Study of compatibility between a 3T MR system and detector modules for a second-generation RF-penetrable TOF-PET insert for simultaneous PET/MRI.

BACKGROUND

Simultaneous positron emission tomography/magnetic resonance imaging (PET/MRI) has shown promise in acquiring complementary multiparametric information of disease. However, designing these hybrid imaging systems is challenging due to the propensity for mutual interference between the PET and MRI subsystems. Currently, there are integrated PET/MRI systems for clinical applications. For neurologic imaging, a brain-dedicated PET insert provides superior spatial resolution and sensitivity compared to body PET scanners.

PURPOSE

Our first-generation prototype brain PET insert ("PETcoil") demonstrated RF-penetrability and MR-compatibility. In the second-generation PETcoil system, all analog silicon photomultiplier (SiPM) signal digitization is moved inside the detectors, which results in substantially better PET detector performance, but presents a greater technical challenge for achieving MR-compatibility. In this paper, we report results from MR-compatibility studies of two fully assembled second-generation PET insert detector modules.

METHODS

We studied the effect of the presence of the two second-generation TOF-PET insert detectors on parameters that affect MR image quality and evaluated TOF-PET detector performance under different MRI pulse sequence conditions.

RESULTS

With the presence of operating PET detectors, no RF noise peaks were induced in the MR images, but the relative average noise level was increased by 15%, which led to a 3.1 to 4.2-dB degradation in MR image signal-to-noise ratio (SNR). The relative homogeneity of MR images degraded by less than 1.5% with the two operating TOF-PET detectors present. The reported results also indicated that ghosting artifacts (percent signal ghosting (PSG) ⩽ 1%) and MR susceptibility artifacts (0.044 ppm) were insignificant. The PET detector data showed a relative change of less than 5% in detector module performance between running outside and within the MR bore under different MRI pulse sequences except for energy resolution in EPI sequence (13% relative difference).

CONCLUSIONS

The PET detector operation did not cause any significant artifacts in MR images and the performance and time-of-flight (TOF) capability of the former were preserved under different tested MR conditions.