Trauma

CT is the most important radiological examination technique in the emergency management of head injuries. The equipment is widely available and the examination is quick, has high sensitivity for fresh bleeding and can reveal oedema. It can also indicate the presence of increased intracranial pressure which manifests itself in the form of compressed cisterns, sulci and ventricles. CT is well suited to the diagnosis of certain fractures especially depressed fractures (Fig. 16) and fractures of the skull base. It is also useful in the examination of multiple injuries. Fig.17 shows in schematic form the commonest traumatic lesions. During examination of the skull and brain, an examination of the cervical spine can be performed and this is important in unconscious patients. Assessment of the brain stem and the posterior fossa is made difficult by beam-hardening artefacts from the dense surrounding bone. If clinically indicated, examination by CT can easily be repeated as required. CT has markedly improved the potential to treat and care for patients with head injury.

MRI is not nearly so important as CT in the emergency assessment of patients with head injury. This is because of certain diagnostic and practical problems. At MRI there are difficulties in detecting fresh bleeding

Figure 18. Linear fracture through squama ossis temporalis (arrows).

and a traumatic subarachnoid haemorrhage can go undetected. The acquisition time is longer than at CT and this leads to greater problems with movement artefacts. In addition, the MR scanner lends itself less well than its CT counterpart to the examination of head injury patients requiring life-support equipment. MRl has, however, certain advantages in the examination of patients in the subacute stage and at later follow-up because of its greater sensitivity in the detection of old bleeding, oedema and other fluid collections. MRI is better suited than CT, both in the acute and in later stages, in the important diagnosis of so called shearing injuries between the grey and white matter. These manifest themselves in the form of small focal changes between grey and white matter and deep axon injuries, both of which can go undetected at CT if they do not have a bleeding component.

To sum up, MRl has a limited role in the emergency examination of head injury patients and CT is the method of choice. In the later stages MRI is superior to CT in providing information of clinical value.

The importance of plain X-rays of the skull in the assessment of fractures has decreased markedly. In general, it can be said that the presence or otherwise of a fracture is less important than intracranial complications resulting from the trauma, in particular bleeding. In addition, with a suitable window setting at CT, precise assessment of both the vault and the base of the skull can be performed. Demonstration of depressed fractures and fractures of the skull base, as well as assessment of the presence of air in fractures involving the sinuses and mastoid air cell system is possible. However, an ordinary linear fracture (Fig. 18) is impossible to detect at CT if it runs more or less parallel to the plane of the slice. In this way certain parts of a comminuted fracture can be overlooked. Intracranial air and foreign bodies can be diagnosed both with plain x-ray

Figure 19. Multiple contusion haematomas with surrounding oedema. Compression and displacement of the lateral ventricles.

of the skull and CT, but CT is more sensitive especially if small amounts of air are involved or if the foreign body is of low density. Plain radiographs of the skull can indirectly demonstrate haemorrhage by showing displacement of a calcified pineal body though the examination can be negative if the bleeding is bilateral or situated basally or in the posterior fossa. This is also true when determining the mid-line with ultrasound (echo- encephalography). Unwanted artefacts can make the examination with ultrasound even more difficult. It should be made clear that both plain X-ray of the skull and echo encephalography can be negative in the presence of serious intracranial complications.

Contusion injuries

Focal traumatic intracerebral lesions are made up of contusion with oedema, with or without a bleeding component, or of a pure haematoma. The injuries are often multiple (Fig. 19) and the sites of predilection are the anterior parts of the frontal and temporal lobes. The oedema is sometimes diffuse. Traumatic haematomas can generally be distinguished from spontaneous haematomas by the fact that they are usually more irregular in outline and in addition that they involve the cortex. They are seldom localized to the basal ganglia and are often multiple. At CT, oedema displays low attenuation and at MRI with T1-weighted images

Figure 20. Large acute left-sided subdural haematoma with a small portion along the falx (arrow). Left lateral ventricle severely compressed and displaced beneath the falx.

low signal and with T2-weighted images high signal. Fresh bleeding displays high attenuation at CT, up to between 50-80 HU, and this decreases post-haemorrhage by 2-3 HU per day. At MRI fresh bleeding is, as has previously been mentioned, difficult to detect. With T2-weighted images, contusion bleeding is demonstrated as an area of low signal within the high signal area of oedema. When the bleeding reaches the subacute stage with formation of methaemoglobin the lesion can, however, be demonstrated well at MRI because of its high signal intensity.

A haematoma can occur some days after trauma and explain sudden worsening of the clinical picture. This is an indication for a repeat CT examamination. Similarly a delayed intracerebral haematoma can arise after surgery for an extracerebral haematoma which, through compression of the hemisphere, prevented development of an intracerebral haematoma.

Shearing injuries resulting in multiple small haematomas or areas of oedema but must be regarded as potentially serious. In the demonstration of traumatic subarachnoid haemorrhage the same rules apply as for a similar spontaneous haemorrhage. Intraventricular bleeding can also occur at trauma.

Figure 21. Right-sided isodense subdural haematoma. The surface of the brain can just be seen (arrows).

Subdural haematoma

Acute subdural haematomas arise through venous bleeding and there is often a co-existing contusion injury. The haematoma is seen between the skull and the surface of the brain and does not deform the latter (Fig. 20). As usual, the collection of blood is hyperdense at CT and the attenuation of the lesion decreases with time. In the subacute stage (after 1-3 weeks) the haematoma is more or less isodense and can therefore give rise to diagnostic difficulties. Visualization of the lesion can be facilitated by injection of contrast medium because a thin membrane takes up contrast to a greater or lesser degree which forms between the haematoma and the surface of the brain. In addition, compression and displacement of the ipsilateral lateral ventricle should lead to the suspicion that the cause is an isodense subdural haematoma (Fig. 21). Assessment of the configuration of the ventricular system is especially important in bilateral isodense subdural haematomas because displacement of the mid-line structures may not occur.

After approximately 3 weeks the haematoma becomes hypodense and the condition is then called chronic subdural haematoma. The haematoma at this stage has varying density because of re-bleeding on one or several occasions, but the hypodensity always dominates (Fig. 22). The shape can become biconvex with formation of multiple membranes.

Figure 22. Right-sided chronic subdural haematoma with formation of membranes which show up clearly after injection of contrast medium. Re-bleeding visible within the haematoma.

Figure 23. Frontal extradural haematoma on the left side.

MRI has higher sensitivity for small subdudral haematomas than does CT once the acute stage has passed.

Extradural haematoma

Extradural haematomas usually arise through rupture of meningeal arteries, usually the middle meningeal artery, but can arise from venous bleeding. Usually a co-existing fracture of the skull is present. The haematoma has a biconvex shape (Fig. 23) and usually respects the sutures because the dura in these areas is especially well attached to the skull. However, the haematoma can detach venous sinuses. Shunting can occur from the haematoma to diploic veins and this can explain the varying pathophysiology of epidural haematomas. The haematoma's density at CT and signal intensity at MRI are similar to those seen with acute subdural haematoma.

Sequelae following skull trauma

Sequelae following skull trauma include CSF fistula with rhinorrhoea caused by a fracture in combination with laceration of the dura and the arachnoid. Another complication is a fistula between the carotid siphon and the cavernous sinus. Hydrocephalus is another posttraumatic complication.

Kjell Bergström and Giuseppe Scotti