|Bonnie Rush, RT(R)(QM)
The technical mandates of mammography are more complex than in any other area of imaging. These include excellent spatial resolution at the lowest possible dose, optimal contrast without significant loss of latitude, imaging of a broad spectrum of tissue types, visualization of adjacent structures with only slight contrast differences, and the ability to compensate for tissue-thickness differences from the chest wall to the nipple.
Where these requirements are concerned, film-screen mammography has reached its peak capability. It has narrow exposure latitude (good image contrast, but susceptibility to underexposure or overexposure); it is subject to film and screen noise that can reduce the ability to visualize high frequency structures adequately; and chemical processing is often the weak link in the optimal conversion of the latent image. Film use is also beset by inefficiencies in its handling, storage and retrieval requiring huge amounts of space and staff time. 1 Post-processing of hard copy images is not possible and thus there is no ability to enhance the image after the exposure.
For film-screen mammography, although spatial resolution potential is 15 to 20 line pairs (lp)/mm this is seldom achieved due to the previously described inherent weaknesses. Under the Mammography Quality Standards Act (MQSA), the minimum required spatial resolution is 13 lp/mm to the parallel and 11 lp/mm to the perpendicular; and film-screen mammography systems now in use meet or exceed these specifications.
The US Food and Drug Administration does not require digital mammography systems to prove that they are superior to film-screen mammography systems. Instead, the FDA requires manufacturers to prove that their equipment performs at least as well as filmscreen mammography. In full-field digital mammography (FFDM), spatial resolution requirements are not mandated, but to detect microcalcifications FFDM technology requires a minimum spatial resolution of 5 to 10 lp/mm. FFDM systems now in use meet this criterion.
Study Evaluates Digital Mammography
by Chris Wolski
Since October 2001, 28 mammography screening centers in the United States and Canada have been participating in a study to determine the diagnostic accuracy of digital mammography versus standard film-screen mammography. During the 3-year duration of the $27-million Digital Mammographic Imaging Screening Trial (DMIST), about 49,000 women are expected to take part. As of September 2003, more than 44,000 have already participated in the study, which will be completed in 2004. The study is being administered by the American College of Radiology Imaging Network (ACRIN) and is being funded by the US National Cancer Institute. The study’s principal investigator is Etta Pisano, MD, of the University of North Carolina, Chapel Hill.
In addition to looking broadly at the effectiveness of digital versus filmscreen mammography, the study will evaluate the four digital mammography systems currently in clinical use or undergoing premarket evaluation in the United States. The fundamental need prompting the study is the ongoing quest for improved cancer detection. Laurie Fajardo, MD, principal DMIST investigator at the University of Iowa, Iowa City, says, “There is clearly room for improvement in both breast-cancer detection and lesion characterization. This major screening trial will help us to determine if digital mammography outperforms standard screen-film mammography in regard to its diagnostic accuracy across all equipment manufacturers.”
The study is screening asymptomatic women using both digital and filmscreen mammography. Each study is read by a participating radiologist, who issues the report for the study. If the examination’s results are positive, the patient will receive follow-up treatment. If the results are negative, the patient has the options of either returning in a year to the participating center for a follow-up examination or providing an examination from another institution for review. Participants must be asymptomatic, must not have breast implants, and cannot be pregnant at the time of the study. All women presenting for screening mammography at one of the 28 participating institutions will be considered for enrollment in the study.
In the initial group of 1,742 participants, the median age was 52 years, with the youngest being 28 and the oldest being 90. About 85% of the initial group was white. Current participants’ characteristics are not yet available. In addition to examining the diagnostic accuracy of digital mammography, the study will analyze the relative cost effectiveness of digital and film-screen mammography. Other secondary aims include assessing the effects of participant characteristics and of reduced false-positive mammograms; conducting retrospective reader studies to determine the effect of soft-copy versus film display on diagnostic performance; and assessing temporal variations in image quality, breast radiation dose, and other quality-control parameters at all participating study sites during the course of the trial. Though the majority of the initial studies have been completed, it may be some time before findings are announced, according to Constantine Gatsonis, PhD, ACRIN’s group statistician.
Chris Wolski is associate editor of Decisions in Axis Imaging News.
