Flashcards in RT CT BASICS Deck (162):
How is the body divided for imaging purposes?
the axial, coronal, oblique or orthogonal, sagittal and transverse planes.
What is the Axial Plane?
An axial plane divides the body into cranial, or superior, and caudal, or inferior, portions. The axial plane often is referred to as the transverse plane,
What is the Coronal Plane?
A coronal plane separates the body into anterior and posterior portions, also termed ventral and dorsal. The midcoronal plane divides the body into equal anterior and posterior halves.
What is the Sagittal Plane?
The sagittal plane divides the body into right and left halves. Midsagittal refers to the plane that divides the body into equal right and left halves.
What is the Oblique Planes?
Oblique planes, also referred to as orthogonal planes, run at a slant through the body.
The Directional Terminology terms used most frequently are:
Anterior – toward the front of the body.
Posterior – toward the back of the body.
Caudal – toward the inferior part of the body (away from the head).
Cranial – toward the upper part of the body (toward the head).
Proximal – toward or above.
Distal – away from or beneath.
Dorsal – the back portion of the body (posterior surface).
Ventral – toward the anterior surface of the body.
Superior – above or on top of.
Inferior – below or underneath.
Lateral – away from the body or sagittal plane.
Medial – toward the body or sagittal plane.
Prone – lying face down on the ventral surface.
Supine – lying face up on the dorsal surface.
Directional Terminology - Anterior
toward the front of the body.
Directional Terminology - Posterior
toward the back of the body.
Directional Terminology - Caudal
toward the inferior part of the body (away from the head).
Directional Terminology - Cranial
toward the upper part of the body (toward the head).
Directional Terminology - Proximal
toward or above.
Directional Terminology - Distal
away from or beneath.
Directional Terminology -Dorsal
he back portion of the body (posterior surface).
Directional Terminology -Ventral
oward the anterior surface of the body.
Directional Terminology -Superior
above or on top of.
Directional Terminology - Inferior
below or underneath.
Directional Terminology - Lateral
away from the body or sagittal plane.
Directional Terminology - Medial
toward the body or sagittal plane.
Directional Terminology - Prone
lying face down on the ventral surface.
Directional Terminology - Supine
lying face up on the dorsal surface.
formation of a blastocyst
which is implanted in the endometrial layer of the uterine wall about seven days after ovulation or around the 21st day of the menstrual cycle.
secretion that comes from the blastocyst to avoid being aborted by the uterus. Pregnancy tests are based on the presence of HCG in a woman’s blood or urine
formation of primary germ layers
During implantation, changes occur to the inner cell mass of the blastocyst, resulting in the formation of primary germ layers.
develops in the inner cell mass as does a two-layered, flattened embryonic disk. The primary germ layers are the ectoderm, endoderm and mesoderm.
All human tissue and organs
Embryonic human development begins at this point. The embryonic stage lasts six weeks, from the beginning of week three through week eight.
Human structures that develop from the ectoderm include the:
1) Epidermis of the skin.
2) Hair, nails and skin glands.
3) Lenses of the eyes.
4) Enamel of the teeth.
5) Adrenal medulla.
6) Receptor cells of the organs of sense.
7) Lining of the oral and nasal cavities, vagina and anus.
Central Nervous System
about 18 to 20 days after conception, the human central nervous system develops from a thickened area of the ectoderm called the neural plate.
The neural plate
develops into a neural crest and a neural tube
The neural tube
forms the human brain and spinal cord
forms the majority of the structures of the peripheral nervous system
The neural tube initially is open at both the caudal and cranial ends.
The cranial end closes around 24 days after conception, and the caudal end closes about two days after that. Differentiation and growth of the neural tube is greatest at its cranial end.
three primary vesicles of the cranial end of the neural tube
the prosencephalon or forebrain, the mesencephalon or midbrain, and the rhombencephalon or hindbrain.
bout a week after differentiation, two of the three primary vesicles, which are they?
the prosencephalon and rhombencephalon, divide.
what develops from the prosencephalon
The telencephalon and the diencephalon
what develops from the rhombencephalon
develops into the metencephalon and the myelencephalon.
the three primary vesicles become what?
five secondary vesicles.
what does the telencephalon develop into?
the cerebral hemispheres and basal ganglia
what does the diencephalon develop into?
the thalamus, hypothalamus, pineal gland, third ventricle and optic tract.
what does the mesencephalon develop into?
ectum, corpora quadrigemina, cerebral peduncles and cerebral aqueduct.
what does the metencephalon develop into?
the pons and part of the medulla oblongata, cerebellum and part of the fourth ventricle
what does the myelencephalon develop into?
the remaining parts of medulla oblongata and fourth ventricle
Structures that develop from the endoderm include the epithelium of the digestive tract, liver, pancreas, urinary bladder, urethra and respiratory tract. The thyroid, parathyroid and thymus glands also are formed by the endoderm.
The mesoderm forms the dermis of the skin; cardiac, skeletal and smooth muscles; connective tissue, including cartilage and bone; the epithelium of the blood vessels, joint cavities and serous membranes; the kidneys, ureters and adrenal cortex; and the epithelium of the female and male reproductive systems.
What is The major bony structure of the head
the cranium, also called the cranial vault.
the cranium, also called the cranial vault.
This structure encases and protects the brain and pituitary gland. The brain and pituitary gland are vital to the integration of the body’s activities and functions.
The skull, or cranium, is covered by the scalp. Ironically, the letters making up the word “scalp” actually stand for the layers that compose it.
