In vivo detection of individual glomeruli in the rodent olfactory bulb using manganese enhanced MRI

Abstract

MRI contrast based on relaxation times, proton density, or signal phase have been applied to delineate neural structures in the brain. However, neural units such as cortical layers and columns have been difficult to identify using these methods. Manganese ion delivered either systemically or injected directly has been shown to accumulate specifically within cellular areas of the brain enabling the differentiation of layers within the hippocampus, cortex, cerebellum, and olfactory bulb in vivo. Here we show the ability to detect individual olfactory glomeruli using manganese enhanced MRI (MEMRI). Glomeruli are anatomically distinct structures ( approximately 150 microm in diameter) on the surface of the olfactory bulb that represent the first processing units for olfactory sensory information. Following systemic delivery of MnCl(2) we used 3D-MRI with 50 microm isotropic resolution to detect discrete spots of increased signal intensity between 100 and 200 microm in diameter in the glomerular layer of the rat olfactory bulb. Inflow effects of arterial blood and susceptibility effects of venous blood were suppressed and were evaluated by comparing the location of vessels in the bulb to areas of manganese enhancement using iron oxide to increase vessel contrast. These potential vascular effects did not explain the contrast detected. Nissl staining of individual glomeruli were also compared to MEMRI images from the same animals clearly demonstrating that many of the manganese enhanced regions corresponded to individual olfactory glomeruli. Thus, MEMRI can be used as a non-invasive means to detect olfactory glomeruli for longitudinal studies looking at neural plasticity during olfactory development or possible degeneration associated with disease.

Fig. 1

(A) Coronal sections of 3D T₁-weighted MRI of a rat before (left) and 24 h after (right) intravenous injection of MnCl₂. Mn²⁺ enhanced spots could be identified in the dorsal (arrows), lateral, and medial portions of the glomerular layer. The scale bar represents 1 mm. (B) The SNR profiles in the upper half of the glomerular layer (the red line in the inserted image) show significant increase in SNR after Mn²⁺ injection (noted the vertical scales of the two profiles are different). Especially higher contrast could be seen at certain locations (arrows) with widths about 100–200 microns, which correspond well with the dimensions of olfactory glomeruli. (C) The SNR profiles in the mitral cell layer (the red line in the inserted image) underneath the glomerular layer in (B) only has general increase in SNR but doesn’t show organized structures.

Fig. 2

(A) T₁-weighted images of a rat before (left) and after (right) euthanasia. (B) The SNR profiles along the glomerular layer (indicated as the red curve in (A)) are similar between live (black line) and dead (gray line). Without Mn²⁺ enhancement, no distinguishable structure can be found in the difference between the two profiles (light gray line).

Fig. 3

(A) Horizontal (top) and coronal (bottom) sections of the gradient echo images after injection of MION show that the boundaries at certain locations in the MEMRI (e.g., the blue arrowheads) are formed by blood vessels surrounding or penetrating the glomerular layer. However, there are also many Mn²⁺ enhanced spots (e.g., the red arrowheads) don’t have visible vessels nearby. (B) Blood vessels can be delineated by subtracting the gradient echo image before MION injection by the image after injection: MEMRI (left); vessel map (middle); vessels overlaid on MEMRI (right). The scale bar represents 1mm. (C) The SNR profile of MEMRI in the glomerular layer (black line; location shown as the red line in (B)) shows that Mn²⁺ enhanced spots usually have blood vessels (gray line) in between. However, the Mn²⁺ contrast doesn’t correlate with the signal change caused by MION.

Fig. 4

The manganese-enhanced spots (arrows) identified in MEMRI (left column) correspond well with the glomeruli shown in the Nissl staining (right column) of the same animal. A and B are the images from two rats. The scale bar represents 1 mm.