High-resolution high-sensitivity small animal SPECT using an electron-multiplying CCD and multiple-pinhole collimation

Abstract

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Objectives: Photodetectors based on electron-multiplying CCDs (EMCCDs) offer improved quantum efficiencies (40 to 90%) over a broader range of wavelengths (400 to 900 nm) and higher intrinsic resolutions (< 100 μm) when compared to photomultiplier tube detectors. The electron gain achieved in the multiplication register of an EMCCD effectively reduces the readout noise to less than 1 electron/pixel, making them sensitive to single photon interactions. Our goal is the development and evaluation of a high-resolution, high-sensitivity EMCCD based small animal SPECT device for murine model studies of human disease. High-resolution is achieved using pinhole collimation, while high-sensitivity is achieved by multiplexing 9 pinholes onto a single EMCCD photodetector. The sensitivity is then further increased by surrounding the source with a ring of up to 12 detector assemblies for a maximum of 108 pinholes.

Methods: Due to the high cost of commercially available EMCCD cameras, we have opted to design and build our own. This reduces the cost per camera by an order of magnitude, thereby allowing us to surround the animal with multiple cameras at reasonable cost. Our camera uses a Texas Instruments Impactron EMCCD model TC253SPD-B0 (7.4 μm square pixels). The EMCCD and preamp are cooled under vacuum to -30 °C using a four stage Peltier cooler with air-cooled heat exchanger. Shuttered lens-coupling is used to image the optical light from the scintillation crystal. Precise clocking for the EMCCD is provided by a National Instruments FPGA (PCI-7811R) that was programmed using the LabVIEW FPGA module (version 8.0). A custom built electronics box contains the driver circuitry and video board for digital conversion of the video signal. The 16-bit video board uses correlated double sampling to optimize signal-to-noise performance and has a maximum rate of 200 kHz, which is adequate for light integration studies. The 3x3 multiple-pinhole collimator (0.5 to 1.0 mm diameter) has been designed such that the projected images do not overlap on the crystal surface, thereby reducing the complexity of source reconstruction.

Results: A fully functional prototype has been designed and constructed. Initial characterization tests show the total noise variance to be less than the typical value (< 29 e- rms, at a gain of 1) with a maximum gain of over 1000.

Conclusions: EMCCD based photodetectors with properly designed multiple-pinhole collimators represent a new generation of detectors for use in small animal SPECT. Our promising initial results indicate a high performance yet cost effective EMCCD based SPECT camera can be built. This technology could be easily adapted to clinical applications.

Research Support (if any): DOD BCRP Idea Award Grant No W81-XWH-0410551