Target Audience

MR scientists who wish to learn about a range of external devices that may be used to track and compensate for patient motion, in order to augment the MRI scan for improved image quality.

Objectives

Patient motion can represent a frequent cause of image degradation in MRI examinations. External devices have been employed in both research and clinical settings towards effective motion compensation strategies. Participants will gain an understanding of the basic physics underlying the operation of a range of external devices, and how they can be used to compensate for bulk rigid-body (e.g. head) motion, as well as physiological (e.g. respiration, cardiac cycle) motion.

Overview

Three general categories of external devices will be reviewed.

1) MR-based markers. The majority of the material will cover prospective motion correction for brain MRI using MR-based markers. Small fiducial markers are rigidly attached to the head, which serve as a proxy for head motion tracking. The positions of these markers inside the MR scanner can be measured with high accuracy/precision, typically using a navigator pulse-sequence, and in a temporal resolution suitable for real-time MRI applications.
Several different MR-based marker designs have been proposed in the literature, including miniature RF-coil designs, MR-visible samples, and other devices such as accelerometers and magnetometers. These design variants include both “wired markers” where each miniature RF-coil is connected to the MR scanner via a traditional coaxial cable, as well as “wireless markers” where the signal is wirelessly transmitted via inductive coupling to the nearby imaging (e.g. head) coil or via WiFi.
When used in a prospective motion correction strategy for brain MRI, the 3D rigid-body transform is calculated that realigns the current marker positions to their reference positions at the beginning of the scan. This transform is then applied in real-time to update the image-volume orientation and position such that it tracks with the motion of the head.

2) Optical cameras. In recent years, commercially available optical camera-based motion compensation solutions are becoming more readily available. Unlike MR-based markers, optical cameras measure patient motion in a 3D coordinate optical reference frame that is not natively aligned with the MR reference frame, and so a cross-calibration procedure is required to determine the coordinate transform between these reference frames. Cross-calibration considerations will be discussed, which is another potentially useful application for MR-based markers.

Acknowledgements

No acknowledgement found.

References

No reference found.