Alexander M. McKinney

Atlas of Normal Imaging Variations of the Brain, Skull, and Craniocervical Vasculature

Part I: Brain -- Brain Introduction -- Cerebellar Tonsillar Ectopia -- Cerebellar Flocculus Pseudomass -- Mega Cisterna Magna and Retrocerebellar Arachnoid Cysts -- Cranial Nerve VII: Normal Contrast Enhancement on Magnetic Resonance Imaging -- Meckel Cave Prominence, Asymmetry, and Petrous Apex Cephaloceles -- Cavernous Sinus Fat -- Pituitary Variations, Artifacts, Primary Empty Sella, and Incidentalomas -- Liliequist Membrane -- Pineal Gland: Normal Size, Appearance, and Enhancement -- Choroid Plexus: Normal Locations and Appearances -- Hippocampi, Caudate Tail, and Ependymal Variants -- Midline Variants of the Septum Pellucidum, Corpus Callosum, and Massa Intermedia -- Dilated Perivascular Spaces -- Enlargement or Asymmetry of the Lateral Ventricles Simulating Hydrocephalus -- Cerebral or Cerebellar Volume Loss Simulating Subdural Hematomas, Hygromas, and Arachnoid Cysts -- Dural Calcifications: Normal Locations and Appearances -- Pseudo-leptomeningeal Contrast Enhancement -- Basal Ganglia: Physiologic Calcifications -- Susceptibility-Weighted Imaging: Concepts, Basal Ganglia Variations, and Artifacts -- Slow-Flow, Asymptomatic Vascular Malformations: Brain Capillary Telangiectasias and Developmental Venous Anomalies -- Brain MRI Pseudolesions: 3.0T Imaging, FLAIR, and Diffusion-Weighted Imaging -- Pediatric Brain: Normal Variations in Development, Maturation, and Myelination -- Part II: Skull -- Skull: Introduction -- Pediatric Skull: Normal Pediatric Sutures on Computed Tomography -- Skull Base Foramina: Normal Variations and Developmental Defects -- Normal Variations and Developmental Anatomy of the Calvarial Sutures and Fontanelles Above the Skull Base -- Emissary Veins, Vascular-Containing Foramina, and Vascular Depressions of the Skull -- Normal Variations in Calvarial Contour, Irregular Ossification/Aeration, and Inward/Outward Projections/Protuberances -- Other “Don’t Touch” Skull Lesions: Arachnoid Granulations, Calvarial Depressions, Hemangiomas, and Intraosseous Lipomas -- Part III: Craniocervical Vasculature -- Craniocervical Vasculature: Introduction -- Aortic Arch and Great Vessel Origin Arterial Variants -- Cervical Carotid and Vertebral Arterial Variants -- Tortuous Cervical and Intracranial Arteries and Basilar-Carotid Dolichoectasia -- Miscellaneous Cervical Venous Variants -- Intracranial Posterior Circulation Variants -- Intracranial Anterior Circulation Variants -- Infundibular Outpouchings of Intracranial Arteries -- Persistent Carotid-Basilar and Carotid-Vertebral Anastomoses -- Variations in the Intracranial Venous System -- Arachnoid Granulations Within the Dural Sinuses -- Artifacts of the Craniocervical Venous System on MRI -- Artifacts of the Craniocervical Arterial System on MRI -- Artifacts on Craniocervical CT Angiography -- Dense Vessels Simulating Thrombosis on Nonenhanced CT.

Dilated Perivascular Spaces

A perivascular space (PVS) or a Virchow-Robin space is a typically narrow tract that is the drainage pathway for the cerebral parenchyma. As there is no lymphatic system within the cerebrum, the PVSs drain interstitial/extracellular fluid peripherally outward to the subarachnoid space (SAS) but are thought not to communicate directly with the SAS. PVSs are also termed Virchow-Robin spaces, which surround small perforating arteries in multiple locations. They have also been described as potential spaces created by leptomeninges that penetrate the cerebral parenchyma and surround blood vessels. The literature suggests that tiny, normal PVSs may be seen in up to 80 % of the pediatric population, while irregular or ectatic PVSs (status cribrosum) may be noted in 1–2 %. However, on standard imaging, slightly prominent PVSs are visible in 33 % of patients greater than or equal to 65 years of age.

