Magnetization Transfer Prepared Gradient Echo MRI for CEST Imaging

Abstract

Chemical exchange saturation transfer (CEST) is an emerging MRI contrast mechanism that is capable of noninvasively imaging dilute CEST agents and local properties such as pH and temperature, augmenting the routine MRI methods. However, the routine CEST MRI includes a long RF saturation pulse followed by fast image readout, which is associated with high specific absorption rate and limited spatial resolution. In addition, echo planar imaging (EPI)-based fast image readout is prone to image distortion, particularly severe at high field. To address these limitations, we evaluated magnetization transfer (MT) prepared gradient echo (GRE) MRI for CEST imaging. We proved the feasibility using numerical simulations and experiments in vitro and in vivo. Then we optimized the sequence by serially evaluating the effects of the number of saturation steps, MT saturation power (B1), GRE readout flip angle (FA), and repetition time (TR) upon the CEST MRI, and further demonstrated the endogenous amide proton CEST imaging in rats brains (n = 5) that underwent permanent middle cerebral artery occlusion. The CEST images can identify ischemic lesions in the first 3 hours after occlusion. In summary, our study demonstrated that the readily available MT-prepared GRE MRI, if optimized, is CEST-sensitive and remains promising for translational CEST imaging.

Imaging of nuclear Overhauser enhancement at 7 and 3 T

“…Thus, here, an MT‐prepared GRE MRI sequence was used at 3 T. Results showed that, even though each MT pulse only generates a small increase in the CEST effect, it could accumulate short T R values over numerous saturation steps. The conclusions drawn here were consistent with those drawn by Dixon and colleagues . In addition, images based on GRE readout at short echo time showed little distortion of the image, rendering it more suitable for applications at 3 T and higher field strengths.…”

inhomogeneity mitigation in CEST using parallel transmission

“…This improvement is comparable to previously published results (14,35). The previously observed effect of excitation flip-angle on CEST contrasts (34) was not clearly observed in our preliminary study (see Supporting Figs. S1 and S2).…”

“…This improvement is comparable to previously published results . The previously observed effect of excitation flip‐angle on CEST contrasts was not clearly observed in our preliminary study (see http://onlinelibrary.wiley.com/store/10.1002/mrm.26624/asset/supinfo/mrm26624-sup-0001-suppinfo.docx?v=1&s=0deb58a8a75b6624b7c7659677c08d9ee2c2269a). Nonetheless, homogeneous excitations were used in our imaging experiments to ensure that the comparison between the two saturation pulses was free from any potential influence caused by an inhomogeneous excitation pattern.…”

“…Excitation flip‐angle has been shown to influence CEST contrasts in small animal imaging at 9.4T . To minimize the influence of the excitation flip‐angle inhomogeneity on the comparison of the pTxCEST and CP‐mode CEST saturation pulses, two‐spoke pulses were used in all CEST experiments to provide homogenized excitations (see http://onlinelibrary.wiley.com/store/10.1002/mrm.26624/asset/supinfo/mrm26624-sup-0001-suppinfo.docx?v=1&s=0deb58a8a75b6624b7c7659677c08d9ee2c2269a).…”

“…The pulse optimizations were restricted to only brain regions (i.e., VOI) by using FSL- BET (33) brain masks generated from the magnitude images of the B 0 and B þ 1 maps. Excitation flip-angle has been shown to influence CEST contrasts in small animal imaging at 9.4T (34). To minimize the influence of the excitation flip-angle inhomogeneity on the comparison of the pTxCEST and CPmode CEST saturation pulses, two-spoke pulses were used in all CEST experiments to provide homogenized excitations (see Supporting Fig.…”