Computationally Efficient Combination of Multi‐channel Phase Data From Multi‐echo Acquisitions (ASPIRE)

Eckstein

Hingerl

et al. 2019

Self Cite

Purpose In this study, we demonstrate the first combination of 3D FID proton MRSI and spatial encoding via concentric‐ring trajectories (CRTs) at 3T. FID‐MRSI has many benefits including high detection sensitivity, in particular for J‐coupled metabolites (e.g., glutamate/glutamine). This makes it highly attractive, not only for clinical, but also for, potentially, functional MRSI. However, this requires excellent reliability and temporal stability. We have, therefore, augmented this 3D‐FID‐MRSI sequence with single‐echo, imaging‐based volumetric navigators (se‐vNavs) for real‐time motion/shim‐correction (SHMOCO), which is 2× quicker than the original double‐echo navigators (de‐vNavs), hence allowing more efficient integration also in short‐TR sequences. Methods The tracking accuracy (position and B0‐field) of our proposed se‐vNavs was compared to the original de‐vNavs in phantoms (rest and translation) and in vivo (voluntary head rotation). Finally, the intra‐session stability of a 5:40 min 3D‐FID‐MRSI scan was evaluated with SHMOCO and no correction (NOCO) in 5 resting subjects. Intra/inter‐subject coefficients of variation (CV) and intra‐class correlations (ICC) over the whole 3D volume and in selected regions of interest ROI were assessed. Results Phantom and in vivo scans showed highly consistent tracking performance for se‐vNavs compared to the original de‐vNavs, but lower frequency drift. Up to ~30% better intra‐subject CVs were obtained for SHMOCO (P < 0.05), with values of 9.3/6.9/6.5/7.8% over the full VOI for Glx/tNAA/tCho/m‐Ins ratios to tCr. ICCs were good‐to‐high (91% for Glx/tCr in motor cortex), whereas the inter‐subject variability was ~11–19%. Conclusion Real‐time motion/shim corrected 3D‐FID‐MRSI with time‐efficient CRT‐sampling at 3T allows reliable, high‐resolution metabolic imaging that is fast enough for clinical use and even, potentially, for functional MRSI.

Section: Methodsmentioning

confidence: 99%

Intra‐session and inter‐subject variability of 3D‐FID‐MRSI using single‐echo volumetric EPI navigators at 3T

Eckstein

Hingerl

et al. 2019

Self Cite

“…

normalΔ

B 0 maps were obtained using the same 3D gradient echo sequence with the following parameters: FOV = 220 mm × 192.5 mm, slice thickness = 1.7 mm, matrix size = 128 × 112 × 80, nominal voxel size = 1.7 × 1.7 × 1.7 mm 3 , GRAPPA = 2, TR = 18.0 ms, TE 1 = 3 ms, and TE 2 = 6 ms. Only TE 3 was B 0 ‐dependent: TE 3,1.5T = 14 ms, TE 3,3T = 12 ms, TE 3,7T = 10 ms, and TE 3,9.4T = 9.5 ms to account for faster phase evolution at higher B 0 . Multichannel data were combined using the ASPIRE coil combination . Phase wraps were unwrapped using UMPIRE, and the Δ B 0 maps were calculated from the difference between phase images .…”

Section: Methodsmentioning

confidence: 99%

“…Multichannel data were combined using the ASPIRE coil combination. 28 Phase wraps were unwrapped using UMPIRE, 29 and the ΔB 0 maps were calculated from the difference between phase images. 30,31 Fat maps were measured only once, on the 3T Prisma, using a turbo spin echo-based Dixon method 32 with the same spatial resolution: FOV = 220 mm × 192.5 mm, slice thickness = 1.72 mm, matrix size = 128 × 112 × 80, nominal voxel size = 1.7 × 1.7 × 1.7 mm 3 , TR = 6090 ms, and TE = 14 ms. 3D T 1 -weighted MPRAGE 33 or MP2RAGE 34 were acquired for anatomic reference and to generate brain masks.…”

Section: Experimental Datamentioning

confidence: 99%

The influence of spatial resolution on the spectral quality and quantification accuracy of whole‐brain MRSI at 1.5T, 3T, 7T, and 9.4T

