PET/CT Takes Aim

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Making A Difference in Cancer, Brain and Cardiac Imaging

When it comes to valuable imaging techniques, it is hard to beat the combination of positron emission tomography (PET) that reveals metabolic processes and cell function, and CT that provides detailed anatomic information. The latest versions of hybrid PET/CT machines merge high-resolution PET technology with multidetector CT scanners to produce fused images that improve various aspects of patient care in oncology, cardiology and neurology.

While clinicians involved in PET/CT imaging are quite impressed with the current capabilities of these systems, they eagerly await future applications now being developed as works in progress. With anticipated new radiotracers, they can only dream of potential benefit to patient care.

In centers around the country, PET/CT has become an accepted workhorse in oncology, but some centers have used this technology for a variety of clinical studies.

Multitalented technology

Marcelo F. Di Carli, MD, FACC, FAHA, chief of nuclear medicine/PET, co-director of cardiovascular imaging at Brigham and Women’s Hospital in Boston and associate professor of radiology and medicine at Harvard Medical School explains that they have been using their GE Healthcare Discovery VCT 64-slice PET/CT system for all applications in PET including oncology, brain and cardiac imaging since May 2006.

“We are looking at cardiac perfusion as a way to diagnose coronary artery disease as well as for CTA [angiography] in connection with the PET scan,” Di Carli says. Not only do they visualize the anatomic extent of coronary artery disease but also they gain instant access to the physiologic significance of that disease. “I say it is the one test that provides you with more than the sum of the parts.”

There are several instances where the PET/CT has proven valuable in analysis of specific neurologic conditions. They use FDG (flurodeoxyglucose) PET for selected circumstances with brain tumors, particularly differentiating radiation necrosis from viable tumor. In some patients who have undergone radiation treatment for a brain tumor, they will have equivocal MRI studies where it is unclear whether recurrence is a factor. The metabolic information from the PET study helps to differentiate tissue types.

Another approved use of FDG-PET is in early diagnosis of dementia, including Alzheimer’s disease. “PET is very sensitive to early changes in brain tissue function, even before clear-cut clinical manifestation such as those individuals with mild cognitive impairment,” says Di Carli. Some experts who study dementia suggest that early diagnosis may hold the key to slowing the progression of the disease.

“For epilepsy, PET has been very successful in both pediatric and adult patients for identifying epileptic foci within the brain,” Di Carli says. Particularly for those patients with intractable epilepsy and who are potential candidates for surgery, PET is specifically approved for that indication. Diagnostic information to identify whether there are either single or multiple foci given the electrical activation of an epileptic cascade is provided by PET used in conjunction with an EEG (electroencephalogram) recording. The CT component helps to identify specific anatomic landmarks.

In the future, Di Carli believes that the use of PET/CT for guiding interventions will emerge. For example, when a patient requires a needle-biopsy of a mass, PET can pinpoint the area that should be sampled. Many tumors have some sections that are necrotic while others contain viable cancer cells. The physiologic information from FDG-PET indicates the portions of the tumor which are most metabolically active and therefore should serve as the area of interest for the biopsy. While this technique is still under study, Di Carli expects that eventually PET will guide interventions such as radiofrequency (RF) ablation or cryoablation of tumors.

Ronald A. Korn, MD, PhD, director of PET/CT and research for Scottsdale Medical Imaging, Ltd. in Scottsdale, Ariz., and past chairman of radiology at Scottsdale Health Care works in a community based private-practice radiology group that provides imaging services in Phoenix and Scottsdale. He explains that their GE Discovery STE PET/CT plays a vital role in the healthcare of their oncology patients, and that they see a large demand for use of this technology in cardiac imaging and a lesser demand for neurologic disease. He considers that they provide the full spectrum of PET/CT imaging and that within their oncology practice PET/CT can bring about changes in patient management as much as 40 percent of the time.

Several of their patients have been helped by the CMS (Centers of Medicare and Medicaid Services) decision to provide reimbursement for PET/CT scans for malignancies that are outside of their approved list of cancers. As a result, they are using PET for many patients with ovarian and uterine cancer as well as other non-CMS indicated malignancies.

“This can really help to plan the surgical removal of cancers, which is really important in women with ovarian cancer. Often conventional imaging does not show the same extent of disease as PET/CT,” Korn explains. When the goal is surgical de-bulking, the additional information provided by PET can reveal hidden disease.

The Scottsdale community is in an area of the country where Valley Fever (coccidiomycosis) is very common. When patients with indeterminate pulmonary nodules on conventional imaging arrive, the PET scan proves valuable in this population, eliminating the need for biopsy in about a third of the patients.

One of the aspects of the Discovery STE that Korn appreciates is its rapid patient throughput with great image sensitivity. They can scan up to 25 patients per day. Technologists describe their Advantage Workstation with Fusion software used in conjunction with the scanner as having a user-friendly interface. This means that not only can they move patients on and off the scanner more quickly, but also the image reconstruction and reprocessing time is reduced, which enhances the technologists work schedule.

Being able to acquire PET scan data more quickly without losing accuracy helps with several classes of patients, including those who are very ill who cannot lie still for long periods of time, as well as pediatric patients where they want to minimize image time but also lower the dose of FDG to reduce radiation exposure. “Our machine has the capability of acquiring breath-hold images, so we can acquire 3D images within 30 seconds, which reduces respiratory artifacts and we find that particularly useful with the indeterminate pulmonary nodules.”

