Metal outside the brain and eye is NOT an absolute contraindication: Magnetic deflection is minimal compared to normal physiologic forces. Cardiac valves), inferior vena cava filters, biliary and vascular stents, IUD's and metallic prostheses are safe unless there is doubt as to positional stability. MRI Policies - Safety and Contraindications . Information is changing all the time. Another useful site is MRI safety.com
This is the MRI equivalent of CT with contrast and uses Gadolinium
which shortens T1 relaxation times. It is useful when there is
suspicion of neoplastic, inflammatory lesions or abscesses. Also useful
for detecting meningeal disease. It is not often used in acute stroke
unless the diagnosis is in doubt.
Apparent Diffusion Coefficient: (ADC map)
Used with DWI. Ischaemic lesions appear dark. If bright this may suggest the DWI increased signal changes are due to T2 shine through and old i.e. false positive.
Identifies pathology in which
there is the breakdown of the blood-brain barrier. Also useful in producing
an angiogram. Tumours or other lesions may show ring-like enhancement.
T1 with Gadolinium will show increased signal with a pituitary tumour,
acoustic neuroma or meningioma.
Susceptibility weighted imaging (SWI)
SWI is an MRI sequence sensitive to paramagnetic compounds which distort the local magnetic field. It can detect blood, iron and calcium etc. and so is useful to detect haemorrhage or blood. Images generate a unique contrast, different from that of spin density, T1, T2, and T2*. It is very sensitive over other sequences. It is not possible to differentiate calcium from the blood. A filtered phase can allow us to distinguish between the two as diamagnetic and paramagnetic compounds will affect phase differently (i.e. veins/haemorrhage and calcification will appear of opposite signal intensity). SWI is very useful in detecting cerebral microbleeds in ageing and occult low-flow vascular malformations, in characterising brain tumours and degenerative diseases of the brain, and in recognizing calcifications in various pathological conditions. The phase images are especially useful in differentiating between paramagnetic susceptibility effects of blood and the diamagnetic effects of calcium. SWI can also be used to evaluate changes in iron content in different neurodegenerative disorders. Reference
Comparing T2*and SWI. Clearly, the right-handed image shows more detail
|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 areas 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 an ischaemic zone.
- There has been increased using MR as a guide to salvageable
tissue using the difference in the volume of the 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.
Imaging Patterns in Haemorrhagic Stroke
- Gradient echo or T2 star. This can show up bleeds especially small microbleeds
- Location of Bleeds in putamen, thalamus, and pons.
- Microbleeds may also be found in the basal ganglia, thalamus, or pons
|Cerebral Amyloid Angiopathy||
- Gradient echo or T2 star can show up bleeds especially small microbleeds
- Haemorrhages more likely to occur in the temporal and
occipital than the frontal and parietal lobes and sites of previous
bleeds. There was a slight bias for the posterior circulation.
- Cerebellum can also be affected.
- Microbleeds as small as 2 mm may be found on GRE or T2*.
Gradient-echo MRI may also show by iron-containing deposits left by old
Other relevant imaging Diagnoses
Non Communicating Hydrocephalus
Radiology of choice is Midsagittal MRI which reveals the drainage pathway in great detail
- Normal - Lateral ventricles small and 3rd ventricle barely visible
- Single lateral ventricle dilated (Univentricular hydrocephalus) = Obstruction of one foramen of munro
- Both lateral ventricles dilated (Biventricular) = Obstruction of both foramen of munro
- Dilation of Lateral + 3rd Ventricle (Triventricular) = Obstruction at level of aqueduct
- Dilation of Lateral + 3rd + 4th = Obstruction at foramen of Magendie and Luschka
- All ventricles are modestly dilated.
- Prominent Subarachnoid spaces and basal cisterns