The proton resonance enhancement for CEST imaging and shift exchange (PRECISE) family of RF pulse shapes for CEST MRI

Abstract

Purpose: To optimize a 100 ms pulse for producing CEST MRI contrast and evaluate in mice.

Methods: A gradient ascent algorithm was employed to generate a family of 100 point, 100 ms pulses for use in CEST pulse trains (proton resonance enhancement for CEST imaging and shift exchange). Gradient ascent optimizations were performed for exchange rates = 500, 1500, 2500, 3500, and 4500 s^-1; and labile proton offsets (Δω) = 9.6, 7.8, 4.2, and 2.0 ppm. Seven proton resonance enhancement for CEST imaging and shift exchange pulse shapes were tested on an 11.7 T scanner using a phantom containing three representative CEST agents with peak saturation B_1,peak = 4 μT. The pulse producing the most contrast in phantoms was then evaluated for CEST MRI pH mapping of the kidneys in healthy mice after iopamidol administration.

Results: The most promising pulse in terms of contrast performance across all three phantoms was the 9.6 ppm, 2500 s^-1 optimized pulse with ˜2.7 × increase in asymmetric magnetization transfer ratio (MTR_asym) over Gaussian, and ˜ 1.3 times over Fermi pulses for the same B_1,peak = 4 μT. This pulse also displayed a large improvement in contrast over the Gaussian pulse after administration of iopamidol in live mice.

Conclusion: A new 100-ms pulse was developed based on gradient ascent optimizations, which produced better contrast compared to standard Gaussian and Fermi pulses in phantoms. This shape also showed a substantial improvement for CEST MRI pH mapping in live mice over the Gaussian shape and appears promising for a wide range of CEST applications.

Keywords: CEST; MRI contrast agent; gradient ascent pulse design; iopamidol.