Figure 4. Optimization of TS - N and BW - Signal.
The SNR (A) and SNR efficiency (B) were simulated as a function of TS effected by changing TD at the WM null regime with the MR parameters as described for (D) and with the higher range of intra-thalamic T1 (dashed) and the lower range of intra-thalamic T1 (solid), similar to Figure 2 and as measured in Figure 1. The signal and signal efficiency were normalized to the maximum values. From the simulations, the signal is expected to increase along TS and the signal efficiency is expected to reach a plateau around TS=6000ms.
At the optimal TS of 6000ms, the map of SNR efficiency as a function of N and bandwidth (C) predicted higher SNR efficiency for higher N and lower bandwidth.
(D): Representative images acquired at three TS=3000ms, 5000ms and 7000ms, with TI computed to stay at the WM null regime in each case. Other parameters fixed at N=200, BW=20KHz, TR=9.8ms, TE=4ms, α =4°, resolution=1mm3, ARC acceleration factor=2.5, NEX=1. Scan time=4.1min, 6.8min, and 9.6min respectively. The images are displayed at the same window and level. The experiments confirmed the gain in signal and contrast with TS. Structures such as the triangle of the lateral dorsal nucleus (LD; arrow and enlarged view at TS=7000ms) only became clearly visible at longer TS.
The WM null regime at TS=6000ms, N=200 and BW=20KHz was preferred for the next steps of the optimization for thalamus segmentation.