Objectives: : Total tissue sodium (Na) content is associated with the viability of cells and can be assessed by Na magnetic resonance imaging. However, the resulting total sodium signal (NaT) represents a volume-weighted average of different sodium compartments assigned to the intra- and extracellular space. In addition to the spin-density weighted contrast of NaT imaging, relaxation-weighted (NaR) sequences were applied. The aim of this study was to evaluate the potential of NaR imaging for tissue characterization and putative additional benefits to NaT imaging.
Materials and methods: : For NaT and NaR imaging, novel magnetic resonance imaging sequences were established and applied in 16 patients suffering from brain tumors (14 WHO grade I-IV and 2 metastases). All Na sequences were based on density-adapted three-dimensional radial projection reconstruction to obtain short echo times and high signal-to-noise ratio efficiency.
Results: : NaT imaging revealed increased signal intensities in 15 of 16 brain tumors before therapy. In addition, NaR imaging enabled further differentiation of these lesions; all glioblastomas demonstrated higher NaR signal intensities as compared with WHO grade I-III tumors. Thus, NaR imaging allowed for correct separation between WHO grade I-III and WHO grade IV gliomas. In contrast to the NaT signal, the NaR signal correlated with the MIB-1 proliferation rate of tumor cells.
Conclusions: : These results serve as a proof of concept that NaR imaging reveals important physiological tissue characteristics different from NaT imaging. Furthermore, they indicate that the combined use of NaT and NaR imaging might add valuable information for the functional in vivo characterization of brain tissue.