In FFDM, however, spatial resolution is not the final indicator of quality. Detective quantum efficiency (DQE) and modulation transfer function (MTF) are terms often used to describe FFDM resolution capabilities. In simplistic terms DQE indicates how much of the information that was originally contained in the object is converted by the imaging medium, resulting in the acquired image. Film has a conversion factor of about 25%, while FFDM has been described as being capable of at least 60%. MTF is the ability of the system to display high-frequency structures (such as microcalcifications) without losing them in the noise of the system. This is affected by spatial frequency (the amount of signal emitted and the resultant frequency needed to display it), by the signal-to-noise ratio, and by dynamic range, which is the contrast capability of the system. Therefore, while spatial resolution is no better than that of film mammography (and may even be worse) digital mammography offers the benefits of decreased noise, and improves contrast resolution by the windowing functions that optimize the available contrast. These features help counteract the lower spatial resolution of digital systems. 2
Criticism of FFDM is usually directed not toward the digital technology itself, which continues to advance, but toward the associated workstation applications. Full resolution at 10 lp/mm requires display capabilities of 4000×5000 pixels but commercially available workstations limit the display to 5 lp/mm at full resolution. The enhanced contrast resolution of FFDM also depends on the imaging display. A 14-bit image displayed on an 8-bit or 10-bit monitor has its contrast capability reduced accordingly. 3 As R. James Brenner, MD, JD notes, “For a busy diagnostic breast center, workstations can become a real bottleneck. The workstations aren’t designed to accommodate multiple image views at full resolution. Moreover, workstations lack a windowing feature whereby a radiologist can switch between two or more patient studies easily. Vendors are aware of the problem and are working on solving it.” 4
Reading soft copy has its own challenges. A facility may be performing 50 exams per day using an FFDM system without a problem. This is probably typical of what happens in the technical arena, but the efficiency gained can be reversed during interpretation, at least initially while they gain experience in reading and manipulating digital images. Radiologists continue to be frustrated by the difficulty of comparing the current soft copy study with prior hard copy images. Even though they like the digital images, they may not want to read soft copy as comparison is difficult. Some institutions are still printing hard-copy images off of the FFDM units because of this problem. There are also the associated lighting and ergonomic issues for the radiologists in reading both hard copy and soft copy images. A PACS reading room may have three types of workstations available: fully digital stations, film-based stations, and hybrid (combination) stations. All of these will probably be needed for some time. 5
MQSA Reauthorization Nowhere in Sight
by Chris Wolski
The fate of the reauthorization of the Mammography Quality Standards Act (MQSA), which expired Sept 30, 2002, is still undecided as the 108th Congress enters the last phase of the 2003 session. MQSA will continue to operate, since, according to Laura Bradbard, acting director of Media Relations for the US Food and Drug Administration (FDA)’s Office of Public Affairs, the authority to certify and inspect mammography facilities does not expire. Reauthorization is tied to the appropriation of funds for MQSA by the Congress. The MQSA was passed in 1992, and falls under the authority of the FDA.
MQSA will continue, but for radiologists, reauthorization has opened the door of opportunity, giving them the chance to press Congress to update the outdated provisions of the act. Federally funded programs are reviewed every 5 years to see whether they are necessary and to make any changes needed to improve them.
The length of reauthorization itself has been a sticking point in the struggle to improve the act, Josh Cooper, director of congressional relations for the American College of Radiology (ACR), reports. The ACR has recommended a shorter 2-year reauthorization period, at which Congress has balked. “There are studies we want in there, and we want to be able to act on those studies quickly, rather than wait for the normal 5-year reauthorization period,” he says. “The shortening of the reauthorization period kind of holds Congress’ feet to the fire.”
The shorter reauthorization period could help financially strapped mammography centers-which are going out of business, according to ACR statistics, at a rate of 40 a month-fare better. “If these studies come back with evidence that will affect our members positively, we want to have the ability to implement those changes immediately, and a shorter reauthorization period will lend itself to accomplishing that goal,” Cooper says.
Another difficulty has to do with how the results of proposed federally mandated training are reported. There has been some suggestion that the individual results of peer-review data be made public (an idea that has left radiologists cold). Leonard Berlin, MD, chair, department of radiology, Rush North Shore Medical Center, Skokie, Ill, says, “There is a federal law for peer review being privileged, and all states have peer-review privilege laws. Peer-review information is not for public consumption. It’s for educational purposes. If [individual] peer review [results] were made public, there would be no peer review. Nobody would do it.” He adds that when he surveyed his colleagues, he found no strong objections to federally mandated testing.