S – skin.
C – connective tissue.
A – aponeurosis.
L – loose connective tissue.
P – periosteum.
Where does the scalp receives its blood supply
from the external carotid artery, which is the reason it can bleed profusely when lacerated.
The term “skull” refers not only to the bones of the cranium, but also to the bones of the facial skeleton. The skull is said to be the most complex bony structure in the body, consisting of 22 bones that are connected by sutures, which are immovable joints.
The eight actual cranial bones
include one frontal bone, two parietal bones, two temporal bones, one ethmoid, one sphenoid and one occipital.
What is the cavity floor divided into?
anterior, middle and posterior regions called fossae.
The frontal bone forms the forehead and superior portions of the orbits. Housed within the frontal bone near its midline are the frontal paranasal sinuses. These sinuses connect to the nasal cavity between the middle and inferior nasal conchae, via the middle nasal meatus. The frontal sinuses usually are visible radiographically by the age of 7.
The paired parietal bones form most of the superior portion and some of the lateral aspects of the cranium. They are located superior to the temporal bone and the posterior aspects of the sphenoid bone.
The two temporal bones are inferior to the parietal bones and form part of the lateral portions of the cranium, as well as its floor or base. Both temporal bones also have a squamous portion that forms the inferolateral aspects of the cranium. A mastoid process is located on the posterior aspect of each temporal bone. Extending medially from the temporal bone is its petrous portion.
Temporal Bones – Petrous Portion
The wedge-shaped petrous portion of the temporal bone forms part of the base of the cranium and houses the organs of hearing. The petrous portion has three chambers: external, middle and inner. The external structures consist of the auricle and the external auditory meatus (EAM) or canal.
The EAM conducts sound to the tympanic membrane, also known as the eardrum, which is located in the air-filled middle ear. The middle ear, or tympanic cavity, also houses three auditory ossicles: the incus, malleus and stapes.
The tympanic membrane amplifies and transmits sound vibrations to the auditory ossicles. The auditory ossicles then convey the sound to the oval window of the inner ear. The inner ear, which is filled with fluid, contains the vestibule, semicircular canals and the cochlea.
The vestibule and semicircular canals control balance and equilibrium, while the cochlea is responsible for hearing. The vestibule is located between the semicircular canals and the cochlea. The semicircular canals are easily identified on CT images because they have three separate passages located at right angles to each other. The round window of the inner ear is located at the basal turn of the cochlea. The round window allows the fluid in the inner ear to shift slightly so that sound waves can be propagated.
This page shows the hearing organs in the coronal plane and the proper orientation of the petrous portion of the temporal bone and its enclosed structures. The petrous portion is said to be the densest and one of the most intricate bones in the body. Because the petrous portion of the temporal bone houses the hearing organs, CT has become the major tool to diagnose pathological conditions involving these structures.
The ethmoid bone is situated between the orbits and forms most of their medial walls. A thin piece of bone called the perpendicular plate extends from the inferior portion of the ethmoid and forms a part of the nasal septum. The middle and superior nasal conchae are considered part of the ethmoid bone.
The ethmoid also contains the ethmoid paranasal sinuses, which are composed of numerous air sacs. Although newborn babies have ethmoidal air sacs, these sacs generally do not appear radiographically until around age 3. The ethmoid sinuses connect with the nasal cavity by way of the superior and middle meatuses.
The sphenoid bone lies at the base of the skull, anterior to the temporal bones. Many imaging professionals describe this bone as appearing “bat-like.” In keeping with that description, the sphenoid bone has two parts referred to as the greater and lesser wings. These wings form the anterolateral parts of the cranium and the lateral walls of the orbits. Embedded within the central portion of the sphenoid bone is the sella turcica, which houses the pituitary gland.
The sphenoid sinuses are also located within the sphenoid bone. These sinuses are situated immediately posterior to the ethmoid sinuses and nasal cavity and communicate with that cavity by way of the sphenoethmoidal recess, which is situated superior to the superior nasal conchae.
The sphenoid is separated from the optic nerves, optic chiasm and pituitary gland by a sliver of bone. Although small sphenoid sinuses may present in newborns, they are most radiographically distinguishable after age 2. This page shows the sphenoid sinuses on a transverse CT image of the head.
The occipital bone is the most posterior of the cranial bones. It forms the posterior part of the cranium and part of the base of the skull. The occipital bone contains a large foramen called the foramen magnum. In addition, on either side of the inferior portion of the occipital bone, there are condyloid processes called the occipital condyles. The occipital condyles articulate with the lateral masses of the first cervical vertebra.
The facial skeleton consists of 14 bones, including:
Two maxillae (the singular term is maxilla).
Two inferior nasal conchae (the singular term is concha).
e maxillae join in the median sagittal plane of the head, forming what many people call the upper jaw. The upper incisors are embedded in the lower portion of the maxillae. Each maxilla has a horizontal part called a palatine process. These processes unite to form the roof of the mouth, also called the hard palate.
A maxillary sinus is encased within the body of each maxilla. The maxillary sinuses connect with the middle meatus of the nasal cavity by way of the hiatus semilunaris. The largest of the paranasal sinuses in adults, the maxillary sinuses occupy most of each maxilla. They are normally present at birth, but become more visible radiographically when a child approaches puberty.
The transverse CT image on this page shows the maxillary sinuses.
The two palatine bones are located posterior to the union of the palatine processes of the maxilla and form the posterior aspects of the hard palate.
