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Purpose/Introduction: 1H FID MRSI can be performed without
localisation and lipid suppression to acquire metabolite maps from the human brain1, 2. However, voxel bleeding can cause the unsuppressed subcutaneous lipid signal to contaminate the spectra. Furthermore, lipid contamination as a result of residual aliasing artifacts can be caused by acceleration schemes such as GRAPPA3. Outer-volume
saturation can be used to reduce this but this leads to long scan-times and high SAR. An alternative is to use global lipid suppression which has lower SAR4. Lipid suppression schemes that use single- or double-inversion recovery (S/DIR) have a lot of SAR. At ultra high field strengths, the high SAR results in longer scan times. In this work, we present a SIR lipid suppression using a low SAR asymmetric adiabatic pulse for 1H MRSI at 9.4T.
Subjects and Methods: A hypergeometric RF pulse was designed for SIR lipid suppression (Fig. 1) at 9.4T to be spectrally selective over the range 0–1.75 ppm. This is an asymmetric adiabatic pulse and the steep transition was place at 1.75 ppm. This pulse has approximately 40 less SAR than conventional asymmetric adiabatic pulses such as the sech/tanh pulse5. A non-lipid suppressed and a SIR lipid suppressed single-slice MRSI dataset were acquired with a Siemens 9.4T human scanner at high resolution (3.125 9 3.125 mm). The FOV was 200 9 200 9 10 mm. For the lipid suppression, the inversion pulse was placed 130 ms before the excitation pulse. The TR for the nonlipid suppressed and lipid suppressed MRSI was 220 ms and 500 ms, respectively.
Results: Lipid maps were calculated as the absolute integral of the spectra over the range 0–1.7 ppm. Figure 2 shows the normalised lipid maps. The remaining lipid signal was a maximum of 30 of the non-lipid suppressed and the mean was 15.46 of the original lipid content. Spectra from the non-lipid and lipid suppressed data are shown in Figure 3. The spectra show a clear reduction in the lipid range while
the rest of the spectra are largely unaffected.
Discussion/Conclusion: The asymmetric inversion pulse allowed us to reduce the lipid content. Furthermore, the TR could be kept short due to the low SAR of the pulse. The lipid content was greatly reduced to a mean of 15 of the original lipid content. We showed an effective method of lipid suppression while keeping the TR short. This is beneficial for reliable acceleration at ultra-high fields, since conventional lipid suppression schemes prolong the TR
and hence add to the scan time3.
