TERRI BRUCKNER

OUTLINE

Overview

Imaging modalities, such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasonography, require the technologist to look at anatomy in the resultant images in a totally different way than they are used to with general radiographs. These technologies create cross-sectional imaging planes, in effect visualizing a slice through the body. Cross-sectional images have the advantage of visualizing anatomic structures without the sometimes confusing superimposition of other anatomic parts. Images are generated in various planes, which makes it crucial for the technologists working with these modalities to have a clear and complete understanding of general anatomic principles. Without a clear understanding of general anatomy, it is difficult to feel confident in the identification of normal and abnormal structures in cross section. The purpose of this chapter is to provide the radiographer who possesses a background in general anatomy with an orientation to sectional anatomy and to correlate that anatomy with structures shown on images from the various computer-generated imaging modalities.

Generally, three major imaging planes exist: axial, coronal, and sagittal. Axial planes (sometimes referred to as transverse planes) transect the body from anterior to posterior and from side to side. In effect, this type of horizontal plane divides the body into superior and inferior portions. Most images generated by CT are examples of axial or transverse planes. When looking at an axial image, it is helpful to imagine standing at the patient’s feet and looking up toward the head. With this orientation, the patient’s right side is to the viewer’s left and vice versa. The anterior aspect of the patient is usually at the top of the image. The coronal plane divides the body into anterior and posterior portions. Coronal planes pass from superior to inferior and from side to side. Images viewed in the coronal plane are similar to radiographs in that the patient’s right side is on the technologist’s left (one can imagine facing the patient while viewing this type of image). Sagittal planes divide the body into right and left portions. These planes pass from superior to inferior and from anterior to posterior. Magnetic resonance (MR) images frequently use the coronal and sagittal planes to present the desired anatomy. CT images may be obtained in the coronal or sagittal planes, or the information from axial images may be reformatted by the computer to obtain images in these planes. Any plane that does not fit the previous descriptions is referred to as an oblique plane. Ultrasonography and MRI of some structures, such as the heart, are generated using oblique planes.

CT uses x-rays to generate images, so the various shades on the images correspond to the gray scale that radiographers are accustomed to seeing. Bones and other dense materials are white, whereas air and lower density materials are closer to black. Fat, muscle, and organs are represented with various shades of gray. Hounsfield units or CT numbers represent the scale of white to black that is used in CT imaging. Lower numbers represent anatomic structures that are more easily penetrated by the x-ray and appear closer to black on the image. Higher numbers are related to more radiopaque structures and are lighter gray or white on the image. Similar to routine radiographs, blood vessels and organs of the digestive system are not easily distinguishable from other structures. To be able to identify these structures more accurately, patients are frequently given a radiopaque contrast medium. Intravascular contrast medium highlights vessels, making them appear radiopaque and whiter on the image. To visualize the gastrointestinal system, patients may be given a contrast agent by mouth or via the rectum. For a full description of CT fundamentals, the reader is referred to Chapter 31.

MRI uses magnetic fields and radiofrequencies to generate images. Anatomic structures are represented on the image with regard to the signal generated from their protons. Structures that produce a strong signal are generally lighter gray or white on the image, and structures that do not generate a strong signal tend to be darker on the image. The signal generated by these structures depends on many things, including the strength of the magnetic fields and the characteristics of the radiofrequencies used. Contrast medium may also be used when performing MRI to change the signal intensity of particular anatomic structures. Gadolinium, air, and fluid may be used as contrast agents depending on the organ of interest and the imaging sequences employed. MRI is discussed in depth in Chapter 32.

The cadaveric sections depicted in this chapter are representative of major organ structures for each of the body regions, and they are depicted from the inferior surface to correspond to the images. All relational terms are used in relation to the body in anatomic position (when a structure is described as being to the right of something, this refers to the patient’s right, not the viewer’s right). The major anatomic structures normally seen when using current imaging modalities are labeled. For each region of the body, a cadaveric section is presented, and representative images are included to provide an orientation to anatomic structures normally seen using the available imaging modalities. The cadaveric sections and diagnostic images do not match exactly; some structures are seen on only one of the illustrations for each body region. Major anatomic structures in each region of the body are reviewed in the following sections to assist identification of the images provided. Systematic review of the bones, vessels, major organs, and muscles begin each section. Selected images are presented in axial, sagittal, and coronal planes to show these structures. In practice, images should be examined collectively because the size, shape, and placement of these structures vary from slice to slice. “Following” a structure is frequently the ideal way to identify it.

