Rapid Quantitative Susceptibility Mapping with Simultaneous Multi-Band Imaging

Nan-Jie Gong^{1}, Hing-Chiu Chang^{2}, Hongjiang Wei^{1}, Mark Sundman^{1}, Nan-kuei Chen^{1}, and Chunlei Liu^{1}

Pulse Sequence

In our preliminary implementation, we acquired a dataset using a 2-band and 2-shot interleaved EPI sequence with a Nova 32-channel phase array RF coil on a 3T scanner. Imaging parameters were: in-plane (sagittal plane) matrix size = 128 *128, FOV = 25.6 * 25.6 cm2, thickness = 3 mm, TR = 2 sec, TE = 25 msec. Magnitude and phase images were reconstructed using the generalized MUSE algorithm to effectively eliminate both in-plane and through-plane aliasing artifacts (6). Figure 1 shows original phase of multi-band acquisition without correction.

Phase Correction

In multi-band phase, larger number of wraps was identified in band2 as compared to band1 in axial plane. Discontinuity of phase between band1 and band2 was also observed in coronal plane. This is very likely resulted from misalignment of k-space across these two bands, which can be corrected by adding a linear phase term to band2. Corresponding equation is:

S_{band1} = S_{band2} × exp[i × (f1 × x+f2 × y+f3)] Equation
1

To eliminate
phase gaps between bands, number of wraps of band2 should be the same as that
of band1 in axial plane. Number of wraps can be represented by sum of phase
gradient along x direction (figure 2(a)), which is denoted as cost function of
number of wraps, cost_{nw}. Phase continuity in axial and coronal plane can be reflect
by sum of phase gradient along z direction in the center adjacent two slices,
which are denoted as cost functions of continuity in axial plane (cost_{ca},
figure 2(b)) and coronal plane (cost_{cc}, figure 2(c)) respectively. Total
cost function can then be written as:

cost= λ × cost_{nw} (band1) / cost_{nw} (band2)+ cost_{ca} + cost_{cc}

where λ is a weighting factor. In equation 1, f1, f2, f3 can be estimated using unconstrained nonlinear optimization through minimizing the above total cost function.

QSM reconstruction

Multi-band phase images corrected using the proposed method were further processed using 3D Laplacian phase unwrap, 3D VSHARP background phase removal (7) and STAR dipole inversion (8). As a comparison, multi-band phase images were further processed using 2D Laplacian phase unwrap across sagittal plane, followed by 2D VSHARP background phase removal and STAR dipole inversion (8).

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2. Liu C, Li W, Johnson GA, Wu B. High-field (9.4 T) MRI of brain dysmyelination by quantitative mapping of magnetic susceptibility. Neuroimage. 2011;56(3):930-8.

3. Balla DZ, Sanchez-Panchuelo RM, Wharton SJ, et al. Functional quantitative susceptibility mapping (fQSM). Neuroimage. 2014;100:112-24.

4. Bianciardi M, van Gelderen P, Duyn JH. Investigation of BOLD fMRI resonance frequency shifts and quantitative susceptibility changes at 7 T. Hum Brain Mapp. 2014;35(5):2191-205.

5. Fan AP, Govindarajan ST, Kinkel RP, et al. Quantitative oxygen extraction fraction from 7-Tesla MRI phase: reproducibility and application in multiple sclerosis. J Cereb Blood Flow Metab. 2015;35(1):131-9.

6. Chang HC, Gaur P, Chou YH, Chu ML, Chen NK. Interleaved EPI based fMRI improved by multiplexed sensitivity encoding (MUSE) and simultaneous multi-band imaging. PLoS One. 2014;9(12):e116378.

7. Li W, Avram AV, Wu B, Xiao X, Liu C. Integrated Laplacian-based phase unwrapping and background phase removal for quantitative susceptibility mapping. NMR Biomed. 2014;27(2):219-27.

8. Wei H, Dibb R, Zhou Y, et al. Streaking artifact reduction for quantitative susceptibility mapping of sources with large dynamic range. NMR Biomed. 2015;28(10):1294-303.

Figure 1. Original
phase of multi-band acquisition without correction. Larger
number of wraps was identified in band2 as compared to band1 in axial plane.

Figure 2.
Number
of wraps can be represented by sum of phase gradient along x direction (figure
2(a)), which is denoted as cost function of number of wraps, cost_{nw}. Phase
continuity in axial and
coronal plane can be reflect by sum of phase gradient along z direction in the
center adjacent two slices, which are denoted as cost functions of continuity
in axial plane (cost_{ca}, figure 2(b)) and coronal plane (cost_{cc},
figure 2(c)) respectively.

Figure
3 illustrates corrected phase using the proposed method. Phase of band1 is
nearly perfectly aligned with phase of band2 in both axial and coronal planes.

Figure
4 illustrated quantitative susceptibility maps derived from uncorrected
multi-band phase, corrected multi-band phase and 2D processed multi-band phase.
The
proposed method markedly improved the image quality with nearly no artifact observable
in the center slices and meanwhile retained the susceptibility contrast
especially across deep gray matter nuclei and surrounding tissues.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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