Mapping tissue sodium concentration in the human brain: a comparison of MR sequences at 9.4Tesla.

Sodium is the second most abundant MR-active nucleus in the human body and is of fundamental importance for the function of cells. Previous studies have shown that many pathophysiological conditions induce an increase of the average tissue sodium concentration. To date, several MR sequences have been used to measure sodium. The aim of this study was to evaluate the performance and suitability of five different MR sequences for quantitative sodium imaging on a whole-body 9.4Tesla MR scanner. Numerical simulations, phantom experiments and in vivo imaging on healthy subjects were carried out. The results demonstrate that, of these five sequences, the Twisted Projection Imaging sequence is optimal for quantitative sodium imaging, as it combines a number of features which are particularly relevant in order to obtain high quality quantitative images of sodium. These include: ultra-short echo times, efficient k-space sampling, and robustness against off-resonance effects. Mapping of sodium in the human brain is a technique not yet fully explored in neuroscience. Ultra-high field sodium MRI may provide new insights into the pathogenesis of neurological disorders, and may help to develop new and disease-specific biomarkers for the early diagnosis and therapeutic intervention before irreversible brain damage has taken place.

Simultaneous single-quantum and triple-quantum-filtered MRI of 23Na (SISTINA).

The low MR sensitivity of the sodium nucleus and its low concentration in the human body constrain acquisition time. The use of both single-quantum and triple-quantum sodium imaging is, therefore, restricted. In this work, we present a novel MRI sequence that interleaves an ultra-short echo time radial projection readout into the three-pulse triple-quantum preparation. This allows for simultaneous acquisition of tissue sodium concentration weighted as well as triple-quantum filtered images. Performance of the sequence is shown on phantoms. The method is demonstrated on six healthy informed volunteers and is applied to three cases of brain tumors. A comparison with images from tumor specific O-(2-[18F]fluoroethyl)-L-tyrosine positron emission tomography and standard MR images is presented. The combined information of the triple-quantum-filtered images with single-quantum images may enable a better understanding of tissue viability. Future studies can benefit from the evaluation of both contrasts with shortened acquisition times.

B0 insensitive multiple-quantum resolved sodium imaging using a phase-rotation scheme.

Triple-quantum filtering has been suggested as a mechanism to differentiate signals from different physiological compartments. However, the filtering method is sensitive to static field inhomogeneities because different coherence pathways may interfere destructively. Previously suggested methods employed additional phase-cycles to separately acquire pathways. Whilst this removes the signal dropouts, it reduces the signal-to-noise per unit time. In this work we suggest the use of a phase-rotation scheme to simultaneously acquire all coherence pathways and then separate them via Fourier transform. Hence the method yields single-, double- and triple-quantum filtered images. The phase-rotation requires a minimum of 36 instead of six cycling steps. However, destructive interference is circumvented whilst maintaining full signal-to-noise efficiency for all coherences.