Diode Integrates PET-MRI Scanning

Originally Published MPMN November/December 2008

BREAKTHROUGHS

Diode Integrates PET-MRI Scanning

Bob Michaels

Until now, medical imaging applications such as positron emission tomography (PET) have required the use of bulky and pricey photo multiplier tubes (PMTs). But those days may be over. Zecotek Photonics Inc. has developed a diode technology that it says will eliminate the need for PMTs and enhance medical imaging capabilities.

The technology, known as a micropixel avalanche photodiode (MAPD), is a photon-counting device that consists of multiple individual avalanche photodiodes connected in parallel. Capable of detecting very small signals, MAPDs exhibit the extremely high gain typical of PMTs (~106) while requiring a power supply of less than 100 V. “MAPDs’ main advantage, is their very small size and power supply requirements in relation to PMTs’ large size and high-voltage power requirements,” remarks Zecotek CTO Stephen Rowe. In addition, the micropixels’ high density ensures linearity of response over a wide dynamic range of input signals, resulting in accurate counting of few or many incident photons. “The tight timing resolution allows accurate time-of-flight measurements and associated mapping accuracy of the photon event,” Rowe adds.

In conjunction with the University of Washington, Zecotek is developing a PET-MRI scanner based on MAPD technology and lutetium fine silicate (LFS) scintillation crystal material. The central challenge to developing a combined PET and MRI scanning device is MRI’s strong magnetic fields, which destroy PET scanners’ photodetection capabilities. Because the electron paths in vacuum tube–based PMTs are relatively large, they are influenced by magnetic fields. In contrast, since MAPDs are semiconductors consisting of individual small P-N junctions, MRI’s magnetic fields do not influence the carrier paths.

A combined PET-MRI scanning device will offer several advantages, notes Rowe: “MRI provides high-resolution, high-contrast morphological imaging of soft tissues. PET provides the evaluation of molecular and metabolic data but provides no anatomical data. A combined PET-MRI system will allow for simultaneous data acquisition.” The combined technology will improve diagnostic accuracy, produce less radiation exposure than PET-CT systems, increase patient throughput, and allow for earlier malignancy detection.

MAPD preproduction testing was completed in April, and mass production is scheduled to begin in the first quarter of 2009. The initial production run will be an 8 × 8 format with 64 MAPDs, each measuring 3 × 3 mm. The 64-chip MAPD array has been designed for use with LFS scintillation crystals in medical scanning applications. In addition, an 8 × 8 array packaging process is also being optimized as a design reference or a standard for other configurations.

Zecotek Photonics Inc., Vancouver, BC, Canada
www.zecotek.com

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