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Publication Date: 2012-Oct-01
Document File: 7 page(s) / 181K

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The invention relates to positron emission tomography (PET) detector systems. Most commercial PET scanners use “block” detectors where complex and expensive photomultiplier tubes (PMTs) are used as light collection systems. This invention proposes novel, low-cost PET detector designs by replacing the expensive PMTs with a combination of solid state photomultipliers (SSPMs) and light concentrator elements. Furthermore, the invention also proposes the use of inexpensive scintillator crystals such as those of bismuth germanate (BGO). The advantages of the proposed designs include high detection efficiency, low noise, high gain, and low timing jitter of photosensors for PET detector applications in a lower cost system.

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The invention relates to positron emission tomography (PET) detector systems. Most commercial PET scanners use “block” detectors, where a block of scintillator crystals are optically coupled so that light from any gamma ray interaction in the block is distributed across the exit face in a controlled manner. The optical coupling of the elements can occur directly in the scintillator block, or it can occur inside a lightguide coupled to scintillator elements. The exit face of the block is coupled to four photomultiplier tubes (PMTs), such as a quad-PMT, as indicated in Fig. 1, or four individual PMTs.

FIG. 1: PMT-based PET block detector and centroid calculation

The crystal in which the gamma ray interacted can be determined by calculating the centroid of the signals of the four photosensors, as demonstrated in Fig. 1. The cost of the PMTs used in block detectors is a significant fraction of the total cost of the detector and is largely independent of their size, so using smaller PMTs would not reduce system costs. This invention discloses lower cost PET detector systems, replacing PMTs which are complex and expensive, with a solid state photomultiplier (SSPM).


The invention discloses novel designs for low-cost PET detector using solid state photomultipliers, wherein:

Figure 1 demonstrates a PMT-based PET block detector and a centroid calculation.

Figure 2 shows solid state photomultiplier (SSPM)-based block-detector to replace the expensive photomultiplier tubes (PMTs) with SSPMs.

Figure 3 presents an SSPM-based block-detector using wavelength-shifting (WLS) fibers.

Figure 4 shows an SSPM-based block-detector using WLS lightguides.

Figure 5 shows an alternative SSPM-based block-detector using WLS lightguides.


The invention presents an alternative to the expensive photomultiplier tubes (PMTs), the solid state photomultiplier (SSPM) which consists of an array of photodiodes that are connected in parallel and operated in Geiger mode, i.e., above their breakdown voltage. Each photodiode, or cell, acts as an independent binary photon counter. When one or more photons strike the SSPM, the number of cells that fire and generate a large packet of charge is proportional to the number of incident photons, when the number of incident photons is less than the number of cells in the SSPM. In order to allow the use of a smaller photosensor area, concentrators comprised of a combination of lightguides, reflectors, and wavelength shifting (WLS) elements are used to couple the scintillator block to the SSPM. The size of the SSPMs and the number of scintillator crystals in the block is determined by the trade-off between system cost and performance. When the size of SSPMs is reduced, the system cost is decreased, but the performance will be compromised as the total light collection efficiency is reduced due to the smaller size of th...