SPECT/CT: Transforming the Diagnostic Process

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This clinical image was acquired using Siemens Symbia SPECT/CT system and shows a rest myocardial perfusion exam of a 47-year-old female with extensive history of coronary disease (angioplasty and stents), who complained of recurring chest pain. Increased collaboration between radiologists and nuclear medicine specialists provides exquisite diagnostic detail as functional metabolic image data provided by SPECT is fused with high-resolution CT anatomic images produced by the latest generation of hybrid scanners. Attenuation correction was originally the benefit of the CT component to enhance the molecular imaging segment, but with the new scanners offering multislice CT capability, improved resolution offers additional diagnostic confidence as to the exact location of identified metabolic activity.

SPECT imaging is accomplished via a gamma camera that acquires multiple 2D images (called projections) from a number of different points, typically every three to six degrees around the patient who has been injected with a radioactive tracer. Software applies a tomographic algorithm to the projections and creates a 3D image data set. Manipulated data can be displayed as thin slices similar to those that are produced in other tomographic techniques such as CT and MRI.

The time required for each projection varies, but between 15 and 20 seconds is the norm, which results in a 15 to 20 minute SPECT scan time. Multi-head gamma cameras can reduce the time required and provide accelerated acquisition times. One of the benefits of SPECT over PET is that typically the radiopharmaceuticals, such as Technetium, used for SPECT offer a relatively slow decay rate compared with FDG (flurodeoxyglucose) used for PET. As a result, handling SPECT agents is simplified.

By combining SPECT with CT images, a number of different diagnostic studies are enhanced, including tumor volume renderings, pinpointing the site of an infection, myocardial perfusion imaging and in the search for distant metastases in advanced forms of cancer. Oncology and cardiology are the primary benefactors of this technology.

Differentiating cancer from benign disease

J. Anthony Parker, MD, PhD, staff radiologist at Beth Israel Deaconess Medical Center in Boston and associate professor of radiology at Harvard Medical School, explains that their Philips Medical Systems Precedence SPECT/CT system, installed in early 2006, has proven valuable for bone scans. Often they conduct one of these studies to look for cancer metastasis, but benign disease also may be revealed.

Different diseases are situated in specific anatomic locations, explains Parker. While a metastatic lesion might be found on the pedicle of the vertebrae for example, degenerative disease would present in a more anterior position. The CT component of the scanner helps to precisely locate the abnormal finding to determine whether the “hot spot” is a malignancy or something less worrisome. They use Technitium99m MDP (methylene diphosphate) for their bone scans.

SPECT/CT also has proven valuable in whole body scans of patients with thyroid cancer. There are several normal locations where clinicians would anticipate finding radioactive iodine uptake (using Iodine 123 and 131) because either it is a normal place to find thyroid tissue, or it is a usual place where iodine is excreted. Then there are particular locations that would be typical for metastatic disease, such as in lymph nodes or lung nodules.

“The beauty of iodine is that it is specific for thyroid tissue, but what that means is that the scans themselves are dots in space,” Parker relates. Being able to determine precisely where those “dots” are located transforms the diagnostic process.

Precedence, that unites a multidetector diagnostic CT with a gantry-free gamma camera, delivers automatically co-registered SPECT and CT images in a dual image dataset, but can be used for independent CT or SPECT imaging studies as well.

Multipurpose imaging

Sandra Rogers, BS, CNMT, NCT, ARRT (R)(N), supervisor of nuclear medicine at Erlan-ger Health System in Chattanooga, Tenn., de--scribes their use of two GE Healthcare Infinia Hawkeye 4 scanners. On the Erlanger Baroness Campus, one scanner is in the Emergency Department (ED) and the other in their Nuclear Medicine department, and they were both installed in the first quarter of this year.

The Infinia Hawkeye in the ED is primarily used for differential diagnosis for patients who present with chest pain. As an  accredited chest pain center, they are accustomed to performing myocardial perfusion studies on this system, especially for any patient with a body mass index (BMI) of 25 to 30 or greater, according to Rogers. “On the heart scans, we do what we call a