Room: AAPM ePoster Library
Purpose: Chemical exchange saturation transfer (CEST) MRI can non-invasively measure Intracellular pH with high spatial and temporal resolution. In CEST, exchangeable protons on proteins can be selectively excited and detected through the transfer of magnetization to bulk water; the rate of which is pH-dependent. A CEST-MRI technique called amine and amide concentration-independent detection (AACID) was previously developed to measure absolute tissue pH. The AACID value is inversely related to tissue pH and heavily weighted to the intracellular compartments. The goal of this study is to monitor the change in tumour pH? over time in a rat C6 glioma model.
Methods: C6 glioma cells were injected into right frontal lobe of fifteen male Wistar rats. CEST-MRI was performed at baseline, 7-11 days, and 14-16 days post-implantation on a 9.4T MRI. CEST images were acquired at saturation frequencies from 1.2-6.6 ppm to create CEST spectra for each pixel in the image and AACID maps were produced.
Results: The average AACID values at day 0 were similar to values in contralateral tissue at days 7-11 and 14-16 post-implantation. The AACID value was significantly lower in the tumour compared to the contralateral region at day 7-11. At day 7-11, the average AACID value was 4.8% lower in the tumour compared to the contralateral side indicating a 0.24 higher pH?. Surprisingly, at day 14-16 the average tumour AACID value was no longer significantly different than the contralateral region.
Conclusion: The difference between tumour pH? and contralateral pH? is expected to increase over time in this model. However, this was not observed potentially due to the temporal stability of the AACID CEST measurement or the emergence of an altered physiological state within the contralateral tissue as tumour size increases. Future work will focus on improving temporal stability by optimizing pulse sequence and coil design.
Funding Support, Disclosures, and Conflict of Interest: The Canadian Institutes of Health Research (CIHR), The Canada First Research Excellence Fund (CFREF)