Hamid Jafroudi

Storage phosphor-based digital mammography using a low-dose x-ray system optimized for screen-film mammography

We are examining the feasibility of performing digital mammography by combining a storage- phosphor image receptor with a highly efficient x-ray system. The image receptor consists of Fuji series HR-V high resolution imaging plates and a Fuji 9000 reader. The x-ray system was developed using multiparameter optimization techniques, with the goal of reducing patient dose as much as possible while retaining acceptable imaging performance. We have measured sensitometric properties, modulation transfer function (MTF), and noise power spectrum (NPS) of the Fuji plates with low-energy x-ray spectra. We have used the measurements, along with information about the x-ray system, to estimate signal-to-noise ratios (SNRs) for objects in a contrast-detail (C-D) phantom. We present the results of our measurements on the Fuji plates, comparisons of calculated and observed C-D diagrams for this system and a conventional system, and comparisons of phantom images and doses for this system to images and doses for a conventional system. We conclude that digital mammography with the system studied is at least feasible since phantom image quality is comparable to that of a conventional system at dose levels that are somewhat lower.

Quality control of storage phosphor digital radiography systems

Quality control (QC) of storage phosphor devices is important in assuring that the image information entered into an Image management and communication (IMAC) system is sufficient for diagnosis. QC of storage phosphor digital radiography systems is complex because of the self-corrective nature of the image-processing software used in these machines. Currently, one must produce hard copy to perform adequate QC. Inspection of images with reject analysis and inspection of cassettes and imaging plates has helped us in our QC program. For those QC tests using control limits, the appropriate settings for these limits are unknown. Starting approximations are given. Recommended tests are described.

Digital mammography: tradeoffs between 50- and 100-micron pixel size

Improvements in mammography equipment related to a decrease in pixel size of digital mammography detectors raise questions of the possible effects of these new detectors. Mathematical modeling suggested that the benefits of moving from 100 to 50 micron detectors were slight and might not justify the cost of these new units. Experiments comparing screen film mammography, a storage phosphor 100 micron digital detector, a 50 micron digital breast spot device, 100 micron film digitization and 50 micron film digitization suggests that object conspicuity should be better for digital compared to conventional systems, but that there seemed to be minimal advantage to going from 100 to 50 microns. The 50 micron pixel system appears to provide a slight advantage in object contrast and perhaps in shape definition, but did not allow smaller objects to be detected.

Image processing in digital mammography

Digital mammography is likely to replace conventional mammography within a few years. In anticipation of this, our group has been exploring the implications of image processing in digital mammography. Some of our findings are reported here.

Dual-energy in mammography: feasibility study

The purpose of this work is to examine the feasibility of dual-energy techniques to enhance the detection of microcalcifications in digital mammography. The digital mammography system used in this study consists of two different mammography systems; one is the conventional mammography system with molybdenum target and Mo filtration and the other is the clinical version of a low dose x-ray system with tungsten target and aluminum filtration. The low dose system is optimized for screen-film mammography with a highly efficient scatter rejection device built by Fischer Imaging Systems for evaluation at NIH. The system was designed by the University of Southern California based on multiparameter optimization techniques. Prototypes of this system have been constructed and evaluated at the Center for Devices and Radiological Health. The digital radiography system is based on the Fuji 9000 computed radiography (CR) system which uses a storage phosphor imaging plate as the receptor. High resolution plates (HR-V) are used in this study. Dual-energy is one technique to reduce the structured noise associated with the complexity of the background of normal anatomy surrounding a lesion. This can be done by taking the advantage of the x-ray attenuation characteristics of two different structures such as soft tissue and bone in chest radiography. We have applied this technique to the detection of microcalcifications in mammography. The overall system performance based on this technique is evaluated. Results presented are based on the evaluation of phantom images.

Imaging performance of a low-dose screen-film mammography system

The purpose of this work is to evaluate the imaging and dose performance of an x-ray imaging system optimized for mammography. The x-ray system design was developed by the University of Southern California and the Center for Devices and Radiological Health using multiparameter optimization techniques. The prototype was built by Fischer Imaging and is now under evaluation at the National Institutes of Health. While previous reports have concentrated on demonstrating the does reduction potential of the system, for this study the x-ray spectrum was modified to maximize imaging performance. Measurements were made to assess the level of imaging performance achieved and to determine the increase in dose. Contrast-detail analysis along with qualitative evaluation of images of conventional mammography phantoms were used to assess imaging performance. Both imaging performance and the dose delivered by the system were compared to those of a conventional mammography system. Because of the current interest in digital mammography, the performance of the optimized system with a storage phosphor plate image receptor, sometimes referred to as computed radiography (CR), was also studied. The optimized system provided significantly better imaging performance than the conventional system with both film-screen and CR detectors. The dose was increased to a level comparable to the average value for conventional systems using grids.

