Abstract
Abstract Optical tracking systems, such as the Moiré Phase Tracking system (MPT), can be used to correct motion prospectively especially during magnetic resonance imaging (MRI, MR) in neurologic application. For that an MRI safe camera is mounted in the MRI bore to detect motion by tracking a specific MPT marker, which is rigidly attached on the subject’s head. To enable prospective updates of the imaging volume´s position and orientation, and therefore to correct motion from the subject, the motion information captured from the camera need to be transformed from the camera into the MR coordinate system. The process of finding the transformation between both coordinate systems is called cross calibration and is essential for the overall motion correction performance. For the procedure of the cross calibration, an MR visible phantom with an attached MPT-marker is measured simultaneously with MRI and the MPT camera in multiple specific alignment positions. To reduce cross calibration errors, it is essential to move the phantom precisely into specified alignment positions. Due to the long and narrow bore design of ultra-high field systems (tunnel length > 3 m), the phantom can not be moved simply from a person leaning inside the scanner bore. Thus, to rotate it after each measurement step, either a technician must work inside the tunnel during the complete period of the cross calibration or the table must be moved in and out of the bore multiple times. To improve this currently established cross calibration procedure, we have developed an MRI safe phantom rotation system, which can be controlled remotely and precisely from outside the MRI bore. Even for ultra-high field imaging, the rotation system is fully MRI compatible. Initial tests were performed at a 7T whole-body MRI system and have proven the benefit of our rotation system.
Highlights
Magnetic resonance imaging (MRI) is frequently used for noninvasive neuroimaging
Due to the long and narrow bore design of ultra-high field systems, the phantom can not be moved from a person leaning inside the scanner bore. To rotate it after each measurement step, either a technician must work inside the tunnel during the complete period of the cross calibration or the table must be moved in and out of the bore multiple times. To improve this currently established cross calibration procedure, we have developed an MRI safe phantom rotation
For the calibration of the tracking coordinate system in relation to the MR coordinate system, an MRI fiducial phantom sphere with specific internal marker structures and with an Moiré Phase Tracking system (MPT) marker that is attached on top of the sphere is used [6]
Summary
At ultra-high field MRI systems, the available signal-to-noise ratio enables images with an isotropic voxel sizes of 250 μm for structural and 150 μm for vascular images [1, 2]. At this high level of resolution, even unintentional small head motions can degrade the image quality [3]. The sphere must be rotated multiple times around two separate rotation axes, each time with a different orientation These rotations are performed manually on a custom-made alignment frame with a technician situated on the system table and working inside the very narrow tunnel (tunnel length 3.80 m, diameter 0.60 m). This method is highly cumbersome and suboptimal with regard to safety restrictions due to the non-ergonomic working position in the confined space, the high noise
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