Medical Device Daily Washington Editor
WASHINGTON – Recent controversies over physician billing of Medicare for imaging services suggest that both doctors and patients rarely handle a piece of scanning equipment they dislike, but exposure to almost any kind of radiation has its hazards and angiography procedures incur some of the longer exposure times to X-rays in all of radiography.
One of the sessions at the 2006 edition of Transcatheter Cardiovascular Therapeutics conference here in the nation’s capital took up that subject, and while the presenters offered a number of tips that can cut exposure for doctor and patient alike during angiography — especially cineangiography — some dilemmas simply will not go away, no matter how skilled the surgeon or how clever the imaging technique.
James Whiting, assistant professor in the Department of Medical Physics and Imaging at Cedars Sinai Medical Center (Los Angeles), discussed the various aspects of image formation that affect the required dose of radiation in conventional cineangiography, the technique that offers a compelling motion-picture view of cardiac function. Whiting emphasized that the operator’s choices affected the dose experienced by both patient and clinical staff, and that the old inverse-square law regarding radiation intensity still holds, regardless of the device novelty.
The modern X-ray system “is almost completely automatic” and does a reasonable job of accounting for the varying thickness of patients’ bodies, Whiting said. Most systems have a feedback mechanism that gauges the intensity of radiation coming through to the image receptor and adjusts the voltage, which affects energy of the photons to provide better penetration but does not increase or decrease the number of emitted photons.
Another output adjustment is to tweak the wattage, thus changing the number of photons but not their energy levels. Many systems generate photons in pulses rather than in a steady stream, and this parameter can also be tweaked to reduce exposure. This feature is often found in fluoroscopic angiography systems and also available in systems employing magnetic resonance technology.
He likened such a system to the operation of a digital camera. The pulse rate corresponds to shutter speed, and the other two measures act as the dimensions of the aperture. As with a long shutter opening, an extended pulse could “produce a blurred image,” Whiting noted.
“It costs a lot in terms of patient exposure to get the perfect picture,” he said, a reality imposed by the inescapable fact that lower doses of radiation tend to carry more signal noise. This phenomenon is also subject to the inverse-square law. “In order to cut the noise in half, you have to increase the dose by a factor of four,” Whiting said.
The factors that affect how much radiation a clinician will induce includes “how much noise you can live with,” he said, but filtration and collimation are two means by which radiation dose can be lowered without sacrificing image quality. Collimation involves the use of multiple projectors that zero in on the area of interest from different angles and hence reduce the exposure to adjacent parts of the body. Many radiographic imaging systems use a three-headed collimator to deliver the needed exposure, but engineers should be wary of positioning collimators in such a way as to allow all three beams to overlap, however small the overlap area.
Two advances offer improved imaging at a lower radiation cost. For example, metal filters are available that can “filter out low-energy photons,” Whiting said, but these filters only slightly change how things are done. They are not quite as energy-level-specific as many would like, and “you can throw out some good photons with the bad,” Whiting lamented.
On the other hand, digital imaging has radically changed the way images can be viewed and offers clinicians a stop-motion feature that allows a temporary cessation of radiation as the recently acquired image or image sequence is played back. Whiting said that image hold is one of the most important features of digital imaging, noting that “if you can train yourself to use last image hold and replay feature, you can save a lot of radiation” that would otherwise land in the patient.
Donald Miller, PhD, with the National Naval Medical Center (Bethesda, Maryland) said the literature on the amount of radiation absorbed by a patient’s skin sufficient to induce lasting damage is about 2 Gy (Gy defined as the absorption of one joule of radiation energy by one kilogram of matter) and that patients hit this threshold “way more often than you think.”
Regarding the sources of treatment-induced radiation damage to the skin, Miller said that “about 80% are due to cardiac interventions, but I believe that’s true because the doses in these [procedures] tend to be higher” than in other uses of X-radiation.
Before taking a patient in for such a procedure, Miller noted, a physician should look at the patient’s medical history [and] the patient’s radiation history” to establish whether his or her risk is elevated due to previous exposure or health problems.
Some diseases, such as hypothyroidism and diabetes mellitus, lower the threshold for various types of damage from radiation, as do some drugs “including some chemotherapy agents” as well as some of the cholesterol statins. Repeated exposure is, of course, also an indication of higher risk of skin effect.
Miller cited “no hard and fast rule” on the maximum amount of radiation that a patient can absorb over a year or a lifetime without suffering from some skin effect. “There is no such thing as a maximum permissible radiation dose,” imposed by any authority, and there is little data for developing formal guidelines.
“Your goal should not to be to minimize the radiation dose” because this precept would demand operating without scanning at all, “but to optimize the radiation dose,” Miller said. When a procedure goes on longer than anticipated and the exposure goes up beyond the planned amount, the doctor has to make an informal risk analysis. Keeping a running tab on total exposure can aid in making decisions about the wisdom of continuing such a procedure.
“If you give a high dose of radiation and you’re very near finishing the procedure but you haven’t accomplished your clinical goal, stopping now means you’ve wasted all that radiation,” Miller said. A physician has to weigh the costs and benefits of finishing the procedure and continuing the radiation versus terminating both.
“Most of the time, the clinical risk [of not finishing] outweighs the radiation risk,” Miller said.