On April 8, 2003, Berlin, along with radiologist David Dershaw, MD, testified to Congress in favor of MQSA reauthorization and the need to review the law to see what provisions might be outdated. Initial versions of the draft legislation include a provision to study which of the regulations may be what Dershaw called “a burden without benefit.” “There are some things that are really almost archaic,” Berlin says. “It’s a burden on everybody. You have to allocate personnel to do this, and personnel time equates with money-and there’s not much money made in mammography.”
Cooper speculates that it is possible that if MQSA funding-which is ending in October and is included in a Labor Department/Health and Human Services bill- is not approved, it could be included in an omnibus appropriations bill at the end of the congressional session.
Chris Wolski is associate editor of Decisions in Axis Imaging News.
DATA AND STORAGE
Despite the availability of image compression and web-based image distribution in other areas of radiology, mammography providers must consider the large size of the data files associated with FFDM and the bandwidth required to move the images swiftly and effectively without creating network havoc. Such traffic would overwhelm the bandwidth available to many PACS networks in existence today. For example, a four-view FFDM study obtained using a large detector with a 70 micron pixel equates to 110MB. Vendors continue to develop improved approaches to data storage and retrieval, and the cost of archiving is declining steadily. Among the newer archive models in use are fully online systems that make mechanical disk jukeboxes unnecessary. 6 A current feasibility study by the US National Institutes of Health to establish a central facility to store digital images is intended to demonstrate the workability and value of centralized storage and retrieval. 7
Compounding storage issues, facilities must still meet MQSA image retention mandates and some state laws may be even more stringent. For MQSA, images must be maintained for 10 years after the patient’s most recent examination at a given facility. For patients that return to a facility regularly (every 1 to 2 years), prior images must be kept for 5 years. Even when a facility provides only digital exams, image storage and retrieval methods must be able to cope with both hard-copy storage and soft-copy archive issues. For the foreseeable future, facilities that provide FFDM will also need to consider hard copy when images need to be transferred to facilities and health care providers outside the facility’s image-distribution network, as well as to patients and their attorneys.
Television and magazine advertising that promotes digital mammography is, meanwhile, generating a level of requests that can be overwhelming. With so much demand, it seems logical to hasten acquisition of FFDM and place the system in service as quickly as possible. The soundness of this investment must be investigated, however.
The Michigan Cancer Consortium (MCC) and its accomplishments have been hailed by the US Centers for Disease Control and Prevention, Atlanta, as a model for the nation. MCC has concluded that study data from the last few years has not proved the superiority of FFDM when compared with standard film-screen mammography. This resulted in the MCC position statement that FFDM costs still outweigh its benefits. 3 This statement buys time for health systems that are weighing whether to spend up to six times as much for a digital system (along with service and software upgrade expenses) as for a conventional mammography unit while they await results of a national study comparing film-screen and digital mammography (see sidebar, page 5).
Despite the MCC position statement, Max Wicha, MD, University of Michigan Comprehensive Cancer Center director, who co-chaired the consortium’s digital mammography statement still stated “All of us believe that, in the future, digital mammography will prove to be better. We will all acknowledge that it is not there yet. Everyone agrees that we should practice evidence- based medicine, and we should not jump on the bandwagon to tout digital mammography as better when we don’t have the proof.” However, Murray Rebner, MD, chief of breast imaging at William Beaumont
Hospital , Royal Oak, Mich, states, “I personally feel that the image quality is superior with digital.” In 2002, the facility invested $2 million to buy four new digital units.
CURRENT DIGITAL ADVANTAGES
Although one study 2 concluded that there was no difference in cancer detection rates, digital mammography had a significantly lower recall rate due to the ability to manipulate the image. This study demonstrated a 10% decrease in the need for repeat imaging where digital mammography was used. Since image contrast can be adjusted on the workstation, there is the ability to reduce the overall radiation dose to the patient which may aid in increased tube life for the unit. Digital mammograms can also be archived in various ways, are easily retrieved, and can be copied exactly. FFDM also encourages the use of off-line interpretation and expert consultation via telemammography. Digital mammograms are an ideal online teaching aid, as well. 2
Another major advantage is in film savings, which can immediately impact the bottom- line. Upon conversion to FFDM, one institution experienced a 35% decrease in film costs during the first year and another projected a $30,000 per year savings.
Due to enhanced throughput a facility was able to reduce the number of mammography units needed from four to three. Using the digital system vs the analog system they perform approximately twice as many examinations in the same amount of time. In the analog room, the facility books one patient every 15 minutes for screening mammography. In the digital room, it books two patients per 15-minute slot, having screened as many as 57 patients in an 8- hour work day. 8
Clearly, the income potential is there to support FFDM for some facilities. Fixed costs for FFDM are high, so spreading those costs over more studies decreases the fixed cost per mammogram and allows return on investment to be achieved more quickly. The investment in digital mammography will be most effective for larger centers performing at least 15,000 mammograms per year. An imaging center that serves a small population will, for this reason, find it difficult to justify the acquisition of FFDM financially.
