SPECT/CT Proving its Potential

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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.

Oncologic clarity

“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