A completed SPECT/MR insert for simultaneous SPECT/MR imaging of small animals

Objectives The goal was to complete the development of a SPECT/MR insert for simultaneous SPECT-MR imaging of small animals.

Methods The development of a 2nd generation SPECT/MR insert for simultaneous SPECT/MR imaging of small animals was completed and fully evaluated. It consists of 5 rings of 19 seamlessly connected 2.54x2.54 cm2, 16x16 pixels cadmium zinc telluride (CZT) detectors. The CZT detectors directly convert incident photons into electric signals and hence are minimally affected by the static magnetic field of the MRI. Two multi-pinhole (MPH) collimators of 18 and 36 PHs were designed with 1.0 and 1.5 mm resolution, respectively. The collimators were made of a cylindrical shell filled with high density tungsten powder and solid tungsten pinhole inserts. A shielded birdcage quadrature transmit/receive RF coil was designed for mouse imaging. It fit inside the MPH collimator to maximize SNR. Accurate system calibration and quantitative sparse-view 3D MPH reconstruction methods were developed for high quality SPECT images.

Results The measured resolutions of the both collimators agreed with their targeted system resolutions and measured detection efficiencies were 172 and 372 cps/MBq, respectively. The predicted Lorenz force effect on the CZT detectors was observed and corrected. With collimator-detector response modeling, the SPECT image resolution exceeded the target system resolution of the MPH collimators. In a simultaneous SPECT/MR phantom study, both the SPECT and the MR images showed minimum image artifacts, and the SPECT images had minimum degradation from the presence of the MRI. By using a linear birdcage coil with long RF shield, the SNR degradation in MR images was reduced to less than 50% compared to that using quadrature coil.

Conclusions We have demonstrated the high-resolution simultaneous SPECT/MR imaging capability of the SPECT/MR insert by using phantom studies. Simultaneous SPECT/MR imaging studies of small animals are underway.

Research Support NIH R01 EB 8730