Nuclear Instruments and Methods in Physics Research A 569 (2006) 563–566 Preliminary performance of optical PET (OPET) detectors for the detection of visible light photons Nam T. Vu à , Robert W. Silverman, Arion F. Chatziioannou Department of Molecular and Medical Pharmacology, Crump Institute for Molecular Imaging, UCLA School of Medicine, A136, 700 Westwood Plaza, Los Angeles, CA 90095-1770, USA Available online 28 September 2006 Abstract The Optical Positron Emission Tomography (OPET) system will provide the capabilities of bioluminescence and PET modalities in one combined imaging system. Bioluminescence light emitted in vivo has a spectrum ranging from 550–700 nm. The optical photon signals detected are approximately two orders lower in amplitude and one order shorter in duration compared to scintillation light pulses from a 511 keV g-event. In this study the performance characteristics of the OPET detectors for bioluminescence imaging were evaluated. The detection uniformity was measured for wavelengths between 400–700 nm and normalization maps were used to correct the acquired images. The spatial resolution along the transaxial and axial directions was 2.30 and 2.38 mm FWHM, respectively. The average dark counts for the detector was 76730 cps at an ambient temperature of 201. r 2006 Elsevier B.V. All rights reserved. PACS: 87.50.Hj; 85.60.Ha; 87.66.Sq Keywords: Multi-channel photomultiplier tube (MC-PMT); Bioluminescence; Optical imaging 1. Introduction The Optical Positron Emission Tomography (OPET) system will provide the imaging capabilities of both bioluminescence and PET modalities in one combined system using the same detector for both modalities [1]. Previous work on OPET focused on the capabilities of the circuit design to readout and distinguishes between the bioluminescence and PET signals [2]. In PET, gamma ray interactions within the scintillator crystal can produce tens of thousands of light photons, which are then subsequently detected with a photomultiplier tube (PMT). In contrast, bioluminescence events are emitted as single optical photons within a cell. This single photon must then traverse through the tissue and scintillator crystal before being detected by the PMT. Gadolinium Silicate (GSO) scintillator crystals used in this study emits light with a spectrum peaked at 440 nm. An example of a commonly used bioluminescence system, firefly luciferase, emits a spectrum with a peak in the green wavelength region (550 nm) [3]. As the light passes through living tissue, the shorter wavelengths are preferentially attenuated yielding a spectrum exiting the mouse that peaks in the red wavelength region (620 nm). It is therefore important to understand and characterize the response of the OPET detectors for single photon counting with particular emphasis on photons near the red wavelength region. This work seeks to expand upon the optical imaging aspect of the OPET detectors and evaluate various performance parameters for bioluminescence imaging. In this paper we report on the OPET detector uniformity, spatial resolution and dark counts for single photon events. 2. Equipment and signal processing 2.1. Photomultiplier tube and crystal array The detector consists of a Hamamatsu 5900-02-64 multichannel photomultiplier tube (MC-PMT) with a multialkali photocathode sensitive to light wavelengths of 300–900 nm, shown in Fig. 1(a). The MC-PMT has 64 ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.08.138 à Corresponding author. Tel.: +1 310 794 9203; fax: +1 310 206 8975. E-mail address: nvu@mednet.ucla.edu (N.T. Vu).