Reducing motion sensitivity in 3D high-resolution T2*-weighted MRI by navigator-based motion and nonlinear magnetic field correction

Abstract

T₂*-weighted gradient echo (GRE) MRI at high field is uniquely sensitive to the magnetic properties of tissue and allows the study of brain and vascular anatomy at high spatial resolution. However, it is also sensitive to B₀ field changes induced by head motion and physiological processes such as the respiratory cycle. Conventional motion correction techniques do not take these field changes into account, and consequently do not fully recover image quality in T₂*-weighted MRI. Here, a novel approach was developed to address this by monitoring the B₀ field with a volumetric EPI phase navigator. The navigator was acquired at a shorter echo time than that of the (higher resolution) T₂*-weighted GRE imaging data and accelerated with parallel imaging for high temporal resolution. At 4 ​mm isotropic spatial resolution and 0.54 ​s temporal resolution, the accuracy for estimation of rotation and translation was better than 0.2° and 0.1 ​mm, respectively. The 10% and 90% percentiles of B₀ measurement error using the navigator were -1.8 and 1.5 Hz at 7 T, respectively. A fast retrospective reconstruction algorithm correcting for both motion and nonlinear B₀ changes was also developed. The navigator and reconstruction algorithm were evaluated in correcting motion-corrupted high-resolution T₂*-weighted GRE MRI on healthy human subjects at 7 ​T. Excellent image quality was demonstrated with the proposed correction method.

Keywords: B(0) field; EPI phase navigator; High resolution; Motion; T(2)*-weighted MRI.

Fig. 1

Diagram of PN and GRE acquisition. (a) Timing of the MRI acquisition sequence. “ADC” denotes the time window when the signal was recorded. Blue color marks navigator acquisition. GRE data were acquired at multiple echo times. (b) Acquisition pattern of fast navigators in k_y-k_z plane. Grey dots represent k-space data in one fast navigator. Black arrows connect consecutive readout lines within one shot. Additional fast navigators (in white dots) were acquired, using a similar but shifted k-space trajectory, one after another until a full navigator was sampled. Red dashed arrows show the varying starting offset for phase-encoding steps used in subsequent TRs for the fast navigators.

FIG. 2

Quality of PN images. Shown are examples of PN images at 4 mm spatial resolution and temporal resolutions of 4.32 s and 0.54 s of the full (a) and fast (b) navigators, respectively. (a) Distortion-corrected full navigator images of the brain. (b) Distortion-corrected fast navigator images. (c) and (d) show the effect of distortion correction: (c) a slice of distortion-corrected image with red contour marking the brain boundary and (d) uncorrected image of the same slice. A yellow arrow points to the most notable difference of the brain boundary in the two cases.

FIG. 3

Motion estimation accuracy from navigator data. Estimated pose changes in reference to the results using 2 mm GRE images in a single subject (a, b) and accuracy of these changes in a group of fix subjects (c-f). Insert in (a) shows definition of rotation and translation axes. (c-f). Columns reflect the RMSEs of estimated rotation (top) and translation (bottom). “Cor.” indicates distortion corrected navigator, and “Uncor.” uncorrected navigator.

FIG. 4

Accuracy of PN-estimated B₀ field changes. (a) B₀ changes of five poses (Fig. 3a) measured using multi-echo GRE images. (b) Difference between 4 mm PN- and GRE-measured B₀ changes, for both single-echo and dual-echo navigators. (c) The navigator image of the slice shown in (a) and (b). (d) Group summary of B₀ measurement accuracy using the navigator in reference to GRE results. Boxes indicate 10–90% percentile interval. Whiskers mark the 2.5% and 97.5% percentiles, respectively.

FIG. 5

Reconstruction results of subject #1 showing image quality improvement using both motion and field estimates from the PN data. The results are reconstructed isotropic 2 mm GRE images (TE=32 ms) from a scan with intentional motion in reference to the results of a static “No Motion” scan. (a) Navigator-measured head motion profile during the motion scan. (b) Representative maps of B₀ changes from three clusters and the change due to respiration in the sagittal view. (c) The global (zeroth-order) B₀ change during the scan as measured by the navigator. The red line represents the respiration belt signal. (d) The first-order terms of the B₀ change measurement. (e) Top row: reconstructed images in different correction modes; bottom row: difference of the magnitude images in reference to the corrected image (MoCo & Lin. B₀) of the static scan. Arrows point to the most dominant artifacts.

FIG. 6

Reconstructed in-plane 0.5 mm GRE images of subject #6 from a scan without intentional motion. These results demonstrated the effect of small motion on the image quality. (a) Navigator-measured head motion profile during the scan. (b) Top row: Reconstructed images with Global B₀-only correction and full correction (one cluster), respectively; bottom row: zoomed-in images in the yellow box. Yellow arrows indicate artifacts inside the brain and red arrows indicate artifacts on the surface of the brain and the skull.

FIG. 7

Reconstructed in-plane 0.5 mm GRE images of subject #6 from a stepwise motion scan. Improvement can be observed with motion and nonlinear B₀ correction. (a) Navigator-measured head motion profile during the scan. (b) Top row: Reconstruction for different correction modes; middle row: difference of magnitude images compared to the reference which was the corrected image in Fig. 6; bottom row: zoomed-in images in the yellow box. Arrows point to artifact which was reduced by nonlinear B₀ correction.

FIG. 8

Reconstructed 0.5 mm in-plane GRE images of subject #8 from a stepwise motion scan. Improvement can be observed with motion and nonlinear B₀ correction. (a) Navigator-measured head motion profile during the scan. (b) Top row: Reconstruction for different correction modes; middle row: absolute difference of magnitude images compared to the reference from a scan without intentional motion; bottom row: zoomed-in images in the yellow box. Arrows point to artifact which was reduced by nonlinear B₀ correction.

FIG. 9

NRMSEs of reconstructed images in different correction modes showing the effectiveness of motion and nonlinear B₀ correction across all subjects. (a) Results of isotropic 2 mm GRE images from scans with the stepwise motion task. (b) Results of in-plane 0.5 mm GRE images from scans with the stepwise motion task. (c) Results of isotropic 2 mm GRE images in the Monte Carlo simulations. (d) Results of isotropic 2 mm GRE images from scans with the stepwise motion task and with measured B₀ changes fitted to spherical polynomials up to orders of 1 to 6. Six clusters were chosen in (d).