had a recent conventional FFDM procedure and prospective PCM in this presumed normal group, and 66 women with actionable findings from mammography who had prospective PCM and FFDM. This group yielded 49 cancers and 17 benign findings.
The strategy of using a cancer-enriched population in such noninferiority studies that assess one imaging technology versus another that serves as the reference standard provides a powerful and efficient tool for the assessment of the performance of the candidate modality. The analysis of performance in this study included relative sensitivity, probability of malignancy assessment, specificity and lesion level analysis, and radiation dose.
FFDM has emerged as the new standard that has essentially replaced screen-film mammography. The development and clinical deployment of FFDM had to overcome formidable challenges from the perspective of technological and clinical implementation. Full confidence in the superiority of this technology over film-screen was not established until the results of several clinical investigations were published . Presently, FFDM systems from about a dozen different manufacturers are commercially available and are in widespread use; some of this equipment is already in the second or third generation. At the time of this report, 86% of all mammography units in the United States are digital and nearly 100% conversion to digital is expected in the next two years.
Digital mammography is performed with x-ray detectors that can be very dissimilar in terms of the detector design, size, shape, construction, signal acquisition and processing, pixel matrix, and dose efficiency. A common approach uses a thin layer of amorphous selenium (a-Se) as primary detector with a pixilated silicon array readout , whereas other approaches use a thin layer of a thallium-doped cesium iodide scintillator with an amorphous silicon (a-Si) pixilated array matrix signal readout . Flat panel a-Se detectors with optical readout represent a more recent version of a-Se detector . Another approach uses the well established computed radiography technology that is based on a storage phosphor in a removable cassette . The study by Cole et al evaluates the clinical performance of a new technological approach to digital mammography that is unique because it uses for the first time an x-ray photon-counting approach to mammography in combination with a slot scanning acquisition. This is accomplished by a lateral movement of the x-ray tube and detector assembly that consists of multiple parallel silicon strips for direct detection of x-rays . The combination of slot scan and photon-counting contributes to efficient x-ray scatter rejection and noise reduction that enable lower dose compared to other FFDM systems .
After the initial introduction of FFDM in the United States in the year 2000, the mean glandular dose (MGD) to the breast was kept at about the same level as with film-screen techniques. In the early days of digital mammography, with limited clinical data available, there was considerable lack of confidence on the degree of dose reduction that could be attained without assuming the risk of compromising the anticipated superiority in detection and in diagnostic imaging . After the initial experience, in recent years the dose in FFDM has been gradually reduced but the degree of dose reduction can vary substantially across the various FFDM technology platforms . As with all x-ray imaging, dose reduction is desirable if it does not negatively affect feature detection or diagnostic performance. In a well-designed and optimized FFDM system, visualization of subtle low-contrast features that are associated with positive findings in mammography is critically dependent on the x-ray quantum statistics of the mammographic image. Therefore, it is the number of detected x-ray photons and anatomic noise that limit the lowest attainable contrast discrimination, and ultimately the visualization of subtle features that may be associated with abnormalities. To attain this objective, detectors with high stopping power (quantum efficiency) and low electronic noise characteristics must be used as in the study by Cole et al and in other FFDM platforms, particularly those using a-Se or a-Si detectors . The use of rhodium and tungsten targets, with higher kVp and x-ray filtration such as rhodium, silver, or aluminum in place of the usual molybdenum filtration, increases x-ray beam transmission though the breast, resulting in lower MGD.
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