Content area
Full text
Received Dec 19, 2017; Accepted Mar 4, 2018
This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. Introduction
1.1. History of Midline Shift as an Imaging Feature
Human head is roughly bilaterally symmetric. Although there are functional differences between hemispheres of the brain, the gross morphology follows the rule [1]. Both cerebrum and cerebellum are symmetric with lobes, ventricles, and deep nuclei of similar size and shape in both hemispheres. Subtle structural asymmetry plays no role in clinical diagnostic neuroradiology [2]. From pathological examinations, physicians have already known that intracranial mass can cause brain shift, followed by herniation, brainstem compression, and death. Therefore, they rely on shift of midline structures for aiding diagnosis from the very beginning of neuroimaging. Shift of calcified pineal gland on plain X-ray was used initially, followed by pneumoencephalography and angiogram [3].
After the invention of ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI), cross-sectional imaging becomes possible with greatly improved resolution and tissue contrast [2, 3]. While the cerebrospinal fluid- (CSF-) containing third ventricle (V3, Figure 1) is more easily identified on US images [4], most authors describe the degree of displacement of the septum pellucidum (SP, Figure 1), a thin membrane between the frontal horns (FHs) of the lateral ventricles, relative to the ideal midline (iML) on CT images [5, 6]. Whether the pineal gland, the V3, or the SP is used, deviation of the given midline structure from the iML is termed midline shift (MLS). Since symmetry plays a key role in radiologic evaluation of the brain, any shift of midline structures is presumed to represent a mass lesion on the side from which the midline is displaced [2]. For practical purposes, there are no acute “sucking” brain lesions that draw the midline toward themselves.
[figure omitted; refer to PDF]1.2. Using Midline Shift as a Quantitative Indicator of Mass Effect to Predict Outcome in Trauma Patients
As early as 1783 Alexander Monro deduced that the cranium is a “rigid box” filled with a “nearly incompressible brain” and that its total volume tends to remain constant [8]. The doctrine states that any increase in...