Comparing Neuromelanin Imagings on the Locus Coeruleus (LC) with TSE, FLASH and MT-GRE Sequences
Poster No:
1751
Submission Type:
Abstract Submission
Authors:
Jiancheng Zhuang1
Institutions:
1University of Southern California, Los Angels, CA
First Author:
Introduction:
The locus coeruleus (LC) is a small nucleus located deep in the pons with high neuromelanin content, which is thought to be related to many neurological disorders, such as Alzheimer's disease and Parkison's disease, and some cognitive changes in aging. Therefore, interest in imaging the LC in vivo has increased recently. The T1-weighted Turbo Spin Echo (TSE) was the first sequence used for LC imaging in vivo (Betts, 2017), but it suffers from relatively low signal-to-noise ratio (SNR) and high specific absorption rate (SAR). An alternative approach is using 3D high-resolution T1-weighted Fast Low Angle Shot (FLASH) MRI to detect the neuromelanin signal in LC, which can provide high spatial resolution in the slice direction (Betts, 2017). Recently, it was found that magnetization transfer (MT) effect may be one source of the contrast displayed in the T1-TSE scans of LC. Thus a gradient echo (GRE) sequence with explicit MT contrast preparation pulse was employed to study the LC (Chen, 2014). However, until today comprehensive comparisons between these three neuromelanin MRI methods together are lacking. Here we aim to provide such comparing of the detectability of the LC in vivo between the applied TSE, FLASH and MT-GRE sequences.
Methods:
The MRI scans to detect LC were performed on six healthy adult subjects. They were scanned on a Siemens 3T Prisma system using TSE, FLASH, and MT-GRE sequences. A 2D TSE sequence was used (Betts, 2017): TE/TR = 12/750 ms, Flip Angle =120 deg, 11 axial slices, voxel size = 0.4 mm x 0.4 mm x 2.5 mm, 2 measurements, and 287 Hz/pixel bandwidth with a scan time of 3 min 44 sec. The 3D FLASH images were acquired using the following parameters: TE/TR = 4.92/20 ms, Flip Angle = 25 deg, voxel size = 0.4 mm x 0.4 mm x 0.8 mm, 210 Hz/pixel bandwidth, 7/8 partial Fourier, and 2 min 9 sec scan duration. The 2D MT-GRE sequence was applied with these parameters: TE/TR = 10/322 ms, Flip Angle = 25 deg, 20 axial slices, voxel size = 0.5 mm x 0.5 mm x 3.0 mm, MT contrast pulses (Flip Angle = 300, 1.5 kHz off-resonance, 10 ms duration), and 260 Hz/pixel bandwidth with a scan time of 2 min 46 sec. In analysis, the regions of interest (ROIs) were selected on the left and right LC areas (Figure 1). The average signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and their standard deviations (SDs) were computed and compared across all subjects among the three sequences respectively.
Results:
The mean SNRs and CNRs of the LC areas were estimated on the acquired LC images (Figure 1). Table 1 showed that the mean SNR and CNR of LC obtained from MT-GRE scans are the highest, the SNR and CNR from TSE scans are the lowest, and the SNR and CNR from FLASH scans are in the middle. As in the result, the lowest CNR obtained by the TSE sequence was expected (Chen, 2014; Betts, 2017). It is due to the relatively low SNR from this sequence, so the longer scanning time is usually needed to achieve sufficient signal level in the LC area. The SNR and CNR obtained from FLASH scans are better than those from TSE, but lower than those from MT-GRE scans. Imaging the LC using the FLASH sequence can have better spatial resolution in the slice direction to assess the topographical distribution of signal intensity changes across the entire rostrocaudal extent of the LC with about the same or even shorter acquisition time than that using the TSE sequence. The SNR and CNR obtained by the MT-GRE scans are the highest among the three methods. This result implies that one possible source of the LC contrast in the TSE images is an incidental MT effect (Chen, 2014).
·Figure 1
·Table 1
Conclusions:
In this study, the explicit MT contrast pulse applied in this MT-GRE sequence was demonstrated to have advantages compared to the TSE and FLASH sequences. Based on these results and methods, we can further optimize these scan sequences to achieve higher SNR/CNR or shorter scanning time for the neuromelanin imaging in practice on the LC or other brain areas in future.
Neuroanatomy, Physiology, Metabolism and Neurotransmission:
Subcortical Structures 1
Novel Imaging Acquisition Methods:
Anatomical MRI 2
Keywords:
Acquisition
Brainstem
MRI
NORMAL HUMAN
STRUCTURAL MRI
Structures
Sub-Cortical
1|2Indicates the priority used for review
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2. Chen, et al (2014). Simultaneous imaging of locus coeruleus and substantia nigra with a quantitative neuromelanin MRI approach. Magnetic Resonance Imaging, 32, 1301.