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Brain and skull base CT scan

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Brain CT scan

Brain CT scan - a method of brain imaging, intracranial spaces, as well as bones and soft tissues. The technique of brain CT scan base on mathematical modeling of the internal structure of the brain and surrounding structures on the results of his X-ray beam irradiation.

By using of the brain CT scan can be visualized by the following structures with their image in different planes:

  • calvarial bones
  • skull base bones
  • paranasal sinuses
  • brain vessels (arteries, veins, venous sinuses)
Brain CT scan allows to visualize some types of the brain parenchyma and its membranes tumor.

 

Skull base CT anatomy

Computed tomography (CT) of the skull base reveals the following normal anatomical structures: a. – Pterygopalatine fossa, b. – Foramen Ovale, c. – Carotid Canal, d. – Incus, e. – Facial Nerve (CN VII), f. – Petroclival synchondrosis, g. – Cochlear Aperture, h. – Groove for the Sigmoid Sinus.

Computed tomography (CT) of the skull base reveals the following normal anatomical structures

  • a. Pterygopalatine fossa. Communications: nasal cavity via the sphenopalatine foramen (1), masticator space via the pterygomaxillary fissure, orbit via the inferior orbital fissure, cavernous sinus via foramen rotundum, middle cranial fossa via the vidian canal (2) and the palate via the greater palatine canal.
  • b. Foramen ovale. An oval shaped opening in the greater wing of the sphenoid (middle cranial fossa). It transmits the mandibular division of the trigeminal nerve (CN V3), accessory meningeal artery, emissary veins, otic ganglion, and occasionally the lesser petrosal nerve.
  • c. Carotid canal. A passage within the petrous temporal bone that transmits the internal carotid artery and sympathetic plexus. Its inferior opening is the carotid foramen located anterior to the jugular fossa. The canal opens into the middle cranial fossa above the foramen lacerum.
  • d. Incus. The incus is the middle ossicle in the ossicular chain articulating with the head of the malleus anteromedially (3) and the stapes inferomedially via its long process (4). It appears as the most lateral of the ossicles because its short process extends posterolaterally.
  • e. Facial nerve (CN VII). On this image the facial nerve has just passed under the lateral semicircular canal and is commencing its descending or mastoid portion. The facial nerve supplies the muscles of facial expression, taste to the anterior 2/3 of the tongue and parasympathetic fibers to salivary and lacrimal glands.
  • f. Petroclival synchondrosis. A synchondrosis between the apex of the petrous temporal bone and the clivus, which is the anterior portion of the occipital bone. This bony junction is the classic site of origin of skull base chondrosarcomas and hence knowledge of this anatomy can assist in formulating a diagnosis.
  • g. Cochlear aperture. Opening of the distal internal acoustic canal that transmits the cochlear nerve into the cochlea. The nerve follows approximately the path indicated by the arrow and is one of four bundles of the vestibulocochlear nerve (CN VIII) within the internal acoustic canal.
  • h. Groove for the sigmoid sinus. The sigmoid sinus (a dural venous sinus) is the continuation of the transverse sinus. It descends medially in an S shaped groove posterior to the mastoid air-cells to the jugular foramen. There is often dominant drainage to one side with the groove being deeper on the dominant side.

 

Indications for brain CT scan

In what cases is carried out brain CT scan examinations:

Soft tissue injuries of the head during brain CT scan are characterized by local thickening them with mild focal increase in the density. This is a sign of blood impregnating of bruised tissue. If slice scan coincides with the lumen of the wound cavity wound of the head or intersects at an angle the wound channel, it is possible to specify the depth of soft tissue damage, determine their blood imbibition and to establish a projection ratio with skull fractures. Subgaleal hematomas easily distinguished from the heads soft tissues injuries and subcutaneous accumulations of cerebrospinal fluid or pus.

Linear skull fractures only clearly visualized on brain CT scan, when accompanied by displacement of the edges towards each other. Bone fragments from depressed skull fractures are defined if they are arranged at an angle to the plane of the slice on brain CT scan. It is also possible to assess the degree and the size of the depressed skull fracture. Optimally selected layer and the width of the "window" on the brain CT scan can determine the depth and direction of the wound channel, topographic correlation between the injuries of soft tissues of the head and depressed skull fractures. In the heads wound channel cavity by a targeted scan detect foreign bodies, bone fragments, air, blood and cerebrospinal fluid accumulation.

Brain CT scan in the axial plane is prescribed case of traumatic brain injury, hemorrhagic stroke, etc.
Brain CT scan in the axial plane is indicated case of traumatic brain injury, hemorrhagic stroke, etc.

Acute subdural hematomas on the brain CT scan are characterized by double-convex, less plano-convex area elevated plane, adjacent to the cranial vault. At the same time is probably the formation of atypical forms of epidural hematoma:

  • sickle-shaped
  • double lenticular
  • nonstandard
  • confluent widespread
  • multiple epidural hematomas adjacent to the vault of the skull on both sides

This kind of form from an epidural hematoma depends on the nature and location of bleeding, amount of extravasated blood. Epidural hematoma over a period of 2 weeks or epidural hematoma, containing incoagulated fresh blood, can have the same brain density. In these cases, the of the brain CT scan can be visualized displaced dura, sometimes only after intravenous injection of contrast agent. The thickness of the epidural hematoma depends on the elasticity of the dura mater, the configuration of the skull bones. This explains the fact that the frontal and occipital-parietal epidural hematoma have the greatest thickness.