As he contemplates the future of PET/CT, Korn believes that the development of new tracer compounds besides FDG will prove valuable. “I imagine a day when people will use a cocktail of different studies to not only probe metabolism but other cellular processes as well.”

In addition to their oncology practice, Korn says they do Rubidium PET/CT scans for myocardial perfusion as a substitute for traditional nuclear medicine scans. “The images are absolutely exquisite and artifacts are reduced to a bare minimum. We’re also beginning to do quantitative analysis to look for coronary flow reserve, which is an important marker of coronary artery disease.”

Their Discovery STE is a 6-slice CT which accomplishes all of the tasks they need. Because they have other CT scanners with higher detector numbers, they can do CTA on other scanners and then compare.

The oncology workhorse

Chaitanya R. Divgi, MD, professor of radiology at the University of Pennsylvania and chief of the division of nuclear medicine and clinical molecular imaging at the Hospital of the University of Pennsylvania in Philadelphia describes that in their use of the Philips Medical Systems Gemini TF about 90 percent of the patients scanned either have cancer or are suspected to have cancer. They do not use this scanner as a primary diagnostic tool, but re-staging and therapy response evaluation have served as the principal indication since it went online for patient care in June 2006.

Divgi relates two significant advantages that they appreciate. The first is that they acquire far better quality PET/CT images in overweight patients. The second is that they are able to acquire very rapid images in normal size patients.

He sees a future application of image adaptive radiotherapy, which is currently considered a works-in-progress. “The advantage of rapid images of excellent quality, doing scans in two minutes vs. 10 minutes, makes this whole thing feasible while patient safety and comfort are not compromised.”

In this scenario, when the patient undergoes radiation therapy, the PET scan would provide biologic characteristics of the tumor. Assuming the tumor will shrink during treatment, they would be able to focus the external beam on a viable portion of the tumor rather than continuing to treat the entire volume. “This approach will allow us to give more dose to the area of the tumor that appears resistant and to contour the beam even more.” Intensity modulated radiation therapy (IMRT) is typically done with CT simulation, but this approach would include the PET data information as well.

Gemini also features time-of-flight technology that precisely identifies the time difference in detection of the two coincident gamma rays to within a billionth of a second (roughly the speed of light) which facilitates calculation of the reconstructed image. This technology enables both faster scans and improved images for obese patients.

Gemini scanners are designed with a gap between the two scanning components. Divgi explains that the design has meant that their patients who are claustrophobic find it easier to be scanned on this machine. Another benefit is that the gap provides access to the patient for interventional purposes. And finally, although they have never done this, Divgi says the two scanners can be disengaged and used independently. He notes that if a center did not have a large PET load but were performing many more CT studies, they could set the PET component aside.

“We’re running this machine ragged, so we don’t have that problem,” Divgi says with a chuckle.

It’s all in the numbers

The Orlando Regional Medical Center uses their Siemens Medical Solutions Biograph 64 PET/CT scanner for a range of oncologic procedures, according to Richard J. Lovas, MD, chief of nuclear medicine who also serves on the staff of M.D. Anderson Cancer Center in Orlando. Because the CT component provides attenuation correction within a minute, scan times are greatly reduced. In addition, the PET component has an improved count rate efficiency, and as Lovas explains, nuclear medicine is a game of numbers.

“The combination of all of this is that we’re doing three-minute PET positions, and with a tall patient, you’ll have six or seven PET positions. If we are looking for melanomas or sarcomas, it’s a total body scan.” He notes they are now getting better images with a greatly reduced scan time, so their patient throughput has improved. Within half an hour, the PET and diagnostic CT scans are completed, and they are able to localize and characterize the lesion more accurately.

For example, in a patient with lung cancer, physicians would look beyond the lung fields for metastasis. If there is a “hot spot” in the neck, the PET could have detected an inflammatory process such as a carotid artery affected by atherosclerosis rather than a lymph node. The PET/CT scan will reveal physiologic and anatomic information about whether the area of concern is localized to adipose tissue or to muscle or exactly where it is.

This center currently uses both of their PET/CT scanners for radiation treatment planning. Their use of respiratory gating assists their targeting for IMRT in soft tissue tumors to reduce “collateral damage” to normal critical structures. In the future, they plan to participate in clinical trials with F-18 MISO (fluoro-misonidazol) to detect hypoxic areas of tumors that are resistant to radiation treatment.

Potential major challenge

Scottsdale Medical Imaging’s Korn is concerned that the reimbursement rates may drastically reduce the future use of PET/CT, especially in smaller communities without a high volume of patients. In the Deficit Reduction Act of 2005, some experts have suggested that reimbursement may be slashed by as much as 70 percent. Last November, the Access to Medical Imaging Coalition asked Congress to reconsider the deep cuts that were made as a result of the DRA. While adding the caveat that this is not his area of expertise, Korn suggests, “We all agree that utilization is a problem in all of medicine, but to really affect this vulnerable population has a greater impact than most.”

Conclusion

The benefits provided by PET/CT scans are beginning to be recognized by a growing number of clinical specialties. By combining the new powerful multidetector CT scanners with improved PET machines, the advantages of offering both functional and anatomic image data has proven helpful in delineating specific problems. Many are hopeful that the recent reimbursement issues will be resolved to permit continued use of this valuable imaging modality.