Two zygomatic bones, or zygomas, are located on each side of the face. These bones form the prominence of the cheeks and the lateral borders of each orbit. The anteromedial wall of each orbit is made up of the two rectangular lacrimal bones. Located posteriorly on each zygomatic bone is a temporal process. These processes join with the zygomatic processes of the temporal bones to form the zygomatic arches.
Inferior Nasal Conchae
We mentioned earlier that the superior and middle nasal conchae are part of the ethmoid bone. Many medical imaging practitioners associate all of the nasal conchae with the nasal cavity because of their location, but remember that only the inferior nasal conchae are truly part of the facial skeleton. These scroll-shaped bones lie on either side of the median sagittal plane of the nasal cavity. Covered by mucous membrane, the nasal conchae warm and moisten the outside air as it enters the nasal cavity.
This coronal CT scan through the face shows the nasal conchae.
The union of the two nasal bones at the median sagittal plane forms the bridge of the nose, while a slender, plow-shaped bone called the vomer forms the most inferior aspect of the nasal septum.
The most inferior bone of the facial skeleton is the mandible. Each side of the mandible, also called the lower jaw, has a posterior, superior portion called a ramus. The rami have superior projections called condyles and coronoid processes. The condyles of the rami join the temporal bones to form the only moveable joint in the skull, the temporomandibular, or TMJ, joint. The lower incisors are imbedded within the superior portions of the mandible.
The cavity created by the cranial bones houses the brain. The brain weighs approximately 3 pounds, or about 2% of a person’s body weight, and is smaller in women than in men. The weight of the brain, however, is not indicative of a person’s intellectual capacity. Rather, the brain has folds called gyriand furrows called sulci. Researchers generally believe that the more sulci and gyri a person has, the greater his or her intellectual capacity.
The brain is composed of well-organized areas of gray and white matter. The white matter is made up of myelinated nerve fibers. Myelin is a white fatty substance. Gray matter is composed of unmyelinated fibers and nerve cell bodies. The nerve cells, or neurons, transmit impulses to other neurons across a small gap called a synapse. The gray areas are regions of synapse.
Sulci and Gyri
The CT image on this page shows the sulci and gyri of the brain on a transverse cut through the upper portion of the brain.
Gray and White Matter
Note the gray and white matter regions in this cadaver segment of the brain.
Three layers of connective tissue called meninges cover the brain and separate it from the cranial vault. The meninges also help protect the brain’s surface. The outermost layer of the meninges is the dura mater, or simply the dura, which is made up of tough, fibrous connective tissue. The term “dura mater” comes from “dura” meaning hard and “mater” meaning mother.
Dura Mater Extensions
Extensions of the dura form four inwardly projecting folds that compartmentalize the brain. The falx cerebri is located between the cerebral hemispheres, while the falx cerebelli is found between the cerebellar hemispheres. The tentorium cerebelli divides the cerebrum and the cerebellum, and the diaphragma sellae forms a bridge over the sella turcica and covers the pituitary gland.
The image on this page is a transverse CT scan of the cerebrum showing the falx cerebri within the longitudinal fissure of the brain.
The tentorium cerebelli is shown here on a reformatted sagittal CT image of the brain. Note its location between the occipital lobe and the cerebellum.
The arachnoid layer is the middle layer of meninges. It is very thin and quite delicate, with minute trabeculae extending from its inner surface. The trabeculae resemble a cobweb, thus the reason for the name “arachnoid.” The arachnoid layer is separated from the innermost meningeal layer by the subarachnoid space. This space contains cerebrospinal fluid, or CSF, and the larger blood vessels in the brain.
There also is a space between the arachnoid membrane and the dura mater called the subtotal space. It contains only a small amount of cerebrospinal fluid to keep the surfaces moist. When the arachnoid layer reaches the area of the venous sinuses, numerous diverticula penetrate and project into the dura.
The pia mater, a thin, very vascular membrane, is the innermost layer of meninges. “Pia” means tender, so pia mater refers to the “tender mother” layer of meninges. The pia mater adheres very closely to the contours of the brain. The pia and arachnoid membranes together are called the leptomeninges. These two layers are very close, especially at the uppermost portions of the gyri.
The pia then separates slightly from the arachnoid to follow the dips of the sulci, while the arachnoid forms a bridge-like structure over the top of the gyri, creating triangular-shaped spaces. These spaces contain cerebrospinal fluid.
Other spaces between the arachnoid and pia mater are called the subarachnoid cisterns. Located near the base of the brain, they contain rather large amounts of cerebrospinal fluid.
Also called the cisterna magna, the cerebellomedullary cistern is formed when the arachnoid layer of meninges crosses the space between medulla oblongata and the inferior surface of the cerebellum. It receives cerebrospinal fluid from the foramen of Magendie, or median aperture of the fourth ventricle. A procedure known as a cisternal puncture is used to sample cerebrospinal fluid from this space when a lumbar puncture is contraindicated.
The pontine cistern is located on the anterior or ventral surface of the pons. This cistern contains the basilar artery and receives cerebrospinal fluid from the foramen of Luschka, or lateral aperture of the fourth ventricle.
Note the location of the pontine cistern anterior to the pons on this transverse CT image.
Cistern of the Lateral Sulcus
The cistern of the lateral sulcus is formed when the arachnoid meningeal layer crosses the gap from the frontal to the temporal lobe. This cistern contains the middle cerebral artery.
The interpeduncular cistern contains the circle of Willis, or circulus arteriosus cerebri. It is located between the paired temporal lobes of the brain.
