Single photon emission computed tomography (SPECT) is a nuclear imaging modality that has been around for almost 50 years. In a sense, SPECT could be characterized as a medical and scientific tool that has waited a long time to be fully appreciated.

The CardioMD fixed 90° nuclear cardiology camera from Philips Medical Systems is designed for the office-based system and can fit into a room as small as 8 x 10 feet.

Introduced in the 1950s and older than most other imaging modalities, SPECT has been a familiar and useful clinical device. But it didn’t come into widespread use until the 1980s.

Several things needed to take place to move SPECT toward broader clinical applications. The technology for creating better images had to advance first. More radioisotopes needed to be developed for use as tracers, and better engineering designs were needed to bring SPECT out of the hospital and into the office.

After some tweaking, software began featuring improved imaging options, and innovations in drugs opened new possibilities. Then another leap—fusion and hybrids, attenuation correction, and, finally, the advent of molecular medicine.

SPECT is a modality that has the advantage of many years’ experience coupled with advances that now are making it an important player in nuclear imaging. As the pharmaceutical industry creates new drugs, the most notable companies manufacturing SPECT systems—Siemens Medical Solutions (Malvern, Pa), GE Healthcare (Waukesha, Wis), and Philips Medical Systems (Bothell, Wash)—are reinventing the hardware portion of the systems (ie, the gamma cameras needed to image and then visualize the drugs in the body).

Recent advances include refinements that make SPECT significantly more user friendly for clinical end-users—both clinicians and patients. The result has been a notable growth in its presence as a member of the imaging community.

In total, about 13,000 SPECT cameras are used in the United States. In 2003, about 1,300 systems, of the approximately 2,000 total worldwide, were added to the US market. Comprising about 65% of the total world market, the United States is an important player in SPECT.

The most common SPECT system sold now, the dual-headed gamma camera, comprises about 80% to 85% of the total US market, according to Plado Kosmaoglou, manager of product and clinical marketing at Siemens Medical Solutions’ Nuclear Medicine Group (Chicago). Traditionally, when the patient is placed in the gamma camera scanner, the detector moves around the patient to cover 360° and create several images. The advantage of the dual-headed detectors is that only a 180° rotation is necessary, cutting imaging time in half.

Toshiba’s t.cam variable gamma camera (above) uses dual detectors with 76°, 90°, and 180° angulation to provide optimal image quality for whole-body, SPECT, and general purpose scanning in a patient-friendly environment.

Matters of the Heart
As with other areas of nuclear medicine, a SPECT image is created by the body’s functioning, not the bones or structure. This distinction from other imaging modalities has contributed to SPECT’s major current role in cardiology. For example, the metabolism of the heart muscle can be easily stimulated, allowing for portions of the heart receiving or not receiving blood to be clearly identified.

“Nuclear medicine is extremely useful in being able to classify coronary artery disease in terms of not seeing the disease itself, but the result of the disease. In other words, how well is the heart muscle being fed with blood?” Kosmaoglou explains. “If you see on the image that it is not being fed, it means you have a problem before [the heart], which is usually in the arteries. Then the patient goes to catheterization in order to find out exactly where and how big the problem is.”

Indeed, most of today’s cardiology studies are performed with SPECT. “Literature out there now shows that cardiac SPECT is a more accurate technique than non-SPECT, and the sensitivity and specificity are better,” says Robert E. Henkin, MD, of the Loyola University Medical Center’s Nuclear Medicine Division (Chicago).

Consequently, a segment of the SPECT market that is likely to continue growing is that dedicated to cardiology. According to F. David Rollo, MD, PhD, of Philips Nuclear Medicine (Andover, Mass), last year alone saw 8 million to 9 million nuclear cardiology procedures performed. Presently, cardiology dominates SPECT, representing about 50% of SPECT procedures. Specifically optimized for the purpose, the SPECT cardiology systems are also more efficient, smaller, easier to use, and more cost effective.

