Synopsis

Iopamidol has been previously used as a CEST based contrast agent to measure pH in mouse kidneys. Here we aim to move this work towards clinical translation by calibrating the pH of ex vivo human blood to the ratiometric values calculated from the two Iopamidol CEST peaks. This is performed in the physiological range for human blood, between pH 6.8 and 8.4.

Purpose

This work aims to develop the use of Iopamidol as a readily available, clinically translatable, CEST contrast agent to determine the pH in vivo in a physiological range. Here we study its effectiveness in concentrated human blood, building on a recent study which found no significant correlations between endogenous CEST signals and pH in human blood¹. Iopamidol has two peaks at +4.2ppm and +5.5 ppm with different exchange rates, which thus evolve differently with pH², and has previously been used to measuring pH in mouse kidneys³. To translate this measure into humans we have characterized the pH dependence of this signal in human blood over a range of RF powers. The normal pH level of blood is 7.6, however certain diseases can cause acidosis, causing it to drop down to pH 6.8 or lower in extreme cases⁴.

Aim

To characterize the CEST properties of Iopamidol at a range of RF powers in concentrated red blood cells as a method of determining pH in human tissues.

Methods

5x6ml blood samples were taken from one healthy volunteer and stored in heparin tubes. All samples were centrifuged to remove blood plasma. Samples were washed 3 times with PBS solutions at pH 6.8, 7.2, 7.6, 8.0, and 8.4, before being re-centrifuged and the PBS removed. 1ml of Iopamidol (300mg/1ml) was added to the samples as a pH dependent contrast agent.

Samples were scanned at 37°C using a 7T Achieva system using a 8ch pTx head coil. Z-spectra were acquired using Semi-CW saturation^5,6 at 5 B₁s (0.33,0.67,1.00,1.33,1.67μT) at 64 off-resonance frequencies between ±100,000Hz (TFEPI readout, voxel size 1x1x3mm). Acquisition of a single spectrum lasted 10mins.

Images were masked and spectra were B₀ and B₁ corrected pixel-wise using in-house MATLAB software. As the B₁ inhomogeneity was relatively high (ranging between 140%-280%), spectra were B₁ corrected to 200% of the nominal B₁ values as to stay within the sampled range. Spectra were analysed using a ratiometric approach to the Iopamidol peaks. The spectra were interpolated using spline fitting between +3ppm to +6ppm. A ratiometric value was obtained by dividing the value of the peak at +5.5ppm by the peak at +4.2ppm, and the saturation powers were obtained.

Results

Figure 1 shows the acquired spectra at a range of pH levels. Figure 2 shows the relationship between the ratio of the Iopamidol peaks with blood pH. We can see that in the physiological range, acquiring a spectrum at relatively low saturation power can provide us with the information we need to determine pH.

Discussion

Figure 2 shows that higher saturation powers are not suitable for determining the pH of blood in the physiological range, even at 1.33μT the calibration curve begins to level out, losing sensitivity. The Iopamidol peak at +5.5ppm is more prominent at higher powers, confirming that it has a faster exchange rate than that of the +4.2ppm peak. Further tests should be carried out to determine the optimal saturation power at this lower range.

Conclusion

Iopamidol is a readily available CT contrast agent which can be administered in humans orally and intravenously. This work has shown that it can be used to measure pH at low RF powers; these powers can be readily accessed even for whole body 7T MRI⁷. Future work will determine the optimum RF power and investigate sensitivity at 3T, and a rapid imaging protocol with RF calibration that takes account of B₁ variation will be established for applications in vivo.

Acknowledgements

References

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2 Sun, P. Z., Longo, D. L., Hu, W., Xiao, G., & Wu, R. (2014). Quantification of iopamidol multi-site chemical exchange properties for ratiometric chemical exchange saturation transfer (CEST) imaging of pH. Physics in Medicine & Biology, 59(16), 4493.

3 Longo, D. L., Dastrù, W., Digilio, G., Keupp, J., Langereis, S., Lanzardo, S., ... & Aime, S. (2011). Iopamidol as a responsive MRI‐chemical exchange saturation transfer contrast agent for pH mapping of kidneys: in vivo studies in mice at 7 T. Magnetic resonance in medicine, 65(1), 202-211.

4 Van Slyke, D. D., & Cullen, G. E. (1917). Studies of acidosis I. The bicarbonate concentration of the blood plasma; its significance, and its determination as a measure of acidosis. Journal of Biological Chemistry, 30(2), 289-346.

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7 Andrew Carradus, Emma Doran, Olivier Mougin, Christopher Mirfin, Hans Hoogduin, Stephen Bawden, Penny Gowland.The 7T z-spectrum from the human liver in-vivo: observing the effects of a meal. Abstract submitted to ISMRM 2019