Time averaging and fitting of nonlinear metabolic changes: the issue of the time index choice applied to 31P MRS investigation of muscle energetics

We present an exact analytical method dedicated to fitting time-dependent exponential-like changes in MR spectra. As an illustration, this method has been applied to fitting metabolic changes recorded by 31P MRS in human skeletal muscle occurring during a rest-exercise-recovery protocol. When recording metabolic changes with the accumulative method, the time averaging of the MR signals implies the choice of a time index for fitting any changes in the features of the associated MR spectra. A critical examination of the different ways (constant, linear, and exponential) of choosing the time index is reported. By numerical analysis, we have calculated the errors generated by the three methods and we have compared their sensitivity to noise. In the case of skeletal muscle, both constant and linear methods introduce large and uncontrolled errors for the whole set of metabolic parameters derived from [PCr] changes. In contrast, the exponential method affords a reliable estimation of critical parameters in muscle bioenergetics in both normal and pathological situations. This method is very easy to implement and provides an exact analytical solution to fitting changes in MR spectra recorded by the accumulative method.     

Dissociation Constants of Protonated Amines in Water at Temperatures from 293.15 K to 343.15 K

The dissociation constants of protonated 2-amino-1-ethanol (MEA), diethanol amine (DEA), triethanol amine (TEA), methyldiethanol amine (MDEA), 2-amino-2-methyl-1-propanol (AMP), 3-dimethylamino-1-propanol (DMAP), tris(hydromethyl)aminomethane (THAM), 2-[2-(dimethylamino)ethoxy]ethanol (DMAEOE) and, 1,2-bis(2-aminoethoxy)ethane (DiAEOE) were determined in the temperature range 293.15 to 343.15 K using a potentiometric titration method. The experimental technique was first validated using as reference the available literature data of MDEA. The dissociation enthalpies of amines were derived from their dissociation constants using the Van’t Hoff equation. Experimental dissociation constants and dissociation enthalpies were discussed in term of amine structure and compared with literature values when available.     

Optimization of residual water signal removal by HLSVD on simulated short echo time proton MR spectra of the human brain

Suppression of the residual water signal from proton magnetic resonance (MR) spectra recorded in human brain is a prerequisite to an accurate quantification of cerebral metabolites. Several postacquisition methods of residual water signal suppression have been reported but none of them provide a complete elimination of the residual water signal, thereby preventing reliable quantification of brain metabolites. In the present study, the elimination of the residual water signal by the Hankel Lanczos singular value decomposition method has been evaluated and optimized to provide fast automated processing of spectra. Model free induction decays, reproducing the proton signal acquired in human brain localized MR spectroscopy at short echo times (e.g., 20 ms), have been generated. The optimal parameters in terms of number of components and dimension of the Hankel data matrix allowing complete elimination of the residual water signal are reported.