Motyka

Hingerl

et al. 2019

Self Cite

Purpose Inhomogeneities in the static magnetic field ( B 0 ) deteriorate MRSI data quality by lowering the spectral resolution and SNR. MRSI with low spatial resolution is also prone to lipid bleeding. These problems are increasingly problematic at ultra‐high fields. An approach to tackling these challenges independent of B 0 ‐shim hardware is to increase the spatial resolution. Therefore, we investigated the effect of improved spatial resolution on spectral quality and quantification at 4 field strengths. Methods Whole‐brain MRSI data was simulated for 3 spatial resolutions and 4 B 0 s based on experimentally acquired MRI data and simulated free induction decay signals of metabolites and lipids. To compare the spectral quality and quantification, we derived SNR normalized to the voxel size (nSNR), linewidth and metabolite concentration ratios, their Cramer‐Rao‐lower‐bounds (CRLBs), and the absolute percentage error (APE) of estimated concentrations compared to the gold standard for the whole‐brain and 8 brain regions. Results At 7T, we found up to a 3.4‐fold improved nSNR (in the frontal lobe) and a 2.8‐fold reduced linewidth (in the temporal lobe) for 1 cm 3 versus 0.25 cm 3 resolution. This effect was much more pronounced at higher and less homogenous B 0 (1.6‐fold improved nSNR and 1.8‐fold improved linewidth in the parietal lobe at 3T). This had direct implications for quantification: the volume of reliably quantified spectra increased with resolution by 1.2‐fold and 1.5‐fold (when thresholded by CRLBs or APE, respectively). Conclusion MRSI data quality benefits from increased spatial resolution particularly at higher B 0 , and leads to more reliable metabolite quantification. In conjunction with the development of better B 0 shimming hardware, this will enable robust whole‐brain MRSI at ultra‐high field.

“…and subsequently unwrapped via fast 2D phase unwrapping available at https://github.com/mfkasim91/unwrap_phase/. The phase offset maps required for the method CEST‐GRE‐1TE were masked and smoothed using a discretized spline smoother [MATLAB function smoothn.m] to provide reliable results even at the brain’s boundaries, as has been shown previously for coil combination and distortion correction . Finally, as the ∆ B 0 maps were not masked, they were smoothed by a spatial Hamming filter before being used for CEST correction.…”

Section: Methodsmentioning

confidence: 99%

A comparison of static and dynamic ∆B₀ mapping methods for correction of CEST MRI in the presence of temporal B₀ field variations

Rodriguez

Dymerska

et al. 2019

Self Cite

Purpose To assess the performance, in the presence of scanner instabilities, of three dynamic correction methods which integrate ∆ B 0 mapping into the chemical exchange saturation transfer (CEST) measurement and three established static ∆ B 0 ‐correction approaches. Methods A homogeneous phantom and five healthy volunteers were scanned with a CEST sequence at 7 T. The in vivo measurements were performed twice: first with unaltered system frequency and again applying frequency shifts during the CEST acquisition. In all cases, retrospective voxel‐wise ∆ B 0 ‐correction was performed using one intrinsic and two extrinsic [prescans with dual‐echo gradient‐echo and water saturation shift referencing (WASSR)] static approaches. These were compared with two intrinsic [using phase data directly generated by single‐echo or double‐echo GRE (gradient‐echo) CEST readout (CEST‐GRE‐2TE)] and one extrinsic [phase from interleaved dual‐echo EPI (echo planar imaging) navigator (NAV‐EPI‐2TE)] dynamic ∆ B 0 ‐correction approaches [allowing correction of each Z‐spectral point before magnetization transfer ratio asymmetry ( MTR asym ) analysis]. Results All three dynamic methods successfully mapped the induced drift. The intrinsic approaches were affected by the CEST labeling near water (∆ ω < |0.3| ppm). The MTR asym contrast was distorted by the frequency drift in the brain by up to 0.21%/Hz when static ∆ B 0 ‐corrections were applied, whereas the dynamic ∆ B 0 corrections reduced this to <0.01%/Hz without the need of external scans. The CEST‐GRE‐2TE and NAV‐EPI‐2TE resulted in highly consistent MTR asym values with/without drift for all subjects. Conclusion Reliable correction of scanner instabilities is essential to establish clinical CEST MRI. The three dynamic approaches presented improved the ∆ B 0 ‐correction performance significantly in the presence of frequency drift compared to established static methods. Among them, the self‐corrected CEST‐GRE‐2TE was the most accurate and straightforward to implement.