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Abstract:
Purpose/Introduction: Subject motion is one of the major problems for MR spectroscopy (MRS) since it often leads to increased linewidth, frequency shifts, reduced peak areas, insufficient water suppression(1), and to a displacement of
the acquisition voxel. Especially if the region of interest is small (like the spinal cord or tumors) even small movements will shift the measurement voxel out of the region resulting in inaccurate results. The detection of patient motion by observing exclusively the spectra is sometimes impossible. Therefore, in this work, 1D navigator acquisitions interleaved with the MRS measurement
are used for real-time patient-motion detection in the spinal cord.
Subjects and Methods: Non-water-suppressed MRS via the metabolite cycling technique at 3T (Achieva, Philips Healthcare, Best, TE/TR 30/2500ms, voxel size 1.2ml, 4*128FIDs) at the cervical level C3-4 (Fig.1) was used in the spinal cord(2). Interleaved navigators (Fig.2) were placed above the MRS-voxel (Fig.1). The software allows real-time data processing so that the navigator
position evaluation can be displayed online (Fig.3). After the approval from the local ethics committee, one volunteer was scanned with (scan 23) and without (scan 1) navigator motion detection. The volunteer was asked to lie still during all measurements except of scans 3. There the volunteer was asked to slowly raise her chin in the middle of the scan of about 2cm and to return to initial position after ~30s (~15 FIDs, ~3 of MRS scan-time). Before and
after each MRS scan axial T2-weighted images were acquired. If these images were shifted more than 1mm the MRS measurement was repeated.
Results: The spectra of scan 12 (Fig.3) show that the addition of navigators to the MRS sequence does not negatively affect the MRS quality. In addition, compared to scans without motion (12) scan 3 shows a different spectral
fingerprint (e.g. increased lipid signal) (Fig.3). Moreover, Fig.3 shows that even small movements are detectable by using navigators.
Discussion/Conclusion: Intra-scan subject movement can lead to a significant distortion of spectra (Fig. 3, scan 3). However, especially in the clinical routine subject movement cannot be eliminated. Thus, motion detection is essential. Interleaved navigators allow precise real-time motion detection for MRS without the need of additional scan-time and additional hardware. It allows early
intervention (e.g. asking the subject to lay still) and to reject motion corrupted FIDs. The use of additional navigators allows the detection of motion in more
dimensions and offers the possibility to update the voxel position.
