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For other articles and previous issues click here. September 27, 2004 PET/CT
Proving a Fruitful Union Already a boon to cancer diagnosis and treatment planning, researchers are looking at the fusion modality’s value in cardiac care and neuroimaging. PET/CT is proving a highly effective fusion technology. With usage rapidly expanding, PET/CT is exercising a major impact in the area of oncology. Though PET and CT are both mature modalities, the PET/CT integration yields new information about cancer that can provide earlier detection, help manage treatment, and reduce unnecessary biopsies and some surgeries. Fused PET and CT can also be created using software systems that combine separately obtained PET and CT images. Most experts in the field believe the hardware approach using combined systems in one exam generally provides improved registration of the two images into a better single image. Software fusion has a flexibility advantage that allows combining different types of DICOM images. This article deals specifically with so-called hardware-based systems. Although hardware-based systems became commercially available in 2001, the hybrid modality has demonstrated its own considerable maturity. Already, researchers are looking at its utility in cardiovascular and neurological imaging applications. In addition, they’re looking to improve the potent combination through new developments such as more specific radiopharmaceuticals. Happily Married Modalities This fusion provides increased sensitivity and specificity in cancer diagnosis and helps reduce unnecessary procedures. Clinicians are better able to identify locations of abnormalities and determine whether or not malignancies have spread to other parts of the body. “PET/CT is a marriage where the two parties are made for each other,” comments Karthik Kuppusamy, PhD, global manager of CT, PET, and nuclear medicine for GE Healthcare of Waukesha, Wisc. “The combination creates a system with excellent clinical sensitivity and specificity. It improves the confidence of the day-to-day reading.” Specifically, PET reads cellular metabolism of glucose by using fluorodeoxyglucose (FDG), a radiotracer injected into the bloodstream. Cancer cells use glucose at a much higher rate than normal cells, and this uptake is detected by PET because FDG is similar in composition to glucose. CT produces cross-sectional x-rays of anatomical structures. Glucose uptake, revealed by PET, will indicate a metabolically active, or malignant, tumor. Alone, CT cannot determine whether a tumor is malignant or benign, but it reveals exactly where the tumor is located. When location information is coupled with the PET information, it provides additional critical information. “PET/CT is not just about diagnosis; it impacts treatment decisions, too,” says Grosso. “It can show if a therapy is working. That’s where the PET part comes in. For instance, if you’re trying to find out if there’s been a response to the chemotherapy, PET will let you know by showing a decrease in the hypermetabolic activity. Oncologists would then know if they’re moving in the right direction.” Decisions about biopsy, surgery, chemotherapy, and radiation therapy are based on PET/CT findings, agrees Hussein M. Abdel-Dayem, MD, director of nuclear medicine service at St. Vincent Catholic Medical Center in New York City. “Furthermore, the PET findings are sometimes directly incorporated into the radiation therapy planning, particularly for those patients getting IMRT [intensity modulated radiation therapy],” he adds. Increasing Usage By the following year, sales of PET/CT systems accounted for 45% of the PET market, according to Frost & Sullivan, the international growth consulting organization, which then predicted that the new hybrid would eventually outsell PET-only systems. In 2003, the American Society of Radiologic Technologists reported that an estimated 225 PET/CT systems were installed. The rapid acceptance of PET/CT is confirmed by sales of GE’s Discovery ST system, an advance over the LS model. The company sold its 100th ST unit in March, a mere eight months after the system’s introduction. GE has predicted installations would reach 400 by the end of this year. Kuppusamy reports that early PET/CT adopters included nuclear medicine departments in larger institutions. Now, he says, installations have extended to hospitals, community centers, and outpatient diagnostic imaging centers. “There is so much evidence that this system allows you to detect cancer earlier, and as that message gets out, the market grows larger,” he comments. R. Edward Coleman, MD, professor of radiology and director of nuclear medicine at Duke University Medical Center in Durham, N.C., estimates that between 80% and 90% of all PET scanners being sold right now are being sold as part of PET/CT systems. There are good reasons for that, he believes. “Besides providing both metabolic and anatomic information, the use of the CT scan for attenuation correction provides improved image quality and shorter scan times. So there is less motion and better quality in diagnostic images,” he says. Expanding Clinical Applications “Ninety percent of all procedures are in oncology,”
says Kuppusamy, but he quickly adds that its use is expanding into
other areas, including neurology and cardiovascular imaging, which
