Magnetic transfer contrast accurately localizes substantia nigra confirmed by histology

Abstract

Background: Magnetic resonance imaging (MRI) has multiple contrast mechanisms. Like various staining techniques in histology, each contrast type reveals different information about the structure of the brain. However, it is not always clear how structures visible in MRI correspond to structures previously identified by histology. The purpose of this study was to determine if magnetic transfer contrast (MTC) or T2 contrast MRI was better at delineating the substantia nigra (SN).

Methods: MRI scans were acquired in vivo from two nonhuman primates (NHPs). The NHPs were subsequently euthanized, perfused, and their brains sectioned for histologic analyses. Each slice was photographed before sectioning. Each brain was sectioned into approximately 500 sections, 40 μm each, encompassing most of the cortex, midbrain, and dorsal parts of the hindbrain. Levels corresponding to anatomic MRI images were selected. From these, adjacent sections were stained using Kluver-Barrera (myelin and cell bodies) or tyrosine hydroxylase (dopaminergic neurons) immunohistochemistry. The resulting images were coregistered to the block-face images using a moving least squares algorithm with similarity transformations. MR images were similarly coregistered to the block-face images, allowing the structures on MRI to be identified with structures on the histologic images.

Results: We found that hyperintense (light) areas in MTC images were coextensive with the SN as delineated histologically. The hypointense (dark) areas in T2-weighted images were not coextensive with the SN but extended partially into the SN and partially into the cerebral peduncles.

Conclusions: MTC is more accurate than T2-weighting for localizing the SN in vivo.

Figure 1

Proton density (PD) and magnetization transfer contrast (MTC) images from the same subject and same axial section at the level of the SN: left side, (A) PD image, right side, (B) MTC image.

Figure 2

Illustration of histology data aligned with block-face and MR images. All images come from the same subject and same axial section at the level of the SN. Histology and MR images are aligned with their corresponding block-face image. The right column illustrates succesful alignment of the KB histology onto the images from the left column. Images from top to bottom: (A) block-face image acquired during sectioning with (B) aligned KB image overlaid, (C) aligned MTC MR image with (D) aligned KB image overlaid, (E) aligned T2 MR image with (F) aligned KB image overlaid. Scale bar, 1 cm.

Figure 3

Aligned KB stain and TH stain images overlaid on corresponding block-face image at 3 levels of the SN. Left column shows overlaid KB images, right column shows TH images. Cerebral peduncles are outlined in blue and the SN is outlined in red. (A) KB and (B) TH axial sections through the caudal portion of the SN at the level of the cerebellar peduncles (black arrow, panel A) (C) KB and (D) TH axial sections at the level of cranial nerve III rootlets (black arrow, panel C). (E) KB and (F) TH axial sections through the rostral portion of the SN at the level of the optic tract (black arrow, panel E). Scale bar, 1 cm.

Figure 4

Illustration of correspondence of cerebral peduncles with hypointense areas and SN with hyperintense bands in MTC contrast MR images. The cerebral peduncles are outlined in blue and SN is outlined in red. The outlines and levels of section are the same as in figure 3. At each level the hyperintense bands correspond well to the SN as determined from the histology. (A) MTC image and (B) overlaid outlines through the caudal portion of the SN. (C) MTC image and (D) overlaid outlines at the level of cranial nerve III. (E) MTC image and (F) overlaid outlines through the rostral portion of the SN. Scale bar, 1 cm.

Figure 5

Illustration of poor correspondence of cerebral peduncles and SN with hypointense areas in T2 contrast MR images. The cerebral peduncles are outlined in blue and SN is outlined in red. The outlines and levels of section are the same as figure 3. At no level do the hypointense areas correspond well to the SN as determined from the histology. (A) T2 image and (B) overlaid outlines through the caudal portion of the SN. (C) T2 image and (D) overlaid outlines at the level of cranial nerve III. (E) T2 image and (F) overlaid outlines through the rostral portion of the SN. Scale bar, 1 cm.