The Breast Center at Houston Northwest Medical Center (HNMC) with 25,000 patients per year, reports that it justified FFDM acquisition on the basis that full return on investment (ROI) would be achieved in less than 3 years. 8 Another high volume center indicated the unit paid for itself within 2.5 years. With a lower patient volume (6500 patients per year) and the addition of computer-aided detection (CAD), one facility determined ROI within 3 years due to the increased reimbursement of mammography when combined with CAD. According to National Consortium of Breast Centers (NCBC), CAD is also a good marketing tool, although it requires approximately five additional minutes of the technologist’s time per examination to input the films into the CAD system. Direct to digital CAD integration with FFDM is now available, eliminating this concern and increasing the viability of CAD.
PREPARING FOR DEMAND
There is no doubt that demand for mammography services is increasing, driving additional procedural volume. The population over age 40 is expected to increase from 66 million in 2002 to 76 million in 2010, and this will also increase mammography demand. The procedural volume for mammography continues to grow at a rate of approximately 7% per year.
There are 9,600 certified mammography facilities in the United States, but the FDA recorded the closing of 160 centers during the first quarter of 2003. As an example of the local impact of this trend, four freestanding breast centers have closed this year in Jacksonville, Fla, alone. Breast imaging centers at two major hospitals there have also discontinued services. The reasons are poor reimbursement and medical malpractice issues. With these closings, delays of three months are now common for screening, as well as diagnostic, examinations. The number of breast imagers is decreasing, and very few are willing to enter the field. FFDM has the potential to obviate these concerns by imaging of up to two times the number of patients as analog units. Mobile vans can meet the needs of rural communities that cannot afford a digital unit locally. Centralized reading rooms can compensate for the rising population demands and decreased breast imagers.
Many women are already enthusiastic advocates of FFDM. They see digital mammography as better technology than what has been available. They want screening with the best tool available, and that, in their thinking, is now FFDM. 8 Due to marketing of FFDM they believe that it also does not require compression. In digital mammography the procedure itself is essentially the same and breast tissue is still compressed in the same way. The obvious difference for the patient is that the technologist does not leave the room to run the films. Women need to know that with film-screen mammography they are receiving just as good a mammogram as with FFDM. Conventional mammography remains a viable method of choice for breast-cancer screening until studies prove that FFDM is superior, and eventually that will happen.
Technological advances in FFDM will outpace those in film-screen mammography -it is only a matter of time. When it becomes public knowledge that the digital system is better than analog systems, women will have a right to demand it. Being prepared for FFDM means considering the future direction of mammography at your facility now. Early detection is the best protection for the woman; planning ahead is the best protection for any institution that will continue to provide mammography services, because the future points squarely at FFDM.
Bonnie Rush, RT(R)(M)(QM), is president of BIS Breast Imaging Specialists, San Diego, Calif, providing consulting and education to the mammography community. She is author and publisher of MQSA Made Easy ([email protected].) This article has been excerpted from FFDM: An Emerging Technology, which she presented at the meeting of the American Healthcare Radiology Administrators in Anaheim, Calif, on August 11, 2003.
- Seibert JA. The digital capture question. Available at www.imagingeconomics.com . Accessed September 23, 2003.
- Lewin JM, Hendrick RE, D’Orsi CJ, et al. Comparison of full-field digital mammography with screen-film mammography for cancer detection: results of 4,945 paired examinations. Radiology. 2001;218:873-880
- Michigan Cancer Consortium provider position paper: digital mammography. Available at: www.michigancancer.org . Accessed September 23,2003.
- . Brenner J. National Consortium of Breast Centers. Available at: www.breastcare.org . Accessed September 23, 2003.
- Weiser JC. PACS perfect. Available at: www.imagingeconomics.com . Accessed September 23, 2003.
- . Shira M. Transitioning to digital mammography. Available at: www.imagingeconomics.com . Accessed September 23, 2003.
- Shira M. Transitioning to digital mammography. Available at: www.imagingeconomics.com/ . Accessed September 23, 2003.
- Smith R. Digital mammography now? Available at: www.imagingeconomics.com . Accessed September 23, 2003.