Evaluations of the Bloch fitting method for the analysis of acidoCEST MRI.

Abstract

Purpose: AcidoCEST MRI measures tumor extracellular pH by fitting chemical exchange saturation transfer (CEST) Z-spectra with modified Bloch-McConnell equations, known as a Bloch fitting method. We evaluated Bloch fitting with Z-spectra and T₁, T₂, B₁, and B₀ MR information with phantoms. We applied the Bloch fitting method to in vivo acidoCEST MRI of a preclinical tumor model.

Methods: We studied 120 phantoms of iopamidol with a range of pH values, concentrations, and T₁ values. We collected 180 Z-spectra for each phantom, along with T₁, T₂, B₁, and B₀ measurements. The data were analyzed with the Bloch fitting method to estimate pH. AcidoCEST MRI was performed with five mice with 4T1 mammary carcinoma using the Bloch fitting method.

Results: Bloch fitting generated accurate and precise pH estimates without also including experimental T₁, T₂, B₁, or B₀ values. Accurate and precise pH estimates of phantoms were achieved with 3-μT saturation power, ≥1-s saturation time, ≥15 mM of iopamidol, and temperature control of 37.0 ± 1.5°C. Similar to our phantom studies, fixing one or more fitting parameters did not significantly alter the extracellular pH estimates from acidoCEST MRI of a 4T1 tumor model, which estimated a tumor extracellular pH of 6.8.

Conclusions: Although fixing Bloch fitting parameters with experimental measurements saved computation time, fixing these parameters did not significantly improve the accuracy or precision of measuring pH in phantoms or within in vivo tumors, Therefore, measurements of T₁, T₂, B₁, and/or B₀ are not needed for acidoCEST MRI.