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Neuroradiology

Brain injury, traumatic

the major cause of death and permanent disability in the first few decades of life. Yearly, head injury mortality rates are estimated as being 25 per 100,000 population. As many as 5-10% of those who survive the initial trauma will experience some degree of residual neurological deficit. CT and MR tomographic imaging spearheaded significant advances in the management of head-injured patients, the former providing a prompt and accessible diagnosis of emergency surgical lesions, the latter diagnostic refinement of particular lesions requiring specific treatment. Traumatic brain injuries are classified as being primary or secondary. Primary lesions are those that arise as a direct result of the initial traumatic force. Virtually, all primary traumatic intra-axial lesions arise as a result of shear-strain deformation of either neurones or blood vessels produced by rotational acceleration of the head. Secondary lesions either arise as sequelae of primary lesions or from the neurological effects of systemic injuries. Primary lesions are classified as primary neuronal injuries, primary haemorrhages (intracerebral, epi- and subdural, subarachnoid and intraventricular) and primary vascular injuries (such as arterial dissection (see dissection arterial) and carotid cavernous fistula) ( Fig.1). Secondary lesions include territorial arterial infarction, diffuse brain swelling and oedema, pressure necrosis due to brain displacement and herniation. Petechial haemorrhages may occur in the tegmentum of the rostral pons and midbrain as a result of stretching or tearing of the penetrating arteries as the upper brain stem is caudally displaced following a transtentorial herniation. Wallerian degeneration may ensue following diffuse brain injury. Primary neuronal injuries of the brain fall into the following categories:

  • diffuse axonal injury (DAI)
  • cortical contusion
  • subcortical grey matter injury

    In all these categories of lesions MRI is superior to CT in terms of sensitivity. This is particularly true for DAI and brain stem lesions. DAI is suspected when severe loss of consciousness starting at the moment of impact is not explained by significant mass lesions. Multiple, small, focal traumatic lesions are scattered throughout the white matter, most lesions sparing the overlying cortex, frequently being located at the grey-white matter interface. Most DAI lesions are nonhaemorrhagic in nature. DAI tends to occur in three fundamental anatomical areas:

  • the lobar white matter;
  • the corpus callosum; and
  • the dorsolateral aspect of the upper brain stem

    DAI may be overlooked on CT examination and it is also likely to be underestimated also on MR where it appears as areas of T2 hyper signal probably representing regions where axonal disruption is confluent enough to allow visualization. MRI evidence of DAI of the corpus callosum is significantly associated with a poor prognosis, callosal lesions representing a marker of DAI in more critical areas of the brain. Cortical contusions, by definition, must primarily involve the superficial grey matter of the brain and due to greater vascularity of grey as compared to white matter, are much more likely to be haemorrhagic than are DAI lesions. Particularly affected are the inferior, anterior and lateral aspects of the temporal and frontal lobes. Due to the haemorrhagic nature and to a frequently larger size cortical contusions are usually detected as haemorrhagic hyperdensities on hypodense areas. Subcortical grey matter injury represents an uncommon diffuse type of injury characterized by multiple petechial haemorrhages primarily localized to the upper brain stem, basal ganglia, thalamus and regions around the third ventricle

    A much higher percentage of these lesions are haemorrhagic than for any other type of primary intra-axial lesion. This is most likely due to the high vascularity of the basal ganglia and thalamus that are supplied by a rich network of perforating vessels. A superficial contusion or laceration in the dorsolateral aspect of the upper brain stem may occur as a result of direct contact with the free edge of the tentorium. Intracerebral haematomas may occur as a consequence of trauma and along with the epi- and subdural haematomas are discussed in the specific sections. See haematoma, subdural, haematoma, epidural and haematoma.

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    Fig.1

    Axial CT images following severe head trauma. Multiple, bilateral haemorrhagic foci of contusion and brain laceration, as well as subarachnoid and interhemispheric subdural blood is seen. Right frontal extracerebral haematoma at the side of the impact, with fracture of the frontal bone.
    Brain injury, traumatic, Fig.1