Prospective real-time head motion correction using inductively coupled wireless NMR probes

Abstract

Purpose: Head motion continues to be a major source of artifacts and data quality degradation in MRI. The goal of this work was to develop and demonstrate a novel technique for prospective, 6 degrees of freedom (6DOF) rigid body motion estimation and real-time motion correction using inductively coupled wireless nuclear magnetic resonance (NMR) probe markers.

Methods: Three wireless probes that are inductively coupled with the scanner's RF setup serve as fiducials on the subject's head. A 12-ms linear navigator module is interleaved with the imaging sequence for head position estimation, and scan geometry is updated in real time for motion compensation. Flip angle amplification in the markers allows the use of extremely small navigator flip angles (∼1°). A novel algorithm is presented to identify marker positions in the absence of marker specific receive channels. Motion correction is demonstrated in high resolution 2D and 3D gradient recalled echo experiments in a phantom and humans.

Results: Significant improvement of image quality is demonstrated in phantoms and human volunteers under different motion conditions.

Conclusion: A novel real-time 6DOF head motion correction technique based on wireless NMR probes is demonstrated in high resolution imaging at 7 Tesla.

Keywords: inductively coupled probes; prospective rigid body motion correction; wireless NMR markers.

Figure 1

(a) Wireless NMR marker with inbuilt sample before and after insulation. (b) Markers on a head shaped phantom (c) Mounting of probes on a human volunteer.

Figure 2

(a) Navigator sequence employed for 3D localization of NMR probe markers. The total time of the sequence is 12 ms. (b) Example navigator magnitude data in the anterior posterior direction acquired in a human volunteer scan.

Figure 3

a: 2D example of a marker triangle in space. Projection peaks along the two axes are back projected and the triangle with minimum side distance (d_i) change from the initial reference side distances (dref_i) is identified as a solution. b. Example of coordinate swapping during backprojection (y coordinate swapped between green and blue markers) leading to an erroneous reflected triangle (in bold) being identified as the initial solution instead of the correct triangle (dotted). Direction of the triangle normal represented by a ✖ for going into and • for coming out of the plane is however reversed allowing for rejection of this solution. Triangle in gray shows an example of a triangle from among the 2925 triangles, which is not in the subset of 36 possible triangles as it misses at least one projection.

Figure 4

Results of marker characterization (a & b) Variation of maker to background contrast (MBC) and marker localization precision with marker orientation with respect to right–left axis (4a) and the anterior–posterior axis (4b). Precision data points are shown only for angles where precision was < 1 mm. (c) Variation of marker signal and MBC on navigator flip angle.

Figure 5

Accuracy and precision measurements of the individual markers and the complete motion detection system measured using synthetic readout direction translations (a&b) and foot head axis rotation (c&d) experiments.

Figure 6

Results of motion correction in rigid corn bundle (Left –right FOV = 256 mm). Significant improvement in image quality is obtained in GRE scan with real time motion correction in the motion case (c & d). Motion traces for the two cases are shown underneath with rotations about the AP, RL and FH axes and translations along the same. The system does not diminish image quality in scans where the object is stationary (a & b).

Figure 7

Results of real time motion correction in 1× 1× 2 mm³ resolution MPRAGE imaging in a human subject. Single slice data is shown with correction of inplane left-right nodding with sharp transitions (7c & 7d) and foot head nodding (7e & 7f). Excellent image quality is restored in the presence of significant head motion shown in Fig 8.

Fig 8

(a –e): Motion traces of head motion in MPRAGE image shown in Fig 7.

Fig 9

Results of real time motion correction in 0.8 mm×0.8 mm×3 mm T2* weighted GRE imaging. (a,b): images showing stationary head without and with motion correction. (c,d) : Motion traces of a and b. (e,f) : images showing continuously moving head without and with motion correction.(g,h) Motion traces of e and f.

Fig 10

Axial (a & c) and coronal (b) slices with markers appearing as a hyper-intense spots (white arrows).