Sensitivity-enhanced Chemical Exchange Saturation Transfer (CEST) MRI with Least Squares Optimization of Carr Purcell Meiboom Gill Multi-echo Echo Planar Imaging

Chemical exchange saturation transfer (CEST) imaging is a novel MRI technique that is sensitive to biomolecules, local pH and temperature, and offers considerable advantages for in vivo applications. However, the magnitude of CEST effect for dilute CEST agents undergoing slow or intermediate chemical exchange is typically small, requiring the use of signal averaging to enhance its sensitivity. Given that T2 -induced signal loss can be normalized by asymmetry analysis, the magnitude of CEST effect is independent of echo time. Therefore, CEST MRI with multi-echo echo planar imaging (EPI) readout should yield the same CEST effect as conventional single echo acquisition. Importantly, CEST multi-echo (CESTme) EPI images can be averaged to enhance CEST MRI sensitivity. The goal of this study was to validate CESTme EPI using a creatine-agarose gel CEST phantom with similar T2 as biological tissue. Using least-squares optimization, we found that the sensitivity of CESTme sequence was significantly higher than that obtained by conventional single echo CEST-EPI acquisition. Specifically, signal-to-noise ratio and contrast-to-noise ratio from the proposed CESTme EPI were approximately equivalent to that obtained by doubling the number of signal averages of the standard single echo CEST MRI sequence. In summary, our results demonstrated CESTme EPI for sensitivity-enhanced CEST imaging.