Cranial Region

Fig. 30-1 is a cadaveric image that can be used to distinguish bone, muscle, and other soft tissue structures. Referring to this image should be helpful in identifying the sometimes confusing shadows on the images. The head can be thought of systematically as being composed of the skull, central nervous system structures, various sensory organs, cranial blood supply, and associated cranial and facial muscles. The bones of the skull are categorized as the 8 cranial bones and the 14 facial bones. The cranial bones include the frontal, occipital, and two parietal bones that surround and protect the external surface of the brain. The other four cranial bones include the ethmoid, sphenoid, and two temporal bones. The frontal bone forms the anterior surface of the skull, with a vertical portion that corresponds to the forehead and a horizontal portion that forms the roof of the orbits. Between the inner and outer layers of the vertical portion of the frontal bone, just superior to the level of the eyes, are the paired frontal paranasal sinuses. The vertex (superiormost portion) of the skull is formed by the paired parietal bones. These roughly square-shaped bones articulate with the frontal bone at the coronal suture, with the temporal bones at the squamosal sutures, with the occipital bone at the lambdoidal suture, and with each other at the sagittal suture.

The posterior aspect of the skull is formed by the occipital bone, which is composed of a squamous (vertical) portion and a basilar portion. The foramen magnum is a large opening within the squamous portion that allows passage of the spinal cord into the brain. The external occipital protuberance is a large prominence on the posterior surface of this bone. Roughly corresponding in position to this landmark is the internal occipital protuberance. The ethmoid bone is found within the cranium and forms the medial walls of the orbits and part of the lateral walls of the nasal cavity. The ethmoid bone is divided into a horizontal portion called the cribriform plate and vertical portions called the perpendicular plate and two labyrinths or lateral masses.

The cribriform plate lies between the orbital plates of the frontal bone and supports the olfactory bulbs (cranial nerve I). The cribriform plate is perforated by many small foramina, which transmit nerves from the nose to this cranial nerve. Projecting superiorly from the cribriform plate is a small ridge of bone called the crista galli, which serves as the anterior attachment for the falx cerebri. Projecting inferiorly from the center of the cribriform plate is the perpendicular plate. This thin strip of bone forms the superior part of the bony nasal septum. Extending inferiorly from the lateral edges of the cribriform plate are the labyrinths or lateral masses. These are perforated by multiple air spaces, which are collectively called the ethmoidal paranasal sinuses. From the medial surface of each labyrinth, two scroll-shaped ridges of bone project into the nasal cavity. These are the superior and middle nasal conchae.

In the center of the base of the skull is the sphenoid bone. This bone is sometimes referred to as the anchor bone of the cranium because it articulates with all of the cranial bones. Thinking of this bone as being composed of a body, two sets of wings, and a pterygoid portion is helpful. The body is the central portion of the bone and contains the easily identifiable landmark known as the sella turcica. The sella turcica forms a cup-shaped depression that surrounds and protects the pituitary gland. The anterior surface of the sella is called the tuberculum sellae, and the posterior portion is called the dorsum sellae. Two posterior clinoid processes project from the superior edge of the dorsum sellae and are attachments for dura mater partitions. Within the body of the sphenoid and inferior to the sella turcica are the paired sphenoidal paranasal sinuses. The lesser wings of the sphenoid are triangular ridges of bone found posterior to the orbital plates of the frontal bone. Directly inferior to the medial edge of each lesser wing is the optic canal, which transmits the optic nerve (cranial nerve II). The larger, greater wings support the temporal lobes of the cerebrum and extend from the body to the external surface of the skull. The pterygoid processes project inferiorly from the body of the sphenoid and form the posterior walls of the nasal cavity.

