头颅常见病变影像学表现
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Intracranial haemorrhage - General principles
Intracranial haemorrhage is a bleed inside the skull. It can be split into intra-axial or extra-axial.
- Intra-axial, bleed within the brain itself (intraparenchymal, intraventricular); or
- Extra-axial, bleed between the brain and the skull (extradural, subdural, subarachnoid)
As opposed to extracranial haemorrhage (outside the skull), i.e. soft tissue bruising/haematoma.
A lucid interval refers to an initial decrease in conscious level (GCS) due to the initial concussion from a head injury, which then improves for a period of time (usually a few hours) before deteriorating again, this time due to a gradually enlarging haematoma causing compression on brain structures. Intracranial haemorrhages can exert mass effect on the brain. Signs include midline shift, effaced (slit-like) ventricles, and herniation of brain structures.
Acute bleeds generally require urgent discussion with neurosurgery.
Appearance on CT:
- Acute (hours to days) - New blood is White
- Subacute (days to weeks) - Grey
- Chronic (weeks to months) - Old blood is Dark
- Acute on Chronic - Layering effect
Epidural haemorrhage
Epidural (also known as extradural) haemorrhage is usually due to head trauma, particularly to the region of the pterion, which tears the middle meningeal artery. This is therefore an arterial bleed.
It lies between the skull and the dura mater.
Appearance: Lens-shaped (lenticular). (as the dura is usually tightly adherent to the skull)
Subdural haemorrhage
Subdural haemorrhage is more common in the elderly and alcoholics, who have cerebral atrophy. There is more strain on the bridging veins, which tear easily with minor trauma, and cause a venous bleed.
It lies between the dura mater and the arachnoid mater.
Appearance: Crescent-shaped.
Subarachnoid haemorrhage
Can occur after trauma, or in individuals with arteriovenous malformations (AVMs), or berry aneurysms (particularly of the Circle of Willis). Patients describe a sudden onset of extremely severe pain (“thunderclap headache”). Blood in the subarachnoid space can cause meningeal irritation and result in symptoms similar to meningitis.
Appearance: High density blood in the sulci, basal cisterns and fissures:
- suprasellar cistern (large pentagon/star shape),
- quadrigeminal cistern (smaller W/smile shape below suprasellar)
Blood may also extend into the ventricles.
Intraparenchymal haemorrhage
Intraparenchymal haemorrhages occur within the substance of the brain itself. Common causes are hypertension, trauma and haemorrhagic stroke.
Appearance: Area of high density within the substance of the brain.
Intraventricular haemorrhage
Intraventricular haemorrhage can be either primary, or secondary to extension from subarachnoid or intracerebral haemorrhage.
Appearance: Small amount of bright blood in the dependent part of the lateral ventricles. There is usually a CSF-blood level, as the denser blood sinks to the bottom.
Ischaemic Strokes / Infarcts
Symptoms of ischaemic strokes vary depending on the blood vessel (and thus area of brain) affected. The Oxford Stroke Classification defines 4 types:
Total Anterior Circulation Stroke (TACS) - Anterior / Middle Cerebral Artery - all 3 of: higher dysfunction, hemiparesis, hemianopia
Partial Anterior Circulation Stroke (PACS) - Anterior / Middle Cerebral Artery - any 2 of: higher dysfunction, hemiparesis, hemianopia
Posterior Circulation Stroke (POCS) - Posterior Cerebral Artery - any 1 of: cerebellar symptoms, loss of consciousness, hemianopia
Lacunar Stroke (LACS) - Small Vessel Disease - any 1 of: hemiparesis, hemiparaesthesia
Appearance:
CT - Low density area, that takes a few hours to develop. The 'stroke window’ helps to see subtle difference in density. Other signs of acute infarct: Loss of differentiation of grey/white matter; Sulcal effacement (oedema); Bright MCA sign. Old infarcts have a lower density than acute infarcts.
MRI - DWI is the best MRI sequence to detect stroke. On this sequence, infarct is a bright area, which can develop in just a few minutes.
