Chemical exchange saturation transfer (CEST) MRI is sensitive to dilute exchangeable protons and local properties such as pH and temperate, yet its susceptibility to field inhomogeneity limits its in vivo applications. Particularly, CEST measurement varies with RF irradiation power, the dependence of which is complex due to concomitant direct RF saturation (RF spillover) effect. Because the volume transmitters provide relatively homogeneous RF field, they have been conventionally used for CEST imaging despite of their elevated specific absorption rate (SAR) and relatively low sensitivity than surface coils. To address this limitation, we developed an efficient B₁ inhomogeneity correction algorithm that enables CEST MRI using surface transceiver coils. This is built on recent work that showed the inverse CEST asymmetry analysis (CESTR_ind) is not susceptible to confounding RF spillover effect. We here postulated that the linear relationship between RF power level and CESTR_ind can be extended for correcting B₁ inhomogeneity induced CEST MRI artifacts. Briefly, we prepared a tissue-like Creatine gel pH phantom and collected multiparametric MRI including relaxation, field map and CEST MRI under multiple RF power levels, using a conventional surface transceiver coil. The raw CEST images showed substantial heterogeneity due to B1 inhomogeneity, with pH contrast to noise ratio (CNR) being 8.8. In comparison, pH MRI CNR of the fieldinhomogeneity corrected CEST MRI was found to be 17.2, substantially higher than that without correction. To summarize, our study validated an efficient field inhomogeneity correction that enables sensitive CEST MRI with surface transceiver, promising for in vivo translation.

Date Published: 17 March 2015
PDF: 6 pages
Proc. SPIE 9417, Medical Imaging 2015: Biomedical Applications in Molecular, Structural, and Functional Imaging, 94171H (17 March 2015); doi: 10.1117/12.2082653

Published in SPIE Proceedings Vol. 9417:
Medical Imaging 2015: Biomedical Applications in Molecular, Structural, and Functional Imaging
Barjor Gimi; Robert C. Molthen, Editor(s)