Prospective real‐time correction for arbitrary head motion using active markers

Abstract

Patient motion during an MRI exam can result in major degradation of image quality, and is of increasing concern due to the aging population and its associated diseases. This work presents a general strategy for real-time, intraimage compensation of rigidbody motion that is compatible with multiple imaging sequences. Image quality improvements are established for structural brain MRI acquired during volunteer motion. A headband integrated with three active markers is secured to the forehead. Prospective correction is achieved by interleaving a rapid track-and-update module into the imaging sequence. For every repetition of this module, a short tracking pulse-sequence remeasures the marker positions; during head motion, the rigid-body transformation that realigns the markers to their initial positions is fed back to adaptively update the image-plane-maintaining it at a fixed orientation relative to the head-before the next imaging segment of k-space is acquired. In cases of extreme motion, corrupted lines of kspace are rejected and reacquired with the updated geometry. Patient movement is a fundamental problem in virtually all in vivo MR applications. Motion induces local field variations, causes erroneous positional encoding of k-space data, and corrupts the spin-excitation history between slices; these phenomena manifest in image-space as misregistrations, blurring, and ghosting. Even a few millimeters of movement during scanning can produce severe artifacts in reconstructed data, thus rendering images unusable. Often, it is subject populations with the highest potential diagnostic benefit in which the utility of MRI is curtailed by motion artifacts. In a study of 17 patients with frontoparietal tumors, data from five had to be rejected due to gross motion artifacts (1). Even among a healthy elderly population, our experience suggests that significant artifacts may appear in 10% to 20% of high-resolution structural brain scans; typically used for diagnostic and morphological analysis, such scans are especially prone to motion artifact due to their longer duration. To address these concerns, a motion-correction strategy for brain MRI is presented.The fact that the head is a rigid-body (to a very close approximation) allows an arbitrary motion to be described by six degrees-of-freedom (6-DOF)-three rotations about a three-dimensional (3D) orthogonal coordinate-system, and three translations. Retrospective motion-compensation methods, such as those used to coregister multiple image volumes in functional MRI (fMRI) studies, are well established. The most popular algorithms (2) determine the 6-DOF via minimization of a least-squares cost function and only correct for interimage motion. Retrospective correction involves interpolation, which can cause image blurring, and is further limited by its inability to fully correct for the influences of through-plane motion on local spin-history.In contrast, prospective strategies compensate for motion in the acquisition stage by keeping the image-plane at a fixed orientation ...

Motion Compensation Strategies in Magnetic Resonance Imaging

“…These devices couple any detected motion back to the MR scanner for prospective correction. Some of the external tracking strategies that have been proposed include using ultrasound guidance, 93 ultrawide band radar, 94 infrared tracking devices, 95 gradientinduced signal in electrodes, 96 active microcoils, 97 or spectroscopy to determine the angle of rotation of a crystal containing deuterium. 98 Although these approaches may allow for real-time motion correction by prospectively adjusting magnetic field gradients and RF pulses, their added complexity may be a significant disadvantage.…”

Self-encoded Marker for Optical Prospective Head Motion Correction in MRI

“…Navigator echos are added to MR sequences to compensate retrospectively and prospectively patient's motion during the scan [3,4,5]. Another MR based motion correction method was introduced by Ooi et al [6], using the response of active markers in form of small coils attached to the forehead of the patient. For a data acquisition independent approach, external tracking systems were proposed.…”

Continuous Magnetic Field Monitoring Using Rapid Re-Excitation of NMR Probe Sets

“…Studying essentially any magnetic field dynamics is no longer limited by probe dephasing and relaxation or restricted to time-consuming snapshot measurements. This capability holds promise for a variety of uses ranging from sequence development, image reconstruction, field control [25], and motion correction [26,27] to system characterization [17,19] and calibration, hardware diagnostics, and quality assurance. Additionally, continuous field monitoring opens up the novel capability of routinely recording entire scans.…”