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Recognizing Some Common Causes of Intracranial Pathology


Advances in neuroimaging have had a remarkable impact on the diagnosis and treatment of neurologic diseases ranging from earlier detection and treatment of stroke to a more timely diagnosis of dementia, from the rapid detection and treatment of cerebral aneurysms to the ability to diagnose multiple sclerosis after a single attack.

Case Quiz 26 Question

This is an image from an unenhanced head CT on a 68-year-old male who fell and struck his head earlier in the same day and has become progressively disoriented. Pertinent clinical history is the current use of anticoagulants for heart disease. What is the diagnosis? See the answer at the end of this chapter.

  • Both CT and MRI are utilized for studying the brain and spinal cord, but MRI is the study of first choice in most clinical scenarios ( Table 26.1 ). Conventional radiography has no significant role in imaging intracranial abnormalities.

    TABLE 26.1
    Imaging Studies of the Brain for Selected Abnormalities
    Abnormality Study of First Choice Other Studies
    Acute stroke Diffusion-weighted imaging (see Table 26.8 ) for acute or small strokes, if available Noncontrast CT can differentiate hemorrhagic from ischemic infarct
    Headache, acute and severe Noncontrast CT to detect subarachnoid hemorrhage MR angiography (MRA) or CT angiography (CTA) if subarachnoid hemorrhage is found in order to detect aneurysm
    Headaches, chronic (with new features) MRI, without and with contrast, or MRI without contrast CT, without and with contrast
    Seizures MRI, without and with contrast. Add thin section images through hippocampi in patients with history of childhood seizures CT, without and with contrast, can be substituted if MRI not available
    Blood Noncontrast CT Ultrasound for infants
    Head trauma Nonenhanced CT is readily available and the study of first choice in head trauma MRI is better at detecting diffuse axonal injury but requires more time and is not always available
    Extracranial carotid disease Doppler ultrasonography MRA excellent study; CTA best for preoperative stenosis evaluation
    Hydrocephalus MRI as initial study CT for follow-up
    Vertigo and dizziness Contrast-enhanced MRI MRA if needed and/or thin section images through internal auditory canals as needed
    Masses MRI, without and with contrast Contrast-enhanced CT if MRI not available
    Change in mental status MRI, without or with contrast CT without contrast

Normal Anatomy ( Fig. 26.1 )

  • We will look at the normal anatomy of the brain using CT.

    Fig. 26.1, Normal Unenhanced CT Scans of the Head.

  • In the posterior fossa, the fourth ventricle appears as an inverted U-shaped structure. Like all cerebrospinal fluid-containing structures on CT, it normally appears black. Posterior to the fourth ventricle are the cerebellar hemispheres, anteriorly lie the pons and medulla oblongata. The tentorium cerebelli separates the infratentorial components of the posterior fossa (cerebellum and fourth ventricle) from the supratentorial compartment.

  • The interpeduncular cistern lies in the midbrain and separates the paired cerebral peduncles (which emerge from the superior surface of the pons). The suprasellar cistern is anterior to the interpeduncular cistern and usually has a five- or six-point star-like appearance.

  • The Sylvian fissures are bilaterally symmetric and contain cerebrospinal fluid (CSF). They separate the temporal from the frontal and parietal lobes.

  • The lentiform nucleus is composed of the putamen (laterally) and globus pallidus (medially). The third ventricle is slit-like and midline. At the posterior aspect of the third ventricle is the pineal gland. Farther posterior is the quadrigeminal plate cistern.

  • The corpus callosum connects the right and left cerebral hemispheres and forms the roof of the lateral ventricle. The anterior end is called the genu and the posterior end is called the splenium.

  • The basal ganglia are represented by the subthalamic nucleus and the substantia nigra, globus pallidus, putamen, and caudate nucleus. The putamen and caudate nucleus are called the striatum.

  • The frontal (also known as anterior ) horns of the lateral ventricles hug the head of the caudate nucleus. The two frontal horns are separated by the midline septum pellucidum. The temporal horns, which are normally very small, are more inferior and contained in the t emporal lobes. The posterior horns ( occipital horns ) of the lateral ventricle lie in the occipital lobes. The most superior portion of the ventricular system are the bodies of the lateral ventricles.

  • The falx cerebri lies in the interhemispheric fissure , which separates the two cerebral hemispheres , and is frequently calcified in adults.

  • The surface or cortex of the brain is composed of gray matter convolutions made up of sulci (grooves) and gyri (elevations). The medullary white matter lies below the cortex.

