Ioannis-Angelos Giapitzakis^{1,2} and Anke Henning^{1,3}

In this abstract, we evaluated the Carr-Purcell behavior of semi-LASER sequence at 9.4T and we studied the influence of different relaxation times between the different moieties of NAA and NAAG using in-vivo acquired from occipital lobe and simulated spectra. The results indicate that differences in the relaxation time between different moieties can affect the fitting results and the metabolites levels. Moreover, an estimated value for the relaxation time of NAA-aspartate moiety is given. This work indicates that more measurements and investigation should be done, studying more metabolites and brain regions.

Simulated NAA-aspartate moiety spectra demonstrate that spin echo sequence with TE of 6.5 ms matches better in-vivo NAA-aspartate data compared to 24 ms because of the inhibition of J-coupling evolution of MC semi-LASER sequence due to its CP behavior. Fig. 2 shows the reduction of the residuals in the range of 2.4 to 2.7ppm when smaller ratios of aspartate moiety are used. The improvement of the residuals in case of the ratios of 0.75 is consistent among different volunteers and significant differences were found (Fig. 3; P < 0.002). Moreover, LCModel results illustrate the improvement of the overall fitting when a ratio of 0.75 is assumed (Fig. 4).

Despite the improvement of the residuals when relaxation is included, it appears the requirement of further optimization (e.g. J-coupling values) since the basis set does not fit perfectly to the NAA-aspartate.

Finally, metabolite concentrations (mM) and Cramer-Raw low bounds (CRLBs) demonstrate potential differences in the result quantifications (Fig. 5), however, without significant statistical difference due to multiple comparisons correction. An estimation of the relaxation time of NAA_{asp} can be exported using the following equations:

$$S_{NAA_{asp}}\propto[NAA_{asp}]\cdot e^{-\frac{TE}{t_{NAA_{asp}}}} [Eq. 1]$$

$$S_{NAA_{ace}}\propto[NAA_{ace}]\cdot e^{-\frac{TE}{t_{NAA_{ace}}}} [Eq. 2]$$

where $$$S_{NAA_{asp}}, S_{NAA_{ace}}$$$ are the acquired signals; $$$[NAA_{asp}], [NAA_{ace}]$$$ the concentrations and $$$ t_{NAA_{asp}}, t_{NAA_{ace}} $$$ the relaxation times of NAA aspartate and acetyl moieties respectively

From Eq. 1 over Eq.2 and $$$[NAA_{asp}] = [NAA_{ace}]$$$ we get

$$\frac{S_{NAA_{asp}}}{S_{NAA_{ace}}}=e^{\frac{TE\cdot t_{NAA_{ace}}-TE\cdot t_{NAA_{asp}}}{t_{NAA_{ace}}\cdot t_{NAA_{asp}}}}\Rightarrow ln(\frac{S_{NAA_{asp}}}{S_{NAA_{ace}}})={\frac{TE\cdot t_{NAA_{ace}}-TE\cdot t_{NAA_{asp}}}{t_{NAA_{ace}}\cdot t_{NAA_{asp}}}} [Eq. 3]$$

Setting $$$ R=\frac{S_{NAA_{asp}}}{S_{NAA_{ace}}} $$$ from Eq. 3 we get:

$$t_{NAA_{asp}} ={\frac{TE\cdot t_{NAA_{ace}}}{1-t_{NAA_{ace}}\cdot lnR}} [Eq. 4]$$

In this work TE = 24 ms, $$$R\sim0.75$$$ and $$$t_{NAA_{ace}}=98$$$ ms^{13}. Thus $$$\bf t_{NAA_{asp}}\sim80.6$$$ ms

The results of this UHF study verify previous studies regarding the CP behavior and J-evolution inhibition of the semi-LASER sequence^{3,14,15}. In this abstract, the influence of different relaxation times between acetate and aspartate moieties in NAA and NAAG metabolites onto fitting results was evaluated. The results highlight the need of careful creation of the basis sets including optimized TEs and relaxation times between different moieties inside metabolites. Despite the improvement of the residuals when relaxation is included, it appears the requirement of further optimization since the basis set does not fit perfectly to the NAA-aspartate.

The reported $$$t_{NAA_{asp}}$$$ is an estimation since it is known that CP-like sequences exhibit longer relaxation times^{16} and the published-used NAA-acetate relaxation time was measured with a STEAM sequence^{13}.

Finally, more studies have to be performed in order to investigate potential differences between different moieties of other metabolites that may affect the quantification results, as well as,in different brain regions.

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**Figure 5: **LCModel quantification results for two different ratios (1 and 0.75) for 9 datasets. Bars indicate the mean values and error bars the standard deviation. Significant differences did not survive after multiple comparisons correction. However, potentials differences are reported: *(*P*< 0.008), **(*P*< 0.004) and ***(*P*< 0.016)

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