Optimized high‐resolution mapping of magnetization transfer (MT) at 3 Tesla for direct visualization of substructures of the human thalamus in clinically feasible measurement time

Abstract

Purpose:To optimize contrast-to-noise and spatial resolution of a FLASH-based magnetization transfer (MT) protocol for visualization of substructures in human thalamus. Materials and Methods:Healthy adults were examined at 3 Tesla with a three-dimensional (3D) spoiled gradient-echo sequence. The signal-to-noise ratio (SNR) was increased by averaging eight bipolar echo acquisitions (mean echo time ϭ 12.3 ms; bandwidth ϭ 370 Hz/pixel). Three isotropic datasets with different weighting (proton density: flip angle/repetition time ϭ 7°/30 ms; T 1 : 20°/30 ms and MT: 10°/48 ms, Gaussian MT prepulse) yielded maps of T 1 , signal amplitude, MT ratio and MT saturation for comparison to MP-RAGE images. Measuring time was 23 min using partial k-space acquisition. First, the SNR of MT saturation maps in thalamus was optimized by means of the excitation flip angle. Then, noise and partial volume effects were traded off by means of the resolution. Finally, the contrast within the thalamus and to adjacent structures was compared between different maps. Results:The optimized MT saturation maps at 0.95 mm isotropic resolution provided the highest contrast. It was most prominent between structures of high axonal content (internal medullary lamina, ventral nuclei) and those containing predominantly neuronal somata (pulvinar, mediodorsal thalamus, geniculate bodies). Conclusion:Semiquantitative MT saturation maps provide an enhanced intra-thalamic contrast. The borders and nuclear groups of the thalamus are reliably delineated; individual assignment of singular nuclei seems feasible.

Imaging for deep brain stimulation: The zona incerta at 7 Tesla

“…Besides the most frequently used heavily T2-weighted fast spin-echo [20,31,38] new sequences including quantitative T1 and T2 imaging [39,40] , T2* mapping [41][42][43] and susceptibility-weighted imaging (SWI) [44] , have been proposed to improve the visibility of the target structures. For the ZI no respective data exists.…”

Optimized 3D Magnetization-Prepared Rapid Acquisition of Gradient Echo: Identification of Thalamus Substructures at 3T

“…Whether SWI images at 3T provide a similar contrast and if the combination of the different contrasts of T1 and SWI further improves segmentation need further investigation. Gringel et al 21 reported that with multiparameter magnetization-transfer maps, improved contrasts can be obtained within the thalamus in a measurement time of 23-25 minutes, but postprocessing is required to calculate the multiparameter maps. The multiecho fast low-angle shot sequence needed is not available as a standard sequence on most scanners; this requirement is currently another drawback for clinical application.…”

Generation of Individualized Thalamus Target Maps by Using Statistical Shape Models and Thalamocortical Tractography

“…[4][5][6][7][8] In parallel to the effort of providing stereotactic atlases that incorporate clinical imaging-based anatomic correspondences, 9 it is desirable to optimize MR imaging sequences and image-processing techniques for the direct identification of thalamic nuclei. [10][11][12] A promising technique to noninvasively chart the human thalamus is diffusion tensor imaging 13 and subsequent probabilistic tractography, which can be applied to map the networks of thalamocortical connectivity. [14][15][16] Such methods are based on the fact that various thalamic nuclei present differential interconnections; therefore, this information can be used to subdivide the thalamus into domains that have different dominant connections with preselected cortical domains.…”