Abstract— A resolution enhanced NaI(Tl)-scintillator micro-SPECT device using a pinhole collimator geometry has been built and tested with small animals. This device was constructed based on a depth-of-interaction (DOI) measurement using a thick scintilator crystal and a position sensitive PMT to measure depth dependent scintillator light profiles. DOI measurement eliminates the parallax error that degrades the high spatial resolution required for small animal imaging. This novel technique for 3-D gamma-ray detection was incorporated into a micro-SPECT device and tested with a 57 Co source and 98m Tc-MDP in mice. We are now investigating the combination of micro-SPECT with 3-D optical imaging. so that the interaction of different biological aspects can be studied by each technique. A simple application permits a 3-D reconstruction of the animal’s skin surface thus enhancing visualization and making possible depth- dependent corrections and a quantitative description of biological processes can be achieved. The combination of micro-SPECT and 3-D optical imaging, including bioluminescence 3-D reconstruction, opens new avenues in multi-modality imaging for biology and medicine. Index Terms -- Image Reconstruction, Single Photon Emission Computed Tomography, Small Animal Imaging, Optical Imaging. 1 Manuscript received October 25, 2004. This work was supported in part by the Cancer Imaging Program (an NCI Pre-ICMIC) 1P 20 CA 86354 and CRDF/ANSF Bilateral Grant #13551 Edward N. Tsyganov is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, phone: 214-648-3689, fax: 214-648- 7513, e-mail: edward.tsyganov@utsouthwestern.edu . Pietro P. Antich is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, e-mail: peter.antich@utsouthwestern.edu Padmakar V. Kulkarni is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, e-mail: padmakar.kulkarni@utsouthwestern.edu Ralph P. Mason is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, e-mail: ralph.mason@utsouthwestern.edu Robert W. Parkey is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, e-mail: robert.parkey@utsouthwestern.edu Serguei Y. Seliounine is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, e-mail: seruei.seliounine@utsouthwestern.edu Jerry. W. Shay is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, e-mail: jerry.shay@utsouthwestern.edu Todd W. Soesbe is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, e-mail: todd.soesbe@utsouthwestern.edu Alexander I. Zinchenko is with the University of Texas Southwestern Medical Center at Dallas, Texas 75390 USA, e-mail: alexandre.zintchenko@cern.ch I. INTRODUCTION A common problem with SPECT is the need for anatomical co-registration, so that signals may be associated with organs and tissues. One approach is the addition of CT, as now provided on the latest clinical SPECT-CT scanners. Another option is optical imaging and this is the approach adopted here. We have developed a NaI(Tl)-scintillator pinhole- collimated μ-SPECT device featuring a novel technique for 3- D position measurement in thick scintillators [1-3]. The device was built and tested on phantoms and small animals. We integrated the micro-SPECT device with a new 3-D optical imaging system that we have also developed. Figure 1 is the photograph of the set-up. Figure 1. Photograph of the device. Left – digital camera, middle – rotating animal holder, right – pinhole and PSPMT. In micro-SPECT, a small pinhole size provides the sub- millimeter resolution required to study small animals such as mice. However, interaction parallax in the detector could degrade the potential pinhole resolution. To minimize this effect, we have initially used a restricted field of view but are currently developing a detector featuring a depth-of- interaction measurement and an extended field of view. To provide the DOI measurement, a Hamamatsu R2486 Position-Sensitive Photomultiplier Tube (PSPMT) is mounted to the base of the detector crystal. The scintillator crystal is blackened on all sides except the side in contact with the photocathode. This eliminates the light with full internal reflections within the crystal, with the width of the scintillation light profile determining the DOI. Simultaneously with μ-SPECT scanning, the animal is photographed with a digital camera for multiple angular positions. Optical imaging has become a powerful tool in biology and medical research. Cooled charge coupled devices (CCD) permit the acquisition of images at very low intensity levels for a wide range wavelengths. Optical imaging is a fast and convenient method of biological interrogation. However, the strong scattering and absorption effects in tissue and complexity of light transport, resulting in the Edward N. Tsyganov, Member, IEEE, Pietro P. Antich, Member, IEEE, Padmakar V. Kulkarni, Ralph P. Mason, Robert W. Parkey, Serguei Y. Seliuonine, Jerry. W. Shay, Todd W. Soesbe, Alexander I. Zinchenko Micro-SPECT combined with 3-D optical imaging 0-7803-8701-5/04/$20.00 (C) 2004 IEEE