The chiasmatic cistern is created by the continuation of the interpeduncular cistern anteriorly and superiorly. This cistern is associated with the optic chiasm.
The cisterna ambiens contains the great cerebral vein and the pineal gland. It occupies the space between the splenium of the corpus callosum and the superior aspect of the cerebellum. This cistern also is referred to as the cistern of the great cerebral vein, the superior cistern or the quadrigeminal cistern.
The pineal gland often appears calcified on skull radiographs and cranial CT and MR images of adults. You can see a calcified pineal gland on this CT image.
The choroid plexus is a specialized vascular structure found in the lateral, third and fourth ventricles. It produces cerebrospinal fluid as the result of filtration and secretion. The primary constituents of CSF are water, glucose, sodium chloride and protein. Brain diseases often are diagnosed by measuring changes in the concentration of these substances in CSF. In addition, the presence of blood and other substances not commonly found in CSF may indicate other disease or traumatic injury.
This page shows an axial CT image through the cerebral ventricles of the brain. Note the choroid plexus within the posterior horns of the lateral ventricles.
The two lateral ventricles are located within the hemispheres of the brain. They are separated by a thin partition at midline called the septum pellucidum. The lateral ventricles drain into the third ventricle by way of the foramen of Monro, or interventricular foramen, and the third ventricle drains into the fourth ventricle via the cerebral, or sylvian, aqueduct.
Cerebrospinal fluid flows into the spaces in and around the brain and spinal cord by way of two openings in the lateral walls and a single opening in the medial wall of the fourth ventricle. The two lateral openings are called foramina of Luschka, while the medial opening is the foramen of Magendie.
The brain can be divided into four distinct regions: the cerebrum, diencephalon, brainstem and the cerebellum.
The cerebrum is the most superior and largest part of the brain. It is divided into left and right hemispheres, which are connected by a central mass of white matter called the corpus callosum.
The corpus callosum is divided into four areas: the genu, rostrum, body and splenium. The genu of the corpus callosum is its most anterior portion, while the rostrum is located inferior and slightly posterior to the genu. The splenium is the most posterior part of the corpus callosum. This page shows the parts of the corpus callosum.
The deep cleft separating the two hemispheres of the brain is called the longitudinal fissure. The midline structures of the brain are located within this fissure, as is the extension of dura mater called the falx cerebri. In CT, when the falx cerebri and other midline structures of the brain deviate from their normal location, it is called “mass effect.” This axial CT image through the cerebrum shows the longitudinal fissure.
Lobes of the Cerebrum
This page lists the functions of the lobes. The frontal lobe is associated with personality and voluntary motor control; the parietal lobes are responsible for peripheral sensations; the occipital lobe controls vision; the temporal lobes manage the sense of hearing, smell and taste; and the insula directs motor and sensory organ function.
Fissures and Sulci
As we mentioned earlier, the cerebral cortex is organized into folds called gyri and furrows called sulci. There also are areas throughout the cerebrum with deeper grooves called fissures. In addition to the longitudinal fissure discussed earlier, there is another important fissure called the sylvian fissure, or lateral sulcus.
The sylvian fissure separates the frontal and parietal lobes from the temporal lobe. The insula is located deep in the sylvian fissure toward the median sagittal plane of the head. This page shows the location of the sylvian, or lateral, fissure.
The central sulcus is the main sulcus within the cerebrum. It separates the frontal lobe from the parietal lobe at the precentral and postcentral gyri. The precentral gyrus is considered the brain’s motor area, while the postcentral gyrus is thought to be the sensory area. Both of these regions can be seen on CT and MR images. There also is a sulcus that separates the parietal lobe from the occipital lobe, aptly named the parieto-occipital sulcus.
As we said earlier, the brain is composed of white and gray matter. Most of the gray matter is located on the surface of the brain, or cerebral cortex, which is only about 1.5 to 5 mm thick. The white matter is found beneath the cortex. Organized masses of gray matter called basal ganglia are interspersed throughout the white matter.
The diencephalon is located centrally within the cerebral hemispheres. A midline structure surrounding the third ventricle, it is composed of the epithalamus, thalamus and hypothalamus. The largest part of the diencephalon is the thalamus.
The thalamus is a mass of gray matter on each side of the third ventricle that creates the ventricle’s lateral walls. It is a major relay station of the afferent, or sensory, pathway, which transmits nerve impulses to the cerebral cortex. The epithalamus is superior to the third ventricle and forms its roof. The pineal gland is a projection off of the epithalamus.
The hypothalamus forms the floor of the third ventricle. The infundibulum, or pituitary stalk, optic chiasm and the mammillary bodies are located in the inferior aspect of the hypothalamus. The mammillary bodies are involved in swallowing reflexes.
The pituitary gland is considered the master gland of the body because it regulates the functions of many other glands through the manufacture and secretion of six major hormones. The pituitary is connected to the hypothalamus by the infundibulum, or pituitary stalk.
This reformatted sagittal CT image shows the pituitary gland.
The brain stem is a small mass of tissue located beneath the cerebrum. It has three major divisions: the midbrain, pons and medulla oblongata. The smallest region is the midbrain, which is located between the diencephalon and pons. Consisting primarily of bundles of nerve fibers, the midbrain surrounds the cerebral aqueduct and is divided into two major parts: the quadrigeminal plate (colliculi) and the cerebral peduncles.
