Reproducibility and Reliability of Quantitative and Weighted T1 and T2∗ Mapping for Myelin-Based Cortical Parcellation at 7 Tesla

Abstract

Different magnetic resonance (MR) parameters, such as R1 (=1/T1) or T2∗, have been used to visualize non-invasively the myelin distribution across the cortical sheet. Myelin contrast is consistently enhanced in the primary sensory and some higher order cortical areas (such as MT or the cingulate cortex), which renders it suitable for subject-specific anatomical cortical parcellation. However, no systematic comparison has been performed between the previously proposed MR parameters, i.e., the longitudinal and transversal relaxation values (or their ratios), for myelin mapping at 7 Tesla. In addition, usually these MR parameters are acquired in a non-quantitative manner (“weighted” parameters). Here, we evaluated the differences in ‘parcellability,’ contrast-to-noise ratio (CNR) and inter- and intra-subject variability and reproducibility, respectively, between high-resolution cortical surface maps based on these weighted MR parameters and their quantitative counterparts in ten healthy subjects. All parameters were obtained in a similar acquisition time and possible transmit- or receive-biases were removed during post-processing. It was found that CNR per unit time and parcellability were lower for the transversal compared to the longitudinal relaxation parameters. Further, quantitative R1 was characterized by the lowest inter- and intra-subject coefficient of variation (5.53 and 1.63%, respectively), making R1 a better parameter to map the myelin distribution compared to the other parameters. Moreover, quantitative MRI approaches offer the advantage of absolute rather than relative characterization of the underlying biochemical composition of the tissue, allowing more reliable comparison within subjects and between healthy subjects and patients. Finally, we explored two parcellation methods (thresholding the MR parameter values vs. surface gradients of these values) to determine areal borders based on the cortical surface pattern. It is shown that both methods are partially observer-dependent, needing manual interaction (i.e., choice of threshold or connecting high gradient values) to provide unambiguous borders.

Improved Visualization of Cortical Lesions in Multiple Sclerosis Using 7T MP2RAGE

“…Recent studies in humans and nonhuman primates have demonstrated the value of T1-based MR imaging for visualizing the myeloarchitecture of the cortex. [23][24][25] Among the different T1 MR imaging techniques, magnetization prepared 2 rapid acquisition gradient echoes (MP2RAGE) provides the best tissue contrast at high spatial resolution while minimizing field inhomogeneities that affect other techniques at 7T. 26,27 We achieved high resolution while preserving image quality by combining multiple repetitions of the same 3D MP2RAGE acquisition.…”

The impact of correction on MP2RAGE cortical T1 and apparent cortical thickness at 7T

“…That is, the variation of T 1 values across the cortex is remarkably low after ${\mathrm{normalB}}_1^{+}$ correction. For a comparison of the intersubject and scan–rescan COV with that of other (quantitative) contrasts (e.g., T 2 *), we refer the reader to Haast et al (2016). The reduction of the superior‐to‐inferior gradient of low‐to‐high T 1 —that led to the initially high COV across regions—resulted in a cortical T 1 distribution that is more comparable with previous studies at lower field strengths and are less affected by ${\mathrm{normalB}}_1^{+}$ effects, revealing the typical myelin‐related cortical pattern (Glasser & Van Essen, 2011; Lutti, Dick, Sereno, & Weiskopf, 2014).…”

“…For the Sa2RAGE, the parameters were: TR/TE = 2,400/0.78 ms, TD 1 /TD 2  = 58/1,800 ms, α 1 /α 2  = 4°/11°, and GRAPPA factor = 2 in PE direction (anterior–posterior) with 24 references lines. See Haast, Ivanov, Formisano, and Uludag (2016) for further details on other scanning parameters. The TR‐FOCI inversion pulse (Hurley et al, 2010) and dielectric pads containing a 25% suspension of barium titanate in deuterated water, placed around the head proximal to the temporal lobe areas (Teeuwisse et al, 2012), were used to improve ${\mathrm{normalB}}_1^{+}$ homogeneity across the brain and locally, respectively.…”

Myeloarchitecture gradients in the human insula serve as blueprints for its diverse connectivity and function

“…Quantitative T1 (qT1) relaxometry data was acquired using a 3D-MP2RAGE sequence (0.8mm isotropic voxels, 240 sagittal slices, TR=5000ms, TE=2.9ms, TI 1=940ms, T1 2=2830ms, flip angle 1=4°, flip angle 2=5°, iPAT=3, bandwidth=270 Hz/px, echo spacing=7.2ms, partial Fourier=6/8). We combined two inversion images for qT1 mapping in order to minimise sensitivity to B1 inhomogeneities and optimize intra-and intersubject reliability (74,75).…”