MRI of human entorhinal cortex: a reliable protocol for volumetric measurement

Abstract

A new protocol for measuring the volume of the entorhinal cortex (EC) from magnetic resonance images (MRI) was developed specifically to measure the EC from oblique coronal sections used in hippocampal volumetric studies. The relative positions of the anatomic landmarks demarcating EC boundaries were transposed from standard coronal sections to oblique ones. The lateral EC border, which is the most controversial among anatomists, was defined in a standard and conservative manner at the medial edge of the collateral sulcus. Two raters measured the EC twice for 78 subjects (healthy aged individuals, very mild AD patients, and elderly patients who did not meet criteria for dementia) to study intra- and inter-rater reproducibility and reliability of measurements. The level of accuracy achieved (coefficients of reproducibility of 1.40–3.86%) and reliability of measurements (intraclass correlation coefficients of 0.959–0.997) indicated that this method provides a feasible tool for measuring the volume of the EC in vivo.

Introduction

The entorhinal cortex (EC), which connects the hippocampal formation (HF) with neocortical regions and supplies it with multimodal information, is considered to be a critical component of the mesial temporal lobe memory system [48], [49], [53], [54]. While the HF has received a great deal of attention in magnetic resonance imaging (MRI) studies of the anatomic pathology that occurs in Alzheimer’s disease (AD) [9], [11], [12], [13], [29], [30], [33], [34], [35], [39], [41], [44], the EC had not been studied using quantitative MRI methods until recently [4], [24], [27], [36], [52]. However, a number of post-mortem histopathological studies based on neuronal counts, as well as the distribution of neurofibrillary tangles, neuropil threads, and senile plaques, have documented the involvement of the EC early in the course of AD [5], [7], [21], [25], [51]. Compared to the EC, the HF of patients dying with relatively mild dementia contains significant, but somewhat less pathology [2], [5], [7], [10], [21], [25], [42]. These data suggest that in vivo EC measurements may have the potential to identify anatomic changes in the earliest stages of AD.

Entorhinal atrophy in vivo in AD patients was initially detected by semi-quantitative visual rating of MR images [14], [20], [43]. Partly due to the fact that the rostral EC boundaries with adjacent structures (primary olfactory cortex, perirhinal cortex) are not demarcated macroscopically, some investigators [3], [14], [36], [45] measured EC volume using only one to three sections from the mid-portion of the structure, where those problematic boundaries are not present. The majority of these studies [3], [14], [45] demonstrated equivalent atrophy of the EC and of the HF in patients with mild to moderate dementia. Using a measure of the EC surface area, Bobinski et al.[4] reported good discrimination between mild AD patients and healthy aged subjects. It was only recently that protocols were published to measure the entire volume of the EC from MR images [24], [27]; Insausti et al.[27] validated their protocol using cytoarchitectonic material. Studies based on the latter protocol have demonstrated an absence of age-related changes in the EC when young and aged control subjects were compared [27], extensive atrophy (40%) in AD patients, and a high level of statistical power in discriminating AD patients from healthy aged individuals [31], [32]. Still, two methodological problems regarding the volumetric measurement of the EC remain.

Published quantitative protocols for volumetry of the entire EC [24], [27] were developed for coronal sections oriented perpendicularly to the line connecting the anterior and posterior commissures (AC-PC), since the borders of the EC have been described histologically using standard anatomic dissections [1], [6], [27], [28], [38], [49], [50]. However, in many laboratories the HF is measured from coronal sections oriented perpendicularly to its long axis, and it is important to measure the EC from the same oblique coronal sections to avoid overestimation of the volume of one of these two adjacent structures at the expense of the other. Therefore, we conducted a pilot study to transpose the relative positions of the anatomic landmarks demarcating EC boundaries from the standard coronal plane to the oblique coronal plane.

The other problem is related to the lateral boundary of the EC, which is not uniformly defined cytoarchitectonically because of individual differences [28], [38]. Some anatomists locate the lateral border at varying points along the medial bank of the collateral sulcus (CS) and consider it as the entorhinal/perirhinal border [1], [28], while others locate it at the medial edge of the CS [49], [50], and consider it as the entorhinal/transentorhinal border [6], [8], [23]. The published protocols for EC measurement differ in definitions of the lateral border, either locating it at the fundus of the CS [4], [24] or constructing it at some point along the CS depending on its length [27]. This controversy reflects different approaches to real anatomic variations between individuals. In the work described herein, the lateral border of the EC was defined in a conservative and standard manner, at the medial edge of the CS for all cases.

In this study, a protocol for measuring the volume of the EC from consecutive oblique coronal sections was developed in order to compare disease-induced changes in the EC and HF. Images obtained from healthy aged subjects, patients with very mild AD and elderly patients who did not meet diagnostic criteria for dementia were used in this work. The protocol for EC measurement is described in detail in this paper, while data comparing the three groups of subjects are reported in the accompanying paper [15].