The temporal bones form part of the lateral walls of the cranium and extend internally to meet the sphenoid and the basilar portion of the occipital bone. Parts of the temporal bone include the squamous, tympanic, mastoid, and petrous portions. The squamous portion is the thin, fan-shaped external part of the bone superior to the external ear. It articulates with the parietal and several other cranial bones. Its articulation with the parietal bone is called the squamous suture. The tympanic portion is the area of the bone surrounding the external ear canal. The zygomatic process arches anteriorly from just superior to the external ear canal. Just inferior to the origin of the zygomatic process is the mandibular fossa, in which the condyle of the mandible is found. On the inferior surface of the tympanic portion of the bone is the styloid process, which serves as an attachment for muscles. Posterior to the ear is the mastoid portion, which is perforated by many small, air-filled cavities. The mastoid portion extends inferiorly to form the cone-shaped mastoid process. The petrous portion of the bone lies within the cranium, normally forming an angle of approximately 45 degrees to the median sagittal plane. This dense ridge of bone surrounds and protects the organs of hearing and balance, the facial and vestibulocochlear nerves, and the internal carotid artery.

The organs associated with the face are surrounded and protected by 14 bones. Each of these bones is paired with the exception of the vomer and the mandible. The lacrimal bones are about the size of a fingernail and are found in the medial wall of the orbit between the maxilla and the labyrinth of the ethmoid bones. The nasal bones form the bridge of the nose and articulate superiorly with the frontal bone, laterally with the maxilla, and with each other in the midline. The zygomatic bones form the inferolateral walls of the orbits. Each of these bones articulates superiorly with the frontal bone, medially with the maxilla, and laterally with the zygomatic process of the temporal bone. The maxilla originates as two separate bones, which ultimately fuse along the midsagittal plane. This large bone forms the inferior surface of each orbit, the lateral walls of the nasal cavity, and the anterior portion of the roof of the mouth. On either side of the nasal cavity, large air-filled maxillary paranasal sinuses are embedded within the bone. The upper teeth are rooted within the alveolar process at the anteroinferior surface of the bone. The maxilla articulates with the nasal bones, the lacrimal bones, the frontal bone, the zygomatic bones, and the palatine bones. The inferior nasal conchae are scroll-shaped facial bones found in the nasal cavity, just inferior to the middle nasal conchae of the ethmoid bone. The posterior portion of the hard palate is formed by the L-shaped palatine bones. The vertical portions of the palatines extend superiorly along the posterior nasal cavity to form a small part of the posterior orbit.

The vomer is an unpaired facial bone that articulates with the inferior surface of the perpendicular plate of the ethmoid. It forms the inferior portion of the bony nasal septum. The mandible, which is also an unpaired facial bone, is formed by a body and two rami. The body comprises the anterior portion of the bone and presents an alveolar ridge in which the lower teeth are embedded. The rami extend superiorly from the body and end in anterior and posterior bony processes. The anterior process at the superior end of the ramus is the coronoid process, where muscles of mastication attach. The posterior process is the condyloid process, which rests in the mandibular fossa of the temporal bone. This articulation, the temporomandibular joint, is the only movable articulation associated with the skull. (Refer to Chapter 20 for review of skull anatomy.)

