The manipulation of RF irradiation to a subject in which a CEST agent has been administered is employed to determine a biological parameter in vivo in conjunction with an MRI imaging process. The exchange rate of the CEST agent is determined through RF manipulation which in turn enables the determination of a biological parameter through standard calibration techniques. The exchange rate of CEST agents is modulated as a function of many biological parameters, such as pH. MRI apparatuses and computer program products configured for determining exchanges rates of CEST agents in vivo are also disclosed.