Flat detectors meet challenges of general and vascular imaging

GE, Toshiba, Philips capitalize on flat detector geometry

Although manufacturers have spent the last decade developing flat-panel detector technology for digital chest and, more recently, cardiac applications, it is only now making its way into other areas. Radiography/fluoroscopy and vascular imaging are two of the most challenging venues because of the need to adapt to a wide spectrum of diseases and patient types and demands for exquisitely detailed images of fine blood vessels.

GE Medical Systems won clearance from the FDA in December for its Innova 4100. Tested on five to 10 patients a day since August at St. Luke's Hospital in Bethlehem, PA, the Innova 4100 has such a large field-of-view it can image the entire chest or the abdomen and pelvis in a single view. The system also allows radiologists to use about 30% less contrast material. It provides such high-quality images that Dr. Hal Folander, chief of radiology, can't imagine going back to older, image intensifier-based systems.

"We do about 7000 interventional procedures a year and have had pretty current equipment. As of July, I was ecstatic with our other systems, but then I started using the flat-panel detector, and it was like waking up in the morning and putting my glasses on," he said. "I could see catheters, wires, and stents much more clearly than I could with the old image intensifiers."

Folander believes, in fact, that flat-panel detector technology will completely replace image intensifiers in angiography and other fluoroscopy-based applications, because solid-state detectors are much more efficient at capturing x-rays and can dramatically improve image quality.

One advantage of flat-panel detector technology is consistently high resolution regardless of FOV. Unlike image intensifiers, whose resolution varies depending on the number of lines in the matrix and the pixel size, flat-panel detector technology maintains a constant pixel size.

And, like manufacturers of digital cameras, makers of flat detectors are following the same path to highly defined resolution-shrink the size of the pixels. Philips Medical Systems' first detectors had a 200-micron resolution. Next-generation cardiac detectors had a resolution of 184 microns. The company's work-in-progress vascular system, which is expected to be commercially available by the end of the year, has a 154-micron pixel resolution, said John Steidley, vice president of cardiovascular imaging systems.

Toshiba Medical Systems' flat-panel detector, also slated for release later in the year, has been designed to deliver 150-micron pixels. The detector will deliver better resolution than the company's CCD-based image intensifiers, said Donald J. Volz, director of the vascular/x-ray business unit, going from a best of 3 line pairs per millimeter, which is comparable to cine film, to 3.3 lp/mm with the flat-panel detector.

Makers of flat-panel vascular systems are also altering the size and geometry of the detector. The largest image intensifiers are around 16 x 16 inches, but because they are round, they can't provide a full 16 inches of imaging, said Folander. Square flat-panel detectors deliver virtually every possible pixel.

GE's Innova 4100 system is 41 x 41 cm, more than twice the size of the 20 x 20-cm detector on the company's first flat-panel x-ray system, the Innova 2000. Because of its configuration, the detector on the 4100 system has more total inches of coverage and consistent image quality, with no distortion from edge to edge.

Toshiba has designed a 9 x 9-inch detector with a matrix of 1500 x 1500 lines, which exceeds the 1000 x 1000-line matrix on most current image intensifiers. The company is also developing a 14 x 14-inch unit, which will have a 2200 x 2200-line matrix.

Philips is adding a new twist to its flat-panel detector: a pivoting design that allows the detector to image at different projection angles during runoff angiography or to image specific areas of the chest, neck, or legs. The detector also has a 30 x 40-cm rectangular shape to improve the acquisition of 3D data sets and enhance the overall quality of images.

"With a square unit, radiologists have to image patients in the center of the detector or as close to the center as possible, because if they come off that center axis, x-rays diverge from the x-ray source and produce unwanted distortion," said Andy Dunne, Philips vice president of nonvascular imaging systems. "Although in principle radiologists could use the edge of a large square detector to image patients, they don't get the same quality as they would with a rectangular unit designed specifically for angiographic applications."

The technology behind flat panels also varies. The Innova 4100 system utilizes GE's digital flat-panel detector technology, a single piece of amorphous silicon with a cesium iodide scintillator, which converts x-rays into flashes of light. These flashes are translated into electrical impulses.

Philips uses the same basic technology, cesium iodide over amorphous silicon, but the scintillator is thicker-550 microns, or about 30% thicker than competing products. This thicker layer converts more x-rays into visible light and enhances contrast, Steidley said. The Philips detector also has a refresh light that flashes 30 times a second to remove unwanted image signals, as well as residual or ghost images, and to provide artifact-free imaging.

Toshiba does not need a scintillator. Its flat panel uses amorphous selenium, which directly converts x-rays into electrical charges, according to Volz.

Just as these detectors vary in some respects, so are they the same in others. They all have compact housing, for example, which increases access to the patient by interventionalists and reduces patients' sense of claustrophobia. They produce higher quality images. And they all instill a sense of curiosity in end users.

St. Luke's Hospital, which is a show site for the Innova 4100, has a waiting list of interventional radiologists who want to see flat-panel detector technology in action. But according to Folander, interventionalists aren't the only ones who will benefit from the new detectors.

"The flat-panel systems are the wave of the future not only for vascular imaging," Folander said, "but for fluoroscopy and multipurpose x-ray imaging, simply because the imaging is so much better."