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U.S. Department of Health and Human Services

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FY 2000 Medical Imaging Evaluation


OST scientists develop consensus evaluation methodology for diagnostic medical imaging systems such as mammography and other film-screen x-ray systems, computed tomography, nuclear medicine, diagnostic ultrasound, magnetic resonance imaging, and digital imaging including fluoroscopy and digital mammography. The goal of the program is to characterize and optimize medical imaging systems and components through the application of quantitative measures of imaging performance and dose. This program also supports development of mammography equipment standards and special procedures and test equipment for MQSA. During 2000, this program contributed to the soft-copy aspects of the premarket guidance document "Information for Manufacturers Seeking Marketing Clearance of Digital Mammography System."

Digital Image Display System Evaluation
Key words: digital radiography, soft-copy display

The purpose of this project is to develop measurement and analysis procedures to evaluate the performance of image display devices for digital diagnostic imaging systems. Currently, the primary technologies used for the display of digital medical images are laser film printers (hardcopy) and CRT-based video monitors driven by computer-controlled digital frame buffers (softcopy). This study provides the scientific basis for regulatory decisions regarding the display subsystems of a variety of digital diagnostic imaging devices. The research will lead to a more efficient review of applications submitted for clearance to market products containing such display systems.

 

During FY 2000, scientists were limited to setting up display hardware and software, and establishing various display measurement capabilities. Existing display hardware and workstation software were supplemented by an additional display system and software upgrades. These elements were reconfigured to provide two dual-monitor display workstations, one of moderate resolution (1728 x 2304), and one of high resolution (2048 x 2560). Both systems were equipped with photometric calibration sensors and software produced by the manufacturers of their respective frame buffers, and both are equipped with a third-party calibration package that can provide independent verification of photometric performance. At the end of FY 2000, a scientific-grade camera and computer-controlled positioning equipment were acquired.

X-ray Physics Laboratory Studies
Key words: x-ray spectroscopy, mammographic grids

The purpose of this project is to evaluate equipment and materials used in medical radiography and in the quality assurance of medical radiography systems; and to support OST research efforts, other Center programs, and the general radiology community, when appropriate. Conducting this project requires establishing and maintaining the capability to generate x-ray beams typical of those used both for mammography and for general diagnostic radiography, as well as state-of-the-art capability for x-ray measurements, including high-resolution x-ray spectroscopy. OST provided technical consultations to the Office of Compliance on questions related to mammography field limitation/primary barrier and "mini C-arm" fluoroscopy systems. The Diagnostic X-ray Standard was extensively revised to address radiation safety during interventional fluoroscopy.

The medical imaging physics community has noted an increasing use of higher-voltage spectra in digital mammography. In response to publication from the University of California, Davis, of Monte Carlo calculations of mono-energetic, mammographic exposure-to-dose conversion factors for an extended range of x-ray energies, OST planned and initiated a series of spectral measurements for the three anode materials used in mammography--molybdenum, rhodium, and tungsten--at tube voltages up to 50 kVp. The measurements are being made at two field positions to allow for inclusion of the influence of the "heel effect" on patient dose in theoretical calculations.

X-ray spectroscopic measurements were employed to evaluate the performance of small-size prototype grids produced using microlithographic techniques. It is expected that such grids will soon be commercialized.

Mammographic Dosimetry Studies
Key words: mammography, dosimetry, optimized mammography system

The purpose of this project is to experimentally verify Monte Carlo predictions of the exposure-to-dose conversion factors for new x-ray sources such as rhodium anode with rhodium filter and tungsten anode with various K-edge filters. This project supports the MQSA mission to promote and maintain minimum dose levels in mammography, and may result in changes to the dose determination procedure for systems using newer x-ray source options. OST scientists began the spectral measurement program described above to extend the range of tube voltages for which experimentally measured spectra are available for the three anode materials used in mammography: molybdenum, rhodium, and tungsten. The scientists performed a small study to estimate phantom doses from the optimized mammography system for several screen-film combinations being considered as replacements for the discontinued products that had been used in earlier experiments.

Imaging System Performance Evaluation
Key words: digital radiography, flat panel detector, CCD-based x-ray imager

OST scientists are extending the quantitative assessment of dose and imaging performance from the analog to digital imaging domain. OST scientists have played a significant role in developing consensus measures of imaging performance that form the basis of the nonclinical device description now required in marketing clearance applications for a variety of imaging devices, and they are now extending these measures to digital imaging systems. Several important measures of imaging performance that are now routinely applied to analog imaging systems are based on assumptions that are violated to one degree or another by digital systems. OST is investigating the validity of these measures for digital systems and is investigating alternate performance measures that are rigorously valid for digital systems.

In one laboratory initiative, the impact of violation of the analog-system assumptions on consensus measures for digital systems was explored through a series of computer simulations. To supplement this work, OST has acquired a flat panel digital detector for experimental verification of the simulation results. In the future, actual human observer data will be compared with predicted results, derived from quantitative measurements on the digital detector, for reading imaging phatoms. It is hoped that this work will help to bridge the gap that currently exists between subjective evaluations using imaging phantoms and objective measures of imaging performance. In the area of analog x-ray devices, OST scientists are evaluating inefficiencies in imaging performance using laboratory measurements. To address the issue of optical coupling inefficiency in the image formation process, which arises in the premarket approval of, some digital imaging devices, OST set up a lens-coupled CCD-based digital imaging system and investigated the effects of system design on overall radiation use efficiency.

Ultrasound Bone Densitometry
Key words: ultrasound, bone density, osteoporosis

OST plays a significant role in the approval of PMAs for ultrasound bone densitometers. This is a new technology that is likely to undergo much technological evolution and regulatory activity in the near future. Currently there is a considerable lack of standardization among devices. Preclinical experiments, clinical trials, and theoretical analysis are important to develop understanding of this technology and to anticipate future trends. This project provides an independent source of data in OST in support of regulatory decision making. OST has explored fundamental mechanisms underlying the interaction between ultrasound and bone. These investigations increase understanding of how and why ultrasound bone densitometry is effective and therefore lead to better and more thorough reviews of these devices. OST contributed a section to a CDRH guidance document on submissions for bone sonometers.