A simple and fast system calibration method for MR-compatible OctoSPECT

Objectives The objective is to develop practical methods for the calibration of detector uniformity and pinhole response of a MR-compatible OctoSPECT system: a SPECT insert designed for use in simultaneous SPECT-MR imaging of small animals.

Methods The OctoSPECT consists of up to 4 rings of eight 2.5x2.5 cm² (16x16 pixels) CZT detectors forming an octagon that fits into 12cm diameter bore of a small animal MR scanner. A collimator sleeve made of heavy metal with about 3.2cm inner diameter has up to 32 pinholes that form projections onto the detectors. The detectors and collimator are insensitive to magnetic fields. A 3cm-diameter cylindrical phantom with uniform distribution of Tc-99m was placed at the axial center of field of view (FOV). List-mode projection data of ~2*10⁴ events per detector pixel were acquired. Energy calibration was applied to the list-mode data using the 140 keV peak of Tc-99m and the X-ray fluorescent peak of the collimator material. Analytical and Monte-Carlo methods were used to simulate OctoSPECT projection data to be divided by the experimental data pixel by pixel to compute the system calibration map which was then applied to subsequent data acquisitions for detector uniformity and pinhole response correction.

Results The integral uniformity in the central FOV of the reconstructed images of a uniform cylindrical phantom is 15% with correction compared to 70% without correction. The reconstruction accuracy was greatly improved for all phantom studies.

Conclusions The proposed method using a uniform cylindrical phantom has shown to be practical and effective in calibration and correction for detector uniformity and pinhole response for the MR-compatible OctoSPECT.

Research Support NIH NIBIB R44 EB 0067112-02