Solid State Photo Multipliers for PET

Positron Emission Tomography (PET) is a functional imaging technique with potential to quantify the rates of biological processes in vivo. The availability of short lived positron-emitting isotopes of carbon, nitrogen, oxygen and especially fluorine allows virtually any compound of biological interest to be labeled in trace amounts and introduced into the body for imaging with PET. The distribution of the tracer is imaged dynamically, allowing the rates of biological processes to be calculated using appropriate mathematical models. PET imaging can provide diagnosis for symptoms of diseases such as Alzheimer’s disease, head trauma, and stroke. PET technology is also playing a prominent and an increasingly visible role in modern cancer research, clinical diagnosis and oncology.

Modern PET camera designs use inorganic scintillator blocks to stop the incoming gamma-rays, coupled to an array of photomultiplier tubes (PMTs) to detect the position of the scintillated light. The overall performance, size, weight, and cost of these clinical PET cameras are strongly influenced by the photomultiplier tubes used. In particular, the relatively low quantum efficiency, high cost, inability to function in high magnetic fields, and bulkiness of PMTs are limiting factors in clinical as well as small animal PET cameras. To overcome the limitations of PMTs in PET imaging, we are exploring an advanced photodetector technology, silicon photomultiplier SSPM for clinical as well as small animal PET applications.

SSPM
RMD has extensive experience in the design and fabrication of Geiger photodiode (GPD) devices. We have developed a complementary metal-oxide-semiconductor (CMOS) compatible design capable of supporting large arrays of Geiger Photodiode (GPD) pixels for fabricating analogous SiPMs, which we refer to as solid-state photomultipliers (SSPMs). SSPMs retain all major advantages of the devices fabricated using MRS structures such as high gain, fast response and low noise. CMOS processing also enables monolithic integration of SSPM devices with other readout elements. This advantage further reduces the cost and simplifies the detector system. In addition, the ubiquitous nature of CMOS technology reduces the cost of fabricating the detector elements in a production environment.

SSPMs for PET
SSPMs offer very high gain and an excellent signal-to-noise ratio. Very high timing resolution, expected from these devices, would enable excellent rejection of randoms and also offer time-of-flight (TOF) capability, which would provide further improvement in the signal-to-noise ratio and image quality. Good energy resolution and scatter rejection should be possible with these devices. Also, because the SSPMs are compact devices, they can be placed on opposite ends of the scintillation crystals to determine the depth-of-interaction (DOI) of 511 keV gamma-rays in the scintillation crystals, as demonstrated in the Phase-I effort. Such DOI information would reduce the parallax error in the resulting PET scanners. Finally, low cost, low bias and modest power requirements, relatively simple electronic read out needs and insensitivity to magnetic fields are all additional promising aspects of SSPMs in PET applications. Hence, the goal of the proposed effort is to perform rigorous investigation of SSPMs (fabricated using CMOS process) for PET.

Active area 1.5mm x .5mm
Total Number of micro-pixels 1156
Micro-pixel area 30µM x 30µM
Micro-pixel pitch 44µM x 44µM
Geometrical Fill Factor 46%
Quench Resistors Ω 144k
PDE @ 500 nm 15%
Dark Count Rate MHz 15(Vx =2V)
Capacitance pF 100
Operating voltage 27.5V to 32V

Package size 12mm x 12mm custom PGA
Active Area 10mm x 10mm
Format 2 x 2 (each 5×5 mm2)
Number of pixels 12769 per quadrant
Pixel size 30µM x 30µM
Fill Factor, FF 46%
Breakdown voltage 27.2 +/- 0.2V
Operating voltage 27.5V to 32.5V
PDE (2 Vx) 15% at 500 nm
Temperature Sensitivity 50 mV /℃
Gain ~106 (Vx = 2V)
Dark Count Rate ~150 MHz (Vx = 2V)