are becoming important strategic areas for PET/CT research and development. “PET/CT cardiology is going to grow very fast,” agrees Dayem. He cites several reasons. Comparing it with SPECT (single photon emission computed tomography) imaging, he says PET/CT eliminates the attenuation correction problem. Also, PET/CT is a quick procedure. A study takes less than 30 minutes. A SPECT study can take four to six hours. In addition, the radiation dose is lower. PET/CT also allows for quantitation of the coronary blood flow, which can’t be done with SPECT. “All of these make it more reliable,” Dayem concludes. “If I were a patient, I would not want SPECT anymore.” Another advantage that Dayem points out is that the CT element enables calcium scoring, which serves as an indication of prognosis. “This allows you to make patient recommendations,” he says. “If the calcium scoring is high even though the perfusion is normal, you might be justified to do some kind of preventive treatment to avoid the progression of the disease and accordingly reduce future interventions and future cardiac risks.” Recent research points to PET/CT’s potential in coronary and cardiovascular diseases. In a study presented at this year’s Society of Nuclear Medicine meeting, researchers from Zurich University Hospital reported that their results demonstrated PET/CT’s potential for assessing CAD. In the study, the researchers evaluated PET/CT image quality for combined acquisition of coronary anatomy and myocardial perfusion. They found that PET/CT proved more reliable and effective in diagnosing CAD compared with coronary angiogram and more accurate than CT angiography. But they also pointed out that these results were preliminary and that more assessment of PET/CT for CAD is still needed. In the neurological area, clinicians have been using PET/CT to look at brain tumors and identify the areas of the brain related to epileptic seizures, and for early detection of dementia disorders like Alzheimer’s disease. This direction of research points to the opportunity for early treatment that would extend an individual’s quality of life. “You could start treatment while the patient is still functioning socially and professionally,” explains Dayem. “Their productive life could be extended for several years before they would need home care. We feel this area is going to be very important.” However, not everyone is optimistic that PET/CT will have a major impact on treatment of conditions such as Alzheimer’s disease. “I don’t think it will have any significant advantages over PET alone, which already does a great job of evaluating brain metabolism,” says Coleman. For that matter, Coleman and others aren’t entirely sold on the advantages of PET/CT for cardiac imaging, either. Its greatest benefit, he feels, is in cancer diagnosis and treatment. “The advantages of PET/CT over PET alone for cardiac imaging haven’t been demonstrated as much as it has been for the oncologic applications,” he says. Expanded Training The big issue, says Coleman, is people who’ve already completed their training who now need to read either CT scans or PET scans. “The question is: What training do they need to read PET/CT?” he says. He indicates that the solution is already being worked on by organizations like the American College of Radiology and the Society of Nuclear Medicine, who are in the process of determining how to train nuclear medicine specialists to read CT scans and radiologists to interpret PET images. “They’re developing guidelines and working to make sure this happens appropriately, but there are still some uncertainties about how individuals already out in the field—both the technologists and radiologists—will get the training so they’ll be competent in performing and interpreting both PET and CT scans,” says Coleman. Future Directions Many observers believe the number of slices a clinician needs depends on the application. For instance, for oncologic purposes, it is generally agreed that the four- and eight-slice systems are sufficient. Systems with 16 slices or more are for those who are interested in the cardiovascular applications. “You really don’t need the higher number of slices for oncologic PET imaging,” says Coleman. “The higher number plays a role in angiographic studies.” For cardiac work, the high-slice systems provide
the user with the necessary higher New Radiopharmaceuticals He indicates that researchers are evaluating F-18 fluoro-L-thymidine (FLT), which looks at cell synthesis. It is believed that FLT could be useful in monitoring response to chemotherapy. Another potential radiotracer is F-18 fluorocholine, which looks at membrane synthesis and appears to work well in prostate cancer, which so far hasn’t been well-served by PET/CT. Development of an appropriate radiotracer would be an important advancement. In research conducted at Duke University, F-18 fluorocholine is outperforming FDG in identifying lesions in prostate cancer patients. If only for its commingling of two especially complementary modalities, PET/CT represents an advance in imaging technology. The combined anatomic-metabolic information moves both modalities significantly beyond prior capabilities. PET/CT should save lives by providing earlier detection of cancers. In addition, it will increase clinicians’ confidence in reading and interpretation and it will reduce the need for certain invasive exploratory procedures. As at least one observer has remarked, the “future of PET/CT is here, now.” Yet, researchers are eager to push its potential, as they look toward possible new applications in the areas of cardiovascular and neurological imaging. Though much of this research is in early stages and some of it remains inconclusive, it is nevertheless producing attention-worthy results. In the meantime, radiology will adjust itself to accommodate this still-evolving hybrid, as technologists and radiologists will be compelled to increase their training to make it available to more patients and researchers will develop new radiopharmaceuticals that will increase its effectiveness. — Dan Harvey is a freelance writer based
in Wilmington, Del., and a frequent contributor to Radiology
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