Time-of-flight PET/CT and PET/MR are adding sizzle to the PET equipment scene, with the driving motivation being fast, faster, and fastest data acquisition.
Faster crystal rise time is the key to 10-minute whole-body imaging made possible with time-of-flight PET/CT, a concept introduced with the Philips Gemini TF scanner at the European Congress of Radiology in February. The device harnesses new crystal material, electronics, and advanced reconstruction algorithms to deliver time-of-flight performance. In place of the gadolinium oxyorthosilicate (GSO) crystal built into the Gemini GXL detector, Philips has put a hybridized crystal constructed from lutetium and yttrium oxyorthosilicate, which the company refers to as LYSO.
For years, vendors have traded barbs over whose detector crystal provided the best response and, consequently, which supported the fastest data acquisition. Siemens' lutetium orthosilicate (LSO) and Philips' GSO each rapidly return to a resting state after emitting a flash of light in response to being struck by a high-energy photon, making them ready to emit another flash in about 60 nanoseconds.
Returning to the resting state in a time-of-flight PET/CT, however, is less important than the time needed to generate a flash of light in response to being hit by a gamma ray, the so-called rise time of the crystal. LYSO has a rise time on the order of 650 picoseconds or 650 trillionths of a second.
The electronics involved must be extraordinarily fast. Those built into the Gemini TF sample the detector once every 25 picoseconds, compared with the previous industry best of 500 picoseconds, according to Jim Cavanaugh, director of global marketing for Philips Nuclear Medicine PET/CT.
Hybrid PET/MR is making the transition from concept to reality. The concept was introduced at the Society of Molecular Imaging meeting in September 2005. The reality, or at least a prototype that will lead to it, was discussed at the International Society for Magnetic Resonance in Medicine meeting in May.
Siemens Medical Solutions announced that a clinical prototype will be installed at Massachusetts General Hospital by year's end. The head-only 3T scanner will feature a PET insert equipped with MR-compatible 3D-avalanche photodiode detectors. Advantages include elimination of registration and postprocessing issues encountered when data are fused from separately performed MR and PET studies, according to Jeffrey M. Bundy, Ph.D., director of MR research and development for Siemens. MR will also be used for PET attenuation correction, much like CT is employed in the current breed of PET/CT hybrids. Radiologists may also want to explore combining functional MRI with PET.