The cerebral peduncles are noticeable in cadaver sections because of a darkly pigmented substance called substantia nigra. Substantia nigra is believed to play a major role in neuromuscular disorders such as Parkinson disease. The substance produces dopamine, a neurotransmitter that controls muscular reflexes. Note the cerebral peduncles on this CT image.
The oval-shaped pons is a band of fibers located between the midbrain and medulla oblongata and posterior to the fourth ventricle. Fibers from the pons extend from the cerebellum to other areas in the brain, but most connect the two cerebellar hemispheres. The basilar artery travels over the anterior surface of the pons. The sagittal anatomical model shows the pons in midline. The axial CT demonstrates the location of the pons in transverse section.
\The medulla oblongata is a conical structure located beneath the pons. Extending from the pons to the foramen magnum, the medulla oblongata is composed of fiber tracts that breach the area between the brain and spinal cord.
It also contains so-called “vital centers” for the regulation of the body activities such as heart rate, respiratory rhythm and blood pressure. The median fissure is located on the anterior surface of medulla oblongata. On either side of the median fissure are small nodule-like structures called pyramids.
The cerebellum is located within the posterior cranial fossa and is situated posterior to the pons and medulla oblongata. The two cerebellar hemispheres are connected by a central structure called the cerebellar vermis. The cerebellar vermis is said to resemble a coiled-up worm. In fact, the term “vermis” is derived from the Latin word “verm,” meaning worm.
As with the cerebrum, the cerebellum is covered with a layer of gray matter called the cerebellar cortex. White matter lies deep within the cortex. Two round, prominent bulges called cerebellar tonsils are located on the inferior surface of the cerebellum. The cerebellum is responsible for controlling muscle tone and coordinating muscular activity.
The brain has 12 pairs of cranial nerves. All except the first and second pair emerge from the brainstem. They pass through foramina located within the skull and transmit nerve impulses to structures in the head, neck and viscera of the body. The cranial nerves are named and numbered in the order that they appear on the inferior surface of the brain. Roman numerals are used as the numbering format.
Some of the cranial nerves, such as the olfactory (I), optic (II), trigeminal (V) and vestibulocochlear nerves(VIII), register smell, sight, hearing or equilibrium, bringing information from the sensory organs to the brain. The trigeminal nerve also controls sensations in the face, scalp and teeth, and contraction of chewing muscles. Other cranial nerves control muscles.
For example, the oculomotor (III), trochlear (IV) and abducens nerves (VI) are associated with various eye movements. The accessory nerve (XI) controls the neck and shoulder muscles, while the hypoglossal nerve (XII) controls the movement of the tongue.
A few the cranial nerves have mixed functions. The mandibular branch of the trigeminal nerve not only registers sensations in the jaw, tongue, lower lip and teeth, but also controls the movement of the chewing muscles. The facial nerve (VII)transmits the sense of taste, directs the contraction of facial and swallowing muscles and controls secretions of the lacrimal and submaxillary glands; the glossopharyngeal nerve (IX) also transmits the sense of taste and manages parotid gland secretions. The vagus nerve (X) carries impulses far beyond the head to the larynx, heart, stomach and intestines, and the bronchial tubes.
Of all of the cranial nerves the trigeminal (V) is the largest. The vagus (X) has the widest distribution, carrying and receiving impulses to help regulate heart rate, blood pressure, digestive secretions and respiration.
Arteries of the Brain
The paired internal carotid and vertebral arteries supply blood to the brain. The internal carotid arteries begin where the common carotid arteries in the neck divide into external and internal branches. The external carotid arteries extend to supply the face and scalp. The internal carotid arteries ascend to the base of the skull where they enter the carotid canals of the temporal bone. They then travel anteriorly within the cavernous sinus and superiorly and posteriorly through the dura mater.
The image on this page is an axial CT scan showing a contrast-enhanced circle of Willis. The internal carotids form the middle and the anterior cerebral arteries. The middle cerebral arteries supply blood to the lateral portions of the cerebrum and divide into numerous branches that travel within the sylvian fissure. The anterior cerebral arteries and their branches pass through the genu of the corpus callosum to supply the anterior portion of the frontal lobe and the medial portions of the cerebral hemispheres.
The short midline anterior communicating artery unites the two anterior cerebral arteries. These arteries supply blood to the orbital contents, and the frontal, parietal and temporal lobes of the brain. An additional branch of the internal carotid arteries travels posteriorly to join the posterior cerebral arteries. In essence, the internal carotid arteries form the anterior portion of the circle of Willis. The middle cerebral arteries are not considered a part of the circle.
The right and left vertebral arteries are branches of the subclavian arteries. They ascend vertically within the transverse foramina located in the transverse processes of the cervical vertebrae, beginning at C6. The arteries pass through the foramen magnum, pierce the dura mater and enter the cerebellomedullary cistern. The two vertebral arteries join to form basilar artery.
The basilar artery travels over the anterior surface of pons, after which it bifurcates into the two posterior cerebral arteries. The posterior cerebral arteries supply blood to the occipital lobe of the brain. These vessels form a configuration called the circle of Willis, or circulus arteriosus cerebri. The circle of Willis is located in the interpeduncular cistern and encloses the optic chiasm and the infundibulum.
The basilar artery is the site of the most common aneurysm found in the brain — the berry aneurysm. The axial CT scan on this page shows a contrast-enhanced basilar artery on the ventral surface of the pons.
On this reformatted sagittal CT image, you can see a contrast-enhanced basilar artery on the anterior surface of the pons.
Circle of Willis (C.O.W.)
The image on this page is an axial CT scan showing a contrast-enhanced circle of Willis.
