MR-compatibility of a high-resolution small animal PET insert operating inside a 7 T MRI

Area of Science:

• Medical Imaging
• Biophysics
• Instrumentation

Background:

• Hybrid PET/MRI systems offer superior anatomical and functional imaging capabilities.
• Integrating positron emission tomography (PET) detectors into magnetic resonance imaging (MRI) scanners presents significant technical challenges, particularly concerning radiofrequency (RF) interference and magnetic field compatibility.

Purpose of the Study:

• To design, construct, and evaluate an MR-compatible full-ring PET insert for a 7 Tesla MRI system.
• To assess the impact of the PET insert on MRI performance and vice versa.
• To validate the performance of the PET detector modules within the MRI environment.

Main Methods:

• A 16-module PET insert with silicon photomultiplier (SiPM) arrays and LYSO crystals was designed, incorporating RF shielding with carbon fibre and copper foil.
• MR-compatibility was assessed through B0 and B1 field mapping and imaging with various pulse sequences (SE, RARE, FLASH, EPI).
• PET detector performance (flood histograms, energy resolution) was evaluated during simultaneous MRI operation, alongside baseline shift analysis.

Main Results:

• The PET insert caused only minor B0 field homogeneity degradation (+0.1 ppm) and no significant changes in B1 fields or general image homogeneity.
• A 9% signal-to-noise ratio (SNR) decrease was observed exclusively in echo planar imaging (EPI) sequences.
• PET detector performance remained unchanged, with only a small, inconsequential baseline shift observed in signals due to MRI gradient switching.

Conclusions:

• The designed PET insert is highly MR-compatible with a 7T MRI system.
• Effective RF shielding and compact electronics minimize cross-interference, enabling excellent performance for both PET and MRI subsystems.
• This integrated hybrid PET/MRI system shows great promise for advanced multimodal imaging applications.