Design and optimization of pulsed Chemical Exchange Saturation Transfer MRI using a multiobjective genetic algorithm

Abstract

Pulsed Chemical Exchange Saturation Transfer (CEST) MRI experimental parameters and RF saturation pulse shapes were optimized using a multiobjective genetic algorithm. The optimization was carried out for RF saturation duty cycles of 50% and 90%, and results were compared to continuous wave saturation and Gaussian waveform. In both simulation and phantom experiments, continuous wave saturation performed the best, followed by parameters and shapes optimized by the genetic algorithm and then followed by Gaussian waveform. We have successfully demonstrated that the genetic algorithm is able to optimize pulse CEST parameters and that the results are translatable to clinical scanners.

Simulation, phantom validation, and clinical evaluation of fast pH‐weighted molecular imaging using amine chemical exchange saturation transfer echo planar imaging (CEST‐EPI) in glioma at 3 T

“…A recent study by Xiao et al implemented a Gaussian saturation pulse in the context of creatine CEST imaging; however, this study did not examine the accuracy of modeling between different pulse shapes on physical phantoms. Yoshimaru et al recently performed a study of pulse shapes and their effect on the CEST effect, in which nontraditional pulse shapes were generated using a genetic algorithm . However, this study did not examine B 1 amplitudes above 1.5 μT and did not perform modeling in tissues with shorter relaxation times than water.…”

“…To our knowledge, this study is one of the first to accurately simu- nontraditional pulse shapes were generated using a genetic algorithm. 37 However, this study did not examine B 1 amplitudes above 1.5 μT and did not perform modeling in tissues with shorter relaxation times than water. Furthermore, they did not test the effects of varying the overall pulse train length, which has ramifications for selecting clinical imaging parameters.…”

Clinical applications of chemical exchange saturation transfer (CEST) MRI

“…However, many CEST MRI studies in human brain and torso have been safely performed with long, low‐power saturation pulses, so that the safety of CEST saturation is no longer a contentious issue. The use of shaped pulses, most often Gaussian shapes, adiabatic pulse shapes, and other pulse shapes optimized for CEST imaging, has been shown to produce almost the same saturation as a square‐shaped pulse and with reduced off‐resonance saturation effects, especially for protons with slower chemical exchange rates. In some cases, shorter saturation pulses have been interleaved with the MR acquisition scheme, which lowers the duty cycle of saturation power and can accommodate rapid endogenous T 2 relaxation, but at the expense of generating less saturation …”

Flip-angle based ratiometric approach for pulsed CEST-MRI pH imaging

“…A major drawback of this irradiation scheme is represented by the high specific absorption rate (SAR) that limit the translation of the preclinical procedures to commercial human MRI scanners. Conversely, the pulsed-CEST imaging scheme addresses the hardware and SAR concerns by exploiting repetitive short RF pulses as irradiation scheme [42][43][44][45][46][47][48][49]. This saturation scheme is commonly applied at clinical level for amide proton transfer imaging [50][51][52][53].…”