The brain is surrounded by three layers of protective membranes called the meninges. From internal to external, they are the pia mater, arachnoid, and dura mater. The pia mater adheres directly to the brain and is composed of a fine network of capillaries and supporting tissue. The arachnoid is a delicate membrane that resembles a cobweb. The subarachnoid space lies between the arachnoid and pia mater. Cerebrospinal fluid (CSF) circulates in this space. The arachnoid does not closely adhere to cerebral structures. As it bridges the gap between various parts of the brain, enlarged regions or cisterns are formed in the subarachnoid space. Some of the more crucial cisterns include the cisterna magna, pontine, interpeduncular, and superior (also known as the ambient or quadrigeminal cistern). The cisterna magna is the largest of these and is found just inside the foramen magnum, between the cerebellum and the medulla oblongata. This cistern receives CSF from the fourth ventricle. The pontine cistern lies anterior to the pons and contains the basilar artery. The interpeduncular cistern is anterior to the midbrain. The infundibulum (stalk) of the pituitary and the vessels of the circle of Willis are seen here. The cistern found posterior to the midbrain is the superior cistern. It surrounds the pineal gland and the great cerebral vein. The most external of the meninges is the double-layered dura mater. The outer layer of dura is attached to the inner surface of the cranial bones. The inner layer can be seen between cerebral structures and in large fissures. Dural sinuses are venous drainage channels formed where the inner dural layers separate from the outer layer. One of the largest dural flaps, the falx cerebri, is found in the longitudinal fissure between the cerebral hemispheres. It extends from the crista galli of the ethmoid to the occipital bone. The tentorium cerebelli extends between the cerebrum and the cerebellum. It attaches to the sella turcica and the internal surface of the occipital bones.

The structures of the central nervous system within the skull include the cerebrum, brainstem, and cerebellum. The cerebrum is the largest of these structures and is divided by the longitudinal fissure into two hemispheres. The hemispheres are connected to each other via a white matter tract called the corpus callosum. This arch-shaped structure is divided into the anterior genu, central body, and posterior splenium. Each cerebral hemisphere is divided into lobes that are named for the most adjacent cranial bone: frontal, parietal, temporal, and occipital. An additional lobe called the insula is buried deep to the temporal lobe. The cerebrum is thrown into numerous folds called gyri, which are separated by small fissures called sulci. The outer surface of the cerebrum consists of a thin layer of gray matter. The central portion of this part of the brain is mainly white matter (formed by myelinated nerve fibers). These fibers, referred to as the corona radiata, connect the gray matter of the cortex to deeper gray matter nuclei deep within each hemisphere. The buried gray matter centers are called basal nuclei or basal ganglia and include the claustrum, putamen, globus pallidus, and caudate nucleus. White matter tracts, or capsules, are found between these gray matter structures. Other gray matter structures found within the central cerebrum include the thalamus and hypothalamus. These form the walls of the third ventricle. Many of these gray and white matter structures are seen on the cadaveric section in Fig. 30-1.

The brainstem is formed by the midbrain, pons, and medulla oblongata. It lies between the cerebrum and cerebellum and serves as a relay for nerve impulses between the spinal cord and these two structures. The midbrain is the most superior of the three. White matter tracts called the cerebral peduncles extend from the anterior midbrain to the cerebrum. Toward the posterior aspect of the midbrain are the corpora quadrigemina, which are formed by two superior and two inferior colliculi that lie just inferior to the splenium of the corpus callosum. The cerebral aqueduct drains CSF from the third ventricle to the fourth ventricle and passes through the posterior portion of the midbrain. The central portion of the brainstem is formed by the pons. It communicates with the medulla, with the midbrain, and via white matter cerebellar peduncles to the cerebellum. The most inferior of the brainstem structures is the medulla oblongata, which is continuous with the spinal cord as it passes through the foramen magnum.

The cerebellum lies in the posteroinferior region of the cranium. Although smaller in size, it is similar in composition to the cerebrum. A midline fissure divides the cerebellum into hemispheres that are connected by a midline vermis. It is also thrown into numerous small folds, here called folia, which are separated by numerous small fissures. The outer surface of the cerebellum is composed of gray matter, with white matter constituting most of the central portion of this part of the brain. Gray matter nuclei can be found here also, although they are difficult to distinguish on images and are not discussed in this chapter.

Four large cavities called ventricles are found in the brain. The ventricles’ major function is to produce CSF; this is accomplished as blood is filtered through capillary networks called choroid plexuses in each of the ventricles. The largest of these chambers are the lateral ventricles, one of which is found in each cerebral hemisphere. The lateral ventricles are divided into body and anterior, posterior, and inferior horns. CSF from these chambers passes into the midline third ventricle through the interventricular foramina. The third ventricle is found between the cerebral hemispheres inferior to the lateral ventricles. The cerebral aqueduct drains CSF from here to the fourth ventricle. This ventricle is found between the cerebellum and the brainstem. Its walls are formed by white matter tracts called cerebellar peduncles, which connect the brainstem and cerebellum. A central aperture and two lateral apertures allow CSF to pass from the fourth ventricle into the subarachnoid space.

