Welcome to Stroke Education.CO.UK

Learning objectives

  • Role of MRI in stroke
  • Limitations of MRI
  • Different MRI sequences

    Introduction

    MRI is incredibly useful in the diagnosis and assessment of both ischaemic and haemorrhagic stroke. We will discuss the indications and uses and limitations of such imaging below. In most stroke services CT is the initial test and has very high sensitivity in detecting haemorrhage. The difficulties of using MRI is the time taken to scan as well as the not being the best place to be if the patient is ill and needs intensive monitoring and is at risk of cardiorespiratory difficulties. The patient also needs tor remain relatively still for longer.

    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
  • DWI of an Anterior cerebral artery infarct

    Haemorrhagic Stroke

  • T2* or Gradient Echo: Certain MRI sequences are excellent at detecting blood. These are often called the "black blood" sequences as blood appears black. Blood products can be seen when they would not be picked up on CT scanning at all. It is useful in acute bleeds but also old bleeds where a rim of blood can often be seen. This can be useful in those who have what looks like an old lacunar stroke and could be infarct or haemorrhage. MRI will show the ring of blood. The following is a list of such sequences.
  • Susceptibility weighted imaging: 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 blood. A filtered phase can allow 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 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

    In addition to this we may want to look at the pipes and in those cases MRA and MRV may prove useful to look for vascular anomalies and other pathologies.


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