Experimental autoimmune encephalomyelitis in the rat spinal cord: lesion detection with high-resolution MR microscopy at 17.6 T

Abstract

Background and purpose: Experimental autoimmune encephalomyelitis (EAE) is an inflammatory demyelinating disorder of the CNS and an animal model of multiple sclerosis. We used high-field MR microscopy at 17.6 T to image spinal cord inflammatory lesions in the acute stage of chronic relapsing rat EAE. We sought to compare lesions detected on MR imaging with histopathologic findings and to quantify the inflammatory lesion load.

Methods: Imaging of fixed spinal cord specimens was performed by using a 3D gradient-echo sequence with a spatial resolution of 35 x 35 x 58 microm3 and a total imaging time of 5.5 hours. Histopathologic analysis was performed by staining axial sections with hematoxylin-eosin or Luxol fast blue to identify cellular infiltration and demyelination.

Results: Clinical signs of EAE occurred on days 10-14 after immunization. On day 22, healthy white matter and gray matter were differentiated by high contrast on T2*-weighted images, with white matter lesions appearing as hyperintense areas in the normal-appearing white matter. Inflammatory lesions identified on histopathologic evaluation were readily detected with MR imaging and vice versa. MR imaging and histopathologic analysis had excellent correlation regarding the extent of white matter lesions. Inflammatory infiltrates of gray matter were not detectable with MR imaging. Using a semiautomatic segmentation of the acquired MR data, we could quantify white matter lesion load.

Conclusion: Ex vivo high-resolution MR microscopy of the spinal cord at 17.6 T allows rapid and highly accurate determination of CNS inflammation by demonstrating virtually all histologically detectable white matter inflammatory lesions.

Fig 1.

Sagittal MR images of rat spinal cord. Arrowheads indicate the cord surface; asterisks, gray matter; white circle, white matter; and arrows, lesions. A, Thoracic cord in a control rat. B–D, Lumbar (B), thoracic (C), and cervical (D) sections in a rat with grade 4 EAE.

Fig 2.

Axial MR and histopathologic images of rat spinal cord. A, Lumbar cord in a control rat. Letter A indicates the corticospinal tract; B, gracile fasciculus; C, lateral funiculus; and D, anterior/ventral funiculus. B–D, Lumbar portion (L3) in a rat with grade 4 EAE. Multiple lesions (1–6) can be identified in the anterior and lateral parts of the white matter. Histologic section (LFB stain) in B shows an inflammatory infiltrate in the gray matter (arrow). Corresponding MR images in C and D, spaced 150 μm, account for the slightly different orientation of the axial plane introduced by histologic processing. Lesions 1–6 in A are shown.

Fig 3.

Lesion correlation in the ventral and lateral columns. Bar in A–D= 250 μm. A and D, Details of the MR image in Figure 2D show a large lesion (1) in the ventral part of the cord (A) and two lesions (2 and 3) in the lateral part (D). B, C, E, and F, Corresponding histologic sections stained for HE (B and E) and LFB (C and F) show the lesions depicted in A and Figure 2B. Note the fingerlike protrusion of lesion 1 (arrowhead) in A and C.

Fig 4.

Lesion correlation in the dorsal column. Bar in A = 250 μm. A, Detail of the MR image in Figure 2B shows only two small lesions (arrows). B and C, Corresponding histologic sections stained for HE (B) and LFB (C) show. only a few inflammatory cells (arrows).

Fig 5.

Lesion load quantification. 3D visualization of the thoracic spinal cord in the animal with grade 4 EAE. Gray matter is green; white matter lesions are blue (arrow); 1 is ventral horn; 2 is dorsal horn. Lesion load in the depicted cord is 16% of the white matter. Note the relative paucity of lesions in the dorsal part versus the ventral and lateral parts of the cord. A, Ventrolateral view. B, Dorsolateral view.