  The thalamus is a subcortical structure of central importance to brain
function, which is organized in smaller nuclei with specialized roles. Despite
significant functional and clinical relevance, locating and distinguishing the
different thalamic nuclei in vivo, non-invasively, has proved challenging with
conventional imaging techniques, such as T$_{1}$ and T$_{2}$-weighted magnetic
resonance imaging (MRI). This key limitation has prompted extensive research
efforts, and several new candidate MRI sequences for thalamic imaging have been
proposed, especially at 7T. However, studies to date have mainly been centered
on individual techniques, and often focused on subsets of specific nuclei. It
is now critical to evaluate which options are best for which nuclei, and which
are globally the most informative. This work addresses these questions through
a comprehensive evaluation of thalamic structural imaging techniques in humans
at 7T, including several variants of T$_{1}$, T$_{2}$, T$_{2}$* and magnetic
susceptibility-based contrasts. All images were obtained from the same
participants, to allow direct comparisons without anatomical variability
confounds. The different contrasts were qualitatively and quantitatively
analyzed with dedicated approaches, referenced to well-established thalamic
atlases. Overall, the analyses showed that quantitative susceptibility mapping
(QSM) and T$_{1}$-weighted MP2RAGE tuned to maximize gray-to-white matter
contrast are currently the most valuable options. The two contrasts display
unique, complementary features and, together, enable the distinction of the
majority of known nuclei. Likewise, their combined information could provide a
powerful input for automatic segmentation approaches. To our knowledge, this
study represents the most comprehensive assessment of structural MRI contrasts
for thalamic imaging to date.
