Development of a dual modality imaging system: a combined gamma camera and optical imager

Several groups have reported the development of dual modality Gamma camera/optical imagers, which are useful tools for investigating biological processes in experimental animals. While previously reported dual modality imaging instrumentation usually employed a separated gamma camera and optical imager, we designed a detector using a position sensitive photomultiplier tube (PSPMT) that is capable of imaging both gamma rays and optical photons for combined gamma camera and optical imager. The proposed system consists of a parallel-hole collimator, an array-type crystal and a PSPMT. The top surface of the collimator and array crystals is left open to allow optical photons to reach the PSPMT. Pulse height spectra and planar images were obtained using a Tc-99m source and a green LED to estimate gamma and optical imaging performances. When both gamma rays and optical photon signals were detected, the signal interferences caused by each other signal were evaluated. A mouse phantom and an ICR mouse containing a gamma ray and optical photon source were imaged to assess the imaging capabilities of the system. The sensitivity, energy resolution and spatial resolution of the gamma image acquired using Tc-99m were 1.1 cps/kBq, 26% and 2.1 mm, respectively. The spatial resolution of the optical image acquired with an LED was 3.5 mm. Signal-to-signal interference due to the optical photon signal in the gamma pulse height spectrum was negligible. However, the pulse height spectrum of the optical photon signal was found to be affected by the gamma signal, and was obtained between signals generated by gamma rays with a correction using a veto gate. Gamma ray and optical photon images of the mouse phantom and ICR mouse were successfully obtained using the single detector. The experimental results indicated that both optical photon and gamma ray imaging are feasible using a detector based on the proposed PSPMT.