Quantitative 19F imaging using inductively coupled reference signal injection

Abstract

This report describes recent efforts on our continuous development of a synthetic signal injection method for quantification of metabolite content in MR spectroscopy and MRI. Previous work showed that conversion of spectral peaks to quantitative units of metabolite content could be achieved with a calibrated synthetic free induction decay generated by an inductively coupled injection coil. This work demonstrates that calibrated synthetic voxels, injected in the same manner, can be used to quantify metabolite content in real (19)F image voxels. Images of vials containing different concentrations of sodium fluoride (NaF) were converted to units of moles by reference to precalibrated synthetically injected voxels. Additional images of vials containing variable sodium chloride (NaCl) demonstrate that the quantification process is robust and immune to changes in coil loading conditions.

Figure 1

The eight pseudo-voxels appear at the top of the image of the seven NaF vials. The configuration of the NaF vials and the loading syringe is illustrated in the middle diagram. The table indicates the NaF concentrations in each of the vials. The signals from vials 6 and 7, containing the two lowest NaF concentrations, were not detectable above the image noise so no attempt was made to calculate these concentrations. Salt concentrations in the loading syringe, sample 0, were varied (0, 0.2 and 2 M) to induce different coil loading conditions.

Figure 2

Pseudo-voxel signal and real signal from the 0.8 M NaF vial as a function of the power setting for the RF amplifier connected to the injector coil. The amplitude of the pseudo-voxel signal increased linearly with RF power. The amplitude of the real signal remained constant.

Figure 3

(a) The graph shows the correlation between known concentrations and the concentrations calculated using equation (1). In comparison, the relative concentrations (b) reflect a substantial drop in signal as the loading of the coil increased. The relative amplitudes acquired with no salt in the loading syringe were about 25% higher than they were at the highest salt concentration. Relative concentrations generate the slopes of 0.99 ± 0.02, 0.94 ± 0.01 and 0.76 ± 0.02 for the loading samples of 0, 0.2 and 2 M NaCl, respectively.