Digital Sensors to Launch Revolution in Medical Imaging

Medical Device & Diagnostic Industry Magazine
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An MD&DI May 1997 Column

R&D HORIZONS

New digital technology promises to improve image quality and give health-care providers faster access to images.

A new breed of digital technology--one that could revolutionize medical imaging--is being hatched in both corporate and academic R&D labs. By capturing x-rays directly, this technology promises several important benefits:

The digital revolution will likely gain only gradual acceptance by the medical community, according to Jean-Pierre Georges, PhD, an executive at one of the companies trying to foment this revolution. Georges, who is general manager of the Image Sensor Products Group of dpiX (Palo Alto, CA), predicts a "slow encroachment" in radiography, largely due to economics. "The cost of a digital radiography system is going to be much more than the cost of a film-cassette system," he says.

But even if that revolution occurs in slow motion, the sales potential is enormous. Most of the 200,000 x-ray machines now installed around the world--radiographic tables, wall mounts, and mobile x-ray units--use radiographic film. These units, replaced with new digital systems listing at just $150,000 each, represent a $30-billion sales opportunity. The radiography and fluoroscopy (R/F) marketplace, with an estimated 28,000 systems worldwide, represents another $14 billion, with R/F systems selling for $500,000 each. The 20,000 mammography systems worldwide offer the potential of $6 billion more, if each new digital system sells for $300,000. And angiography and cardiac catheterization systems, of which there are about 20,000 in the world, have price tags well over $1 million each, representing more than $20 billion. All told, the total market for digital systems is potentially $70 billion.

The Senographe DMR from GE Medical Systems is one of the first x-ray-based mammography systems in the United States that allow digital mammography. This technology offers improved penetration of dense breast tissue.

The electronic components that will convince many owners of film-based systems to buy new digital systems are disarmingly simple both in appearance and in principle. No thicker than a pizza box and light enough for a child to carry, these flat-panel sensors come in several types, but work in a similar manner--recording individual x-rays, and transforming and then transmitting the resultant energy to computer workstations, where images of bones and blood vessels, lesions and cancers take shape.

Development efforts involve companies and universities around the globe. But much of the work is going on in the United States. Huge brain trusts, connected to some of the deepest pockets in the medical device industry, have formed to address the challenge.

General Electric Medical Systems (GEMS; Milwaukee), the largest manufacturer of x-ray equipment in the world, has three groups collaborating on the development of these sensors--one near its Midwest headquarters, another at the GE Corporate R&D Center (Schenectady, NY), and the third at EG&G (Sunnyvale, CA), a major developer and vendor of electronic components. Similarly, Trex Medical (Danbury, CT), a leading manufacturer of mammography equipment, is combining research and development resources within the company with those of its parent, multinational conglomerate ThermoTrex (San Diego). Like GE, ThermoTrex is collaborating with another company, the Xerox Palo Alto Research Center (PARC) in California, to develop high-performance sensor arrays for digital x-ray.

Following in their wake are several contenders varying in size and market, including Sterling Diagnostic Imaging (Newark, DE), a leading vendor of radiographic film and electronic networking equipment; Fischer Imaging Corp. (Denver), a vendor of mammography equipment; Hologic (Waltham, MA) and Lunar Corp. (Madison, WI), two leading developers of bone densitometry equipment to diagnose osteoporosis; and SwissRay International (Hitzkirch, Switzerland), a manufacturer of general radiographic equipment.

While many OEMs are now developing the technical know-how to build digital sensors, most will not want to get into the mass production of these components. That is where manufacturers such as dpiX, EG&G, and OIS Optical Systems (Northville, MI) come into the picture.

OIS has received an order from a major medical imaging company to deliver 300 flat-panel sensors in the third quarter of this year. The sensors to be shipped are capable of both radiographic and fluoroscopic imaging, according to John McGill, director of business development for sensors. McGill refused, however, to indicate how the sensors would be used or who is buying them. "We have agreed not to divulge the identity of our customers or their application at this time," he explains. But that information should be available relatively soon. The imaging product into which the sensors will be integrated is scheduled for market introduction by the end of 1997 or at the beginning of 1998.

The order for OIS sensors, which has an estimated worth of more than $1.5 million, will be the largest known delivery of flat panels for medical applications. But OIS is not alone in delivering commercial product to the medical industry. DpiX is currently fielding smaller orders from a variety of OEM vendors and expects to have shipped "several hundred" sensors before the end of this year, according to Georges.

