ADAC pushes high-energy envelope with attenuation correction for MCD

June 25, 1997

MCD/AC may support cardiology applicationsADAC Laboratories isn't slowing down its effort to enhance the clinical utility of its Molecular Coincidence Detection (MCD) technology, which enables nuclear medicine physicians to image PET radioisotopes

MCD/AC may support cardiology applications

ADAC Laboratories isn't slowing down its effort to enhance the clinical utility of its Molecular Coincidence Detection (MCD) technology, which enables nuclear medicine physicians to image PET radioisotopes like fluorodeoxyglucose (FDG) with a standard gamma camera. At this month's Society of Nuclear Medicine meeting, the Milpitas, CA, company announced its work on adding an attenuation-correction capability to MCD, thus making MCD's image quality closer to that of PET.

ADAC was the first nuclear medicine vendor to begin marketing a coincidence-detection technique when shipments of MCD started last year. Interest in coincidence detection-both from ADAC and other vendors-is widely credited with having sparked a resurgence in gamma camera purchasing this year. At the SNM show, ADAC reported that it has installed 40 MCD upgrades and has orders for 40 more.

One of the problems with coincidence detection, however, is that it is affected by several types of artifacts that inhibit image quality, one of which is photon attenuation. Coincidence detection works by collecting photons produced at 180 angles from a scintillation event in the body. Some of these photons pass relatively unobstructed through the body, while others have to travel through layers of tissue, bone, and other obstructions, creating artifacts that appear similar to anatomical defects on images.

Attenuation correction is already used in PET, and ADAC decided to apply it to MCD as well, according to Ian Farmer, senior vice president and general manager of ADAC's nuclear medicine business. MCD/AC uses an externally mounted source of cesium-137, with an energy level of 662 keV, to help collect the data needed to compensate for attenuation. The principle is roughly similar to ADAC's Vantage attenuation-correction technique for SPECT, although it uses a different radioactive source.

While an MCD study is in progress, the cesium-137 source is transmitting radioactivity through the patient, which is collected by the camera's opposing detector head. These data give the camera an anatomical map of tissue densities in the body, and, when merged in an iterative reconstruction algorithm with emission data from the FDG radioisotope, gives clinicians a corrected image.

ADAC's work on the technique has been conducted in cooperation with Methodist Medical Center in Peoria, IL. In one image displayed at the SNM meeting, an image of a patient with an apparent heart defect was revealed to be normal after the application of attenuation correction. In another example, a lung tumor image was far clearer after MCD/AC was applied.

"What begins to become apparent is that deep structures within the body, such as the diaphragm, start to become clearly visible, and the anatomical body outline is now clear," Farmer said. "These are important aspects of positron imaging that allow the physician to locate the specific area for the particular tumor and to clearly visualize its size. This will allow us to see results that up to this point have only been available with PET systems and attenuation correction."

Coincidence detection has been limited to oncology studies, due to the impact of diaphragmatic attenuation, but ADAC believes that MCD/AC could enable the use of coincidence techniques for cardiology as well. It will also improve the visualization of organs and structures deeper in the body, such as in the abdomen and mediastinal areas.

ADAC has applied for 510(k) clearance for MCD/AC, Farmer said. Like MCD, it will be available on ADAC gamma cameras fitted with the company's Epic detectors, which have count rates up to 2.4 million counts per second, high enough to handle both the emission data from the FDG and the transmission data from the cesium-137 source. The company has not yet set a price of upgrading to MCD/AC, which will be available on both new and installed cameras.

The release of MCD/AC will complete ADAC's artifact-correction package for coincidence detection, Farmer said. MCD/AC includes provisions for scatter correction, another common artifact. Geometric or resolution recovery correction, which is used for SPECT imaging, is not needed for coincidence detection because the technology does not use collimators, which cause geometric artifacts.

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