pH imaging

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Abstract
This work presents a review of pH measurement methods and applications in cancers. Magnetic resonance spectroscopic imaging and magnetic resonance imaging measurements indicate that pH gradients of up to 1.0 pH unit can exit within 1-cm distance. Although measurement of blood pH can indicate systemic problems, it cannot pinpoint the lesion or quantitatively assess the magnitude of excursion from normal pHe. Hence, there is need to develop pHe measurement methods with high spatiotemporal resolution. The two major approaches being investigated include magnetization transfer (MT) methods and relaxation methods. pH-dependent MT effects can be observed with endogenous signals or exogenously applied chemical exchange saturation transfer (CEST) agents. While endogenous signals have the advantage of being fully noninvasive and relatively straightforward to apply, they lack a full biophysical characterization and dynamic range that might be afforded by future CEST agents. pH-dependent relaxivity also requires the injection or infusion of exogenous contrast reagents. In both MT and relaxographic approaches, the magnitude of the effect, and thus, the ability to quantify pHe, depends on a spatially and temporally varying concentration of the CR. A number of approaches have been proposed to solve this problem and once it is solved, pH imaging methods will be applicable to human clinical pathologies.