Just as in other areas of the body, arteries carry blood to the brain and veins drain the blood away. However, the venous system of the brain is somewhat different from normal veins in the body. The brain’s venous system is primarily composed of the dural, or venous, sinuses, superficial cortical veins and the deep veins of the cerebrum.
All the veins of the head drain into the dural sinuses, which in turn empty into the internal jugular veins. While the other veins in the body have valves to keep the blood flowing in the correct direction, the dural sinuses do not.
The dural sinuses include superior and inferior sagittal, straight, transverse, sigmoid, cavernous and petrosal parts.
Superior Sagittal Sinus
The superior sagittal sinus is located in the median plane between the falx cerebri and the calvarium, or cranial bones. Beginning at the crista galli, it runs the entire length of the falx and ends at the occipital protuberance. The superior sagittal sinus often appears as a triangular-shaped structure at the superior and inferior portions of CT and MR images of the head. Note the superior sagittal sinus anteriorly and posteriorly on this axial CT image of the head.
Inferior Sagittal Sinus
The inferior sagittal sinus, which is much smaller than the superior sagittal sinus, extends posteriorly, immediately beneath the free edge of the falx cerebri. The inferior sagittal sinus joins the great vein of Galen (the great cerebral vein) to form the straight sinus. The straight sinus travels along tentorium cerebelli until it reaches the occipital protuberance, where it continues as the lateral sinus.
Although the lateral sinuses are continuations of the straight sinus or superior sagittal sinus, they usually form a common area at their origin called the confluence of sinuses. The confluence of sinuses is located at the level of the occipital protuberance. The lateral sinuses branch into transverse and sigmoid sinuses.
The sigmoid sinus is a continuation of the transverse sinus. The sigmoid sinus follows an S-shaped path that loops over the petrous and mastoid portions of the jugular foramen, where it continues as the internal jugular vein.
A large venous sinus called the cavernous sinus is located on each side of the body of the sella turcica. The cavernous sinus receives venous blood from the ophthalmic and middle cerebral veins and is drained by the small petrosal sinuses that empty into the sigmoid sinus or jugular vein. The cavernous sinus not only contains venous structures, but the internal carotid artery also enters the sinus via the foramen lacerum, turns sharply and exits the sinus to enter the subarachnoid space.
You can see the internal carotid arteries within the cavernous sinus on this coronal CT image. The abducens cranial nerve (VI) is closely related to the internal carotid artery as it travels through the cavernous sinus. From superior to inferior, the oculomotor (III) and trochlear (IV) nerves, and the ophthalmic and maxillary divisions of the trigeminal nerve (V) also are associated with the cavernous sinus.
Structures Within the Facial Skeleton
Several nonosseous structures are located within the skeletal bones of the face. In this portion of the module, we will focus on the orbits and their contents, and structures posterior to the oral and nasal cavities. In addition, we will discuss the salivary glands.
The bony orbit is described as pyramid shaped. Its apex is the most posterior aspect, the base its most anterior aspect and its walls are shaped like triangles. The medial walls are formed by the lacrimal bone and the orbital plates of the ethmoid and palatine bones. The lateral walls are formed by the zygomatic bones and the greater wings of the sphenoid bone.
The orbital plates of the frontal bone make up the roof of each orbit, while the floor is formed by the maxillae and a small portion of zygomatic bones. This image shows the bony orbits. You can see their position within the facial skeleton and the anatomical contributions of the bones that form the orbits.
Organs of Sight
The orbits contain the organs responsible for vision. The primary organs of sight are the bulbus oculi, which are commonly known as the eyeballs. Each eyeball is roughly spherical in shape and 2 to 3 cm in diameter. The walls of the eyeball are composed of three layers called tunics: the external or fibrous layer, middle or vascular layer and innermost or retinal layer.
Each bulbus oculi has several parts. You can see the bulbus oculi and the lens of the eye distinctly on this axial CT scan through the orbits. The optic nerves also are visible. The ganglion cells of each eyeball join to form the optic nerve. The optic nerve emerges from the eye posterior to the optic disc, which is located in the center of the posterior aspect of bulbus oculi. The optic nerves continue posteriorly until they cross at the optic chiasm, or area of decussation. From the area of decussation, the optic nerves continue through the brain until they reach the occipital lobe, which is the visual cortex and is responsible for sight.
The eye moves using six muscles that are inserted in the sclera. These muscles are the medial, lateral, superior and inferior rectus, and the superior and inferior obliques. You can see the eye muscles and the optical disc on this coronal CT image. In some cases, the ophthalmic artery can be seen on a diagonal course through this same section, and is sometimes mistaken for one of the extraocular muscles.
Three pairs of salivary glands are found in the lower portion of the facial skeleton: the parotid, submandibular and sublingual. Although considered part of the digestive system or gastrointestinal tract, these glands are located in the facial skeleton. They are responsible for secreting fluid that moistens food. The secretions contribute to the taste of food and aid in swallowing. In addition, the salivary glands produce amylase, a substance that begins carbohydrate digestion.
The parotid glands are the largest and most superiorly situated of the salivary glands. These glands are positioned between the mastoid portion of the temporal bone and the ramus of the mandible. They can be palpated by placing the index and middle fingers just anterior and inferior to the ear lobe.
The major duct of the parotid gland is the Stensen duct. This duct communicates with the oral cavity through an opening in the mouth near the upper second molar. Note the parotid glands on this axial CT image.
