Parallel detection of MRI and 1H MRSI for multi-contrast anatomical and metabolic imaging.

Abstract

Purpose: MRI and MRSI provide unique and complementary information on anatomy, structure, function, and metabolism. The default strategy for a combined MRI and MRSI study is a sequential acquisition of both modalities, leading to long scan times. As MRI and MRSI primarily detect water and metabolites, respectively, the small frequency difference between resonances can be exploited with frequency-selective RF pulses to achieve interleaved or parallel detection of MRI and MRSI, without an increase in total scan time.

Methods: Here, we describe the pulse sequence modifications necessary to allow acquisition of T₁ and T₂-weighted MRI and B₀/B₁ mapping in parallel with MRSI. In general, the MRSI module, including water suppression, can be used unmodified. MRI methods are executed in 3D using 3- to 4-ms frequency-selective Gaussian RF pulses with acceleration along the third dimension through repetitive small-angle nutation or multi-spin-echo acquisitions.

Results: Phantom experiments demonstrated artifact-free 3D MRIs. MRSIs in the absence or presence of MRI elements were identical in sensitivity and spectral resolution (line width) and showed consistent water suppression. Parallel MRI-MRSI was applied to the brains of tumor-bearing rats in vivo. High-contrast, high-sensitivity metabolic MRSI data at 8 μL nominal resolution was acquired in parallel with 3D T₁-weighted, T₂-weighted, and B₀/B₁-weighted MRIs for an overall scan duration of 30 min.

Conclusion: Multi-contrast MRIs and MRSI can be acquired in parallel by utilizing the small frequency difference between water and metabolites. This opens the possibility for shorter overall scans times, or the acquisition of higher-resolution or additional contrast MRIs.