Important Points

  • On an unenhanced CT scan of the brain, anything that appears white will generally either be bone (calcium) density or blood, in the absence of a metallic foreign body ( Table 26.2 ).

    TABLE 26.2
    CT Densities
    Hypodense (Dark) Isodense Hyperdense (Bright)
    Fat (not usually present in the head) Normal brain Metal (e.g., aneurysm clips or bullets)
    Air (e.g., sinuses) Some forms of protein (e.g., subacute subdural hematomas) Iodine (after contrast administration)
    Water (e.g., CSF) Calcium
    Chronic subdural hematomas/hygromas Hemorrhage (high protein)

  • Physiologic calcifications that may be seen on CT of the brain:

  • Normal structures that can enhance after administration of iodinated intravenous contrast :

    • Venous sinuses

    • Choroid plexus

    • Pituitary gland and stalk

  • Metallic densities in the head can cause artifacts on CT scans. Dental fillings, aneurysm clips, and bullets can all cause streak artifacts .

Mri and the Brain

  • In general, MRI is the study of choice for detecting and staging intracranial and spinal cord abnormalities. It is usually more sensitive than CT because of its superior contrast and soft-tissue resolution. It is, however, less sensitive to CT in detecting calcification in lesions or in evaluating cortical bone , which appear as signal voids with MR. It may be contraindicated in some patients with pacemakers.

  • MRI is more difficult to interpret in part because the same structure or abnormality may appear differently on the same study, depending on the pulse sequence, the scan parameters, and the fact that MRI is more variable in its depiction of differences that occur over the time course of some abnormalities (e.g., hemorrhage) than is CT.

  • Initial evaluation of an MRI of the brain might start with a T1-weighted sagittal sequence of the brain. On this sequence, the brain looks more like the anatomic specimens or diagrams that you are more accustomed to seeing ( Fig. 26.3 ). Many structures in the brain are paired , so remember to compare one side with the other on axial scans of the brain ( Fig. 26.4 ).

    Fig. 26.3, Normal Midline MRI.

    Fig. 26.4, Normal MRI of the Brain, T1 and T2.

  • Table 26.3 summarizes the signal characteristics of various tissues seen on MRI.

    TABLE 26.3
    Signal Characteristics of Various Tissues Seen on T1-Weighted and T2-Weighted MRI Scans
    Bright on T1 Dark on T1 Bright on T2 Dark on T2
    Fat Calcification Water (edema, CSF) Fat
    Gadolinium Air Calcification
    High protein Chronic hemorrhage Hyperacute hemorrhage Air
    Subacute hemorrhage Acute hemorrhage is isointense to hypointense on T1 Late subacute hemorrhage Early subacute hemorrhage
    Melanin Water (edema, CSF) Chronic hemorrhage
    Acute hemorrhage
    High protein

Head Trauma

  • Traumatic brain injuries extract a huge cost to the patient and society, not only as a result of the acute injury but for the long-term disability they produce. In the United States, motor vehicle accidents account for nearly half of traumatic brain injuries.

  • Unenhanced CT is the study of choice in acute head trauma. The primary goal in obtaining the scan is to determine whether there is a life-threatening, but treatable, lesion. One of the principle advances in the advent of CT scanning of the head was its ability to detect surgically amenable lesions in a timely fashion.

  • Initial CT evaluation of the brain in the emergency setting focuses on whether there is (1) mass effect and (2) blood.

    • To determine whether there is mass effect, look for a displacement or compression of key structures from their normal positions by analyzing the location and appearance of the ventricles , basal cisterns, and the sulci.

    • Blood will usually be hyperattenuating (bright) and might collect in the basal cisterns, Sylvian and interhemispheric fissures, ventricles, subdural or epidural spaces, or in the brain parenchyma (intracerebral).

Skull Fractures

  • Skull fractures are usually produced by direct impact to the skull, and they most often occur at the point of impact. They are important primarily because their presence implies a force substantial enough to cause intracranial injury.

  • In order to visualize skull fractures, you must view the CT scan using the bone window settings that optimize visualization of the osseous structures ( Fig. 26.5 ).

    Fig. 26.5, Epidural Hematoma, Brain and Bone Windows.

  • Skull fractures can be described as linear, depressed, or basilar.

Linear Skull Fractures

  • Linear skull fractures are the most common, and their primary importance lies in the intracranial abnormalities that may have occurred at the time of the fracture, such as an epidural hematoma. Fractures of the cranial vault most likely occur in the temporal and parietal bones (see Fig. 26.5B ).

