Simultaneous quantification of PCr, Cr, and pH in muscle CEST-MRI

CEST-MRI allows sensitive in vivo detection of PCr and Cr in muscle. However, the accurate quantification is difficult due to overlapped "peaks" from multiple solutes and mixed contributions from fractional concentration ( $f_b$ ) and exchange rate ( $k_b$ ). This study aims to achieve simultaneous and accurate mapping of PCr, Cr, and pH in muscle.

A two-step quantification method was proposed, by considering the co-existence of PCr and Cr in muscle and their dynamic transition. Firstly, exchangeable protons resonating at +2.6 ppm ( ${\mathrm{PCr}}_{2.6}$ ) were quantified using our previous gQUCESOP. In the second gQUCESOP for resolving parameters at +1.9 ppm, we included both Cr's and another exchangeable guanidino proton of PCr resonating at +1.9 ppm ( ${\mathrm{PCr}}_{1.9}$ ), with $f_b$ and $k_b$ for ${\mathrm{PCr}}_{1.9}$ estimated from ${\mathrm{PCr}}_{2.6}$ estimation in the first step. The method was validated by simulation and phantom study. In vivo rat experiments were performed at 9.4T, with pH measured also by ³¹P-MRS.

Simulation suggested an over-estimated $f_b$ and an under-estimated $k_b$ of Cr if including a non-neglectable content of PCr. For a phantom with mixed PCr and Cr, the proposed method allowed accurate calculation of both concentrations and pH. For in vivo rat scans performed before and right after euthanasia, our methods achieved coincided $f_b$ and $k_b$ with literatures. Furthermore, the pH values from ³¹P-MRS, $k_b$ of ${\mathrm{PCr}}_{2.6}$ , and $k_b$ of Cr could verify each other.

The proposed method is promising for quantifying the $f_b$ and $k_b$ for both PCr and Cr in skeletal muscular tissue.