The brain is a highly metabolic organ, and to function well it needs a rich blood supply. Four major arteries supply the brain and its related structures: the two internal carotid arteries and the two vertebral arteries. The internal carotid arteries supply the anterior structures of the brain. After passing superiorly through the neck, these arteries enter the skull via the carotid canals in the petrous portion of the temporal bones. After exiting the petrous portion, the internal carotid arteries pass along the lateral aspect of the sella turcica, ultimately dividing into the anterior and middle cerebral arteries. The posterior communicating artery arises from the internal carotid just before this bifurcation. The anterior cerebral arteries pass anteriorly and superiorly to the longitudinal fissure, where they curl around the external aspect of the corpus callosum and supply the anterior portion of the brain. The middle cerebral arteries pass laterally to the lateral fissures, where their branches supply the middle portion of the brain. The posterior communicating arteries pass posteriorly to join with the branches from the vertebrobasilar arterial system. The vertebral arteries traverse the neck in the transverse foramina of the cervical spine and enter the posterior skull via the foramen magnum. These arteries pass superiorly along the anterior aspect of the medulla and at the base of the pons join to form the basilar artery. At the superior aspect of the pons, the basilar artery splits to form the posterior cerebral arteries. A unique arterial anastomosis exists in the brain to protect it from sudden loss of blood supply. This vascular connection is called the circle of Willis. The blood supply to the anterior brain is connected to the blood supply for the posterior brain as the posterior communicating arteries extend from the internal carotid arteries to the posterior cerebral arteries. This communication lies in the interpeduncular cistern, just anterior to the midbrain.

Venous drainage in the cranium is accomplished by two systems: cerebral veins and dural venous sinuses. The dural sinuses are created by gaps formed between the inner and outer layers of the dura mater. These gaps are found in the areas where the dura invaginates between the various structures of the brain. The superior sagittal sinus is found in the superior border of the falx cerebri, and the inferior sagittal sinus is found in its inferior margin. The channel formed where the falx cerebri meets the tentorium cerebelli is the straight sinus. This sinus is a continuation of the inferior sagittal sinus as it joins with the great cerebral vein. The transverse or lateral sinuses are found along the lateral aspect of the tentorium cerebelli as it meets the occipital bone. At the level of the petrous portions of the temporal bones, the transverse sinuses curl medially and inferiorly and become known as the sigmoid sinuses. As the sigmoid sinuses pass out of the cranium via the jugular foramina, these vessels change names again and become the internal jugular veins. One of the major veins within the skull is the great cerebral vein. This large venous structure is found in the superior cistern, and there it joins the inferior sagittal sinus to form the straight sinus.

Many muscles are associated with the face, only a few of which are referred to in the following sections. The temporalis muscle is found on the external surface of the squamous portion of the temporal bone. Its inferior attachment is to the coronoid process of the mandible. On the external surface of the mandibular rami are the masseter muscles, and on the internal surface of the rami are the pterygoid muscles. These muscles all are associated with moving the mandible and with swallowing.

A lateral skull radiograph is used here for localization of the imaging plane in this section (Fig. 30-2, A), and a sagittal MR image (Fig. 30-2, B) is used for localization of MRI cross sections of the brain. CT imaging for the cranium may be performed with the gantry parallel to or angled 15 to 20 degrees to the orbitomeatal line. Angling the gantry of the CT scanner allows for imaging of the brain without excess radiation to the eyes. MRI of the cranium generally results in images that are parallel to the orbitomeatal or infraorbitomeatal plane. More details on patient positioning for CT are provided in Chapter 31, and information on patient positioning for MRI is provided in Chapter 32. Because the imaging planes may be different for CT and MRI, some variation exists in the anatomic structures visualized on corresponding illustrations in this section. Seven identifying lines represent the approximate levels for each of the labeled images for this region.

