Originally Published MPMN November/December 2009
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Silicon Photomultipliers’ Sharper Image Could Enhance MRI/PET Applications
Photomultipliers’ crosstalk-suppression methods lead to high linearity and fast response times, which could enable them to replace avalanche photodiodes in combined MRI/PET applications.
Modern medicine is continually struggling to improve imaging technologies that can generate sharp images of organs while providing information on metabolic activity at the same time. This goal is being achieved by combining magnetic resonance imaging (MRI) and positron-emission tomography (PET) applications into a single platform. But the key to improving combined imaging techniques is the development of detectors that can work with both MRI and PET while providing high sensitivity, rapid response time, and low power consumption. Rising to meet this challenge, PerkinElmer Inc. (Waltham, MA; www.perkinelmer.com) is working to market silicon photomultipliers (SiPMs), special crosstalk-suppression technology originally developed by a team of astronomers at the Max Planck Institute of Physics (Munich, Germany; www.mpp.mpg.de).
In PET applications, photomultiplier tubes (PMTs) are the standard detection technology. However, PMTs are unsuitable for magnetic imaging applications because they are susceptible to magnetic fields. In addition, they are based on fragile vacuum tubes, which can only be assembled manually, have high voltage bias requirements, and cannot be exposed to ambient light.
Until now, the main alternative to PMTs for combined PET/MRI applications has been avalanche photodiodes (APDs). Compact solid-state detectors, APDs can operate in a classical linear mode with a relatively high noise level, or in the Geiger mode as a high-speed photon counter over a modest photon-count range, explains Jürgen Schilz, product-line leader, photon detection solutions at PerkinElmer. “SiPMs address this issue by using a single chip containing several hundred to thousands of micro-APD cells coupled to a common signal-output terminal. Each micro-APD is operated in the Geiger mode, whereupon an arriving photon can trigger the cell, leaving the surrounding cells untriggered and ready to collect other arriving photons.”
In contrast to APDs, PerkinElmer’s crosstalk-suppression methods lead to higher linearity and faster response times, according to Schilz. “APDs are indeed on the way into mass production for PET imaging, but the use of SiPMs will improve system performance even more,” he adds. “SiPMs as solid-state detectors are a further step toward high-definition combined MRI/PET imaging.”
The new SiPM design, according to Schilz, is innovative because it reduces crosstalk to a record low level. The component also offers high detection efficiency and an excess noise factor very close to one, yet it can resolve individual photons even while measuring a flux close to hundreds of photons. “Such a design shows great promise as an almost ideal sensor for a variety of crucial low-light-level applications,” Schilz remarks.
For molecular imaging techniques, a major advantage of SiPMs is their immunity to magnetic fields, enabling the concurrent deployment of MRI and PET in a single system. “The detectors’ reduced crosstalk capability is especially advantageous,” Schilz adds, “since the scintillators that are coupled to SiPMs effectively act as a mirror for the silicon-generated secondary photons that normally boost crosstalk.”
Of all its likely applications, possibly the main one driving SiPM technology into higher volumes and to higher specification levels is molecular imaging, Schilz says. “Beside an anticipated cost advantage of SiPM over PMTs, the governing factors are their two to three times higher photon detection efficiency, better time and amplitude resolution, compact size, and overall reduced system complexity.”
Still in the developmental stage, SiPMs will not see commercialization for a few years. “We anticipate SiPMs being used in combined MRI/PET imaging equipment in three years or so,” Schilz concludes. “In the meantime, avalanche photodiodes will do the job.”
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