SPECT/CT: Transforming the Diagnostic Process

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 partial scan and only open the window on the CT to the size of the heart so that they [the patients] are not getting unnecessary radiation.”

Besides cardiac imaging, including routine rest/stress tests, they also use their Hawkeye system for OctreoScans for specific tumors such as gastrinomas, neuroblastomas and some colon cancers, because that radiotracer targets a certain type of tumor cell. Once they have located a “hot spot,” they narrow the window on the CT to provide the precise anatomic information that is required.

Additionally, they perform white blood cell studies to look for a cause in a Fever of Unknown Origin (FUO). Rogers explains that first they draw some of the patient’s blood, label it with Indium 111 and then inject it back into the patient. Wherever white cells would accumulate (as they would in a specific infected area), a “hot spot” emerges. Once again, the CT provides precision in locating the target.

Rodolfo Nuñez, MD, assistant professor of nuclear medicine at the University of Texas M.D. Anderson Cancer Center in Houston, is using the Siemens Medical Solutions Symbia T6 True Point SPECT/CT system exclusively for oncology studies. While bone scintigraphy is one of the primary indications, they also do parathyroid scans.

A diagnosis of a parathyroid tumor can be challenging, because they are unable to discern whether a “hot spot” on SPECT is a thyroid nodule or on the parathyroid. With diagnostic quality multidetector CT, they can differentiate one from another. “When they are doing minimally invasive surgery, with a very small incision, the surgeon must know the precise location of a lesion,” explains Nuñez.

They’re also using SPECT/CT to guide a new therapy called Selective Internal Radiation Therapy (SIRT) to treat metastatic liver tumors that are considered inoperable. Here’s how it works: Microspheres (SIR—Spheres by SirTex Medical), approximately 32 microns in size, contain a radioactive element called yttrium-90. They are delivered by an interventional radiologist via a catheter through the hepatic artery to the arterial section of the liver. These microspheres become implanted in the tumor, and remain in place for approximately 14 days, slowly releasing radiation to the tumor.


Establishing a diagnostic tree in cardiology

James R. Corbett, MD, professor of Radiology and Internal Medicine at the University of Michigan Medical School and director of Cardiovascular Nuclear Medicine at the University of Michigan in Ann Arbor, has been utilizing the Siemens Symbia TruePoint T6 systems for myocardial perfusion studies, with a usual patient load of 15 to 25 studies per day, since 2004. He describes their process for assessing heart muscle viability beginning with a rest SPECT perfusion exam. If there isn’t adequate information, they may proceed to a PET scan without repeating the perfusion study. For the SPECT study, they use technetium -99m, sestimibi or tetrofosmin, and for PET they use fluorine-18 FDG.

The benefits of the SPECT/CT hybrid scanner are quite impressive for all patients but especially obese patients, and they have used it for patients well over 400 pounds. “It’s a combination of much improved attenuation correction as well as 3D iterative algorithms that are used to reconstruct the SPECT data,” explains Corbett. “The combination of those factors plus fully integrated scatter correction and resolution recovery makes for clearly superior images.”

In terms of their process, he says they perform the CT segment first, and attempt to capture that information in a single breath hold.

“As an ancillary benefit to this technology, in addition to the high-quality SPECT images and the ability to perform coronary calcium scans that are fundamental to its cardiac applications, there are some extra-cardiac findings that are found on some of the CTs. This is a high-quality CT scanner,” he adds. “We don’t image the entire thoracic volume, but it does cover the axial volume including the heart. It’s not rare to find other things in the lungs, breasts and dome of the liver that are important diagnostic information.” Radiologists read the extra-cardiac images.


SPECT/CT comes into its own

Similar to the scenario when PET/CT first burst onto the scene, as multidetector CT has been enhanced with expanded SPECT capabilities, new clinical applications have been developed to maximize the benefits of combined anatomic/physiologic images to the clinical management of a variety of patient conditions.