The cranial CT image seen in Fig. 30-3 represents a CT slice obtained through the frontal and parietal bones, and Fig. 30-4 is a corresponding MR image. The cortex, or outer layer of gray matter, can be differentiated from the deeper white matter. The numerous gyri, or convolutions, and sulci are shown and are surrounded by the darker appearing CSF in the subarachnoid space. The cerebral hemispheres are separated by the longitudinal cerebral fissure. Invaginated in this fissure is a fold of dura mater, the falx cerebri. The superior sagittal sinus, which passes through the superior margin of the falx, follows the contour of the superior skull margin. In cross section, the anterior and posterior aspects of this sinus can normally be seen in the midline deep to the bony plates when the patient has been given an intravenous contrast agent and appear as triangular expansions near the bones easily seen on MR images. Two of the five cerebral lobes are seen (frontal and parietal). The corona radiata is the central tract of white matter in the cerebrum and is darker than the cortex on the CT image. This section passes through the superiormost portion of the corpus callosum, which separates the anterior and posterior portions of the falx cerebri.

Fig. 30-5 is an axial CT slice through the superior portions of the lateral ventricles; Fig. 30-6 is the corresponding MR image. Visualized bony structures on the CT scan include the frontal bone and the two parietal bones. The falx cerebri is seen within the longitudinal fissure. The frontal lobes and parietal lobes of the cerebrum are shown. In the center of each image, the lateral ventricles are easily seen because of the dark appearance of the CSF circulating within each. In the posterior portions of the ventricles, the contrast-filled capillary network of the choroid plexuses also is visualized. A thin membrane called the septum pellucidum can be seen separating the ventricles. The corpus callosum is an arch-shaped structure; in cross section at this level, only the anterior genu and the posterior splenium can be seen. The caudate nuclei lie along the lateral surfaces of the ventricles and tend to follow their curves. Because these nuclei are composed of gray matter, they are the same shade of gray as the cortex on MR images (see Fig. 30-6). Several contrast-filled vascular structures are visible. The anterior cerebral arteries lie within the longitudinal fissure just anterior to the genu of the corpus callosum. A few branches of the middle cerebral arteries are seen near the lateral aspect of the skull on the CT scan. The anterior and posterior portions of the superior sagittal sinus are seen in the periphery of the falx cerebri. The inferior sagittal sinus lies in the internal edges of the falx. The thin strips of muscle seen on the external surface of the frontal bone correspond to the superior edges of the temporalis muscles.

The axial sections through the mid-portion of the cerebrum show many of the central structures of the cerebral hemispheres (Fig. 30-7 is a CT image, and Fig. 30-8 is an MR image). Images at this level pass through the frontal bone, greater wing of the sphenoid, and squamous portion of the temporal bones. The posterior portion of the skull comprises the top portion (squamous portion) of the occipital bone at this level. The falx cerebri is shown within the longitudinal fissure, with the superior sagittal sinus best shown in the midline of the anterior and posterior margins of this membrane. In the CT image, the genu of the corpus callosum is found between the anterior horns of the lateral ventricles; however, the posterior portion of this slice is inferior to the level of the splenium. The MR image shows the genu and the splenium. At this level, the MR image shows the frontal, temporal, and occipital lobes along with the insula (fifth lobe or island of Reil), which is deep to the temporal lobe at the lateral fissure. Because of its orientation, the CT image shows the insula, frontal, and temporal lobes; the posterior aspect of the skull in this image is occupied by the cerebellum.

