|Acute Ischaemic Stroke||
- T2 Weighted Imaging and FLAIR show increased signal 'bright' which peaks at 7 days and may persist for a month.
- Diffusion weighted imaging is the most sensitive
sequence for acute ischaemia as it shows the diffusion restriction
(reduced Brownian motion) of extracellular water due to imbalance
caused by cytotoxic oedema within minutes. It can remain bright for up
to 3 weeks. Some of the bright area may be viable. Vasogenic oedema can
also give a bright appearance. Chronic Infarction is not bright on DWI.
- ADC map is initially 'dark' low signal with cytotoxic
oedema (acute ischaemic stroke) and then increases in signal later on.
Vasogenic oedema increases water diffusion and gives a bright
appearance on the ADC map and this is called 'T2 shine through'. With
time the DWI shows decreased signal intensity and the ADC shows
increased ADC values.
- Gradient echo or T2 star It may also be useful in the
very early detection of acute thrombosis and occlusion involving the
middle cerebral (MCA) or internal carotid artery (ICA). This may show
as a hypointense (dark) signal within the MCA or ICA, often in a
curvilinear shape. Note that the diameter of the hypointense signal is
larger than that of the contralateral unaffected vessel. This finding
is called the susceptibility sign, and it is analogous to the
hyperdense MCA sign described for CT imaging.
- Perfusion weighted imaging requires fast MRI
techniques to quantify the amount of MR contrast agent reaching brain
parenchyma after an IV bolus. Allows construction of maps of cerebral
perfusion. This can show ischaemic zone.
- There has been increased using MR as a guide to salvageable
tissue using the difference in the volume of brain with restricted
diffusion in the DWI and that with high signal in the flair. When
haemorrhage is suspected the sequence of choice is MRI with Gadolinium
- Typically large vessel stroke or strokes
- Multiple lesions : Anterior and posterior circulation and bilateral
- Strokes of different age
- Typically < 1.5 cm diameter usually subcortical hypodensity
- Within thalamus, caudate, entire subcortex and brainstem especially pons
- Occluded small penetrating arteries
|Basilar Artery Occlusion||
- Hyperdense Basilar artery in front of pons
- Confirm with MRA/CTA/DSA
- Axial Use Fat suppression T2 shows cross section of artery with thrombus
- CTA/DSA or CEMRA shows vessel with obstruction or subtotal obstruction
- MRI or CT may show downstream infraction from occlusion or thromboembolism
|Cerebral/Vertebral venous sinus thrombosis||
- Generalised parenchymal oedema
- Cerebral Haemorrhage
- Cerebral infarction which does not fit to typical arterial territory
- Thrombus may be seen even on NCCT within occluded sinuses and veins
- Empty sinus or Delta sign - may be seen on CT images post
contrast. The sign consists of a triangular area of enhancement or high
attenuation with a relatively low-attenuating centre on multiple
contiguous transverse CT images obtained in the region of the superior
- CT may be normal or show mild to advanced periventricular small vessel disease
- Multiple areas white matter T2-hyperintensity and lacunar
infarctions concentrated in the anterior temporal lobes and in the deep
white matter of the frontal and parietal lobes. There is relative
sparing of the occipital lobes.
- Anterior temporal pole and external capsule lesions have higher sensitivity and specificity for CADASIL.
- A DSA is typically normal not identifying any significant large vessel disease.
|Primary angiitis of the CNS||
- The Digital subtraction angiogram or CEMRA shows lumen irregularities in distal cerebral arteries.
- CT shows widespread infarct like lesions
- MR shows multiple cortical and subcortical infarct-like lesions that cross vascular boundaries
- Lesions are most prominent in the parieto-occipital region and basal ganglia.
- Lesions have a migrating pattern over time, with appearance, disappearance, and reappearance.
- Variable degrees of generalised cerebral and cerebellar atrophy may also be seen.
- Basal ganglia calcifications may also be seen.
- CTA and MRA are usually normal.
- DSA in the acute phase may show dilated cortical arteries with prominent capillary blush and no arterial occlusion.
|Posterior reversible encephalopathy syndrome (PRES)||
- Marked cerebral oedema which is often widespread
- Focused predominately in the cortical and subcortical grey
matter of the parietal and occipital regions but may also be seen in
frontal and inferior temporal-occipital junction and cerebellum.
- MRI (DWI) has shown that the areas of abnormality represent vasogenic oedema which follows arterial territories
- DSA has shown diffuse vasoconstriction as well as focal
vasoconstriction, vasodilation, and even a string-of-beads appearance
consistent with vasospasm or arteritis.
- There is reduced brain perfusion in regions of PRES.
Radiology of choice is Midsagittal MRI which reveals the drainage pathway in great detail