Multiband Fast Spin Echo on portable low-field systems

Abstract

Purpose: To improve the SNR efficiency of Fast Spin Echo (FSE) using RF-encoded multiband imaging and to develop techniques that correct artifacts from non-Carr-Purcell-Meiboom-Gill (CPMG) magnetization arising from system imperfections.

Methods: Optimal refocusing pulse $0-\pi$ band phase modulations that satisfy the CPMG condition were calculated for different multiband factors. Fast recovery was used to enhance T2 contrast. Quadratic phase refocusing pulses, phase encode and slice select gradient reshaping, and zeroth-order phase calibrations were tested to reduce residual non-CPMG magnetization. Image contrast of T2-weighted and T1-weighted imaging and SNR improvements were evaluated in phantom and in vivo experiments on an in-house portable 110 mT system.

Results: The peak $B_1$ of multiband refocusing pulses using the obtained optimal $0-\pi$ phase modulations varies closely with the expected increase of $\sqrt{N_{bands}}$ . Modulations satisfying the CPMG phase condition provide similar peak $B_1$ reductions to previously proposed subband modulation schemes. Quadratic phase increment reduced shading from residual non-CPMG magnetization, and bipolar phase encoding reduced residual ghosting. Zeroth-order RF phase calibrations on the excitation and tipup, and slice select reshaping that minimized residual phase on 2D maps calculated from even and odd echo images improved contrast with long T2 species. RF-encoded multiband FSE improved SNR by a factor close to the theoretical expected improvement of $\sqrt{N_{bands}}$ .

Conclusion: RF-encoded multiband imaging is a viable approach for improving SNR efficiency on low-field portable systems.

Keywords: Fast Spin Echo; concomitant fields; low‐field portable MRI; multiband RF pulse design; non Carr‐Purcell‐Meiboom‐Gill artifacts.