Comparison of human septal nuclei MRI measurements using automated segmentation and a new manual protocol based on histology

Abstract

Septal nuclei, located in basal forebrain, are strongly connected with hippocampi and important in learning and memory, but have received limited research attention in human MRI studies. While probabilistic maps for estimating septal volume on MRI are now available, they have not been independently validated against manual tracing of MRI, typically considered the gold standard for delineating brain structures. We developed a protocol for manual tracing of the human septal region on MRI based on examination of neuroanatomical specimens. We applied this tracing protocol to T1 MRI scans (n=86) from subjects with temporal epilepsy and healthy controls to measure septal volume. To assess the inter-rater reliability of the protocol, a second tracer used the same protocol on 20 scans that were randomly selected from the 72 healthy controls. In addition to measuring septal volume, maximum septal thickness between the ventricles was measured and recorded. The same scans (n=86) were also analyzed using septal probabilistic maps and DARTEL toolbox in SPM. Results show that our manual tracing algorithm is reliable, and that septal volume measurements obtained via manual and automated methods correlate significantly with each other (p<.001). Both manual and automated methods detected significantly enlarged septal nuclei in patients with temporal lobe epilepsy in accord with a proposed compensatory neuroplastic process related to the strong connections between septal nuclei and hippocampi. Septal thickness, which was simple to measure with excellent inter-rater reliability, correlated well with both manual and automated septal volume, suggesting it could serve as an easy-to-measure surrogate for septal volume in future studies. Our results call attention to the important though understudied human septal region, confirm its enlargement in temporal lobe epilepsy, and provide a reliable new manual delineation protocol that will facilitate continued study of this critical region.

Figure 1

Example of CH1-CH2 (septal) delineation on 200-um-thick coronal cresyl violet-stained sections of the formalin-fixed, celloidin-embedded brain from a 52 year old male who died without neurological disease. A: hemispheric section at the level of the globus pallidus (GP), caudate nucleus (CN) and putamen (Pu). The contours of the CH1 and CH2 nuclei are outlined in white. Because the major portion of the CH1 and CH2 septal nuclei is located on the level of the globus pallidus, the anterior GP extent has been used as a landmark for MRI tracing protocol. Note the strictly medial location of these nuclei. B. Low magnification (lens 2.5x) view of region demarcated by black rectangle in Panel A. C. Medium magnification (lens 40x) view to verify topography, borders and cytoarchitecture of CH1 and CH2 nuclei. CH1 (top) is composed mainly of small to medium size round neurons, whereas CH2 (bottom) has larger hyperchromatic neurons with prominent nucleus and dark nucleolus.

Figure 2

Manual definition of septal region on T1 template in MNI space (Collins et al., 1994) with tracing protocol landmarks shown. The y coordinates indicate distances from the anterior commissure in mm in the rostro-caudal directions. The left hemisphere is left in the image. The septal region on the template is present on four 1 mm thick coronal slices (B,C,D,E) and is shaded in red. A. Slice anterior to the septal nuclei in which globus pallidus is not visible and no septal gray matter is apparent. B. 1^st slice traced in which the most anterior portion of septal region, shaded in red, is defined by the anterior-most appearance of bilateral globus pallidus (blue arrows, labeled GP). C. 2^nd slice traced in which the crossing fibers of the anterior commissure (green arrow, labeled AC) are incompletely visualized. . D. 3^rd slice traced in which crossing fibers of the anterior commissure (green arrow) are again visible but still appear incomplete medially. Yellow lines show the maximal septal thickness measurement. E. 4^th and final slice traced. Because the anterior commissure is fully visible in this slice (green arrow), it serves as the inferior border of tracing. F. This slice was not considered to include any septal gray matter because the region had intensity identical to white matter, and CSF (purple arrow) was visible between the columns of the fornix. See text for details of tracing rules.

Figure 3

Colocalization of manual and automated septal ROI's in a control subject. Manual ROI is yellow, automated ROI is blue, and overlap is green. The automated ROI extends several slices anterior and posterior to those presented, with only a small number of voxels on each of these slices.

Figure 4

Plot showing correlation of manual and automatic septal measurements in patients and controls.