BBI Contributing Editor

CHICAGO, Illinois – Because picture archiving and communication systems (PACS) are the fastest-growing part of medical imaging, developments in this sector (covered in last month's issue of The BBI Newsletter) tend to overshadow other news at the annual meeting of the Radiological Society of North America (RSNA; Oak Brook, Illinois). But a great deal else that is worth noting is happening in the medical imaging arena.

Medical imaging procedures in the United States during 2000 were estimated to number almost 400 million, a figure that has been increasing by about 8% per year for the past five years. Two fundamental reasons underlie this growth: the new role of imaging in diagnostic medicine and technological developments.

Where imaging procedures once were ordered after completion of a history and physical, they now often are ordered as the first step in diagnosis. Under the effects of managed care, harried physicians often find it faster and more effective to seek definitive findings from imaging or lab tests than to work through a series of guesses about what may be wrong with a patient. Furthermore, imaging techniques have improved so that they more often provide accurate and definitive answers.

It is well-known that the worldwide imaging field is dominated by a handful of companies that offer products in all or nearly all modalities. GE Medical Systems (Waukesha, Wisconsin) leads the field, followed by Siemens (Iselin, New Jersey), Philips (Shelton, Connecticut), Marconi (Cleveland, Ohio), Toshiba (Tustin, California), Hitachi (Twinsburg, Ohio), Shimadzu (Torrance, California) and a number of more specialized companies.

Computed tomography (CT), a technique that uses X-rays to reconstruct images of cross-sectional slices of the body, has undergone vast changes in the last decade, first with spiral scanning and then with multiple rows of detectors. Previously, the X-ray source in CT machines was tethered to the frame by its power leads. In spiral scanning, the leads are replaced by slip rings so that the source can rotate continuously, thus bringing the time necessary to collect sufficient data for a single slice down to one second. Adding multiple rows of detectors (up to four) means that sufficient data for a single slice can be collected in a fraction of a second – as many as 32 slices per second. Furthermore, the data are not constrained by the mechanical motion in the manner that they had been in conventional CT. The result is that spiral CTs can rearrange the data, creating new slices of different thickness, position or orientation.

Table 6-
Medical Imaging Procedures in the U.S., 2000
Modality Procedures
X-ray 270 million
Ultrasound 75 million
Computed tomography 29 million
Magnetic resonance 14 million
Nuclear medicine 11 million
Total 399 million
Source: The BBI Newsletter

The increased speed is valuable not so much for reduced imaging time as it is for making possible images of moving structures like the lungs, heart and flowing blood. These developments have opened new fields to CT examination, and they are part of the reason for growth in the number of imaging examinations. All of the companies mentioned above (except Hitachi in the U.S.) offer these products at prices generally in the range of $700,000 to $1.1 million.

Machines that produce so much data present a problem to radiologists who must scrutinize the images. In a conventional presentation, where the slices are arranged side by side on a film or monitor, the time necessary to examine, say, 100 or 200 slices can be excessive. Alternative presentations are cine mode and 3-D. In the former, the slices are presented one after another at full resolution on a monitor under operator control so that the radiologist can stop whenever something anomalous comes into view. In the latter, the data are combined to create, in two dimensions, what looks like a 3-D rendering of internal structures.

This field has come a long way in the last decade. All the CT companies offer 3-D software, some of which they have developed internally and some of which they have bought from companies such as Vital Images (Plymouth, Minnesota) and Mitsubishi Electronics/Real Time Visualization (Concord, Massachusetts) and Cedara Software (Mississauga, Ontario). At one time, 3-D reconstructions were exceedingly time-consuming, but modern systems can produce 3-D images in a fraction of a second.

Open-field MR grows in popularity

Magnetic resonance (MR) has moved from the esoteric realm it once occupied to mainstream medical practice. The largest magnets permitted by the FDA for ordinary use are those with fields of 2.0 tesla or less. All of the companies mentioned offer machines with fields of 1.5 tesla, which are said to produce the most detailed images. However, a divergent trend became apparent a few years ago when patients showed a marked preference for lower-field, open machines, which induce less claustrophobia. The older high-field machines confined the patient in a tunnel hardly large enough to hold a big person, and some 5% to 15% of patients could not be examined because of claustrophobia. New high-field machines flare at the ends, diminishing the sense of claustrophobia, while lower-field machines use magnets above and below the patient supported by posts. The latter are open on four sides, and, though they have lower fields, they have proven to be popular.

Fonar (Melville, New York), always a maverick, showed a 0.6-tesla machine with two permanent magnets side by side so that the patient could be imaged while standing between them. Some joint problems reveal themselves only while the patient is in a weight-bearing stance, and this machine is aimed at such cases.

Developments in X-ray centered mainly around new digital capture techniques. Heretofore, if one wanted to capture X-ray images in digital form without using film, the only technique available was computed radiography (CR), which substitutes a reusable phosphor plate for the film. After exposure, the data on the plate is read out and digitized in a special reading device originally developed by Fuji (Stamford, Connecticut) but now offered by Agfa (Ridgefield Park, New Jersey) and Eastman Kodak (Rochester, New York). Lumisys (Sunnyvale, California), a small company that made film digitizers, also offers a low-cost, tabletop CR reader, but Kodak acquired the company and will presumably add the reader to its line.