The anterior and temporal horns of the lateral ventricles are seen on the CT scan, whereas the anterior and posterior horns are visible on the MR image. Within each posterior horn is a portion of the choroid plexus, which appears bright owing to the presence of contrast medium in the capillaries. The heads of the caudate nuclei lie along the external surfaces of the anterior horns of each lateral ventricle. Several areas of gray matter can be discerned on the CT image deep within the white matter of the cerebrum and constitute the basal nuclei. The MR image contrast has been enhanced so that the deep gray matter structures can be seen on it as well. The major components of the basal nuclei seen at this level are (from lateral to medial) the claustrum, lentiform nucleus (composed of the putamen and globus pallidus), and caudate nucleus. The lentiform nucleus is separated from the caudate nucleus and thalamus by a tract of white matter known as the internal capsule. These sections pass through the superior portion of the midline third ventricle. The thalamus, which serves as a central relay station for sensory impulses to the cerebral cortex, forms its lateral walls. The plane of the CT image passes through the structures of the midbrain. The anterior portions of the midbrain include the cerebral peduncles (white matter tracts that connect the cerebrum and the midbrain). The dark circular area at the posterior edge of the midbrain is the CSF-filled cerebral aqueduct. This passage connects the third and fourth ventricles and allows the circulation of CSF. A contrast-enhanced vessel, the great cerebral vein, is found just posterior to the third ventricle and the splenium of the corpus callosum on the MR image. It passes through the upper portion of the superior cistern. The pineal gland is also found in this cistern but is not clearly visualized in either image. This is an important radiographic landmark because of its tendency to calcify in adults. Branches of the middle cerebral artery are visible within the lateral fissures, and the anterior cerebral arteries can be seen in the anterior portion of the longitudinal fissure on the MR image.

Fig. 30-9 is a CT image that passes through the frontal lobe, pons, and cerebellum; Fig. 30-10 is the MR image, which passes through the superior portions of the orbits, the midbrain, and the occipital lobes. Bony structures visible in the CT image include the frontal bone, the temporal bones, and the occipital bone. Within the temporal bones, the black air-filled structures represent the mastoid air cells. The internal protrusion of bone in the center of the occipital bone is the internal occipital protuberance. The area of signal void between the eyes on the MR image corresponds to the frontal sinuses. Frontal and temporal lobes of the cerebrum are shown on the CT image, whereas the frontal, temporal, and occipital lobes of the cerebrum and the insula are shown on the MR image. The CT scan passes just inferior to the midbrain, and the MR image passes through the level of the midbrain. The large dark area in the center of the CT image is the interpeduncular cistern. This is an enlarged area in the subarachnoid space containing CSF. The optic chiasm and the circle of Willis normally lie within the interpeduncular cistern. The pituitary stalk and some of the vessels that contribute to the circle of Willis are visible on this image. The pons lies posterior to the cistern. The cerebellum lies within the posterior fossa of the skull between the pons and the occipital bone. The large dark region between the pons and cerebellum is the CSF-filled fourth ventricle. The temporalis muscles are seen on the external surfaces on either side of the cranium. On the MR image, the cerebral peduncles form the anterior portions of the midbrain, and the corpora quadrigemina forms the posterior portion. The small black circle anterior to the colliculi is the CSF-filled cerebral aqueduct. Posterior to the midbrain is the cerebellum, which is surrounded by the tentorium cerebelli. The dark region anterior to the midbrain is the interpeduncular cistern, and the region posterior to the midbrain is the superior cistern. On the MR image, within the interpeduncular cistern, the optic tracts, hypothalamus, inferior portion of the third ventricle, and mammillary bodies can be seen. Several important vascular structures can be seen at this level.

The CT image is just superior to the internal carotid arteries and shows the origins of the left anterior and middle cerebral arteries, at the anterior edge of the interpeduncular cistern. The anterior cerebral arteries pass from their origin toward the longitudinal fissure in the midline of the brain; the middle cerebral arteries course from their origins toward the lateral fissures. The CT image also shows the bifurcation of the basilar artery into the two posterior cerebral arteries. These vessels can be seen just anterior to the pons. The circle of Willis is an important vascular structure found in this region of the brain. Although it does not lie in the same plane as the imaging plane, much of the circle can be seen on the MR image. The bright anterior vascular structures represent the bifurcation of the internal carotid arteries into the anterior and middle cerebral arteries. The posterior cerebral arteries are seen here originating between the cerebral peduncles. The posterior and anterior communicating arteries are not seen in this image because they are not at the same level as the other vessels. The posterior portion of the superior sagittal sinus is located near the internal occipital protuberance; the straight sinus can be seen in the edge of the tentorium cerebelli, just posterior to the cerebellum on the MR image.

