ADAC Debuts Work at SNMon Attenuation Correction

Technique Clears up Cardiac SPECT Images

Attenuation artifacts caused by human anatomy have long been aproblem for nuclear medicine specialists doing cardiac SPECT.Anatomical structures such as the diaphragm, breasts and anteriorwall of the heart can create artifacts that resemble perfusiondefects, complicating the interpretation of cardiac SPECT images.

Every problem demands a solution, however, and nuclear medicinevendors have been forthcoming. Vendors have discovered that attenuationartifacts can be corrected using an external source of radiationthat permits gamma cameras to simultaneously transmit and emitgamma rays during SPECT scans.

Triple-head vendors Siemens and Picker pioneered simultaneoustransmission/emission attenuation correction techniques at lastyear's Society of Nuclear Medicine meeting (SCAN 8/11/93). Pickerreceived Food and Drug Administration clearance last week forits version of the technique (see story, page 1).

Transmission/emission techniques give triple-head vendors ameans to counter increasing competition from adjustable dual-headgamma cameras in the cardiac SPECT segment. Dual-head manufacturerswere not to be caught flat-footed, however. At this month's SNMmeeting, both ADAC and Toshiba will show their work in developingtransmission/emission attenuation correction for dual-head cameras.

ADAC's technique relies on two external sources of gadoliniummounted on the gantry of Genesys Vertex, the company's variable-angledual-head gamma camera. With Vertex's heads configured in the90º position, the gadolinium source transmits radiation throughthe patient's body and to the detector heads on the other side.The resulting data are used to construct a transmission map thatshows the different tissue densities in the body, according toIan Farmer, ADAC's director of nuclear medicine marketing.

The Vertex computer then uses special algorithms that filterout attenuation artifacts when reconstructing images acquiredthrough conventional SPECT imaging. The resulting image featuresimproved contrast resolution, especially in patients with heavy,dense tissue that can cause serious signal attenuation.

"Today with perfusion imaging, the physician has to readthrough the attenuation artifact. For patients with significantbreast or diaphragmatic attenuation, this requires a lot of expertiseand experience on the part of the physician," Farmer said."With attenuation correction, those artifacts get removed.That improves specificity, reduces false positives and makes thediagnosis more accurate."

ADAC claims its technique is superior to those being used bytriple-head vendors because it allows faster patient throughput.ADAC uses parallel-hole collimators for both emission and transmission.This allows Vertex to acquire 180º worth of data with only90º of rotation, Farmer said.

ADAC also plans to introduce a new workstation at the SNM meeting.Pegasys MD is a low-cost workstation based on ADAC's Pegasys viewingsoftware. It uses Sun Sparc 5 architecture with Unix and XWindowsoperating systems. It is designed for clinicians who wish to reviewnuclear medicine images in their office or viewing room, Farmersaid. ADAC will begin shipping Pegasys MD in July.