title: Respiration gating and Bloch fitting improve pH measurements with acidoCEST MRI in an ovarian orthotopic tumor model.
abstract: We have developed a MRI method that can measure extracellular pH in tumor tissues, known as acidoCEST MRI. This method relies on the detection of Chemical Exchange Saturation Transfer (CEST) of iopamidol, an FDA-approved CT contrast agent that has two CEST signals. A log10ratio of the two CEST signals is linearly correlated with pH, but independent of agent concentration, endogenous T1relaxation time, and B1inhomogeneity. Therefore, detecting both CEST effects of iopamidol duringin vivostudies can be used to accurately measure the extracellular pH in tumor tissues. Pastin vivostudies using acidoCEST MRI have suffered from respiration artifacts in orthotopic and lung tumor models that have corrupted pH measurements. In addition, the non-linear fitting method used to analyze results is unreliable as it is subject to over-fitting especially with noisy CEST spectra. To improve the technique, we have recently developed a respiration gated CEST MRI pulse sequence that has greatly reduced motion artifacts, and we have included both a prescan and post scan to remove endogenous CEST effects. In addition, we fit the results by parameterizing the contrast of the exogenous agent with respect to pH via the Bloch equations modified for chemical exchange, which is less subject to over-fitting than the non-linear method. These advances in the acidoCEST MRI technique and analysis methods have made pH measurements more reliable, especially in areas of the body subject to respiratory motion.
authors.affiliation: Biomedical Engineering, University of Arizona, Tucson, AZ.
authors.forename: Kyle M
authors.initials: KM
authors.lastname: Jones
authors.affiliation: Department of Medical Imaging, University of Arizona, Tucson, AZ; Cancer Biology Program.
authors.forename: Edward A
authors.initials: EA
authors.lastname: Randtke
authors.affiliation: Department of Medical Imaging, University of Arizona, Tucson, AZ; Cancer Biology Program.
authors.forename: Christine M
authors.initials: CM
authors.lastname: Howison
authors.affiliation: Biomedical Engineering, University of Arizona, Tucson, AZ; Department of Medical Imaging, University of Arizona, Tucson, AZ; Cancer Biology Program; University of Arizona Cancer Center, Tucson, AZ.
authors.forename: Mark D
authors.initials: MD
authors.lastname: Pagel
keywords: Bloch fitting; CEST MRI; cancer; pH; respiration gating