Quantifying the fractional concentrations and exchange rates of small‐linewidth CEST agents using the QUCESOP method under multi‐solute conditions in MRI signals

Abstract

To develop a novel method for quantifying the fractional concentration (f b) and the exchange rate (k b) of a specific small-linewidth chemical exchange saturation transfer (CEST) solute in the presence of other unknown CEST solutes. Theory and Methods: A simplified R 1ρ model was proposed assuming a small linewidth of the specific solute and a linear approximation of the other solutes' contribution to R 1ρ. Two modes of CEST data acquisition, using various saturation offsets and various saturation powers, were used. The f b and k b of the specific solute could be fitted using the proposed model. In MRI experiments, using either singlesolute or multi-solute phantoms with various creatine concentrations and pHs, the f b and k b values of creatine were calculated for each phantom; the f b and k b values of phosphocreatine in rats' skeletal muscles were also evaluated. Results: The fitted f b value of creatine from the phantoms were in excellent agreement. The fitted k b value of creatine from the phantoms coincides with that from the literature, as do the f b and k b values of phosphocreatine in skeletal muscles. Conclusion: The proposed approach enables us to quantify the f b and k b values of a specific small-linewidth solute in the presence of other unknown solutes. K E Y W O R D S CEST, chemical exchange saturation transfer, creatine, multiple solutes, phosphocreatine, quantitative CEST analysis 1 | INTRODUCTION Chemical exchange saturation transfer (CEST) has the potential to characterize low-concentration solutes with labile, exchangeable protons in an NMR/MRI field. 1-6 The CEST technique provides contrast on the basis of the chemical-exchange (CE) parameters (ie, the fractional concentration f b , exchange rate k b , transverse relaxation time R 2b , and spectral offset) of specific CEST solute(s). The f b of various solutes are given more attention in relevant studies. 7-11 The k b of some solutes are also widely studied, as the microenvironmental properties (eg, pH) could be indirectly reflected. 12-14 How to cite this article: Wang Y, Chen J-F, Li P, Gao J-H. Quantifying the fractional concentrations and exchange rates of small-linewidth CEST agents using the QUCESOP method under multi-solute conditions in MRI signals.

A generalized QUCESOP method with evaluating CEST peak overlap

“…. Identical to our previous study, 19 q 1j Δω i þ q 2j represents a linear 2 θ i,j can be accurately calculated. Then R1ρ Δω i , B 1j À Á can be fitted using the R 1ρ model with the linear approximation in Equations ( 2) and ( 3) to obtain the unknown parameters, namely, the CE parameters (f b , k b , and R 2b ) of the solute of interest, q 1j and q 2j (j = 1, 2, … n).…”

“…The R 1ρ relaxation model and the linear approximation due to the assumption of weak overlap have been described in a prior publication. 19 When a continuous-wave (CW) rectangular saturation pulse is utilized, the experimentally acquired Z-values ( Z) can be converted to the corresponding R 1ρ values ( R1ρ ) by using the R 1ρ relaxation model 3,9,19 :…”

“…where R water 1ρ and R ex are the contributions of water and the solute of interest, respectively. R ex has been given in some previous reports 3,9,19 :…”

“…For each phantom, maps of B 1 , T 1a , and T 2a were also acquired, and the details were identical to those described in our previous report. 19…”

“…Various methods have been developed for quantifying the fractional concentration f b and/or exchange rate k b of specific CEST solutes. [12][13][14][15][16][17][18][19][20] Some of these approaches have been successfully applied under in vivo conditions.…”