Brain Tumours
Primary brain tumours are extremely rare. The brain is a relatively rare site for metastases compared to the liver or the lungs, but they are far more common than primary brain tumours. Cancers that most commonly metastasise to the brain include melanoma, lung, breast, renal and colon cancers. They can be asymptomatic, discovered incidentally; or they may cause symptoms due to their mass effect, such as cranial nerve palsy, seizures, or various other neurological signs.
Appearance:
The appearances on CT and MRI vary but they are usually heterogenous (not a uniform texture) solid lesions. Sometimes the lesions themselves aren’t seen but the secondary vasogenic oedema or other sign of mass effect such as midline shift is seen. Tumours generally enhance with IV contrast as they are vascular. On MRI diffusion weighted imaging, they are bright due to high cellularity.
The main differential for this appearance is an infective lesion such as a brain abscess.
Small Vessel Disease
As we age, the small arterioles in the brain become more affected by arteriosclerosis. These small arterioles supply the subcortical, periventricular, and lacunar areas. These areas thus become ischaemic and low density on CT. This can lead to vascular dementia.
Appearance: Generalised low attenuation of ischaemic white matter. Frequently associated with age-related general cerebral atrophy, signs of which are enlarged ventricles and widened sulci.
Alzheimer's Disease
Alzheimer's disease is the commonest type of dementia, causing memory impairment, loss of language skills and disorientation. This is due to the accumulation of amyloid plaques and neurofibrillary tangles. There is disproportionate atrophy of the brain, typically involving the hippocampus and the temporo-parietal cortex.
Appearance: Disproportionate hippocampal atrophy on CT or MRI.
Venous Sinus Thrombosis
Venous sinus thrombosis is very important but can be easily missed. It usually presents with headaches and neurological signs. Most occur in patients with prothrombotic risk factors, particularly young women. It can progress to a cerebral infarct and secondary haemorrhage.
Appearance: CT venogram - A filling defect within the contrast-filled venous sinuses ('empty delta sign’). A high index of suspicion for this pathology is needed, as it is difficult to see on plain CT without contrast.
Cerebral Aneurysm
It is important to identify and treat these as a subarachnoid haemorrhage and cerebral haematoma from a ruptured aneurysm can cause sudden death. There are most commonly found involving the vessels of the Circle of Willis.
Appearance: Localised dilatation along one of the cerebral vessels.
Possible associated findings: Aneurysm clips (metallic artefact), Old bleeds, Burr holes (defect in skull)
MRA showing an aneurysm at the bifurcation of the right middle cerebral artery.
Skull Fractures
Types of skull fracture: Linear (uncomplicated), Depressed (pushed in), Diastatic (suture widening), Basilar (base of skull)
Basal skull fractures must not be missed in head trauma. Signs are bruising around the eyes (raccoon eyes) and over the mastoid process (Battle's sign), as well as CSF (i.e. clear fluid) leakage from the nose (rhinorrhoea) and ears (otorrhoea).
Appearance: It is easiest to see fractures on the bone window setting. Skull fractures may be particularly hard to spot if undisplaced, and may be mistaken for a suture, so it is important to 'follow' the line to see if it runs in a suture line. Facial fractures may bleed into the maxillary sinus, causing opacification or a fluid level.
Cervical Spine Fractures
A cervical spine X-ray is often performed to clear the cervical spine before removal of head blocks. The junction of C7 and T1 must be seen. If inadequate, cervical spine CT will be required. If a head CT is being done, the C-spine should be scanned at the same time (instead of doing X-rays).
Appearance: Cervical spine fractures are important to stabilise as they may cause paralysis. The classical 'hangman’s fracture’ is a fracture of C2 that involves both pedicles.
Multiple Sclerosis
This is a chronic demyelinating disease of the central nervous system (brain and spinal cord) which usually affects young females. There is no definitive diagnostic test for MS. The diagnosis may be made when two distinct neurological episodes with no other cause occur. MRI and CSF analysis are the most useful tests.
Appearance: Bright plaques on MRI (T2 or FLAIR sequences). FLAIR is a modified T2 where fluid (CSF) signal is suppressed, so bright plaques are more obvious next to the ventricles (which are now dark). Classically periventricular with a finger-like appearance in sagittal section. Plaques can also be found in the cerebellum, brainstem, and spinal cord. If IV contrast is used and the plaques enhance, this indicates current disease activity.
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来源：神经病学医学网