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T 1 map reveals structural organization within gray matter of marmoset sensorimotor cortex. T 1 (ms). Posterior ends of lateral sulci. T 1 map (resolution: 0.35 mm isotropic), shown every other coronal slice in posterior to anterior order (#1 to #12). #6. 2050. #1. 1750. #7. #12.
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T1 map reveals structural organization within gray matter of marmoset sensorimotor cortex
of lateral sulci
T1 map (resolution: 0.35 mm isotropic), shown every other coronal slice in posterior to anterior order (#1 to #12).
T1 map matches cortical myeloarchitecture
Cortical myeloarchitecture is obtained by staining of brain slices for myelin (lower T1 = higher myelination = darker in image).
Flattened Cortex View
Single Slice View
Myelin staining (from Ref.7)
Physical flatteningof cortical gray matter peeled post-mortem
Digital flatteningof the slab 0.7-1.3 mm away from gray-white matter boundary.
T1 map (acquired by EPI)
Slice thickness: 0.5 mm, posterior to anterior order
Based only on the T1 map, cyan lines were drawn to demarcate 2 of the 3 major low-T1 bands within SI, and the medial side of SII.
Arm electrical stimulation
Significance of fMRI BOLD response to each of the two stimulation condi-tions was computed (as coherence).
Leg electrical stimulation
Structural MRI-measured T1 Map Reflects Functional Topography in Primary Somatosensory Cortex of Awake Non-human Primates
Junjie V. Liu, Nicholas A. Bock, Ara Kocharyan, Yoshiyuki Hirano and Afonso C. Silva
Cerebral Microcirculation Unit, LFMI, NINDS, National Institutes of Health, Bethesda, MD, United States
Area 3b in primary somatosensory cortex (SI) of non-human primates comprises myelin-rich bands, which are innervated by thalamocortical fibers contralateral to peripheral inputs,. These bands are separated by myelin-poor gaps, which are innervated by corticocortical fibers, including interhemispheric projections (Ref.1). The functional implications of such myelination-related spatial organization of fibers are unclear.
Here we study the correspondence between myelination pattern and functional BOLD response, both of which are measured non-invasively using magnetic resonance imaging (MRI).
We show that the contralateral SI response is organized into a topographic map that matches the myelination pattern, whereas the ipsilateral SI response does not follow topography but rather centers near the myelination-poor gaps.
Ipsilateral response centers on low-myelination band of SI
L arm stim
R arm stim
The center of ipsilateral response (e.g. response to Left arm stim. in Left SI) is significantly more medial, closer to a low-myelination band, compared to the center of contralateral response (e.g. response to Right arm stim. in Left SI).
Co-activation analysis (see below) indicates that the ipsilateral response is not likely to be driven directly by contralateral SI high-myelination regions.
Functional MRI of awake marmosets
A model of functional pathway projecting to ipsilateral SI
Trial-to-trial co-activation map
(measures the degree of two regions activating together)
Two trained awake adult marmosets were used for fMRI. After received a daily acclimation process and habituated to a sham scanner bore (3 weeks), during each fMRI session the animal was strapped in prone position with a light body-restraining harness, and its head was secured by head posts. The animal was monitored by a camera and appeared relaxed.
FMRI scans used EPI, TE/TR = 20/2000 ms, Ernst flip angle, Matrix: 96 (left-right) × 72 (dorsal-ventral). FOV: 32 × 24 mm, 1-shot, bandwidth: 227kHz. Twelve slices (thickness: 0.5 mm). Effective resolution: 0.5 mm isotropic.
Each fMRI scan comprises a series of blocks of stimulation; each block (40 s) comprises 8 s of stimulation pulses (1.5 mA, 400 us, repeated at 40 Hz) followed by 32 s without stimulation.
Each block delivered unilateral electrical stimulation of either the arm or the leg, via a pair of contact electrodes (diam: 8 mm).
Seed region: Contra SI
T1 map matches the somatotopic map derived by functional MRI
Structural MRI: 2D and 3D T1 mapping
T1 mapping was conducted in two ways: 3D volumetric mapping and 2D multi-slice mapping. The only difference between 3D and 2D T1 maps was that the latter had geometric distortions caused by EPI. 3D T1 maps were directly comparable to brain slices stained for myelin. 2D T1 maps were directly comparable to functional MRI response maps because they were measured by the same EPI paradigm.
We used a linear T1-mapping algorithm that takes very short imaging time (just 3 inversion times needed) and very short post-processing time, and is largely insensitive to RF/coil inhomogeneities. The conventional algorithm is to fit data acquired at different inversion times (TI) to the nonlinear function S = A+Bexp(-TI/T1), whereas our linear algorithm solves T1 directly from data at the 3 TIs:
3D T1 mapping is conducted using the standard magnetization-prepared rapid gradient-echo (MPRAGE) sequences, which are available on many MRI scanners and routinely used in both clinics and research. Specific details:3 runs of MPRAGE at inversion time of 150, 1400, 4700 ms respectively. Matrix: 108 (left-right) × 90 (dorsal-ventral) × 48 (anterior-posterior). Voxel size (resolution): 0.35 mm isotropic. TE/TR: 2.65/9.3 ms. Flip angle: 9˚. Number of segments: 3. Segment delay: 5000 ms. Nine repeats for each run.
In-vivo T1 mapping reveals myelination pattern in area 3b that reflects the spatial organization of thalamocortical and corticocortical fibers. FMRI responses in contralateral area 3b, driven mainly by thalamocortical inputs, are organized by somatotopy such that highly myelinated bands comprise representations of major body parts. Surprisingly, ipsilateral and contralateral SI representations of the same body part (e.g. arm) do not match. FMRI responses in ipsilateral area 3b, driven mainly by corticocortical projections, center near the myelin-poor gaps, not the myelin-rich bands.
The method presented here is a non-invasive alternative to classical histology and electrophysiological methods that map neural receptive fields. The large spatial coverage of functional MRI facilitates the detection of weak ipsilateral responses.
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