Meanwhile, Varian Associates (Palo Alto, CA), a supplier of x-ray tubes and a major vendor of radiotherapy equipment, has jumped in as a value-added reseller of these flat panels. Varian is building electronics around sensors manufactured by dpiX for sale to OEMs that are interested in assessing the potential of this technology. These augmented panels, based on LAST (large area sensing technology), began shipping to OEMs for evaluation in March. According to Dave Gilblom, general manager of Varian Imaging Products, the LAST panels can do both fluoroscopy and radiography. "They have enough dynamic range so that you can run in fluoro, flip a switch, and do a radiographic exposure," Gilblom says.

The LAST panels represent one type of digital sensor--the amorphous silicon array. This type of flat-panel sensor uses thin films of silicon integrated with arrays of photodiodes. These photodiodes are coated with a scintillator material that emits light photons when struck by x-rays, which in turn generate electrical signals in the photodiode.

These compact, LAST lightweight devices could replace not only the film cassettes used in standard radiography but also the bulky image intensifiers found in every general-purpose R/F system, as well as in angiography and cardiac catheterization systems. No challenge is greater, however, than the one presented by mammography. To reach a resolution equivalent to film, pixels need to be under 100 µm, and preferably near 50. One company, GE Medical Systems, claims to have overcome that hurdle.

On March 6, the company demonstrated a fully digital diagnostic mammography system to lawmakers and their staff on Capitol Hill as part of a federal effort to develop a mobile breast-care center. Conquering breast cancer is a national priority--and digital mammography may be a stepping stone to achieving that goal. Mona Theobald, marketing and product sales manager for women's imaging at GE, notes that the sensors that are being incorporated into the company's digital mammography system ultimately could be applied to any product line--fluoroscopy as well as radiography, angiography as well as cardiac catheterization products. "The most challenging technology is mammography, because of the spatial resolution requirements," says Theobald. "If you do well in that kind of environment, you will do well in other areas. Radiography would certainly be a logical next step," he adds.

Amorphous silicon arrays represent only one possible approach to the digital x-ray. Among other approaches are selenium-based arrays. These have the advantage of directly converting x-rays into electrical signals, rather than going through the intermediate step of using a scintillator to produce flashes of light. Instead, they absorb the x-rays directly into selenium photoconductors, which generate a large number of electron-hole pairs per absorbed x-ray. These electron-hole pairs written on these arrays are then read out and converted into electrical signals.

Introduced in the United States three years ago by Philips Medical Systems, the Thoravision digital radiography system performs chest imaging.

Philips Medical Systems (Shelton, CT) was the first company to introduce a selenium-based system, called Thoravision, which began selling in the United States about three years ago. Thoravision uses a thin film of selenium mounted on a drum to record the impact of x-rays as electrical charges.

The charges, read by a probe that scans the surface of the drum, are translated into electrical signals that are transferred for computer processing. Sterling Diagnostic is taking a different tack, developing a system called Direct Radiography (DR), which can be used in place of film cassettes in existing radiography systems. In this way, DR promises to preserve the owner's basic investment yet provide the benefits of digital capture. The electronic array directly converts x-rays into analog voltage, which is then converted into digital signals. DR, which is initially being designed to fit 14 * 17-in. cassette-style buckys, is scheduled for commercial release in 1998.

Selenium also holds promise for both radiography and fluoroscopy. John A. Rowlands, associate professor of medical biophysics at the University of Toronto, has developed a detector that provides real-time readout at up to 30 frames per second. Such instantaneous reporting is essential for radiography as well, says Rowlands. "Instead of batch processing films and tying up a room for 20 minutes or more--or taking a chance and sending the patient home--you can be sure the image has been taken properly."

Also near at hand are systems based on charge-coupled devices (CCDs). These arrays record light created by the impact of x-rays on a scintillation material, such as photostimulable phosphors. Lunar Corp. unveiled a new product at the end of April incorporating such a CCD-based sensor. The product, called PIXI, measures bone density in the heels of patients as an indicator of weakening bones due to disease, such as osteoporosis. The x-ray image, created on a scintillation screen composed of sodium iodide, is optically focused onto a single CCD chip.

The PIXI from Lunar Corp. measures bone density in the heels of patients using a CCD-based sensor.

