Nuclear medicine physicians would be the first to admit that the resolution of their color maps could be better. Despite the ability of radioisotope tracers to home in on likely areas of malignancy, poor spatial resolution can hinder precise localization of pathology.
The addition of CT can sharpen the picture, aiding tumor localization and surgical planning. Collection of both sets of imaging data in the same exam increases the likelihood of 1:1 mapping and improved diagnostic accuracy. Thus it is not surprising that the advent of hybrid scanners is helping to raise the status of nuclear imaging in oncology, said Prof. Ora Israel, director of nuclear medicine at the Rambam Medical Center in Haifa, Israel.
"Some clinicians used to look down on us and misspell 'nuclear' as 'unclear' medicine," she said. "But we are trying to look for very specific tracers or probes for a particular pathological process. The more specific you are, the less background signal you have. It is not a matter of having an easy life, but of giving a precise answer to what referring physicians are asking."
The combination of FDG-PET and CT data is fast becoming established as a powerful strategy for cancer diagnosis, staging, and follow-up. But there is room for SPECT/CT in oncological imaging as well. Many institutions do not have ready access to FDG, due to its expense and short half-life. SPECT tracers can be cheaper, easier to acquire, and, in certain tumors, more accurate than FDG.
"There are some instances where single photon-emitting radiopharmaceuticals are predictably more sensitive and specific; for instance, when looking at endocrine-related tumors that express somatostatin receptors," said Kirk Frey, Ph.D., chief of nuclear medicine and director of PET at the University of Michigan Medical Center. "The octreotide series of single photon-emitting peptide tracers is far more specific for identifying the tumor phenotype than FDG-PET. If we were looking for a neuroendocrine tumor based on a patient's biochemistry and symptomatology, we would do an octreotide SPECT scan."
Researchers are experimenting with combining SPECT and CT data for a number of oncology applications. Common areas of interest include iodine-131 for thyroid tumors, technetium-99m MIBI for parathyroid imaging, indium-111 octreotide and iodine-131 MIBG for neuroendocrine tumors, In-131 ProstaScint for prostate cancer, and I-123 MIBG for neuroblastoma. Experts agree that adding CT information to these SPECT results helps rule out false positives and aids specificity. Performing the two exams while the patient remains on the table increases the likelihood of good image registration.
"The question comes up, Why not just do a good CT and read it side by side? In the abdomen in particular, it just can't be done," said Dr. Stanley Goldsmith, director of nuclear medicine at New York Presbyterian Hospital/Weill Cornell Medical Center. "There is too much movement. The bowel inflates and deflates, and this can distort the position of neighboring organs like the liver."
The arrival of multislice SPECT/CT will doubtless clarify the value of hybrid imaging in these areas. Researchers are also eager to test the modality's capabilities in more novel applications. Dr. Richard Wahl, director of nuclear medicine/PET at Johns Hopkins University School of Medicine, plans to use the institution's new 16-slice SPECT/CT scanner to continue trials in radioimmunotherapy assessment. Doctors at Johns Hopkins regularly use radiolabeled antibodies to treat patients with lymphoma. Scanning patients post-therapy with SPECT/CT should show the quantity of antibody probe reaching the tumor and the amount taken up by normal tissue. This helps determine the dose of normal radiation the patient should receive.
Prof. Dr. Torsten Kuwert, chair of nuclear medicine at the University of Erlangen in Germany, is lining up a variety of clinical oncology projects for his department's SPECT/CT system. Installation of the multislice hybrid has allowed doctors at Erlangen to try a new protocol that Kuwert calls SPECT-guided CT. Bone scintigraphy is often used to detect possible areas of metastases, but the scans are so nonspecific that further radiological follow-up is always needed when abnormal uptake is present. Using the hybrid scanner, a patient undergoes SPECT imaging alone; if abnormality is detected, the patient remains on the table for a focused CT scan of the suspect region.
"This will greatly enhance the specificity of skeletal scintigraphy, shorten the diagnostic process, and reduce the radiation burden to the patient," Kuwert said.
The theoretical advances of combining SPECT and CT for cancer imaging seem clear, but experts remain divided about the impact of image fusion in practice. The ongoing debate about the value of merging SPECT and CT results in imaging parathyroid adenomas illustrates this division. A study presented at the European Congress of Radiology in March added weight to arguments that fusing Tc-99m MIBI SPECT and CT data can improve sensitivity and specificity for identifying parathyroid adenomas.1 But research from M.D. Anderson Cancer Center in Houston, published the previous month, stated that the addition of CT added little value to SPECT scanning unless the tumor was ectopic.2
Few practitioners consider the matter resolved as yet. Surgeons, in particular, are eager to have a CT anatomic road map when they are considering patients for surgery, said Michael O'Connor, Ph.D., a professor of radiologic physics at the Mayo Clinic in Rochester, MN.
"At Mayo, every patient scheduled for parathyroid surgery has a Tc-99m MIBI and an I-123 scan of the thyroid and parathyroid region, and we pick out the adenomas. If you have a high-quality CT that you can overlay with that, the surgeon can see the trachea, the esophagus, and exactly where the adenoma is located," he said.
Dr. Michael Hartshorne, chief of imaging at the New Mexico VA Health Care System, agrees that surgeons facing tricky oncological resections are likely to welcome SPECT/CT. Sites with lower patient throughput, in particular, will appreciate the benefits of morphing functional and anatomic data.
"Maybe SPECT/CT doesn't help with imaging parathyroid adenomas if you are at an institution where the surgeon performs parathyroid surgery three times a day and is quite familiar with it," he said. "But when you're in Albuquerque and your surgeon performed parathyroid surgery six months ago and would like another shot, he needs all the help available."
The real advantages of combined SPECT/CT for parathyroid imaging may be realized only with high-resolution CT, given the small size of parathyroid adenomas, said Dr. Marius Horger, a radiologist at the Eberhard-Karls University in Tubingen, Germany. The same is likely to be true for deep-seated abdominal lesions.
"We had a project initially, using a basic SPECT/CT system in combination with antibodies to localize gastrointestinal carcinomas, and there you need very good image quality. We performed this on a few patients and then dropped the study because it was clear that the image quality was not good enough," Horger said.
1. Kovacs P, Rapf K, Prommegger C, et al. Accuracy of CT-SPECT image fusion in the pre-operative detection of parathyroid adenomas. Presented at ECR 2005, Vienna:146.
2. Gayed IW, Kim EE, Broussard WF, et al. The value of 99mTc-sestamibi SPECT/CT over conventional SPECT in the evaluation of parathyroid adenomas or hyperplasia. JNM 2005;46(2):248-252.