The combination of single-photon emission computed tomography (SPECT) with computed tomography (CT) into a single modality has provided interpreting clinicians with greater diagnostic certainty, expanded their capability to localize disease, reduced patient exam time, and delivered images and reports to referring physicians that better assist in their delivery of quality care.
The hybrid technology, which was introduced commercially by GE Healthcare in 1999, has seen steady deployment as Siemens Medical Solutions and Philips Healthcare have also launched SPECT/CT systems during the past decade. Peer-reviewed research published during this time has demonstrated improved sensitivity (superior disease localization) and specificity (exclusion of false-positives due to physiological tracer uptake) for a variety of clinical indications such as oncology, cardiac, and infection/inflammation imaging, which has also bolstered demand for the modality in clinical practice.
“The inherent physical perturbations of radionuclide imaging due to photon attenuation, scattered radiation, and partial volume effects result in relatively poor spatial resolution, and the absence of a detailed anatomical context for image interpretation can limit the overall usefulness of functional imaging,” observed the authors of SPECT/CT research published in the journal Clinical Radiology (March 2008). “Information provided by structural and functional imaging in tandem is, however, complementary.”
Although co-registration of SPECT and CT images taken on the respective modalities in different exams is possible using side-by-side comparison or through the use of software tools, artifacts can be introduced due to patient motion or other events that cause image misregistration.
“Hybrid imaging with sequential acquisition of structural (CT) and functional data in the same imaging session without alteration in patient position overcomes many of the shortcomings and enables more accurate co-registration,” according to the researchers.
One of the oncologic indications where SPECT/CT has proved its imaging primacy is in the visualization of endocrine neoplasms.
“Hybrid SPECT/CT combines accurate anatomic localization and functional characterization of endocrine neoplasms in one examination and is a major advance in the management of selected patients with endocrine malignancy,” reported the authors of a recent study in the American Journal of Roentgenology (March 2008). “Within endocrine imaging, SPECT/CT improves diagnostic accuracy by aiding localization, defining functional significance, and excluding sites of physiologic uptake.”
This is particularly true for poorly differentiated thyroid tumors, which are much more aggressive and are associated with a poorer prognosis. Because these tumors are unable to concentrate iodine, their detection with radioiodine scintigraphy is difficult, making patient management more challenging.
FDG-PET imaging has shown some success with this indication; however cost and availability of the technology has limited its usefulness. SPECT/CT, which has been approved for a wider combination of radiopharmaceuticals, such as 111-Inpentreotide, 201-Tl chloride, and 99mTc MIBI, has demonstrated that it can improve the detection and localization of these poorly differentiated tumors, according to the authors.
In a study of 71 patients with thyroid cancer undergoing SPECT/CT (European Journal of Nuclear Medicine and Molecular Imaging, October 2004), researchers demonstrated that SPECT/CT increased diagnostic accuracy in 57 percent of patients by identifying sites of additional disease, correctly characterizing physiological tracer uptake, precisely locating areas of pathological uptake to the skeleton, and differentiating between uptake in remnant tissue in the thyroid bed from activity within local nodal disease.
Hemalatha Rao, MD, who uses a GE Hawkeye 4-slice SPECT/CT system in her practice as chair of nuclear medicine at Coney Island Hospital, a multi-site community medical center serving southern Brooklyn, has found the modality extremely useful for endocrine neoplasms.
“The utilization of SPECT/CT has improved our diagnostic capabilities in patients presenting with thyroid cancer,” she says. “It is particularly useful for patients who come in after surgery, where the anatomy can be distorted. Before, we were able to determine that something was in the neck; but now we can tell specifically if it is in the thyroid gland, the lymph node, the muscle, or the bone.”
Her experience correlates with research conducted at Johns Hopkins University in Baltimore and published in the Journal of Nuclear Medicine (July 2007). Clinicians at the institution compared SPECT/CT, SPECT, and planar imaging to determine whether dual-phase imaging is advantageous for parathyroid exams.
“Early SPECT/CT in combination with any delayed imaging method was statistically significantly superior to any single- or dual-phase planar or SPECT study for parathyroid adenoma localization,” they report. “Localization with dual-phase acquisition was more accurate than with single-phase Tc99m-sestamibi scintigraphy for planar imaging, SPECT, and SPECT/CT.”
The technology has also benefitted patients and the facility by reducing the amount of time needed to conduct studies; thereby increasing throughput and enabling more patients to receive exams.
“Depending on the indication, exams could take upward of four hours,” she notes. “However, with SPECT/CT we’re able to conduct studies in approximately 20 minutes.”
Another oncologic indication which is seeing currency with SPECT/CT is prostate cancer imaging. Radioimmunoscintigraphy using capromab penedetide, marketed as ProstaScint by its developer Cytogen, is a relatively new technique that has demonstrated improved detection of lymph node involvement in patients at high risk of extra-prostatic disease.
ProstaScint is a murine monoclonal antibody conjugated to a linker-chelator that is directed against the cytoplasmic epitope of the glycoprotein prostate specific membrane antigen, which is highly expressed by patients presenting with prostate adenocarcinoma.
ProstaScint imaging with SPECT/CT is facilitated by labeling it with Indium-111.
