Flat panels deliver more dose than image intensifiers

Flat-panel technology may be exposing fluoroscopy patients to more dose than the image intensifiers they were designed to replace, despite claims to the contrary. The problem is being made worse by the way flat-panel systems are being used.

A comparison of flat-panel and II-based interventional systems found a wide variation in dose depending on operating parameters, such as cine or fluoroscopy and large or magnified field-of-view. In all cases, however, flat panels delivered a higher radiation dose during fluoroscopic procedures, according to Dr. Michael Van Lysel, an associate professor of medicine and medical physics at the University of Wisconsin-Madison.

Claims of dose savings have come from apples to oranges comparisons between image intensifiers with thinner phosphors and flat panels with thicker phosphors, Van Lysel said. Dose savings claims are valid, however, when the systems have phosphor layers of varying thickness.

"But the dose saving is due to the thicker phosphor on the flat-panel system, not the flat- panel system itself," he said.

Since digital detectors entered commercial use nearly a decade ago, proponents have talked up the dose efficiency that might be realized by the new technology. This potential, however, is largely achievable by flat panels designed to deliver static images as opposed to competitive offerings, such as computer radiography plates. A different situation exists with fluoroscopy.

"If you talk to medical physicists today, they would say that, in general, flat-panel detectors have delivered on at least dose parity but more commonly dose reduction in radiographic applications," said Barry Belanger, clinical research manager for interventional x-ray at GE Healthcare. "Fluoroscopy is more challenging for a flat-panel detector because it must operate at such low doses. When you are dealing with a very small amount of radiation, your detector must have very low electronic noise so it doesn't contaminate the signal."

One of the inherent problems in flat panels used in fluoroscopy is the electrical noise coming from the wires that run the length of the detectors, according to John Grady, founder of Ayer, MA-based Grady Research.

"Each hit of an x-ray photon generates a signal of about 600 electrons, but the noise in the line and the amplifier are 3000 electrons," Grady said. "The only way you can overcome that noise is to essentially double the radiation going to the detector."

But the problem doesn't necessarily mandate a higher dose, Belanger said. As long as the flat-panel system reduces the capacitance and impedance of readout lines, this kind of noise can be minimized.

Contributing to the dose problem is the desire for image quality. Trade shows and customer demonstrations, which often are produced under ideal conditions or may involve excessive dose, may help create unrealistic expectations for operators.

"The beautiful images that you see are often taken from recording runs, where the dose rate may be five times higher," Grady said.

Operators also need to become aware that added dose may be creeping in through system settings as well.

"There is a push to produce higher and higher image quality, and the only way to do that with these systems is to turn up the dose," Van Lysel said. "Doses on newer systems appear to be set higher."

How much higher is anyone's guess.

"In order to put a number on this, you need to do a very careful experiment in which you define exactly what you are comparing," he said. "Or if you want to go the other route, you would need to find out the doses in several labs. Without conducting a survey that measures many different labs, we won't know what the doses are."

Hope still prevails that technology can be improved to reduce noise further, thereby lessening the need for increased dose as well as providing other advantages.

"We continue to look for opportunities to get the noise down further because it will help us do other things, such as read out the data faster," Belanger said. "So we see value in continuing to work on the set of parameters that affects the fluoroscopic DQE."