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Advantages and limitations of prospective head motion compensation for MRI using an optical motion tracking device

: Dold, C.; Zaitsev, M.; Speck, O.; Firle, E.; Hennig, J.; Sakas, G.


Academic radiology 13 (2006), Nr.9, S.1093-1103
ISSN: 1076-6332
ISSN: 1878-4046
Fraunhofer IGD ()
motion artefact; brain imaging; motion tracking; image quality; motion registration; infrared based technology; magnetic resonance imaging; MRI

Patient motion remains a significant problem in many magnetic resonance (MR) imaging (MRI) applications, including functional MRI (fMRI) (1 and 2) and cardiac and abdominal imaging, as well as conventional acquisitions. Many techniques are available to reduce or compensate for bulk motion effects, such as physiological gating, phase-encode reordering, fiducial markers (3), special algorithms (4), fast acquisitions, image volume registration, or such alternative data-acquisition strategies as projection reconstruction, spiral, and Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction (PROPELLER) (5 and 6). Navigator echoes are used to measure motion with one or more degrees of freedom (df) (7). The motion then is compensated for either retrospectively or prospectively. An orbital navigator (ONAV) echo captures data in a circle in some plane of k-space, centered at the origin (8, 9 and 10). These data can be used to detect rotational and translational motion in this plane and correct for this motion. However, multiple orthogonal ONAVs are required for general three-dimensional (3D) motion determination, and the accuracy of a given ONAV is affected adversely by motion out of its slice.
Methods capable of correcting for head motion in all 6 df have been proposed for human positron emission tomography brain imaging (11). These methods rely on accurate measurement of head motion in relation to the reconstruction coordinate frame. Implementing a similar technique in MRI presents additional challenges. Foremost, the tracking system and MRI system have to be compatible. High magnetic fields of 1.5 Tesla or greater in MRI systems require that the tracking camera system be positioned at sufficient distance from the MRI system to ensure proper function and safety. Additionally, efforts need to be taken to ensure radiofrequency (RF) screening on the tracking hardware. fMRI also proves challenging because of the high spatial accuracy (root mean square [RMS] < 0.3 mm) required by the complete measurement chain with a small latency time of the tracking system. A precise relationship between spatially varying magnetic field gradients and spatial tracking information is necessary to compensate for motion artifacts. Our initial trials using an external tracking system to compensate for movement artifacts in MRI are published elsewhere (12, 13, 14, 15 and 16).