Neuroimaging Indications Tutorial Notes

Neuroimaging Indications  The below table shows pathologies and their recommended imaging technique Pathology Trauma Vascular: stroke, subarachnoid haemorrhage, arterio-venous malformation Tumour Inflammation/infection, demyelination, abscess, meningitis Congenital and metabolic   Imaging Technique CT CT, CT angiography, MRI, angiography CT, MRI CT, MRI MRI Acute stroke cannot be seen on CT scans until 24 hours later They can be seen on MRI immediately Trauma  May be a result of primary or secondary lesions  Primary lesions include: o Skull fracture or scalp haematoma o Extracerebral haemorrhage o Intra-axial lesions e.g. diffuse axonal injury, contusion, parenchymal haemorrhage  Secondary lesions include: o Herniations o Traumatic ischaemia/infarction o Diffuse cerebral oedema o Hypoxic injury  Below is a CT scan of an extradural haematoma       The blood is located outside both the brain and dura This is a medical emergency It is due to bleeding of the middle meningeal artery – blood vessel supplying the dura Injury/fracture to the temporal region (pterion) can injure the artery and cause bleeding CT windowing means you can use the same image to view both the bleed and the fracture – different densities just need to be set Below is a CT scan of a subdural haematoma        The bleed next to the brain and within the dura The potential space between the dura and arachnoid mater has become real in the scenario of pathology Subdural haemorrhage is a venous haemorrhage due to rupture of bridging veins between the pia and the dura Usually a disease of older people May be a result of anticoagulation therapy or minor injury – presentation is much slower than extradural haematoma Early in the course of bleeding, blood appears white on CT As the blood becomes older and is metabolised, it becomes less dense and becomes darker – it may then appear like CSF  The below image shows subarachnoid haemorrhage    It is common in the setting of trauma Bleeding is in the subarachnoid space – blood can travel into the sulci The most common cause is ruptured cerebral aneurysm  The below image shows trauma resulting in contusion of the brain itself   The area of contusion is typically the site of physical trauma e.g. frontal lobe The below image shows herniation of the brain towards the falx       The pressure from the extradural haematoma has pushed the brain towards the midline The falx can be seen in this scan It is a very tough dural structure that can damage the delicate brain tissue A majority of permanent brain injury is a result of herniation As well as herniating towards the midline, the brain may also herniate downwards through the foramen magnum This compresses the spinal cord and may cause autonomic and spinal difficulties Vascular  Stroke: acute vascular episode leading to neurological deficit  80% occur due to infarction from the thrombotic or embolic event  Cardiac embolisms are common causes (embolism has already bypassed the lungs)  The left image on the below CT scan shows a patient presenting with acute stroke (< 24 hours)       Although pathology is present, it is not evident on the above scan CT is usually performed as intracerebral haemorrhage is a differential diagnosis of stroke symptoms Intracerebral haemorrhage is an emergency that requires surgical intervention The right image was taken 24 hours later The oedema of the right hemisphere is now visible The below image is a few days later   The patient had developed significant swelling of the brain The bone had been removed to alleviate some of the pressure and prevent the brain from swelling inward and herniating across the midline  The below images show a comparison of a stroke patient between CT and MRI    MRI shows many more details than CT MRI shows stroke as soon as 15 minutes following the event MR angiography can be used to view blood vessels of the brain Angiography  The below image shows CT angiography of blood vessels of the brain      There is a visible occlusion in the middle cerebral artery that would lead to stroke and infarction of the area of the brain supplied by that vessel Angiography is performed using a digital x-ray machine The x-ray takes an image of the skull as contrast is being injected into the blood vessels The image of the skull is subtracted from the blood vessels, which can then be viewed clearly Angiography can be completed with CT and MRI as well Vascular  Intracranial haemorrhagic strokes make up ~15% of cases  The below image shows an acute haemorrhagic stroke on CT    Unlike embolic strokes, haemorrhagic strokes may be viewed on CT Surgical interventions can evacuate the haemorrhage and correct the symptoms Non-traumatic subarachnoid haemrrohage accounts for the remaining 5% of stroke presentations  The below image shows subarachnoid haemorrhage    There is blood instead of CSF present in the subarachnoid space In the absence of trauma, subarachnoid haemorrhage is usually due to cerebral aneurysm The below image shows another CT scan of subarachnoid haemorrhage  While the blood appears to be within the brain, it is in fact in between the temporal lobe and insula Blood has pooled in the fissure between these two spaces  The below image shows a CT scan of an arterio-venous malformation (AVM)   This is a much less common problem that stroke There is a developmental abnormality in the vessels of the brain – this can be viewed with contrast on CT This malformed vessels can haemorrhage into the brain Glue can be injected into an AVM to stop it from bleeding    Carotid Ischaemia  May be a result of embolic disease originating in the internal carotid artery  Presents as a stroke or TIA  Is imaged with duplex ultrasound of CT angiography  Is treated with stenting or endarterectomy Tumour  Can be either intra-axial or extra-axial  In either category, there are primary and secondary forms  The below image is a CT scan of a glioblastoma  The only thing visible at this stage is the oedema surrounding the tumour and compressing sections of the brain The right image is after injecting dye – this enhances the borders of the tumour slightly Glioblastomas are aggressive tumours that are almost never cured – very rapidly fatal  The below image shows an MRI of a cerebral lymphoma       There is much more detail on MRI than CT (compare with previous tumour) MRI has different weighting (T1 and T2) This can help determine the composition of tumour material, based on its density in relation to fat and water This may help in the diagnosis of tumour and treatment plan Inflammation and Infection  May be a result of: o Demyelination e.g. MS o Vasculitis o Encephalitis o Cerebritis/abscess o Meningitis  The below MRI shows MS       MRI shows MS lesions and plaques very well The above is a sagittal section The corpus callosum can be seen running over the third ventricle MS has a typical presentation – effects the pericallosar white matter (white matter adjacent to corpus callosum) There is a radiating appearance from the corpus callosum This is to do with the nature of MS disease progress – it is a venular disease that affects perivenular tissues  The below is a CT scan of cerebral abscesses as a result of infection  There is pus and enhancement around the abscess  The below image is an MRI of herpes encephalitis    It typically affects the hippocampus Patients with this disease begin to lose their memory Short term memory is impaired first, as patients are unable to learn things they do not already know  Tuberculosis meningitis is shown in the image below  It affects areas surrounding the brain   The inflammation appears to be in the subarachnoid space However, the inflammation is actually in the pial surface, as it surrounds the brain and the sulci Summary  CT is a good screening tool for acute presentations of trauma and neurological symptoms  However, it cannot view all pathologies and may be used to rule certain pathologies out  MRI is typically used as a follow-up, diagnostic imaging technique  Once it has been ascertained that an abnormality is present, MRI provides much better detail