Radiofrequency labeling strategies in chemical exchange saturation transfer MRI

Abstract

Chemical exchange saturation transfer (CEST) MRI has generated great interest for molecular imaging applications because it can image low‐concentration solute molecules in vivo with enhanced sensitivity. CEST effects are detected indirectly through a reduction in the bulk water signal after repeated perturbation of the solute proton magnetization using one or more radiofrequency (RF) irradiation pulses. The parameters used for these RF pulses—frequency offset, duration, shape, strength, phase, and interpulse spacing—determine molecular specificity and detection sensitivity, thus their judicious selection is critical for successful CEST MRI scans. This review article describes the effects of applying RF pulses on spin systems and compares conventional saturation‐based RF labeling with more recent excitation‐based approaches that provide spectral editing capabilities for selectively detecting molecules of interest and obtaining maximal contrast.

Estimating the synaptic density deficit in Alzheimer’s disease using multi-contrast CEST imaging

“…To investigate molecular specificity and enhance the detection sensitivity of the exchange species of interest at 9.4 T, we initially used numerical simulation based on modified Bloch-McConnell equation for a 9-pool model [ 41 ]. The model contained water pool at 0 ppm, fast exchanging amine solute pool at 3 ppm, symmetric semisolid component pool centered at 0 ppm or asymmetric semisolid component pool centered at -2.3 ppm, guanidinium protons of Cr and PCr at 2.0 ppm and 2.64 ppm, respectively, Amide protons at 3.5 ppm and nuclear overhauser enhancement (NOE) at -3.5 and -1.6 ppm, respectively.…”

“…The model contained water pool at 0 ppm, fast exchanging amine solute pool at 3 ppm, symmetric semisolid component pool centered at 0 ppm or asymmetric semisolid component pool centered at -2.3 ppm, guanidinium protons of Cr and PCr at 2.0 ppm and 2.64 ppm, respectively, Amide protons at 3.5 ppm and nuclear overhauser enhancement (NOE) at -3.5 and -1.6 ppm, respectively. These simulation parameters of the multi-pool model were obtained from the literature [ 41 – 44 ]. For the simulation, the block pulse saturation amplitude was varied from 0.5–8.5 μT and for each experiment, the saturation pulse duration was varied from 500 ms– 6100 ms.…”

“… The Glu-CEST contrast is shown at 3.00 ppm, PCr-CEST contrast at 2.64 ppm and Cr-CEST contrast at 2.0 ppm. The simulation parameters of the multi-pool model were obtained from the literature [ 41 – 44 ]. Each metabolite distribution map was obtained by taking the Z-spectra difference at the frequency of the metabolite of interest with and without the respective pool.…”

Hyperpolarized Xenon-129 Chemical Exchange Saturation Transfer (HyperCEST) Molecular Imaging: Achievements and Future Challenges

“…Chemical exchange saturation transfer (CEST), introduced by Ward et al [76], is a rapidly evolving molecular MRI technique that relies on various reporters and is typically employed to indirectly detect contrast agents following magnetization transfer from selectively excited exchangeable sites [77][78][79][80][81]. The fundamental principles of CEST [82] are based on the saturation transfer from the solute protons to the bulk protons engaged in the chemical exchange process [83].…”