The laws of physics cannot be broken—but they can be bent. New detectors for SPECT and SPECT/CT systems, along with algorithms that shift the signal-to-noise ratio toward improved resolution, are cutting patient radiation in half during myocardial perfusion studies while actually increasing image quality, when compared with preceding generations of equipment.
This is an extraordinary break from the traditional seesaw battle between image quality and dose, whereby gains in one were achieved only by sacrificing the other.
“Physics principles still hold, the number of counts per second still determines the quality of the image, but the equipment now has improved so much that we can cut the radiation dose and get the same information density as before,” said Dr. James K. O'Donnell, director of the nuclear imaging division at Case Western Reserve University, University Hospitals of Cleveland, and Case Medical Center.
The major vendors of SPECT/CT equipment have taken distinctive routes to achieving this win-win scenario. Siemens through its line of SPECT/CTs with multiple capabilities and price points; GE with high-efficiency detectors built on cadmium-zinc-telluride (CZT) sensors; and Philips in a SPECT/CT built around a flat-panel x-ray detector.
And these efforts do not represent the only technological progress in this area. Advances in cardiac MR, PET/CT, and even CT as a one-stop shop for cardiac assessment may reshape the way myocardial perfusion is assessed.
CT angiography is a good screen for coronary artery disease, since the absence of stenoses in itself rules out the possibility of perfusion defects. A sufficiently advanced PET/CT, generating 64 or more slices per rotation, can adequately identify stenosis in the coronaries while its positron imaging component does at least as well as SPECT in identifying perfusion defects. MR provides very exact data on myocardial perfusion (although the temporal resolution of the modality is not up to assessing the coronaries).
These advances are all the more important given the ongoing struggle to obtain enough technetium to satisfy the annual demand for 10 million myocardial perfusion studies. Supply of the radioisotope was cut drastically in May 2009 when the nuclear reactor that serves as a major provider of the technetium generator molybdenum was shut down for repairs. The shortage worsened earlier this year when another major supplier went on a six-month maintenance hiatus.
The new breed of superefficient hardware and software technologies in nuclear cardiology is helping to make the most of whatever supply can be obtained, as SPECT fights to stave off challengers. Following the most traditional route to SPECT/CT, Siemens combines a high-performance gamma camera (Symbia) with either of two CTs. The primary distinctions among its TruePoint products are the number of gamma detectors (one or two) onboard the Symbia component and number of CT slices per rotation (six or 16).
“We have improved image quality and radiation sensitivity so that we can decrease dose,” said Dr. Ron Petrocelli, chief medical officer for Siemens molecular imaging.
