Quantification of phosphorus metabolites from chemical shift imaging spectra with corrections for point spread effects and B1 inhomogeneity.

A method is described for quantifying phosphorus metabolites in tissue using spectra localized with surface coils and chemical shift imaging (CSI) and assuming that metabolites are uniformly distributed within a well-defined volume. An analytical expression is developed that yields a single numerical correction factor that takes into account the excitation and receiver profiles of the coil, T1 saturation, and point spread effects associated with Fourier transformation of CSI data. An external phosphorus standard is used to calibrate instrument gain and the B1 profile of the coil. For spherical samples, point spread effects can modulate the signal intensities of three-dimensional CSI spectra from -32% to +54%, depending on the voxel size. Measurements of phantoms of known concentrations showed systematic variations of +/- 10% and random errors of +/- 5%. We have used this method to measure the concentration of phosphocreatine in the thigh muscle of normal volunteers.