SPECT/CT's Expansion into Radiology
 | |
| Hybrid SPECT/CT scanners are helping to bridge the gap between nuclear medicine and radiologic technologists, as seen here at Bristol Royal. |
Dispelling the myth of ‘unclear medicine’
Over the years, nuclear medicine has been plagued with inherently noisy images that lacked anatomical information, but offer valuable physiological information. Prior to the emergence of hybrid imaging systems like SPECT/CT, radiologists attempted to fuse SPECT and CT images. Due to the time differences between the cross-sectional study and the SPECT study, misregistration issues were significant. With newly integrated SPECT/CT systems, this is no longer the case. The technology now automatically co-registers SPECT and CT images acquired in a single imaging session, within seconds or minutes of each other.
The previous lack of anatomical information could sometimes make interpretation suspect, but the key has been the marriage of nuclear medicine images with CT, something more familiar to radiologists, says Donald Neumann, MD, PhD, department of nuclear medicine, Cleveland Clinic Imaging Institute in Cleveland, Ohio. Now, SPECT/CT is a valuable complement to standard imaging techniques in clinical nuclear radiology. In coupling morphologic images with nuclear medicine images, the radiologic community has been able to determine where things are located anatomically—a comfort zone for radiology—as opposed to just functional data.
Since the introduction of SPECT/CT, there has been a surge of interest from radiology residents in the field of nuclear medicine, just as there is a resurgence of nuclear medicine within the radiologic community in general. The challenge has been to bridge the gap between the nuclear medicine and radiology clinical communities, and bring them closer together, says Jerry Froelich, MD, director, nuclear medicine and molecular imaging at the University of Minnesota, Fairview. “Hybrid SPECT/CT takes the sensitivity of the nuclear medicine procedure and brings it down to an anatomical and more molecular level,” Froelich notes.
Applications proving useful
SPECT/CT is primarily proving to be most useful due to its ability to provide complementary sources of information; abnormalities shown during the radiopharmaceutical uptake in SPECT imaging can be anatomically localized with CT and transversely, CT can show anatomic abnormalities which can draw attention to subtle abnormalities of tracer uptake on SPECT, according to an article in the September 2008 issue of the American Journal of Roentgenology. With such specificity and ability to correlate anatomic and functional information, the clinical applications of SPECT/CT can be useful within nuclear medicine, radiology, cardiology, neurology and oncology—to name a few.
Within cardiology, myocardial perfusion SPECT imaging has emerged as a well-established tool in evaluating patients with known and suspected coronary artery disease (CAD), allowing physicians a means of determining both morphology and physiology in one imaging session. “Previously, if we didn’t have [attenuation] correction, we might have identified a diaphragmatic defect as a real defect, potentially misdiagnosing CAD,” says Sankaran Shrikanthan, MD, nuclear medicine, Cleveland Clinic. “By using SPECT/CT for attenuation correction, we increase the specificity of cardiac exams, ruling out false defects versus true positive defects.” The ability to do a rapid attenuation correction scan has opened the door to study larger patients who in the past have been difficult to evaluate.
Additionally, using SPECT/CT, physicians can add calcium scoring to the diagnostic work up in evaluating a patient’s risk of developing CAD. This capability is something that Joanna Galpin, superintendent radiographer, nuclear medicine, at the Bristol Royal Infirmary in England, anticipates being able to soon capitalize on.
The hospital, part of University Hospitals Bristol NHS Foundation Trust, recently installed the Symbia T16 TruePoint SPECT/CT from Siemens Healthcare. The hybrid system combines a variable angle dual-detector SPECT scanner with a 16-slice CT for rapid, accurate attenuation correction and expanded coverage in CT scans. The new system can carry out calcium scoring for detailed images of the coronary arteries in less than 20 seconds, irrespective of patient heartrate. While they are still performing all cardiac SPECT imaging on a dual-headed SPECT camera, Galpin says they will eventually transition over those exams to the T16, specifically for calcium scoring for coronary patients.
SPECT/CT also is excellent benefits in aiding in oncology diagnosis. “We get a clear picture of where a hot spot is in the body, and where precisely the uptake is of the nuclide,” Galpin adds.
In completing a series of lymphangiograms using SPECT/CT to look for lymphatic drainage, clinicians at Royal Bristol can accurately stage cancers and eliminate the need for total axilliary lymphatic dissection in suspected breast cancers prior to surgery. This will eliminate the occurrence of lymphadema in patients, she notes.
Imaging patients with prostate cancer is another area where SPECT/CT is gaining ground. Prostate cancer has traditionally presented a challenge, since accurate characterization of the disease is crucial for treatment planning and patient management. Non-invasive SPECT imaging using 111In-labeled capromab pendetide can increase the diagnostic accuracy while decreasing the number of false positives. “We cannot do prostate cancer imaging without SPECT/CT anymore,” says Froelich. “Now, we can do SPECT/CT with ultra-thin, 3-mm slices, looking for tiny lymph nodes within the pelvis. I would never want to go back to the old techniques.”
SPECT/CT also is used more frequently in patients with known or suspected sites of inflammation or infection, with the assistance of radiolabeled leukocytes. “Often times, the question is not about whether a patient has an infection but rather, where is the infection and how can we address that site?” says Neumann. “Is it related to an abscess, pneumonia or possibly an infected thrombus in a false lumen in an aortic dissection? SPECT/CT is a valuable tool for diagnosis and patient management.”
Additionally, it can provide enhanced localization and staging of neuroendocrine tumors and neuroblastomas, using 111In octreotide and 123I MIBG for the SPECT and co-registering it with CT.
Imaging parathyroid adenomas is increasing in utilization, too, Froelich says. Typically, patients are injected with Tc99m Sestamibi and then, based upon the pattern of washout (over several hours), surmise the probable location of an adenoma. Now, using an oral 123I, and then an injection of Tc99m Sestamibi, clinicians can do an iodine-sestamibi scan with two energy windows and an immediate CT. The iodine and the sestamibi are subtracted, leaving behind only the parathyroid tumors, which are localized by the CT. “We believe this is going to cut down patient exam time from three hours to less than one hour as well as significantly increase our sensitivity for detection and pre-operative localization of the adenomas,” Froelich says.
Neumann notes that many physicians do not see the distinction between radiology and nuclear medicine, simply because radiology can be characterized as medical imaging. “No matter what modality you are using, it’s all imaging. We are all looking at the same patient, just using different techniques,” he says.