Imaging research maps elusive area of brain

TORONTO and MONTREAL – A collaborative breakthrough between Sunnybrook Health Sciences, University Health Network, University of Toronto and McGill University is unlocking the brain’s elusive higher-cognition hub and opens a new frontier for clinical neuroscience.

For decades, neuroscientists believed that the claustrum – a mysterious, super-thin sheet of neurons deep inside the brain, famously speculated to coordinate human consciousness – was simply too thin to be reliably captured in a living human using MRI.

“Despite the challenge of the claustrum’s physical profile being incredibly daunting for medical imaging, we were able to demonstrate that submillimeter MRI is fully capable of mapping this elusive structure in living humans,” said Dr. Kâmil Uludağ (pictured), senior author of the study and a senior scientist at Sunnybrook Research Institute and the Krembil Brain Institute. “It was like finding a sub-millimeter needle in a haystack.”

The pioneering study, published in the journal PNAS, combined a state-of-the-art 7-Tesla (7T) MRI with a newly engineered, 3D histological “gold standard” model to achieve this milestone.

Why the claustrum eluded science
The claustrum is a highly connected subcortical structure that coordinates massive networks across the brain, but imaging it presented three major hurdles:

• The Geometry: It is a curved, undulating sheet often only a few hundred microns thick.
• The Location: It is tightly sandwiched between other major brain structures (the putamen and the insula), and broaches on the amygdala, separated only by thin bands of white matter.
• The Resolution Barrier: On conventional 3-Tesla (3T) MRI scans, spatial limitations cause “partial voluming” (averaging the claustrum with surrounding tissues). Historically, this led to highly inconsistent, fourfold discrepancies in measurements, leading many to believe the structure was entirely inaccessible in vivo.

The breakthrough: A two-pronged approach
To establish a definitive anatomical foundation for the field, the research team bridged the gap between histology and ultra-high field MRI:

1. Building the 3D “gold standard” reference
The researchers manually segmented the ultra-high resolution dataset from the BigBrain Project – the first openly accessible, microscopic resolution 3D model of the human brain – to construct the first continuous, 3D histological model of the human claustrum at a staggering 100-micron resolution.

This interactive map revealed that while the claustrum is paper-thin, it spans over 5 cm front-to-back and top-to-bottom and has a total volume twice that of the substantia nigra, a critical structure in the midbrain that can now be reliably imaged.

2. Testing the limits of 7-Tesla imaging
The team compared this histological benchmark against 7T MRI scans at three different high spatial resolutions: 0.5 mm, 0.7 mm, and 1.0 mm isotropic.

“At 0.5 mm isotropic resolution, which approaches the best resolution that can be acquired from living participants, MRI can isolate the claustrum from its neighbouring structures and ultimately resemble the structure as revealed by histology,” said Navona Calarco, lead author of the study and a researcher at the Krembil Brain Institute at University Health Network.

“While the claustrum’s fragmented ventral ‘puddles’ extending into the temporal lobe remain a challenge, 7T MRI reliably captures the dense dorsal ‘core’ of the claustrum, which houses its primary sensory-motor and frontal connections, and underlies the most exciting speculations about claustral function.”

Why this matters for future medicine
By proving that the claustrum is a viable target for high-resolution imaging, this study opens new opportunities for human brain mapping, neuroimaging and future clinical neuroscience.

“Together, these findings overturn the view that the human claustrum is inaccessible to MRI and establish a foundation for future functional and clinical investigation in the living human brain,” said Dr. Uludağ, also a senior scientist at the Krembil Brain Institute at University Health Network and a professor in the Department of Medical Biophysics with the Temerty Faculty of Medicine at the University of Toronto.