We have developed a CEST MRI method that can measure pH using ioversol, a contrast agent that is clinically approved for X-ray imaging and has been repurposed for CEST MRI studies. Using ioversol as a CEST agent, we have measured pH over a range of 6.0 - 7.8 pH units by a novel ratiometric pH MRI method, in a concentration-independent manner. We also have used this agent and CEST MRI method to measure the extracellular pH (pHe) within the liver of healthy SD rats.Extracellular pH (pHe) is a indicator for tumor growth, invasion, and metastatic potential, and also contributes to chemoresistance. Chemical exchange saturation transfer (CEST) is a novel contrast mechanism for magnetic resonance imaging (MRI) that exhibits an intrinsic advantage to monitor pH levels, due to the exchange-rate pH-dependence of labile proton pools. MRI-CEST pH-responsive agents are probes able to map pH in the microenvironment in which they distribute. In this study, we have developed a CEST MRI method that can measure pH using ioversol, an X-ray contrast agent that containing a single set of amide protons, both in vitro and in vivo.

In vitro: Phantoms of ioversol which ranged in pH (6.0 - 7.8), concentration (10 - 60 mM), T1sat (1 - 5 sec), and RF saturation (1.5 - 9ut) were tested. CEST spectra were acquired using EPI sequence (TR, 6 seconds; TE, 4.1 milliseconds; NEX, 2; field of view [FOV], 3 × 3 cm; slice thickness, 3 mm; matrix, 64 × 64). The RF saturation offset was varied from +8 ppm to -8ppm.

In vivo: Male SD rats were 6 to 8 week of age, and their weights were 180g to 220g. Eight mice received the same dose of ioversol of 4g I/kg body weight (BW), slowly injected into the tail vein. We acquired liver CEST images at two RF power levels (1.5 ut and 6.0 μT) before and 5 min after ioversol injection.

CEST spectra of 30mM ioversol solution at pH of 6.0-7.8 showed CEST effects at 4.3ppm (RF saturation power = 1.5-9 μT , T1sat = 5 s, T = 310 K, Bo = 7 T) (Figure 1). A ratio of these CEST effects was correlated with pH (Figure 2c), which was independent of concentration (Figure 2d). Figure 3 shows liver CEST-MRI following ioversol injection and T2WI liver MRIand CEST difference map between pre-/post injection at 1.5 μT and 6 μT. Ioversol distributes well in the extracellular space of the liver allowing the detection of good levels of saturation transfer (ST). (3b) Ratio of RF power mismatch (RPM) pH mapping showed only liver signal displayed in color on grayscale image to highlight effects.These results show that ioversol can be considered as a MRI-CEST contrast agent. In this study, we also proved that a novel ratiometric pH MRI method based on the analysis of CEST effects under different radio applied to ioversol. To our knowledge, this is the first study of ioversol CEST MRI of liver. Further studies are ongoing to use ioversol as a MRI CEST agent for in vivo pH mapping of the tumor region in a hepatoma model, and monitoring tumor acidosis and aggressiveness. Measuring the pHe of a tumor and normal organs may aid in optimizing the treatment for each individual patient, and therefore also support personalized medicine.