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Skull, Scalp, and Meninges Overview
Karen L. Salzman, MD
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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, yet 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 images 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 fibroadipose 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 (Calvarium)

  • The calvarium 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 calvarium, 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 calvarium. 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.
    • Leptomeninges

      • The leptomeninges are formed by the arachnoid and pia. The arachnoid is loosely attached to the dural border cell layer. Pathologies often affect both the arachnoid and dura together, and the 2 areas cannot be easily differentiated on imaging.
      • The arachnoid is a thin, nearly transparent layer of meninges closely applied to the inner (meningeal) dura. It forms the outer margin of the subarachnoid space (SAS). It does not enter the sulci or fissures except along the falx where it dips into the interhemispheric fissure. Trabeculae extend from the arachnoid across the SAS to the pia and are invested with a thin pia-like layer. The SAS is a cerebrospinal fluid (CSF)-filled space between the arachnoid and pia.
      • The pia is a thin, delicate membrane closely applied to the brain. It covers vessels and trabeculae in the SAS and lines the perivascular spaces.
      • Perivascular (Virchow-Robin) spaces are normal variants. They appear as interstitial fluid-filled, pial-lined spaces that accompany penetrating arteries and veins.
      • Arachnoid Granulations

        • Arachnoid granulations are normal extensions of the SAS and arachnoid through the dural wall and into the venous sinuses. They are covered with arachnoid cap cells and venous sinus endothelium. CSF drains through the endothelium into the venous sinus. The most common locations for arachnoid granulations are the superior sagittal sinus and transverse sinuses. These normal variants are important "pseudolesions" to recognize, as they may be misdiagnosed as pathology. They are CSF density or intensity on imaging and do not enhance. They are often associated with bone changes on CT, particularly in the occipital bone. They are also well visualized on CTA/CTV and MR.

        Differential Diagnosis

        • The following differential diagnosis lists are provided to help organize the most common scalp, skull, and meningeal lesions.
        • Scalp Masses
        • Calvarial Thickening
        • Calvarial Thinning
        • "Hair on End"
        • Lytic Skull Lesion
        • Sclerotic Skull Lesion
        • Diffuse Dura-Arachnoid Enhancement
        • Pial (Leptomeningeal) Enhancement
        Related Anatomy
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        Related Differential Diagnoses
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        References
        Tables

        Tables

        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, yet 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 images 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 fibroadipose 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 (Calvarium)

        • The calvarium 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 calvarium, 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 calvarium. 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.
          • Leptomeninges

            • The leptomeninges are formed by the arachnoid and pia. The arachnoid is loosely attached to the dural border cell layer. Pathologies often affect both the arachnoid and dura together, and the 2 areas cannot be easily differentiated on imaging.
            • The arachnoid is a thin, nearly transparent layer of meninges closely applied to the inner (meningeal) dura. It forms the outer margin of the subarachnoid space (SAS). It does not enter the sulci or fissures except along the falx where it dips into the interhemispheric fissure. Trabeculae extend from the arachnoid across the SAS to the pia and are invested with a thin pia-like layer. The SAS is a cerebrospinal fluid (CSF)-filled space between the arachnoid and pia.
            • The pia is a thin, delicate membrane closely applied to the brain. It covers vessels and trabeculae in the SAS and lines the perivascular spaces.
            • Perivascular (Virchow-Robin) spaces are normal variants. They appear as interstitial fluid-filled, pial-lined spaces that accompany penetrating arteries and veins.
            • Arachnoid Granulations

              • Arachnoid granulations are normal extensions of the SAS and arachnoid through the dural wall and into the venous sinuses. They are covered with arachnoid cap cells and venous sinus endothelium. CSF drains through the endothelium into the venous sinus. The most common locations for arachnoid granulations are the superior sagittal sinus and transverse sinuses. These normal variants are important "pseudolesions" to recognize, as they may be misdiagnosed as pathology. They are CSF density or intensity on imaging and do not enhance. They are often associated with bone changes on CT, particularly in the occipital bone. They are also well visualized on CTA/CTV and MR.

              Differential Diagnosis

              • The following differential diagnosis lists are provided to help organize the most common scalp, skull, and meningeal lesions.
              • Scalp Masses
              • Calvarial Thickening
              • Calvarial Thinning
              • "Hair on End"
              • Lytic Skull Lesion
              • Sclerotic Skull Lesion
              • Diffuse Dura-Arachnoid Enhancement
              • Pial (Leptomeningeal) Enhancement