Brain CT scan in the axial plane is indicated case of brain abscess, spontaneous cranial cerebrospinal fluid leak (CSF liquorrhea), etc.

Acute subdural hematomas on the brain CT scan are characterized by sickle area of homogeneous density increase. In most cases, subdural hematomas spread to the entire hemisphere of the brain. Often subdural hematomas may be bilateral, but also spread to the interhemispheric fissure and the cerebellum tentorium.

Depending on the severity of destructive changes, signs of cerebral edema, as well as the amount of extravasated blood, on the brain CT scan there are 4 type of injury lesions.

Diffuse axonal injury (DAI) is characterized by a general increase in brain volume due to swelling (congestion), or generalized edema, and often small focal hemorrhages in the corpus callosum, stem and periventricular structures.

Brain CT scan allows to judge the brain's response to traumatic brain injury (TBI) - the severity of edema, swelling, dislocation processes, size and degree of deformation subarachnoid cisterns, basal cisterns and ventricular system.

During the initial deformation suprasellar cistern (due to displacement hippocampal or direct gyri) in its cavity revealed additional protrusion. During a rough deformation of this cistern due to the caudal dislocation and / or displacement of one or both gyrus of the hippocampus in its cavity defined by only a few fragments cistern.

During expressed disturbances of intracranial volumetric ratios occurs dislocation hydrocephalus (due to cerebral aqueduct inflection, compression of the caudal 3 ventricle part and interventricular foramens).

The role of the brain computed tomography (CT) in the diagnosis of acute traumatic brain injury (TBI) is extremely high. It is significant during consequences of craniocerebral injury. Depending on the nature and severity of the changes in the brain tissue, intrathecal space and ventricular system, post-traumatic focal and diffuse changes detected by the brain CT scan, divided into 3 degrees: mild, moderate, severe.

Cerebral arachnoiditis on the brain CT scan appears uneven expansion of subarachnoid fissures, forming different shapes and sizes of cystic cavities or fuzzy rendering them in some areas. There the expansion, deformation or absence of the cranial base cisterns. In chiasmal areas can be detected isolated and communicating with the cerebrospinal fluid spaces cystic cavity.

Subarachnoid cysts are characterized by well-defined zone of low density. Most of these cysts are located in the lateral fissure, at least on the surface convexital hemispheres, and even in the posterior fossa. In most cases, the cysts have a funnel shape with a vertex facing towards the brain. Usually the cyst wall formed by atrophied cortex, white matter of the brain and its membranes.

Chronic subdural hematomas visible like a sickle modified density zones adjacent to the surface of the cerebral hemispheres. There are four types of hematomas: a reduced, equal with the brain, increased and mixed density. The length of the hematoma varies from 5 to 20 cm. Sometimes during the brain computed tomography (CT) visualized the outer and inner layer of the hematoma capsule.

Subdural hygroma appear as areas of low density, located convexital in the subdural space.

Post-traumatic hydrocephalus is characterized by a spherical expansion of the ventricular system, especially the anterior horns, often with periventricular edema. At the same time convexital subarachnoid spaces are not visualized.

Posttraumatic stroke is characterized by a zone of low density, which, in contrast to local edema, a more precise borders and formed into autonomous areas of vascularization pools of blood supply of compressed cerebral vessels.

During pneumocephalus on the brain CT scan revealed clearly defined areas of low density with a coefficient of absorption equal to air density.

Great importance of the brain CT scan has in the diagnosis of post-traumatic inflammatory complications. In the area of encephalitis revealed heterogeneous areas of lowering density of edematous brain tissue. Encephalitis area gradually increased, may appear more areas of reduced density in the center and the periphery.

Often the focus of encephalitis bordered strip of moderate increase density. At this stage it corresponds perifocal zone of hyperemia. Later, at her side during the formation of an abscess appeared contours of his membrane. At the beginning the shell slightly visible, it is not closed, visualized only by intravenous administration of contrast material. The most demonstrative abscess in the stage of formed capsule. During this period, it is characterized by a clearly defined area of increased density, which has an annular shape. The introduction of contrast agent uniformly enhancing the density of the capsule, contributes a good visualization of the abscess in the area surrounding edematous substances.

Subdural empyema characterized by sickle area of reduced density. With intravenous administration of contrast agent noted increased density the edge of squeezed hemisphere, which outer contour emphasizes the inner edge of the empyema. Sometimes pus can have the same density of the brain.

Epidural empyema lentiform characterized by thick-walled formation, which is usually visualized after intravenous contrast enhancement. Given the frequent combination of epidural and subdural accumulation of pus, it is believed that any inappropriate epidural empyema volume effect indicates a combination of epidural from subdural accumulation of pus.

Brain CT scan can produce a number of thin sections, perform a three-dimensional reconstruction of the study area of the brain, identify the vasculature (angiography) and even individual nerve trunks and blood vessels after injection of a special contrasting drug. This reconstruction provides invaluable assistance to the neurosurgeon in planning the operation and for subsequent post-operative monitoring.