Purpose: The Bloch-McConnell equations can be used to fit CEST spectra to estimate the proton chemical exchange rates (kₑₓ) and pH of a sample. This study aims to test the improvement of the fitting algorithm when the fitting parameters are fixed with T₁, T₂, B₁ and B₀ experimental values.
Methods: Two-hundred iopamidol samples were prepared with 5 concentrations (5-50 mM), 5 T₁ values (0.4-3.9 sec) and 8 pH levels (6.25-7.30, equally spaced). Thirty-six CEST scans were performed on each sample using a FISP acquisition sequence and a cross-combination of six saturation powers (0.5-6 μT) and six saturation times (0.5-6 sec) with a Bruker 7T preclinical scanner. The set of 36 CEST scans was repeated at five temperatures (31-43 °C). In addition, T₁, T₂, B₁ and B₀ maps were also acquired. Image analysis and Bloch-fitting was performed with MATLAB.
Results: The changes in the iopamidol amide contrasts in Z-spectra with CEST agent concentrations, T₁ values, sample pH values, saturation powers and saturation times match well with previous studies. Our Bloch fitting algorithm can detect the increase of amide kₑₓ with increasing sample pH and temperatures. Reducing the number of parameters fitted in the Bloch-fitting process by fixing the parameters with experimentally measured values does not significantly affect the fitting results of mid-to-high pH (>6.55) iopamidol samples, but can increase the sensitivity to the pH changes of low pH (<6.55) samples. Including T₁, T₂ and B₁ experimental information also significantly reduces the computation time.
Conclusion: Our Bloch-fitting algorithm is capable of detecting the changes of kₑₓ of iopamidol protons at different pH levels. The inclusion of experimental information in the Bloch-fitting process improves the accuracy and precision of the fitting result and shortens the fitting time.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the National Institutes of Health grant no. 1R01CA169774. S.Z. would like to thank the support from the Odyssey Program and Cockrell Foundation Award for Scientific Achievement at The University of Texas MD Anderson Cancer Center.