FOCAL CENTRAL NERVOUS SYSTEM LESIONS
Magnetic resonance imaging (MRI) usually confirms the location of the lesion (anatomical diagnosis) and often substantiates the tentative pathological diagnosis. The most frequent focal pathological processes in the central nervous system are vascular accidents, tumors, malformations, and infections. Vascular accidents include infarcts and hemorrhages, porencephalic cysts, and schizoencephaly. Tumors include neoplasms and aneurysms. Malformations include encephalocele and myelomeningocele. Infections include congenital or acquired, viral or bacterial processes.
CENTRAL NERVOUS SYSTEM INFARCTS
nervous system infarcts may occur in the brain, brainstem, cerebellum,
or spinal cord. They may involve the arterial or the venous systems.
Infarcts may be ischemic or hemorrhagic. Arterial infarcts are produced
by hypoperfusion, embolic or thrombotic phenomena, or by vasospasm.
Venous infarcts are usually thrombotic in nature.
Figure 244.1.— [A] MRI of the brain demonstrating a large infarct in the distribution of the left middle cerebral artery; [B] B-mode ultrasonography demonstrating a thrombus at the origin of the internal carotid artery; [C] MRA of the brain demonstrating a narrow internal carotid artery and absence of the middle cerebral artery.
a normal MRI or CT within the first 24 hours after the onset of clinical
manifestations does not eliminate the possibility of an ischemic infarct
because ischemic central nervous system parenchymal changes may not
be detected by these studies during this period. Diffusion-weighted
imaging may reveal ischemic CNS parenchymal changes earlier than other
infarcts are usually detected within the first 24 hours after the onset
of symptoms by any of the above mentioned modalities. Computed tomography
shows blood better than MRI in the first 24 hours after an event. Magnetic
resonance arteriogram may demonstrate the flow abnormality earlier than
MRI or CT in ischemic infarcts. B-mode
ultrasonography should be used to diagnose arterial infarcts if transportation
to the MRI area is not possible. B-mode ultrasonography is very useful
in premature neonates with periventricular leukomalacia. The studies
of choice to diagnose arterial infarcts are MRI and MRA of the appropriate