ROC study of screen-film mammography and storage phosphor digital mammography: analysis of nonconcordant classifications and implications for the approval of digital mammography systems

A recently completed ROC study of digital mammography using a 100 micron pixel storage phosphor receptor showed that digital mammography and conventional screen film mammography were essentially equivalent in areas under the ROC curve. In this study, there were 24 biopsy proven breast cancer cases, 25 benign biopsy cases and 48 clinically normal breast images each with matched screen film and storage phosphor images. Fifteen of the 24 cancer cases were 10 mm or less in size. Of these 10 presented with microcalcifications as the sign of disease. Six radiologists not involved with the research program and without prior experience with digital mammography and who met qualification criteria under the Mammography Quality Standards Act of 1992 served as readers. This poster looks at the cases in which there was variance between the radiologists ROC classification system for the digital and screen film system in order to analyze case specific discrepancies that may indicate benefits or deficits of the digital system. Aspects of the ROC ratings are also analyzed including an evaluation of the different thresholds used by radiologists on the digital and screen film systems, the distribution of ROC ratings in normal and abnormal cases, the effect of using different gold standards of proof on the results and the effect of substituting an ACR BIRADS category agreement study as proposed by the FDA compared to the ROC study outcome.

Digital mammography: an evaluation of the shape of microcalcifications

Microcalcifications can be identified on mammograms in approximately 50 - 55% of breast cancer cases. Three factors affect the ability to use the presence of microcalcifications as a sign of cancer. They must be seen (conspicuity), their shape must be assessed (to differentiate benign and malignancy associated shapes) and they should be countable since the greater the number of clustered calcifications, the more likely they are malignancy associated. Concern has been expressed that digital systems with their inherently worse resolution would not allow adequate shape information to be captured. Using a 100 micron pixel size storage phosphor system we randomly selected 20 cases, 10 benign and 10 showing malignancy on biopsy and asked four radiologists to assess the calcifications present comparing the original screen film and the digital images and using the screen film biopsy specimen radiograph as ground truth. The preferences were mixed with some radiologists preferring screen film and others the digital images. Whatever their preferences, the radiologists were unable to use the shape criteria to distinguish benign and malignant cases in this case sample.

Multiparameter optimization of mammography with alternative x-ray sources

The conventional x-ray source for mammography, with a molybdenum (Mo) anode and Mo filter, works well for breasts of low to moderate x-ray attenuation, but is not readily adaptable to the production of higher x-ray energies that are more suitable for imaging breasts of higher attenuation. Accordingly, alternative sources with anodes of rhodium (Rh) and tungsten (W) have been developed to improve the efficiency of the examination for thick or radiographically dense breasts. We have applied previously developed multiparameter optimization techniques to imaging systems using these alternative x-ray sources. Since these sources are intended to improve mammography of high-attenuation breasts, optimizations were performed for a range of breast thicknesses. Since high attenuation is generally associated with high scatter, optimizations for each source were done with a high-ratio, air-interspace grid similar to the one developed in our previous work. Preliminary results have been obtained for optimized system configurations using a W-anode source with Mo, Rh, and aluminum (Al) filters, and for a Mo-anode source with Rh filtration. These results indicate that the alternative sources studied can significantly improve the efficiency of mammography of high-attenuation breasts.

Digital mammography: the effects of decreased exposure

It has been stated that digital mammography will reduce the exposure required for mammography. This poster explores the effects of decreased exposure on the information present in digital mammography. In general, the digital system performed better than screen film mammography with lower exposures. With the usual exposures used for screen film mammography, performance was equal. With high exposures sufficient to result in a dark film (OD 1.5), the digital system performed better than screen film with very small test objects. Proposals have been made to decrease the tube loading required for slot scanning devices by increasing KVP. This would result in their being less object contrast due to the decreases in the absorption coefficient of calcium compared to water at higher KVP. This poster looks at the potential for correcting the loss in object contrast that would result from the use of high contrast look up tables. It was found that in the tested system, one could restore the information in one of the two test objects used (but not the other), but that the image processing methods used would result in an image that radiologists would probably find inadequate for interpretation.