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Skull, Scalp, and Meninges Overview
Overview
Understanding the anatomy of the skull, scalp, and meninges is key to formulating a correct imaging diagnosis. Several important differential diagnoses are based on location. However, each of these locations requires a different imaging approach.
For example, CT is often the best imaging modality for lesions of the skull and scalp. When faced with a complex skull base lesion, a combination of bone CT and contrast-enhanced MR is often required for optimal imaging. MR with contrast is the best imaging modality for meningeal processes.
Scalp
The scalp is made up of 5 layers including the dermis (skin), subcutaneous fibro-adipose tissue, epicranium and muscles, subaponeurotic areolar tissue, and pericranium. The first 3 layers are firmly connected and surgically act as a single layer.
The majority of scalp lesions are not imaged, as the area is easily accessible to both visual and manual inspection. Imaging becomes important when a scalp lesion is malignant or has a vascular component that could alter the surgical approach.
Skull Vault (Calvaria)
The calvaria is composed of 5 bones: Frontal, parietal, occipital, temporal, and sphenoid (greater wings) bones that are primarily connected by the major sutures, including the coronal, sagittal, and lambdoid sutures. The metopic suture is variably seen in adults.
There are many normal variants of the skull. These must be recognized to prevent misdiagnosis and unnecessary biopsy. Some of the most common skull normal variants include arachnoid granulations, vascular grooves from the meningeal arteries and veins, venous lakes, emissary veins, parietal thinning, asymmetric marrow (particularly in the petrous apex), aerated clinoid processes, and accessory sutures.
Meninges
Dura
The dura (or pachymeninges) is a thick, dense, fibrous connective tissue that is made up of 2 layers: An outer (peri- or endosteal) layer and an inner (meningeal) layer. These outer and inner layers are closely adherent and apposed except where they separate to enclose the venous sinuses.
The outer layer forms the periosteum of the calvaria, tightly attached to the inner table, particularly at the sutures. The inner layer folds to form the falx cerebri, tentorium, and diaphragma sellae. It also divides the cranial cavity into compartments. On imaging, the dura usually shows smooth, thin enhancement (< 2 mm).
The dura forms 2 important potential spaces. First, the epidural space is located between the dura and the inner table of the calvaria. Important lesions of the epidural space include hemorrhage related to trauma and infection causing an empyema, a rare but potentially lethal complication of sinusitis. Second, the subdural space is the potential space between the inner (meningeal) layer of the dura and the arachnoid. A traumatic subdural hematoma is the most common process to affect the subdural space (more accurately, it probably collects within the border cell layer along the inner margin of the dura). The subdural space may also be affected by infection, either a subdural effusion related to meningitis or a subdural empyema related to meningitis in a child or sinusitis in an adult.
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