Types of Detectable Abnormalities

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COMPUTED TOMOGRAPHY WITHOUT CONTRAST MEDIUM ENHANCEMENT

Computed tomography is an excellent study to evaluate for acute intracranial hemorrhage, particularly in the subarachnoid space ( Fig,.23:3 ). Acute hemorrhage is detected as high attenuation owing to clot retraction with separation of the high-density erythrocytes from lower density plasma. Unclotted active bleeding may be detected in epidural hematomas as a relative lucency.

Edema is identified as low attenuation owing to the presence of increased water content. Cytotoxic edema associated with infarct or diffuse anoxia is seen as low density in the gray matter, representing abnormal accumulation of intracellular water ( Fig 2..3..-.4 ). Vasogenic edema is the abnormal accumulation of extracellular water in the white matter seen as "fingers" or fronds" of low attenuation following white matter tracts.

CT is valuable in detecting mass effect including subfalcine,

Figure 23-3 Middle-aged woman presenting with what she reported as the worst headache of her life and third nerve palsy. Acute subarachnoid hemorrhage is present in left-sided basal cisterns secondary to rupture of a posterior communicating artery aneurysm. Central lucency represents the aneurysm.

Figure 23-4 Thirty-four-year-old woman status post cardiorespiratory arrest and resuscitation. CT shows severe diffuse cerebral (cytotoxic) edema with loss of white-gray matter differentiation and compression of ventricles and subarachnoid spaces secondary to anoxia.

descending transtentorial (uncal), ascending transtentorial, and transsphenoidal herniations (see Chapter.! ). Cerebellar tonsillar herniation can be missed secondary to beam hardening artifact at the level of the foramen magnum.

Evaluation of ventricular size can be done by CT. Enlargement of the temporal horns out of proportion to the lateral ventricular bodies is helpful in recognizing hydrocephalus, as is dilatation of the ventricles with relative effacement of sulci. CT can be used to follow ventricular size after shunting.

CT is very useful for detecting intracranial calcifications, such as those seen in congenital infections, vascular lesions, metabolic disease, and neurocutaneous disorders. The location and distribution of calcifications is key in differentiating these various causes. The identification of calcification in a neoplasm aids in differential diagnosis.

Although congenital brain malformations can be seen on CT, MRI is the modality of choice owing to direct multiplanar imaging with visualization of structures such as the corpus callosum and improved sensitivity for migrational abnormalities. CT is complementary in the evaluation of encephaloceles and meningomyeloceles in regard to identifying bone defects.

CT is preferred for evaluating acute fractures of the calvarium, skull base, and spine. Fractures are identified as lucencies without sclerotic margins. These features aid in differentiating fractures from vascular grooves or sutures. In the spine, CT can demonstrate bone impingement on the central canal. Epidural hematomas are occasionally but

Figure 23-5 Twenty-three-year-old man status post motor vehicle accident. Bone windows from a lumbar CT reveal a burst fracture of LB, Soft tissue window demonstrates an anterior epidural hematoma at the L1 level.

not consistently identified ( Fig, 23-5 ). The spinal cord is not directly visualized.

CT and MRI provide complementary information about neoplasms of the vertebrae, particularly in the case of hemangioma if the MR signal characteristics are atypical (.Fia 23-6 ).

Infection of the spine is better evaluated by MRI, although CT can provide information about bone destruction.

Many pathological processes of the extracranial head and neck can be identified by CT. High-resolution thin-section CT is the test of choice for evaluation of the middle ear

Figure 23-6 Fifty-five-year-old man with asymptomatic mass lesion identified on chest x-ray. CT myelogram reveals hyperdense spiculated lesion of the T7 vertebral body with soft tissue components in the paraspinal and epidural space consistent with hemangioma.

structures and paranasal sinuses owing to its exquisite bone detail highlighted by air. CT affords better evaluation of the paranasal sinuses, although the intracranial extent of neoplasm or infection is best evaluated by MR. CT and MR can both be used for the evaluation of the orbits, with the orbital fat providing excellent contrast for the optic nerve and extraocular muscles with either technique. CT provides information about calcification, which may be seen in tumors, drusen, and vascular lesions.

COMPUTED TOMOGRAPHY WITH CONTRAST MEDIUM ENHANCEMENT

In general, it is preferable to perform MRI than to perform CT with contrast medium enhancement. MRI is more sensitive to subtle changes in tissue signal and is superior to CT in initial as well as follow-up evaluation of enhancing lesions such as neoplasms, infection, and vascular lesions. MRI affords evaluation of masses in multiple planes and is especially useful in surgical or stereotactic irradiation planning. Meningeal disease is better evaluated by MR and can be missed on CT owing to volume averaging with the skull and also the normal bright enhancement of the meninges.

Contrast medium-enhanced CT is useful in patients who cannot undergo MRI and when MRI is not readily available. Contrast medium aids in the detection of intracranial lesions that may otherwise be poorly delineated or invisible on unenhanced CT. Patterns of enhancement can be helpful in differential diagnosis of intracranial masses.

CT is not as sensitive in the detection of white matter and neurodegenerative disorders as MRI and should not be used as the primary imaging modality. If MRI is contraindicated, CT may identify white matter lesions secondary to demyelination or dysmyelination, and contrast medium enhancement is occasionally useful in differential diagnosis.

Figure 23-7 One-year-old male with tuberous sclerosis. CT without contrast reveals periventricular calcifications, the largest at the left foramen of Monro.

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