Depressed Skull Fractures

  • Depressed skull fractures are more likely to be associated with underlying brain injury. They result from a high-energy blow to a small area of the skull (e.g., from a hammer), most often in the frontoparietal region, and are usually comminuted. They may require surgical elevation of the depressed fragment when the fragment lies deeper than the inner table adjacent to the fracture ( Fig. 26.6A ).

    Fig. 26.6, Skull Fractures.

Basilar Skull Fractures

  • Basilar fractures are the most serious and consist of a linear fracture at the base of the skull. They can be associated with tears in the dura mater with subsequent CSF leak, which can lead to CSF rhinorrhea and otorrhea. They can be suspected if there is air seen in the brain (traumatic pneumocephalus), fluid in the mastoid air cells, or an air-fluid level in the sphenoid sinus ( Fig. 26.6B ).

Facial Fractures

  • CT is the imaging study of choice for evaluating facial fractures. Multislice scanners allow for reconstruction of the images in the sagittal and coronal planes so the patient does not have to be repositioned in the scanner.

  • The most common orbital fracture is the blow-out fracture, which is produced by a direct impact on the orbit (e.g., a ball strikes the eye). The impact causes a sudden increase in intraorbital pressure leading to a fracture of the inferior orbital floor (into the maxillary sinus) or the medial wall of the orbit (into the ethmoid sinus). Sometimes the inferior rectus muscle can be trapped in the fracture, leading to restriction of upward gaze and diplopia as presenting symptoms.

  • Recognizing a blow-out fracture of the orbit ( Fig. 26.7A ).

    • Orbital emphysema. Air in the orbit from communication with one of the adjacent, air-containing sinuses, either the ethmoid or maxillary sinus.

    • Fracture through either the medial wall or floor of the orbit.

    • Entrapment of fat and/or extraocular muscle that projects downward as a soft-tissue mass into the top of the maxillary sinus.

    • Fluid (blood) in the maxillary sinus.

    Fig. 26.7, Facial Bone Fractures.

  • A tripod fracture, usually a result of blunt force to the cheek, is another relatively common facial fracture. This fracture involves separation of the zygoma from the remainder of the facial bones through the combination of separation of the frontozygomatic suture , fracture of the floor of the orbit, and fracture of the lateral wall of the ipsilateral maxillary sinus ( Fig. 26.7B ).

Intracranial Hemorrhage

  • Skull fractures may be accompanied by intracranial hemorrhage and/or diffuse axonal injury.

  • There are four types of intracranial hemorrhages that can be associated with head trauma ( Fig. 26.8 ):

    • Epidural hematoma

    • Subdural hematoma

    • Intracerebral hemorrhage

    • Subarachnoid hemorrhage (discussed with aneurysms)

    Fig. 26.8, Hemorrhages and Herniations.

Epidural Hematoma (Extradural Hematoma)

  • Epidural hematomas represent hemorrhage into the potential space between the dura mater and the inner table of the skull ( Table 26.4 ).

    TABLE 26.4
    The Meninges
    Layer Comments
    Dura mater Composed of two layers, an outer periosteal layer that cannot be separated from the skull and an inner meningeal layer; the inner meningeal layer enfolds to form the tentorium and falx.
    Arachnoid The avascular middle layer is separated from the dura by a potential space known as the subdural space.
    Pia mater Closely applied to the brain and spinal cord, the pia mater carries blood vessels that supply both; separating the arachnoid from the pia is the subarachnoid space; together the pia and arachnoid are called the leptomeninges.

  • Most cases are due to injury to the middle meningeal artery or vein from blunt head trauma, typically from a motor vehicle accident.

  • Almost all epidural hematomas (95%) have an associated skull fracture, frequently in the temporal bone. Epidural hematomas may also be caused by disruption of the dural venous sinuses adjacent to a skull fracture.

  • Recognizing an epidural hematoma:

    • They appear as a hyperintense, extraaxial, biconvex, lens-shaped density most often found in the temporoparietal region of the brain ( Fig. 26.9 ).

      Fig. 26.9, Epidural Hematoma.

    • Because the dura is normally fused to the calvarium at the margins of the sutures, it is impossible for an epidural hematoma to cross suture lines (subdural hematomas can cross sutures).

      • Epidural hematomas can cross the tentorium, but subdural hematomas do not.

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