Eva Alonso Ortiz^{1}, Ives R. Levesque^{1,2}, and G. Bruce Pike^{3}

In this

Multicomponent T2* decay was simulated for WM at 3T and 7T. T2* components were generated for MW (12%), EW (38%), and AW (50%), with relaxation times at 3T equal ^{7}, respectively. At 7T, the relaxation times were correspondingly set to 6 ms, 30 ms, and 40 ms ^{8}. Another simulation was performed in which the EW and AW T2* values were switched. Additional component fractions (MWF = 5% and 20%) were simulated to reflect a range of myelin density from low myelination to densely myelinated. The frequency offsets for each of the three components were calculated using the Hollow Cylinder Fiber Model (HCFM) ^{9}, for five $$$\theta$$$’s ranging from 0 to $$$\pi/2$$$ in steps of $$$\pi/8$$$. Complex signals were generated for each orientation with 64 echoes, TE1 = 2.4 ms and echo spacing = 1.2 ms. Two-hundred

1. MCMT2*: rNNLS multi-component fitting of a T2* distribution to the signal magnitude, with a 25 ms cutoff time for MW.

2. 3CMT2*: 3-component T2* fitting to the signal magnitude:

$$ s=A_{MW}e^{-t(1/T^*_{2,MW})}+A_{EW}e^{-t(1/T^*_{2,EW})}+A_{AW}e^{-t(1/T^*_{2,AW})} $$

3. 3CCT2*: 3-component T2* fitting with $$$\delta f$$$ terms to the complex T2* signals:

$$ s=A_{MW}e^{-t(1/T^*_{2,MW}+i2\pi \delta f_{MW})}+A_{EW}e^{-t(1/T^*_{2,EW})}+A_{AW}e^{-t(1/T^*_{2,AW}+i2\pi\delta f_{AW})} $$

The HCFM model predicts $$$\delta f_{EW}$$$ = 0, therefore it was assumed to be zero. The MWF (defined as the ratio of MW to total water) and fit residuals obtained using the three different methods were compared and the stability of the fits with respect to SNR was evaluated.

1. Du, Y., et al., Fast multislice mapping of the myelin water fraction using multicompartment analysis of T2* decay at 3T: a preliminary postmortem study. Magnetic resonance in medicine, 2007. 58(5): p. 865-870.

2. Hwang, D., D.-H. Kim, and Y. Du, In

3. Lenz, C., M. Klarhöfer, and K. Scheffler, Feasibility of in vivo myelin water imaging using 3D multigradient-echo pulse sequences. Magnetic resonance in medicine, 2012. 68(2): p. 523-528.

4. Andrews, T., et al., Testing the three-pool white matter model adapted for use with T2 relaxometry. Magnetic resonance in medicine, 2005. 54(2): p. 449-454.

5. Lancaster, J., et al., Three-pool model of white matter. Journal of magnetic resonance imaging : JMRI, 2003. 17(1): p. 1-10.

6. van Gelderen, P., et al., Nonexponential T(2) decay in white matter. Magn Reson Med, 2012. 67(1): p. 110-7.

7. Nam, Y., et al., Improved estimation of myelin water fraction using

8. Sati, P., et al., Micro-compartment specific T2* relaxation in the brain. Neuroimage, 2013. 77: p. 268-78.

9. Wharton, S. and R. Bowtell, Fiber orientation-dependent white matter contrast in gradient echo MRI. Proc Natl Acad Sci U S A, 2012. 109(45): p. 18559-64.

Figure 1: Mean (red) ± std (blue) MWF for a range of fiber
orientations, using MCMT2*, 3CMT2*,
and 3CCT2*
at 3 T (left) and 7 T (right). The true MWF is
shown in green. The 95% confidence interval for the mean is shown in pink.

Figure 2: Average residuals (over 200 iterations of noise)
± std for θ = 0 (top) and θ = π/2 (bottom) using MCMT2* (black), 3CMT2*
(blue), and 3CCT2* (red), at 3 T
(left) and 7 T (right). The MCMT2*
data points (black) at 3 T for θ = π/2 (bottom left plot) overlap with the 3CMT2* data points (blue).