Cervical Carotid and Vertebral Arterial Variants

This chapter covers congenital vascular variations in branching or anomalous entry points of vertebral artery (VA) and internal carotid artery (ICA) vasculature. These include common carotid artery (CCA) origin of the VA, external carotid artery (ECA) origin of the VA, duplication of the VA, anomalous entry site of the VA into the vertebral transverse foramen, hypoplastic VA, hypoplastic ICA, and origin of ECA branches from the ICA. Tortuosity that lacks abnormal branching or site of entry is covered in Chap. 34.

Artifacts on Craniocervical CT Angiography

With the advent in the late 1990s of multichannel, multidetector (also called “multislice”) technology for CT, and its subsequent implementation, the performance of CT angiography (CTA) has been augmented in terms of coverage, speed, and dose efficiency. Overall, perhaps more than 95 % of examinations are interpretable and have adequate contrast attenuation (>150 HU), and more than 85 % have adequate difference in contrast density (>50 HU) between arteries and veins for the purpose of CTA. However, there are a number of artifacts that can limit CTA evaluation and potentially lead to false interpretation. Kim et al. describe these artifacts in detail, but for the purposes of discussion here, these are the most common: n n nStreak artifact from dense contrast material in adjacent veins, such as within a subclavian vein or brachiocephalic vein that may obscure visualization of an adjacent common carotid artery or vertebral artery n n nStreak artifact or decreased attenuation horizontally due to the shoulders n n nStreak or beam-hardening artifact from metal such as dental amalgam n n nContrast reflux into neck veins n n nFlow-related artifacts due to the high speed of multislice CT acquisition n n nMotion artifacts

Tortuous Cervical and Intracranial Arteries and Basilar-Carotid Dolichoectasia

Tortuosity most often occurs in the elderly or in the setting of atherosclerosis, but it also occurs in the uncommon patient with a collagen vascular disorder. The vertebral arteries can be tortuous at their origins or throughout the remainder of their course in the cervical and intracranial portions, and they can be particularly troublesome for interpretation or procedures in several scenarios.

Artifacts of the Craniocervical Arterial System on MRI

Three-dimensional time-of-flight (3DTOF) MRA utilizes the concept of flow-related enhancement (FRE) to produce bright signal on a gradient echo acquisition, where fresh spins in blood flowing into (and preferentially perpendicular to) the slice are unsaturated and exhibit strong signal prior to being saturated. Arteries do not always follow a perpendicular orientation, however. Particular examples include tortuous internal carotid arteries (ICAs) in the cervical region, vertebral arteries (VAs) near their origin, or the normal horizontal course of the petrous ICAs. When travelling horizontally within a slice, the phenomenon of in-plane saturation may reduce FRE and thus decrease the visualized flow of vasculature. This phenomenon is overcome on contrast-enhanced MRA (CEMRA), as the T1-bright gadolinium usually overpowers the saturation effect. Note that the FRE effect is also useful with 2D or 3DTOF MR venography, most commonly in the transverse sinuses with an axial acquisition, or within the superior sagittal sinus with a sagittal acquisition. It is also important to point out that 3D acquisitions can be susceptible to saturation if a larger volume is obtained, or if the technique of multiple overlapping thin slab acquisition (MOTSA) is not used properly. Hence, most facilities utilize overlapping slabs to decrease in-plane saturation for noncontrast cranial MRA, but saturation may still occur if flow occurs within the same plane as the plane of acquisition.

