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

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

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.

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.

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.