Two-pulse biexponential-weighted 23Na imaging

- Biexponential-weighted contrast of sodium is accomplished with a new sequence design.
- New sequence uses two instead of three RF-pulses and thus generates reduced SAR.
- SNR improved and further improved by reducing repetition time at high field (7 T).
- Less correlation time weighting than with TQF imaging and double-readout sequence.

A new method is proposed for acquiring 3D biexponential-weighted sodium images with two instead of three RF pulses to allow for shorter repetition time at high magnetic fields (B0 ⩾ 7 T) and reduced SAR.The second pulse converts single- into triple-quantum coherences in regions containing sodium ions which are restricted in mobility. Since only single-quantum coherences can be detected, an image acquired after the second pulse is intrinsically single-quantum-filtered and can be used to generate a biexponential-weighted sodium image by a weighted subtraction with the spin-density-weighted image acquired between the pulses.The proposed sequence generates biexponential-weighted sodium images of in vivo human brain with 140% higher SNR than triple-quantum-filtered sodium images and 4% higher SNR than a biexponential-weighted sequence with three RF pulses at equal acquisition time and with 1/3 lower SAR. As SAR is reduced, accordingly repetition time can be spared to obtain even higher SNR-time efficiency. In comparison to a difference image generated from two images of a double-readout sequence, the proposed two-pulse sequence yields about 14% higher SNR.Our new two-pulse biexponential-weighted sequence allows for acquisition of full 3D data sets of the human brain in vivo with a nominal resolution of (5 mm)3 in about 10 min.

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