Quantitative MRI to Study the Structure of the Human Midbrain

The anatomy of the brain nuclei located in the midbrain are challenging in view of their small size and limited available signal-to-noise-ratio. 9.4T offer distinct advantages, with greater contrast effects between structures and sufficient SNR even for voxel sizes of a few hundred micrometers.

1. Superior colliculus; 2. Periaqueductal gray; 3. Nucleus of the oculomotor nerve; 4. Cerebral aqueduct; 5. Red nucleus (parvocellular part); 6. Substantia nigra (pars compacta); 7. Substantia nigra (pars reticulata); 8. Medial geniculate body (dorsal subnucleus); 9. Medial lemniscus; 10. Central tegmental tract; 11. Frontopontine tract; 12. Pyramidal tract; 13. Parietotemporopontine tract; 14. Oculomotor nerve; 15. Brachium of the superior colliculus; 16. Brachium of the inferior colliculus. For more details see pp. 234-235, Fig 6.32A,B, Section II Structure of Spinal Cord and Brain Parts in Nieuwenhuys R, Voogd J, Huiijzen C van; Human Central Nervous System - A synopsis and atlas, 4th ed, Springer, Berlin-Heidelberg-New York 2008.

Depicting the exact anatomy of the brain nuclei located in the midbrain are challenging in view of their small size and the limited available signal-to-noise-ratio. Another challenge is posed by the inhomogenous excitation field that requires development of alternative quantification strategies than at clinical fields [1]. Once overcome, ultra-high fields offer greater contrast effects between structures and sufficient SNR even for voxel sizes of a few hundred micrometers. For instance, we found that the combined use of several quantitative MRI parameters provides a mean to study the layer specific anatomy of the Superior Colliculi in vivo, a brain structure which serves as a gateway for several sensory modalities. Our quantitative MRI data [2] exhibited features reminiscent of local microstructure previously described in the histology literature. These features could be reproducibly observed in 14 subjects. Distinct variation of R2* values were observed 2mm below the SC surface, likely reflecting the myelinated fibers in the superficial optic layer (layer III). At a depth matching the intermediate gray layer (IV) composed of multipolar neurons, a second increase in R2* was paralleled by a paramagnetic shift in QSM suggesting the presence of an iron-rich layer. In collaboration with the group of Dr. Ress at the Baylor College of Medicine in Houston, we are currently exploring the possibility to observe these effects in 3D using a standardized coordinate system [3].

1.
Hagberg GE, Bause J, Ethofer T, Ehses P, Dresler T, Herbert C, Pohmann R, Shajan G, Fallgatter A, Pavlova MA, Scheffler K.:
Whole brain MP2RAGE-based mapping of the longitudinal relaxation time at 9.4T.
Neuroimage 2017;144(Pt A):203-216.
2.
] Loureiro JR, Himmelbach M, Ethofer T, Pohmann R, Martin P, Bause J, Grodd W, Scheffler K, Hagberg GE:
In-vivo quantitative structural imaging of the human midbrain and the superior colliculus at 9.4T.
Neuroimage. 2018;177:117-128.
3.
Truong P, Kim JH, Savjani R, Sitek KR, Hagberg GE, Scheffler K, Ress D:
Depth relationships and measures of tissue thickness in dorsal midbrain.
Hum Brain Mapp. 2020;41(18):5083-5096.  
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