Visualization of intra-thalamic nuclei with optimized white-matter-nulled MPRAGE at 7T

Abstract

Novel MR image acquisition strategies have been investigated to elicit contrast within the thalamus, but direct visualization of individual thalamic nuclei remains a challenge because of their small size and the low intrinsic contrast between adjacent nuclei. We present a step-by-step specific optimization of the 3D MPRAGE pulse sequence at 7T to visualize the intra-thalamic nuclei. We first measured T1 values within different sub-regions of the thalamus at 7T in 5 individuals. We used these to perform simulations and sequential experimental measurements (n=17) to tune the parameters of the MPRAGE sequence. The optimal set of parameters was used to collect high-quality data in 6 additional volunteers. Delineation of thalamic nuclei was performed twice by one rater and MR-defined nuclei were compared to the classic Morel histological atlas. T1 values within the thalamus ranged from 1400ms to 1800ms for adjacent nuclei. Using these values for theoretical evaluations combined with in vivo measurements, we showed that a short inversion time (TI) close to the white matter null regime (TI=670ms) enhanced the contrast between the thalamus and the surrounding tissues, and best revealed intra-thalamic contrast. At this particular nulling regime, lengthening the time between successive inversion pulses (TS=6000ms) increased the thalamic signal and contrast and lengthening the α pulse train time (N*TR) further increased the thalamic signal. Finally, a low flip angle during the gradient echo acquisition (α=4°) was observed to mitigate the blur induced by the evolution of the magnetization along the α pulse train. This optimized set of parameters enabled the 3D delineation of 15 substructures in all 6 individuals; these substructures corresponded well with the known anatomical structures of the thalamus based on the classical Morel atlas. The mean Euclidean distance between the centers of mass of MR- and Morel atlas-defined nuclei was 2.67mm (±1.02mm). The reproducibility of the MR-defined nuclei was excellent with intraclass correlation coefficient measured at 0.997 and a mean Euclidean distance between corresponding centers of mass found at first versus second readings of 0.69mm (±0.38mm). This 7T strategy paves the way to better identification of thalamic nuclei for neurosurgical planning and investigation of regional changes in neurological disorders.

Direct Visualization of Anatomic Subfields within the Superior Aspect of the Human Lateral Thalamus by MRI at 7T

“…Also the La.m appears as a rather broad and diffuse band in the quantitative susceptibility map, notwithstanding the extremely high 0.4 3 -mm 3 spatial resolution of the underlying MR images. The white matter-nulled MPRAGE images of Tourdias et al 19 explicitly indicated the nuclei ventral lateral anterior and posterior in the dorsal thalamus; however, they did not depict a structure similar to the D.o.i/Z.im.i boundary shown here. The coarser appearance of the La.m and of boundaries between the nuclei in their axial images is probably a consequence of the isotropic 1-mm 3 resolution.…”

“…Ultimately, an inversion recovery approach proved to be most suitable. The popularity of gradient echo imaging at 7T [17][18][19] is driven, to a high degree, by the possibility of covering the whole brain in a couple of minutes in compliance with specific absorption rate regulations. The applied IR-TSE sequence, with adiabatic inversion and full 180°refocussing pulses, allows us to measure only a small brain slab within legal specific absorption rate limits.…”

“…[17][18][19] Abosch et al 17 and Deistung et al 18 presented promising images directly dorsal to the ACPC The MR images were acquired with a resolution of 0.5 2 ϫ 3 mm 3 (IR-TSE, TI ϭ 500 ms) and 0.5 2 ϫ 2.5 mm 3 (TSE). In the bottom row, which is identical to the top row, arrows point to vessels, and arrowheads, to bright spots.…”

“…Indeed, the first encouraging results have been reported by using SWI, quantitative susceptibility mapping, and MPRAGE imaging. [17][18][19] This feasibility study aims to investigate the usefulness of inversion recovery turbo-spin-echo (IR-TSE) MR imaging at 7T in revealing anatomic detail within the thalamus. This task comprises both the optimization of the MR imaging acquisition and the assignment of visualized structures to thalamic anatomy.…”

Thalamic structures and associated cognitive functions: Relations with age and aging

“…Recently, enhanced visualization of intra-thalamic nuclei has been demonstrated with ultra high field MRI imaging at 7T (Saranathan et al, 2014; Tourdias et al, 2014). Optimization of the 3D MPRAGE (magnetization-prepared rapidly-acquired gradient echo) enabled 3D delineation of 15 substructures, including the mediodorsal nucleus, pulvinar, ventral lateral posterior nucleus, lateral dorsal nucleus, and the mammillothalamic tract.…”