AcidoCEST-UTE MRI for the Assessment of Extracellular pH of Joint Tissues at 3 T Objectives The goal of this study was to demonstrate feasibility of measuring extracellular pH in cartilage and meniscus using acidoCEST technique with a 3-dimensional ultrashort echo time readout (acidoCEST-UTE) magnetic resonance imaging (MRI). Materials and Methods Magnetization transfer ratio asymmetry, radiofrequency (RF) power mismatch, and relative saturation transfer were evaluated in liquid phantoms for iopromide, iopamidol, and iohexol over a pH range of 6.2 to 7.8, at various agent concentrations, temperatures, and buffer concentrations. Tissue phantoms containing cartilage and meniscus were evaluated with the same considerations for iopamidol and iohexol. Phantoms were imaged with the acidoCEST-UTE MRI sequence at 3 T. Correlation coefficients and coefficients of variations were calculated. Paired Wilcoxon rank-sum tests were used to evaluate for statistically significant differences. Results The RF power mismatch and relative saturation transfer analyses of liquid phantoms showed iopamidol and iohexol to be the most promising agents for this study. Both these agents appeared to be concentration independent and feasible for use with or without buffer and at physiologic temperature over a pH range of 6.2 to 7.8. Ultimately, RF power mismatch fitting of iohexol showed the strongest correlation coefficients between cartilage, meniscus, and fluid. In addition, ratiometric values for iohexol are similar among liquid as well as different tissue types. Conclusions Measuring extracellular pH in cartilage and meniscus using acidoCEST-UTE MRI is feasible. Received for publication January 31, 2019; and accepted for publication, after revision, April 3, 2019. Drs Ya-Jun Ma and Rachel High contributed equally to this work. Conflicts of interest and sources of funding: The authors gratefully acknowledge grant support from the VA Rehabilitation Research and Development Service (Merit Award I01RX002604), VA Clinical Science Research and Development Service (Merit Award I01CX001388), and National Institutes of Health (2R01AR062581, 1R01AR068987, and 1R21AR073496). The authors declare no conflicts of interest. Supplemental digital contents are available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Web site (www.investigativeradiology.com). Correspondence to: Eric Y. Chang, MD, Radiology Service, VA San Diego Healthcare System, 3350 La Jolla Village Drive, MC 114, San Diego, CA 92161. E-mail: ericchangmd@gmail.com. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.