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Abstract:
Purpose/Introduction: One of the main goals at ultra-high magnetic fields is to take advantage of the increased SNR to improve the spatial resolution [1]. However, the long scan durations together with the high resolution amplify the problem of involuntary subject motion, even for experienced subjects. For further boosts of the voxel size to below 10 nl, some way to correct for motion is required.
Subjects and Methods: An optical tracking system (Metria Innovations), based on an in-bore camera and a single motion marker which allows for detection of all translation and rotation parameters was used [2]. Five subjects were placed inside a 16 channel transmit/31 channel receive coil array [3] and padded to avoid movements to a large degree. The motion correction marker was attached to a bitebar
[4], custom-made for each volunteer and placed to protrude from the coil housing. Two ultra-high resolution 3D gradient echo experiments with acquisition weighting were performed with a spatial resolution of (120 9 150 9 500) lm3 (9 nl). Each scan acquired two echoes with echo times of 7.4 ms and 15.8 ms. A total of 62,400 scans were
recorded within 26 min. Motion correction was applied for only the second image.
Results: The motion tracks of the subject with the largest motion are shown in Fig. 1. Even though motion was suppressed to a large extent by efficient padding and the use of experienced subjects, the remaining variations only due to breathing had an amplitude of up to 0.8 mm in the direction of strongest motion. In the resulting images
(Fig. 2), this already lead to strong artifacts. All images showed sufficient SNR in spite of the high resolution. The
long TE-data also has excellent contrast and shows remarkable details, while the low TE images, in spite of the higher SNR, are of weaker quality due to a significant lack of contrast. The images with motion correction are largely free of artifacts.
Discussion/Conclusion: The high SNR at ultra-high field makes it possible to obtain images with very high spatial resolutions, showing an unprecedented amount of detail in brain structures. Although we were able to obtain artifact-free images from some of our most experienced subjects even without motion correction, the data shown
here indicate that reliable imaging with these resolution requires an efficient motion correction technique.
