In Vivo Brain MR Imaging at Subnanoliter Resolution: Contrast and Histology

This article provides an overview of in vivo magnetic resonance (MR) imaging contrasts obtained for mammalian brain in relation to histological knowledge. Emphasis is paid to the (1) significance of high spatial resolution for the optimization of T1, T2, and magnetization transfer contrast, (2) use of exogenous extra- and intracellular contrast agents for validating endogenous contrast sources, and (3) histological structures and biochemical compounds underlying these contrasts and (4) their relevance to neuroradiology. Comparisons between MR imaging at subnanoliter resolution and histological data indicate that (a) myelin sheaths, (b) nerve cells, and (c) the neuropil are most responsible for observed MR imaging contrasts, while (a) diamagnetic macromolecules, (b) intracellular paramagnetic ions, and (c) extracellular free water, respectively, emerge as the dominant factors. Enhanced relaxation rates due to paramagnetic ions, such as iron and manganese, have been observed for oligodendrocytes, astrocytes, microglia, and blood cells in the brain as well as for nerve cells. Taken together, a plethora of observations suggests that the delineation of specific structures in high-resolution MR imaging of mammalian brain and the absence of corresponding contrasts in MR imaging of the human brain do not necessarily indicate differences between species but may be explained by partial volume effects. Second, paramagnetic ions are required in active cells in vivo which may reduce the magnetization transfer ratio in the brain through accelerated T1 recovery. Third, reductions of the magnetization transfer ratio may be more sensitive to a particular pathological condition, such as astrocytosis, microglial activation, inflammation, and demyelination, than changes in relaxation. This is because the simultaneous occurrence of increased paramagnetic ions (i.e., shorter relaxation times) and increased free water (i.e., longer relaxation times) may cancel T1 or T2 effects, whereas both processes reduce the magnetization transfer ratio.

Cell layers and neuropil: contrast-enhanced MRI of mouse brain in vivo

Contrast-enhanced T₁- and T₂-weighted MRI at 9.4 T and in-plane resolutions of 25 and 30 µm has been demonstrated to differentiate between neural tissues in mouse brain in vivo, including granule cell layers, principal cell layers, general neuropil, specialized neuropil and white matter. In T₁-weighted MRI of the olfactory bulb, hippocampus and cerebellum, contrast obtained by the intracranial administration of gadopentetate dimeglumine (Gd-DTPA) reflects the extra- and intracellular spaces of gray matter in agreement with histological data. General neuropil areas are highlighted, whereas other tissues present with lower signal intensities. The induced contrast is similar to that in plain T₂-weighted MRI, but offers a 16-30-fold higher contrast-to-noise ratio. Systemic administration of manganese chloride increases the signal-to-noise ratio in T₁-weighted MRI to a significantly greater extent in principal cell layers and specialized neuropil than in granule cell layers, whereas gadolinium-enhanced MRI indicates no larger intracellular spaces in these tissues. Granule cell layers are enhanced no more than general neuropil by manganese, whereas gadolinium-enhanced MRI indicates significantly larger intracellular spaces in the cell layers. These discrepancies suggest that the signal increase after manganese administration reflects cellular activity which is disproportionate to the intracellular space. As a result, principal cell layers and specialized neuropil become highlighted, whereas granule cell layers, general neuropil and white matter present with lower signal intensities.

Myelin mapping in the central nervous system of living mice using contrast-enhanced magnetization transfer MRI

This work compares magnetization transfer (MT) MRI of living mice with contrast-enhanced MT MRI using intraventricular administration of gadopentetate dimeglumine (Gd-DTPA), systemic administration of MnCl2, and both. In MT MRI at 9.4 T, the contrast-to-noise ratio (CNR) between white matter (WM) and gray matter (GM) increased by 85% after Gd-DTPA injection into the lateral ventricle. When applied in conjunction with manganese-enhanced MT MRI (117 μm isotropic resolution, 6 min measuring time), Gd-DTPA boosted the CNR increase from +56% to +117%. Additional T1 measurements at 2.35 T revealed that intraventricular Gd-DTPA shortens the T1 of GM much more than that of WM, which corresponds to estimated extracellular spaces of 26% in GM and only 15% in WM. These results explain the additional MT contrast enhancement by Gd-DTPA and demonstrate that the T1 shortening by intracellular Mn2+ is well complemented by extracellular Gd-DTPA. The data suggest a high myelin and low water content to hinder access of hydrophilic paramagnetic agents, so that the resulting differential accumulation effectively reduces the MT saturation in water-rich tissues and thereby facilitates the mapping of myelin-rich tissues. Finally, a 156% CNR increase between GM and WM for contrast-enhanced MT MRI at 9.4T using both Gd-DTPA and manganese allowed for 60μm isotropic resolution (102 min measuring time), which delineated myelinated fibers and layers even within GM areas such as the thalamus and cerebellar cortex. Improved MT contrasts were also seen in the cervical spinal cord.