Another factor driving nuclear cardiology is efforts at reducing the number of unnecessary procedures performed. Among those cardiac catheterizations that were based on symptoms and family history but lacked a nuclear cardiology procedure, about 40% to 50% resulted in normal studies, Rollo says. Conversely, for patients referred for a catheterization based on results from a nuclear cardiology procedure, the quantity of normal studies drops to fewer than 10%.

The e.cam Signature Series from Siemens Medical Solutions offers high-definition dynamic digital detectors (HD4) to deliver high image quality and reliability.

SPECT also is a relatively cost-efficient modality. Its gamma cameras don’t require the shielding and room preparation necessary for other modalities, such as CT and MRI. The SPECT equipment is also less expensive. Additionally it doesn’t require the expensive disposable parts required, for example, by the CT X-ray tube. This cost advantage has made it an attractive imaging modality, especially for certain specialties, including cardiology.

Kosmaoglou estimates that departments using dedicated cardiology SPECT could break even within 2 or 3 years at approximately three patients a day. A busy department could see a higher number of patients, reaching the break-even point more quickly, he suggests.

Most of these systems are purchased for use in private cardiology practices. Reimbursement for procedures performed with these imaging systems also are very favorable at this time, Kosmaoglou says, which adds to the systems’ attractiveness.

“[The patient has] immediate access to the availability of nuclear cardiology [for when] the diagnostic cardiologist says, ‘This looks like cardiac disease,’” Rollo explains. “What [the cardiologist] would like to have is the diagnosis within minutes. Then, if the patient has cardiac disease, he/she can be sent to the hospital; if the patient doesn’t have the disease, he/she can be sent back to the referring physician, replete with the information that the physician needs to look for other causes for the pain.”

Attenuation correction is a sophisticated solution for potential false results in SPECT studies. The correction compensates for tissues or materials that lie between the detector and the organ of interest, which cause the absorption of photons emitted by the organ. Particularly important in decision-making in cardiology, this burgeoning area has significant promise. New and better software continues to be developed to address the issue. And gamma systems incorporate methods designed to reduce this effect. For example, the Infinia Hawkeye from GE Healthcare was introduced last year; it performs a new and quite effective attenuation correction, according to the manufacturer.

On to Oncology
The second major clinical use for SPECT is oncology. “What [physicians] are looking for is that a tumor has a much higher intensity function than the rest of the body does. It consumes more energy because it grows very fast,” Kosmaoglou explains. With the correct radiopharmaceutical, the SPECT image will show the tumor as a center that consumes much more of this drug than its surroundings.

Estimates of SPECT use for oncology range from 30% to 50%. The balance is made up of general imaging—lungs, kidneys, brains, and thyroids. Brain scans in particular are an important application. “Cerebral perfusion studies must be performed with SPECT, because there is no other way to do them,” says Henkin of Loyola University.

Two significant advances in SPECT have been the combination of functional and anatomical views through fusion and hybrid systems. Time has shown that it makes sense for diagnosticians to see combined information received from both functional modalities (like SPECT) and anatomical modalities (like CT).

Supporting this practice has been image fusion, which occurs when images from two separate scans—SPECT and CT, for example—are mixed together via a computer. One image, then, enables physicians to see both the functional information of nuclear medicine and the anatomical information of CT. This process has become an important diagnostic advance, used primarily for oncology studies at present.

The Infinia Hawkeye joins a nondiagnostic CT with a SPECT gamma camera to combine anatomical and functional imaging. A major contribution of this system is its localization of disease as well as improved image quality, notes Michelle Heying, general manager of global nuclear medicine at GE Healthcare.

And this month, Philips is launching the SkyLight, a SPECT/CT device that brings a new hybrid into the market to further evolve this innovative approach.

The Future of SPECT
SPECT is revealing its strengths both clinically and in the marketplace. As a less-expensive technology compared to other modalities, SPECT also offers lower setup costs for facilities. Additionally, SPECT has the ability to be narrowly applied and optimized for specific organs—the heart, for example, with cardiology. So the question arises: What does the future hold?

