Measuring Extracellular pH Within Tumors Using CEST MRI

pH is one of the key microenvironmental factors in the development of tumors. Tumor cells are often viewed as high lactate and H⁺ producers.¹ Extracellular acidosis represents a threat to cell survival by modifying the intracellular pH (pHi), wherein a 0.1 pHi variation can disrupt multiple biological functions.² Measurements of tumor extracellular pH (pHe) may be useful for diagnosis of clinical tumors as well as for preclinical studies of cancer biology. Efficacy of a weak-base or weak-acid drug may be affected by tumor pHe, and it may also effect normal organs. Therefore, measuring pHe of tumor and normal organs may offer insight in predicting/evaluating treatment effect and facilitate treatment optimization. Chemical Exchange Saturation Transfer (CEST) MRI has been proven by multiple studies to be a practical, non-invasive method to track tumor acidosis.

CEST MRI measures the water signal change after a long, low power RF (radiofrequency) irradiation pulse is applied at a resonance frequency different from water. In CEST imaging, a chemical agent containing protons exchangeable with water is used. When the irradiation pulse is applied at its specific resonance frequency, the detectable magnetization from such a proton is reduced (or “saturated”). Meanwhile, rapid chemical exchange of this proton with a proton in a nearby water molecule occurs, transferring the saturation to the water proton and leading to a reduction in water signal. In an actual experiment, a series of irradiation pulses with different frequency offsets is applied, and a spectrum of normalized water saturation is acquired for analysis. CEST has taken MRI into new territory in which chemical agents with low concentration can be detected. Furthermore, CEST MRI using a variety of contrast agents has been used in clinical and preclinical studies to assist in tumor staging, tumor metabolite imaging, tumor pH imaging, and tumor reporter gene imaging.³

Fig. 1 – Chemical Structure of Iopamidol

The CEST effect of an amide proton is pH dependent because the exchange of an amide proton with water is base catalyzed under physiological conditions. The contrast agent we utilize for CEST pH imaging, iopamidol (Isovue, Bracco Imaging) (Fig.1), a clinically approved CT contrast agent, contains amide groups generating two CEST effects at different irradiation frequencies. Their ratio can be used to measure pH in a way that is independent from agent concentration, endogenous T1 relaxation time, and incomplete saturation.⁴ In an in vivo CEST-based pH measurement, we acquire a collection of CEST images (one of them in Fig. 2, left) with a series of predefined frequency offsets, and a CEST spectrum is generated based on a user-defined ROI. Then, 4.2ppm peak and 5.6ppm peak are extracted mathematically (Fig. 2, center) and their amplitudes are used to derive the pH value based on a formula calibrated previously using a phantom with known acidities. In addition to numerical pH readout for the selected ROI, a pH map (Fig. 2, right) is also generated offering pH values spatially distributed at pixel level in the ROI. This may provide useful information in evaluating heterogeneity in tumor acidosis.  In this example, since the kidneys are in the same view, the pHe values of the kidneys can also be obtained for evaluation.

Fig. 2 – Tumor pH Analysis Based on CEST MRI

We have a high-field strength (7.0T) Bruker Biospec MRI system capable of acquiring high quality images, and an in-house developed program that provides fast and accurate data analysis. We are validating tumor acidosis CEST imaging studies for customers.

Contact us to learn more about tumor pHe imaging and advance your oncology research with this cutting edge solution.