Normal Variations in Calvarial Contour, Irregular Ossification/Aeration, and Inward/Outward Projections/Protuberances

This subsection focuses on perceived abnormalities of osseous overgrowth or prominent protuberances that are within normal limits or are not atypical in certain age groups (e.g., benign hyperostosis frontalis in middle-aged females). This also includes perceived abnormalities of calvarial contour or multifocal irregularities in adults that may simulate disease. Many disease states that cause diffuse bony overgrowth are not covered here except as an occasional comparison case, since there can be a myriad causes/diseases for this appearance that range from malignant neoplastic (e.g., diffuse metastatic disease), benign (e.g., fibrous dysplasia), or metabolic (e.g., Paget disease or thalassemia).

Other “Don’t Touch” Skull Lesions: Arachnoid Granulations, Calvarial Depressions, Hemangiomas, and Intraosseous Lipomas

This subsection describes a few miscellaneous normal variants of the skull that can mimic lytic lesions. These include the often encountered pacchionian granulations, calvarial depressions, and the quite uncommon intraosseous lipomas. Pacchionian (arachnoid) granulations on CT could appear nearly identical to calvarial depressions from vascular channels; indeed, this distinction can be arbitrary and only based on the observation that calvarial depressions are filled with a clearly contrast-enhancing vascular channel, while arachnoid granulations are most commonly (but not always) filled with the T2-bright signal intensity of cerebrospinal fluid (CSF) on MRI, which suppresses/darken on FLAIR, uncommonly, these exhibit internal enhancement because of vasculature contained within. Intraosseous lipomas can be differentiated by their characteristic appearance and density on CT, although MRI can also help confirm that they are normal variants in questionable situations. Finally, calvarial hemangiomas can overlap with intraosseous lipomas if a hemangioma is small and T1-bright; however, larger hemangiomas have increasing enhancement, can be expansile, and can simulate lytic lesions such as metastases.

Normal Variations and Developmental Anatomy of the Calvarial Sutures and Fontanelles Above the Skull Base

This section focuses on the variations in sutural anatomy and development of the calvarium above the skull base that can mimic disease or cause confusion, particularly in the setting of blunt trauma. This includes the major cranial sutures, the anterior and posterior fontanelles in infants and young children, asymmetric or atypical configurations of the skull that may occur with suture closure, and normal foramina (such as emissary foramina) that can occur within the upper calvarium. The persistence of these sutures and fontanelles and their normal variations in adults can potentially cause difficulty in interpretation. Other variations can occasionally be problematic to identify on standard axial NECT images, such as normal intrasutural bones, positional plagiocephaly, and bathrocephaly. In such instances, three-dimensional (3D) volume-rendered (VR) reconstructions from thin axial NECT can readily identify such normal variations. For this reason, many pediatric neuroradiologists have advocated that thin (≤1-mm thickness) axial NECT images with bone windows should be standard for reviewing infants and young children with trauma, and that three-dimensional reconstructions should accompany them. As stated in prior chapters, the persistence of normal sutures can often be distinguished from fractures based on reviewing the 3D reconstructions of the cranium that are usually quite easy to produce on standard CT vendors’ consoles. Also, if there is solely a question of fracture, extremely low-dose protocols can be obtained combined with iterative reconstruction to delineate such fractures, potentially using one third to one tenth of a standard axial NECT radiation dose to the head. Another point regarding NECT 3D reconstructions is that spiral (rather than sequential) acquisitions are optimal as source data, since sequential acquisitions can suffer from “stair-step” artifacts when reconstructed into 3D. Spiral scans also take less time to scan but often have a mildly higher dose for the same slice thickness, thereby decreasing motion effects to some degree.

Skull Base Foramina: Normal Variations and Developmental Defects

This chapter is devoted to the normal variations in foramina at the skull base. Because these foramina can simulate fractures, lytic lesions, and cephaloceles based on their location, one can recognize the importance of being familiar with these variants. Although these foramina can be particularly prominent and problematic in younger children, their persistence in adults can cause confusion. Therefore the intent and methodology utilized here are to follow the normal anatomy and staying power of these variants from the pediatric to adult ages. A number of other skull base foramina are not covered in this text; those that are not mentioned either are not typically confused with other abnormalities or do not have significant asymmetry (e.g., optical canal, superior orbital fissure, carotid canal).