Fig. 30-11 is a CT image through the sella turcica and the posterior fossa. The MR image (Fig. 30-12) passes through the center of the orbits, the tops of the ears, the pituitary and center of the sella turcica, and the cerebellum. The MR image shows the nasal bones, visible in the anterior skull. Between the eyes the ethmoidal sinuses are seen, and the cribriform plate of the ethmoid bone is seen. The sphenoidal sinuses lie posterior to the ethmoidal sinuses. The sella turcica and dorsum sellae are seen surrounding the pituitary gland. Several cranial bones are visible on the CT scan. The anterior clinoids of the sella turcica and the greater wings of the sphenoid are seen. The roof of the sella is formed by the lesser wings, anterior clinoids, and posterior clinoids. The temporal bone constitutes most of the lateral portions of the skull, and the petrous ridges can be seen on the CT image extending toward the median sagittal plane. The black air spaces near the lateral aspect of the petrous portions of these bones correspond to mastoid air cells, and the air spaces farther medial are associated with the internal structures of the ear. On the CT image, the frontal and temporal lobes of the cerebrum are visible, along with the pons and cerebellum. The dark region between the sella turcica and the pons is the pontine cistern, filled with CSF. The lower region of the fourth ventricle is seen between the pons and the cerebellum. On the MR image, both globes are visible within the orbits. Rectus muscles lie along the medial and lateral walls of each. The optic nerves are seen in the centers of the posterior orbits passing from the eyes toward the brain via the optic canal. The temporal lobes are found lateral to the sella turcica, resting in the middle cranial fossa. The pons lies posterior to the sella, and the cerebellum is seen filling the posterior cranial fossa. The edges of the tentorium cerebelli can be seen faintly between the temporal lobes and the cerebellum. The dark region anterior to the pons corresponds to the CSF-filled pontine cistern in which the contrast-filled basilar artery is easily visualized on both the CT and MR images. The dark region between the pons and the cerebellum is the superior region of the fourth ventricle. On the CT image, the contrast-filled basilar artery lies between the sella and the pons. At this level in the MR image, the internal carotid arteries lie lateral to the body of the sphenoid bone in an almost horizontal orientation. The confluence of sinuses can be seen just anterior to the internal occipital protuberance on the MR image. The confluence is the region where the superior sagittal sinus and the straight sinus meet the transverse sinuses. The transverse sinuses are seen on the MR image at this level. On the external surface of the skull in both images, the temporalis muscles lie along the temporal bones. The auricle, or cartilaginous portion of each ear, lies external to the temporal bone.

The sectional images through the lower cranium show the inferior portions of the cerebrum, brainstem, cerebellum, and associated major skeletal structures (Fig. 30-13 is a CT image, and Fig. 30-14 is an MR image). The CT image shows the frontal sinuses and the roofs of the orbits. The greater and lesser wings of the sphenoid bone are shown. The optic foramina (canals) can be seen between the greater and lesser wings. The optic chiasm and cavernous sinus can be seen posterior to the optic foramen. The petrous and mastoid portions of the temporal bones are shown dividing the middle and posterior cranial fossae. The maxilla, maxillary sinuses, and nasal bones are seen in the anterior skull on the MR image (note the mass within the right maxillary sinus). The zygomatic bones form the lateral walls of the orbits, and the maxillae form the medial walls. The perpendicular plate and vomer form the bony nasal septum seen in the center of the nasal cavity. Posterior to the nasal cavity, the sphenoidal sinuses are seen between the lower aspects of the greater wings. Both petrous ridges extend toward the midline; these are seen as dark areas on the MR image because of the lack of signal from this dense region of bone. Extending into the right petrous ridge is the external auditory canal. Just anterior to the canal is the condyle of the mandible resting in the mandibular fossa. Mastoid air cells lie posterior to the external acoustic meatus.

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