The submandibular glands are sometimes referred to as the submaxillary glands. This gland can be palpated as a small lump medial to the body and angle of the mandible. Its major duct, the Wharton duct, opens into the oral cavity near the frenulum of the tongue. This reformatted coronal CT image of the face shows the submaxillary glands.
The smallest and most deeply located of the salivary glands are the sublingual glands. These glands are found in the floor of the mouth beneath the mucous membrane of the oral cavity. United anteriorly at the frenulum of the tongue, the two sublingual glands open into the oral cavity via a larger duct called the Bartholin duct and several smaller ducts referred to as the Rivinus ducts.
On occasion, the sublingual glands may open into the Wharton duct, the major duct of the submandibular glands. This coronal CT image of the face shows the sublingual glands.
The pharynx extends from the base of the skull to the sixth cervical vertebra. Therefore, it is considered one of the structures within the facial skeleton, but it also continues into the neck. The pharynx is about 12 cm long, with walls composed of mucous membrane covered by overlapping constrictor muscles. The pharynx is divided into three parts: nasal, oral and laryngeal.
The nasal portion of the pharynx, appropriately termed the nasopharynx, is located posterior to the nasal cavity, extending from the base of the skull to the soft palate. It is an open space that communicates with the nasal cavity through the choanae. The eustachian tubes from the ears open into the nasopharynx from the lateral walls. An aggregation of lymphoid tissue known as the pharyngeal tonsil can be found within the posterior wall of the nasopharynx.
The oropharynx, or oral portion, is located posterior to the mouth. The oropharynx begins at the soft palate and extends inferiorly to the tip of the epiglottis. The lateral walls of the oropharynx are lined with lymphoid tissue called the palatine tonsils, commonly referred to as just the tonsils. The lingual tonsils, another aggregation of lymphoid tissue, are located in the anterior walls of the oropharynx.
The laryngopharynx, or laryngeal portion, begins at the superior border of the epiglottis and travels inferiorly to the cricoid cartilage. The cricoid cartilage is located at the union of the larynx and the trachea at the C6 vertebral level . The walls of the laryngopharynx extend laterally around the opening to the larynx, forming the piriform recesses.
A space, known as the retropharyngeal space, is located between the cervical vertebrae and the musculature and the fascia surrounding the pharynx. The space is filled with loose connective tissue that allows movement of the esophagus, larynx, pharynx and trachea. This movement is necessary for swallowing. The retropharyngeal space is closed to the skull superiorly but opens into the thoracic cavity inferiorly. The opening allows infections in the space to move directly into the mediastinum. The pharynx extends the length of the neck before becoming the esophagus at the sixth cervical vertebra.
The region of the body referred to as the neck is located between the thorax and head. Seven cervical vertebrae, the skeletal components of the neck, support the cranium. The neck also contains structures that pass from the head into the thorax and the abdominal cavity.
The cervical vertebrae are convex anteriorly. Although similar to the other vertebrae, cervical vertebrae have some interesting and unique characteristics. The first cervical vertebra, or C1, is also called the atlas. It is an oval, ring-like structure that does not have a body or spinous process. The lateral aspects of the atlas include lateral masses with indented articular facets.
The lateral condyles of the occipital bone fit into these indentations. This articulation, which supports the cranial and facial skeleton, allows the back-and-forth movement of the skull when we nod yes. The atlas also has inferior facets or processes that articulate with the second cervical vertebra.
This axial CT scan shows the atlas. The parts of this particular vertebra are labeled on the image.
The second cervical vertebra, called the axis, is unique among other vertebrae in that it has a superiorly projecting process called the odontoid process, or dens. The dens projects through the vertebral foramen of the atlas. Along with muscular support, this articulation allows the pivot-like motion of the skull when we shake our heads side to side. Notice the lateral portions of the atlas (C1) and the odontoid process of the axis (C2) on this coronal CT scan through the neck.
This axial CT image shows the dens of the axis (C2) projecting through the atlas (C1).
One of the most distinctive characteristics of the cervical vertebrae is that they have holes within their transverse processes. These holes, or transverse foramina, provide a passageway for the vertebral arteries and other essential structures.
The spinous processes of the cervical vertebrae also are unique, in that, when present, they have bifid tips, meaning that they are forked in appearance. Sometimes the spinous process is not bifid, but is long and pointed. This configuration allows the seventh cervical vertebra at the base of the neck to be palpated easily. C7 often is referred to as the “vertebra prominens.” The bifid process of a cervical vertebra is shown on this axial CT scan through the neck.
The vertebra prominens (C7) is shown on this reformatted sagittal CT image.
In addition to the cervical vertebrae, muscles provide additional support for the structures of the neck. Neck muscles usually are divided into anterior and posterior triangles. The triangles are further described as being located anterior or posterior to the sternocleidomastoid muscle. Extending from the sagittal plane of the neck to the anterior border of the sternocleidomastoid muscle, the muscles of the anterior triangle also help form the floor of the mouth.
The base of the anterior triangle is formed by the lower border of the mandible, while its apex is the manubrium of the sternum. All of the muscles of the anterior triangle are attached to the hyoid bone. This axial CT image shows the major muscles of the anterior triangle.
Just as the neck muscles are divided into two groups, the muscles of the anterior triangles also are described as either infrahyoid or suprahyoid. As the name suggests, the infrahyoid muscles are inferior to the hyoid bone. These muscles pull the hyoid bone and larynx inferiorly during swallowing, and return them to their normal position afterward.
