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

Several groups have been reporting on the development of dual modality gamma camera/optical imaging system, a useful tool for understanding biological processes in experimental animals. While the previously reported dual modality imaging instrumentation employed separated gamma camera and optical imager, we designed a new detector using a PSPMT that is capable of imaging both optical photons and gamma rays for combined gamma camera and optical imager. The proposed combined system consists of parallel-hole collimator, array type crystal and position sensitive (PS) PMT. The top surface of collimator and array crystals left open to allow optical photons to reach to the PSPMT. The detector was designed to be operated in a light tight box. Pulse height spectrum and planar images of Tc-99m phantom and mini-Derenzo phantom were obtained to evaluate the gamma imaging performance. Pulse height spectra and planar images of black mask having various hole sizes with green LED were obtained to estimate the optical imaging performance. A mouse phantom containing optical and gamma ray source was imaged to assess the imaging capability of the system. Sensitivity, energy resolution and spatial resolution of the gamma camera image acquired with Tc-99m were 1.1 cps/kBq, 23% and 2.1 mm, respectively. Rod sizes from 4.8 to 2.4 in the mini-Derenzo phantom were clearly resolved. The spatial resolution of the optical image acquired with the LED was 3.5 mm. Each hole location of the mask was accurately delineated. Gamma ray and optical photon images of the mouse phantom were successfully obtained using the single detector. The preliminary results indicated that both optical photon and gamma ray imaging is feasible using a detector based on a PSPMT proposed in this study.