The principle is the same as that used by image intensifiers found on fluoroscopy systems. The Lunar solution, however, is a lot less costly. "Image intensifiers are relatively expensive--they're also big, bulky, and heavy," says James Hanson, PhD, vice president of marketing at Lunar. "And, depending on how you make them, the image intensifiers may not have the same image quality."

To cover larger areas of the body, some companies are binding several CCDs together. These mosaics cover a larger area, but gaps in the coverage result from where the modules join. Also, fabrication can be expensive, with cost rising for each new module added. Such arrays are currently being developed by academic and commercial R&D groups, including at least one at Trex Medical. The problems accompanying this approach rise with the number of modules used, which tends to limit the utility of the technology to relatively small fields of view.

An alternative is to optically couple the CCDs. SwissRay International has developed a CCD product that can replace film cassettes in existing systems. Called the Add-On Bucky, the product integrates a scintillation screen with four CCD arrays, each focused on a segment of the screen with an optical lens. Scintillation events, resulting from x-rays striking the screen, are recorded by a CCD and transformed into electrical signals.

Several companies, including Fischer Imaging, are developing slot-scanning sensors based on CCD technology primarily for use in full-breast digital mammography. In this approach, a narrow fan-shaped beam scans tissue in step with a slot-shaped CCD detector. As the detector sweeps the anatomy, an electronic image is built up. The scanned-slot approach has the advantage of minimizing the amount of x-ray scatter in the image by preventing many of the photons scattered in the breast from striking the detector.

CCDs are attractive because they are both widely available and relatively low in cost. Building detectors that cover any sizable part of the anatomy, however, is relatively complex and expensive.

The ideal solution would be to produce integrated electronic arrays. That is why the industry is now emphasizing the development of amorphous silicon or selenium arrays. The trick is to develop a fabrication process that will produce these flat panels at a cost low enough to be attractive to a medical community growing increasingly sensitive to cost.

Xerox PARC, the R&D arm of dpiX, is working on a solution. As part of a joint R&D project with ThermoTrex Corp. and TPL, Inc. (Albuquerque), Xerox is designing a hybrid fabrication process that integrates both amorphous and polysilicon thin-film devices into the same electronic array.

This combination promises to reduce external contacts and thereby lower the cost of fabricating these sensors. "One of the major costs of amorphous silicon arrays is the attachment of the several thousand external connections coming off these plates so you can read the data out," says Jim Boyce, research area manager for Xerox PARC. "With polysilicons, we can envision incorporating a lot of the electronics on the plate itself so we would have only a couple of connections. Do that and you can greatly reduce the packaging costs."

But even if flat-panel sensors become affordable, other challenges threaten to slow the adoption of digital x-ray systems. The high-resolution monitors needed to view digital mammograms at the same resolution as is seen with film on lightboxes, for example, will either be unavailable or too expensive for healthcare practitioners when the first digital mammography systems enter the marketplace. "We're probably not going to see interactive gray-scale display systems at 4K [a display matrix of 4000 * 4000 pixels] for quite a while, maybe for more than five years," says Samuel Dwyer, MD, professor of radiology at the University of Virginia in Charlottesville. "So we are faced with the situation of having a digital system print images on laser film in order to look at them. That's not taking advantage of the digital side of things."

High-performance monitors will be required for all digital x-ray systems, although not necessarily at the level of performance needed for dedicated mammography systems. Engineers at Sterling Diagnostic Imaging, which hopes to have its Direct Radiography product on the U.S. market soon, believe they have developed a workstation that will meet the needs of radiologists.

"We are talking about a monitor with between 7 and 8 million pixels," says Gary Sadow, vice president of electronic imaging at Sterling. "That will give us a direct mapping to the DR image so you will end up with a very-high-quality image display."

The need for better resolution monitors to handle digital x-ray images, says Sadow, could drive medical imaging as a whole to greater heights of image quality. The elevation of x-ray-based equipment to such new heights would be a welcome change, after it has been overlooked for so long in the shadows of radiology. Magnetic resonance imaging and computed tomography have dominated sales for much of the last two decades. But with the technical challenges behind them and manufacturing solutions apparently near at hand, the vendors of x-ray equipment could enter the next millennium on a par with these glamour technologies.

Copyright ©1997 Medical Device & Diagnostic Industry