The infrahyoid muscles include the omohyoid, sternohyoid, sternothyroid and thyrohyoid. The suprahyoid muscles are superior to the hyoid bone. This muscle group includes the digastric, geniohyoid, mylohyoid and stylohyoid. These muscles elevate the hyoid bone during swallowing, or if the hyoid bone doesn’t move, the muscles open the temporomandibular articulations.
The anterior triangle muscles of the neck are interesting in that they contain the carotid sheath. Remember that the carotid sheath is composed of a carotid artery, either common or internal depending on the level; the internal jugular vein; and vagus nerve.
The posterior triangle muscles are located posterior to the sternocleidomastoid muscle and extend from its posterior border to the trapezius muscle. The base of the triangle is made up of several muscles, including the levator scapulae; anterior, middle and posterior scalenes; and the splenius capitis.
The apex of the posterior triangle is formed by the junction of the sternocleidomastoid and the trapezius. Notice the association of the selected muscles with the sternocleidomastoid and trapezius muscles.
Organs of the Neck
Although some organs belong strictly to the neck such as the thyroid and parathyroid glands, other organs are continuations of structures located within the cranium and face, such as the pharynx. Remember that the pharynx actually originates posterior to the nasal cavity and continues inferiorly as the oropharynx and laryngopharynx, ultimately transitioning into the esophagus at C6.
Essentially an air passageway, the larynx also functions as the major organ of voice production. The larynx normally begins at the level of third cervical vertebra (C3) in men; in women and children, the organ is normally positioned more superior to C3. The larynx culminates at the level of C6. The organ is made up of nine cartilages connected by ligaments, which compose its skeleton. Some of the cartilages are paired, while others are unpaired.
The arytenoid, corniculate and cuniculate cartilages are paired, while the thyroid, cricoid and epiglottic cartilages are unpaired. Of these, the arytenoids, thyroid and cricoid are considered hyaline cartilages, which can calcify later in life. The others are considered elastic in nature.
The largest of the cartilages, the thyroid cartilage, forms the anterior wall of the larynx . Most often, the thyroid cartilage is simply referred to as the Adam’s apple. This sagittal CT reformation of the neck shows the opening into the larynx.
One of the other organs within the neck region is the thyroid gland. The thyroid gland is the largest endocrine gland in the body. The gland is composed of two lobes connected by the thyroid isthmus. The isthmus passes across the second and third rings of the trachea. The lobes of the gland are situated lateral to the inferior part of the larynx and the superior part of the trachea. The lobes also relate to the esophagus on their posterior aspects.
The thyroid gland is responsible for the production of vital hormones that regulate normal growth and metabolism. The gland requires the mineral iodine to synthesize some of these hormones. Two parathyroid glands are situated in the posterior and inferior portions of each thyroid lobe. They generally are embedded within the lobar tissue. This axial CT scan through the thyroid gland shows both the lobes and the isthmus.
There is a strong vascular system in the neck region that includes both major and minor arteries and veins. Let’s talk about the major arteries first, starting with the vertebrals, since they were discussed briefly earlier in the module. The left and right vertebral arteries originate at their respective subclavian arteries. The vertebrals ascend through the neck, enclosed by the transverse foramina of C6 to C1. Notice the location of the vertebral arteries within the transverse foramina of the cervical vertebrae.
This sagittal CT reformation shows the vertebral artery within the transverse foramina of the cervical vertebrae. After exiting the transverse foramina at C1, the vertebral arteries travel through the foramen magnum, traverse the dura mater and arachnoid meninges and enter the subarachnoid space within the cerebellomedullary cistern.
When they reach the inferior aspect of the pons, the vertebral arteries join to form a single artery called the basilar artery. As mentioned earlier, the basilar artery passes over the anterior surface of pons, and forms the posterior portion of the circle of Willis.
The sagittal CT reformation of the neck on the left shows the basilar artery on the anterior surface of the pons. The axial CT image on the right shows the basilar artery in the interpeduncular cistern.
The next major artery found in the neck is the carotid. The right common carotid artery begins as a branch off the brachiocephalic artery posterior to the sternoclavicular articulation. The left common carotid artery branches directly off of the aortic arch. Both common carotids ascend the neck in what is called the carotid sheath.
Note the location of the carotid artery within the carotid sheath on this contrast-enhanced axial CT image of the neck. The carotid artery is medial to the larger of the vascular structures shown here. The lateral vessel is a jugular vein.
You can see the carotid arteries on this contrast-enhanced reformatted coronal CT image of the neck.
The common carotid arteries ascend the neck until they reach the superior border of the thyroid cartilage, where they divide into the internal and external carotid arteries. The common and internal carotid arteries are dilated at that division. The dilation is known as the carotid sinus, and it contains receptors that regulate blood pressure.
The major venous structures of the neck are the internal and external jugular veins. You might remember that the dural or venous sinuses drain blood away from the head. These sinuses empty into a common sinus, the sigmoid sinus. The sigmoid sinus then drains blood into the internal jugular veins. As they travel inferiorly, the internal jugular veins pass posterior to the sternocleidomastoid muscles.
The veins then turn anteriorly to a point posterior to the sternal end of the clavicle, where they connect to the subclavian veins and then form the brachiocephalic veins. The internal jugular veins are the largest vascular structures of the neck region. In most people, the right internal jugular is larger than the left. The external jugular veins have two major branches: anterior and posterior.
The external jugulars receive blood from structures in the face, neck and scalp, and the contents empty into the appropriate subclavian vein, which then form the brachiocephalic veins. These veins then drain into the superior vena cava.