The new techniques for digital X-ray capture are called direct radiography (DR). GE Medical has FDA approval for both a chest unit and a mammographic unit, while the other large companies are working on DR products. Hologic (Bedford, Massachusetts) acquired the selenium-based DR system originally developed by DuPont (Wilmington, Delaware), which will no doubt be incorporated in mammographic units from Trex (Danbury, Connecticut), which Hologic acquired several months ago. Trex also was developing a silicon-based DR product. Swissray International (New York) markets a direct radiography system that focuses an image of a scintillating screen on CCD detectors. Devices using the same principles are available from Cares Built (Keyport, New Jersey) and Canon (Irvine, California). A new company, Edge Medical Devices (Hackensack, New Jersey), showed an innovative detector at RSNA with a mechanically scanned readout, developed in Israel. It offers resolution comparable to other DR devices but is expected to cost only half as much.

The cost of direct radiography has been its principal drawback. Those that use large arrays of transistors are difficult to manufacture without significant blemishes and those that use optics are bulky and hard to align. For example, a conventional top-of-the-line mammography unit costs around $90,000, while a DR mammography system costs around $400,000. For this reason, DR has not made much headway in the market, but that picture can be expected to change gradually over the years to come as manufacturing techniques improve and production costs fall with volume.

There is some controversy about the relative merits of computed radiography and direct radiography. A few studies have shown that DR is technically superior to CR, but this is by no means a universal opinion. Because the DR device stays in the X-ray machine, there is no need to carry a cassette to a reader as there is with CR (though even CR can be configured with an internal reader and transfer mechanism), but this is not a decisive advantage. Thus, it appears that CR will continue for some years to hold its place as the favored means for capturing digital X-ray images, but DR will make steady inroads.

Crisis situation for mammography

Mammography is a field that appears headed for crisis. The demand for mammographic services has steadily increased over the decades as more and more women heed the admonitions of the National Cancer Institute (Bethesda, Maryland) and the American Cancer Society (Atlanta, Georgia) to have annual or biennial screening examinations. Close to 30 million mammographic procedures are performed in the U.S. each year. However, the number of radiologists who read mammograms is steadily decreasing as fewer radiologists choose this field and many leave it. Reading mammograms is one of the most challenging tasks for a radiologist; the rewards are limited as Medicare sets reimbursement at $67 for a screening mammogram (one made when the patient is asymptomatic) and $81 for a so-called diagnostic mammogram (one made when there are reasons to suspect cancer); and the risk of malpractice suits in this volatile area is substantial.

Even though some hospitals regard mammography as a loss leader that will make up for its economic shortcomings by creating profitable referrals for treatment, radiology groups find that their mammographers do not contribute as much to the group as the other radiologists so that mammographers reap less income or more scorn (or both) from their better-paid colleagues. As it is today, waiting times up to three months for a screening mammogram are common. The only solution appears to be increasing payments, and that will probably happen, but it will not turn the situation around anytime soon.

In the longer run, computer-aided diagnosis (CAD) could help, but there are high hurdles to cross. R2 Technology (Los Altos, California) markets a computerized system based on pioneering work from the University of Chicago. In the FDA-approved version, the mammographic films are placed on a light box, and a small monitor mounted below highlights areas that a computer indicates are suspicious. But a mammographer must still make the interpretation so that this does little to alleviate the shortage of mammographers. According to published papers, the device is as accurate as a skilled mammographer, but the FDA has shown considerable reluctance to turn over responsibility for interpretation to a computer. One day, however, CAD may offer a better solution to the crisis in mammography.

PET in nuclear medicine lead

In nuclear medicine, positron emission tomography (PET) was the leader. This is a consequence of recent approvals by the Health Care Financing Administration (Baltimore, Maryland) to pay for PET examinations to detect cancer, especially metastases. Siemens introduced a PET scanner combined with CT. The advantage of these dual devices is that, while PET is quite sensitive for detecting cancers, its spatial resolution is poor. CT, on the other hand, has less sensitivity but provides excellent spatial resolution. The combined images provide the best of both. Having just acquired SMV America (Twinsburg, Ohio), GE Medical Systems was able to show a similar product. Having acquired ADAC Labs (Milpitas, California), Philips was also able to show a PET scanner.

In ultrasound, technical advances were overshadowed by acquisitions. The last major independent, Acuson (Mountain View, California), was acquired by Siemens, and Agilent (formerly Hewlett-Packard's Medical Products Division; Andover, Massachusetts) was acquired by Philips. These developments place ultrasound firmly in the grip of the major imaging companies, repeating the situation that was true in 1980, when most of the small ultrasound companies had been acquired by larger companies until Acuson burst on the scene, creating a new standard for ultrasound imaging.

As each RSNA rolls around, observers speculate that one or another of the imaging modalities will be eclipsed by new developments. But the December 2000 show in Chicago's sprawling McCormick Place convention center, like its predecessors, demonstrated that each modality has unique virtues, and technical developments only strengthen them.