A study reported in the journal Clinical Prostate Cancer (May 2005) demonstrated the incremental value of fused ProstaScint SPECT/CT in prostate cancer and suggested that the technique may have a valuable role in identifying primary, metastatic and recurrent disease, in guiding therapy and in the monitoring of treatment response.
David Davidson, a certified nuclear medicine technologist and clinical coordinator of the nuclear medicine department at the Medical University of South Carolina (MUSC) in Charleston, S.C., says that his facility is using its SPECT/CT technology (a GE Hawkeye and three Siemens Symbia TruePoint systems) for ProstaScint imaging.
A study conducted at MUSC and Walter Reed Army Medical Center in Washington, D.C., published in Reviews in Urology (August 2006, supplement), stated that the co-registration of fused CT and SPECT in ProstaScint imaging was of great benefit.
“The much clearer fused scans have been correlated with step-sectioned evaluation of prostate specimens to demonstrate an 80 percent overall accuracy, with sensitivity of 79 percent, specificity of 80 percent, positive predictive value of 68 percent, and negative predictive value of 88 percent for detection of cancer within the prostate gland,” the authors report.
The preliminary study was used to guide focal brachytherapy for intermediate- to high-risk prostate cancer patients, for whom the treatment field was altered on the basis of increased signal in areas of the prostate on the image.
“The 7-year follow-up data for 239 patients demonstrated superior results across all risk categories compared with the 5-year meta-analysis of brachytherapy patients (96 percent vs. 87 percent for low risk, 87 percent vs. 74 percent for intermediate risk, and 73 percent vs. 50 percent for high risk),” the authors wrote. “Furthermore, patients with positive scan results outside the pelvis showed a 9-fold increase in biochemical disease recurrence, regardless of risk category.”
The MUSC work demonstrated the utility of fused SPECT/CT images in managing prostate cancer patients, according to the researchers.
“These data suggest that the scan results can be used both to predict better outcomes on the basis of the absence of distant signal intensity and to direct increased dosimetry to focal areas of increased uptake within the prostate gland,” they wrote. “These results have stimulated the use of focal image guidance to deliver intensity-modulated external beam radiotherapy (EBRT), with encouraging results. On the basis of these findings, focal image-guided cryotherapy for localized prostate cancer is now underway at several sites.”
Obesity impacts cardiac, inflammation utilization
Diabetes afflicts approximately 21 million people, or 7 percent of the population in the United States, according to the Centers for Disease Control (CDC). According to Linda S. Geiss, the chief of the CDC’s diabetes surveillance, diabetes (concurrently with obesity) has grown in the U.S. for the past 15 years and shows no sign of slowing down; the CDC estimates that the diabetes epidemic is growing by 5 percent annually.
Rao has seen first hand the impact of the obesity and diabetes epidemic at Coney Island Hospital. The majority of the work performed on SPECT/CT by her department is conducted on cardiac patients as well as infection/inflammation imaging for diabetes patients.
“A significant number of heavy patients present for cardiac disease SPECT/CT exams,” she says. “With our cardiac exams we are able to avoid a lot of unnecessary cardiac catheterization procedures on these patients.”
In addition, the facility is seeing an increase in the number of bone scans it is conducting with the technology.
“With the bone scans it is very crucial to use SPECT/CT,” she notes. “It used to be that people with diabetic feet would have a gallium scan and it would light up everywhere. We couldn’t rule out osteomyelitis, so they would have to have an MRI and CT. They would go through a very difficult diagnostic route, and in many cases, there still would not be a definitive answer. With SPECT/CT, we’re able to provide a very specific diagnostic answer to our referring clinicians.”
Davidson’s institution in South Carolina has also experienced a significant increase in cardiac presentations for molecular imaging. So much so that MUSC has dedicated its recently acquired Symbia TruePoint SPECT/CT systems (one T2 and two T6 models) for deployment at a newly constructed cardiology and digestive diseases hospital.
“We’re performing a lot of myocardial perfusion imaging using Cardiolite [Lantheus Medical Imaging] technetium Tc99m sestamibi,” he said. “Now that we’ve opened up our new facility, we’re really concentrating on our SPECT/CT cardiac imaging.”
He reports that clinicians at the facility have found tremendous benefit to the attenuation correction available with the CT portion of the hybrid technology.
“There’s different ways of performing attenuation correction for cardiac studies,” he says. “One is with a sealed source, which is not quite as good as doing it with a CT. The CT portion of the SPEC/CT has better imaging characteristics and is not as limiting as the sealed-source method. In addition, sealed sources have to be replaced every 12 to 18 months due to their half-life, which poses an additional problem. It also just doesn’t have the strength of the CT for the morbidly obese patient. We feel that the CT method for attenuation correction is far superior to what we’ve used in the past.”
As word about the capabilities of SPECT/CT has spread among the clinical specialties representing the continuum of care—oncologists, cardiologists, orthopedics, and surgeons—served by Coney Island Hospital and MUSC, both Rao and Davidson noted that their referring physician base has specified SPECT/CT exams with greater frequency.
“Once they saw the reports and images, they’re all demanding that we perform SPECT/CT studies for them now,” Rao says.