“One of the real advantages of [this modality] is the large number of SPECT cameras that are available,” Philips’ Rollo says. Virtually every hospital has SPECT as do several thousand outpatient diagnostic centers. “We have 13,000 SPECT cameras in the United States; therefore, the issue of access to care allows anyone to have a SPECT scan available to them,” he says. By comparison, about 600 PET systems exist, and most of them are in fairly large hospitals.

Another advantage, Rollo notes, is the large number of radiopharmaceutical agents already available as well as the number of new agents that are in the process of being cleared by the FDA for specific applications. Citing the more than 4 million bone scans performed each year for a variety of reasons, he says that by far, “the most sensitive method of looking [at a bone] is the use of the bone agents and the unique advantages of SPECT bone scans. Plus, we have some new software that allows us to see even the smallest areas of abnormality much better, and we can localize them better than we ever have in the past.”

Rollo notes that Philips has a number of new cerebral imaging agents that are in development and will be cleared soon. The conditions addressed include Alzheimer’s disease, Parkinson’s disease, and attention deficit disorders. “Obviously, to have an accurate diagnosis and, therefore, be able to apply therapy becomes extremely important to patients. Heretofore, most of what we’ve done is a neurological examination, a family history, and then [explained why we think a patient has a certain disease],” he says. “We want to have an agent that allows physicians to make a picture of the brain and have a pattern that allows them to specifically determine the disease type and then treat the patient appropriately. Better yet, we’d like physicians to be able to say, ‘No, you don’t have the disease’ and then look for other causes.”

The forces that are moving medicine toward molecular diagnoses and treatments will undoubtedly include SPECT. “The SPECT world is changing,” says Jody Garrard, product manager for gamma cameras at Philips Medical Systems. “We’re no longer just taking pictures; we’re trying to provide the doctors with more information than just whether it’s positive or negative for, let’s say, cancer. We want to give quantification; we want to give quantifiable data that can calculate how much radiation delivery could kill this tumor.” Garrard cites the SkyLight as a product that addresses these informational needs.

The relationship between gamma camera companies and pharmaceutical companies is also likely to change, with greater incentives to work cooperatively and in tandem. The creation of GE Healthcare from GE Medical Systems and Amersham Health, for instance, just might herald a new paradigm for the future.

A good example of that collaboration among drug and camera companies is with Philips. The company is looking to use the ability of a gamma camera to separate agents with different energy windows and then characterize tumors with “essentially a battery of tracer tests,” Garrard says. SPECT can image at a range of different energies, which allows separation of different tracers. Injecting the patient with a combination of tracers, each at a different energy and each tagged to a different array of pharmaceuticals, provides information that will give the physician better data in order to tailor the treatment.

“We’re going to be a lot more creative in the way that we apply these energy windows, in order to get the most information possible,” Garrard says. She also adds that Philips is focusing on an acquisition protocol that optimizes image quality and concurrently provides accurate quantitative results. By using multiple isotopes, the patient is able to undergo just one procedure while the physician obtains optimal image and quantitative information.

Loyola University’s Henkin believes that SPECT has a lot of potential. “The biggest advances could stand to be in SPECT because the pharmaceuticals for SPECT are easier to make and easier to label, and a lot of researchers are [exploring with this modality].”

In fact, Henkin believes that during the next 5 years, much more will be happening in SPECT than has occurred to date. Citing a new class of drugs in clinical trials now that will measure cell death, he notes that the organs or tumors will be imaged using SPECT. These will be a very powerful class of drugs that, he believes, will change how medicine looks at tumors and cardiac disease.

Perhaps this new century, with molecular medicine close on the horizon, portends a new and significant role for nuclear medicine. And maybe SPECT will be an important part of that future. It certainly looks promising.

Ellen Zagorin is a contributing writer for Medical Imaging.