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MR Imaging for Stroke


Learning objectives

  • Development of MRI imaging
  • Indications for MRI
  • Contra-Indications for MRI imaging
  • Normal MRI appearance
  • MRI and stroke appearance

MRI Basics

Unlike CT, which requires ionising radiation, MRI is based on the interaction between radio waves and hydrogen nuclei in the body in the presence of a strong magnetic field. There is no ionising radiation and no known safety issues. MRI produces higher quality images than CT. In CT scans must be in the plane of the gantry, that is, axial or semi-coronal. In MRI, one is able to acquire images directly in any plane, that is, the usual axial, sagittal, coronal, or any other.

Both modalities focus on the properties of a volume element or "voxel" of tissue. This is represented in 2-D form or picture elements or "pixels". The pixel intensity in CT reflects the electron density but in MRI it reflects the density of hydrogen, generally as water (H20) or fat. To be more exact, MR signal intensity reflects the density of mobile hydrogen nuclei modified by the chemical environment, that is, by the magnetic relaxation times, T1 and T2, and by motion.

A hydrogen nucleus is a single proton. It has a positive charge and spins and so generates a small magnetic field (a "magnetic moment") which are usually randomly distributed. These magnetic moments align when placed in a larger magnetic field. With MRI the magnetic field across the body-sized sample is intentionally made non-uniform by superimposing additional magnetic field gradients that can be turned on and off rapidly. Activation of these additional magnetic fields results in a net gradient in the strength of the magnetic field across the body which is necessary for spatial localisation and imaging. The essential components of an MR imaging system include a large magnet which generates a uniform magnetic field, smaller electromagnetic coils to generate magnetic field gradients for imaging and a radio transmitter and receiver and its associated transmitting and receiving antennae or coils. In addition to these fundamental components, a computer is necessary to coordinate signal generation and acquisition and image formation and display.

When the body lies within a strong magnetic field, it becomes temporarily magnetised. This state is achieved when the hydrogen nuclei in the body align with the magnetic field. When magnetised, the body responds to exposure to radiowaves at a particular frequency by sending back a radiowave signal called a "spin echo". This phenomenon (NMR) only occurs at one frequency (the "Larmor frequency") corresponding to the specific strength of the magnetic field. The spin echo signal is composed of multiple frequencies, reflecting different positions along the magnetic field gradient. When the signal is broken into its component frequencies (by a technique called a "Fourier Transform"), the magnitude of the signal at each frequency is proportional to the hydrogen density at that location, thus allowing an image to be constructed. Thus, spatial information in MRI is contained in the frequency of the signal, unlike X-ray-based imaging modalities such as CT.

Sometimes MRI is not possible

Brain MRI usually involves lying within a very tight space for between 10 and 30 minutes. Access to the patient is difficult. It is incredibly noisy and communication with radiographers is difficult. One has to be realistic and consider the shortest protocol that will get the important clinical information needed. Some do not tolerate it at all. It is a common paradox and frustration that those in whom it would be of most value can't have one due to one reason or other. There are some open scanners available for those who are claustrophic but access is often difficult.

Reasons why MRI not possible
  • MRI scanner unavailable, Swan-Ganz catheter
  • Brain aneurysm clips - check with manufacturer,Deep brain stimulator
  • Ocular or other metallic foreign bodies (skull X-ray can help to exclude)
  • Unable to monitor safely in MRI environment - patient too ill
  • Extreme claustrophobia
  • Pacemaker/AICD or recent surgery and clips or metallic foreign bodies.
  • Insulin pumps, neurostimulators, cochlear implants, etc. may be de-programmed
  • Patient unable to lie flat - musculoskeletal or cardiorespiratory reasons
  • Patient kyphosis or obesity
  • Bullets or gunshot pellets near great vessels or vital organs such as lungs, heart or brain
  • Relative C/I in early pregnancy- data lacking
  • Cognitive impairment - sometimes sedation considered if agitated

Examples

T2 FLAIRT2
DWIADC
SWIT1 Sagittal

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

MRI with Gadolinium

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.

MR Angiography

Scan Sequences

Relaxation times

  • T1 relaxation time: Time taken for 63% of longitudinal magnetisation to recover in a tissue. T1 is short in fat and long in water and proteins. Contrast causes T1 shortening.
  • T2 relaxation time: Time taken for 63% of transverse magnetisation to be lost in a tissue. Liquids have long T2 and large molecules a short T2.
  • T2* based on T2 decay and dephasing due to inhomogeneities in the magnetic field. Relevant in Gradient echo imaging.
  • Weighting T1 and T2 weighting can increase tissue contrast

T1 weighted

  • Scanning parameters are set (short TR/short TE)
  • Dark : CSF and anything with increased water (oedema, tumour, infection, infraction, haemorrhage or flow void or calcification).
  • Bright: Fat, subacute haemorrhage, melanin, protein rich fluid, slow flowing blood, gadolinium, laminar necrosis of an infarct. White matter is brighter than grey. Myelin is light grey. Grey matter is grey. T1 is better for showing anatomy. An acute stroke will be hypointense.

T2 Weighted

  • Dark: Calcification, Blood products, protein rich fluid, flow void.
  • Bright: Anything with increased water e.g. CSF, Oedema, tumour, Infarct, inflammation, infection, subdural collection, methaemoglobin in subacute bleed. CSF is white and brain tissue is darker and more intermediate. Fat is dark. Myelin is dark grey. Grey matter is brighter than white matter. Good for identifying pathology. T2 is bright due to water or any oedema in pathology. For example a fresh infarct with oedema will show up bright or hyperintense after a few days becoming most obvious after a few months. T2 weighted hyperintensities may be seen within 6 hours and are present in 90% by 24 hours.

T2* "star"

  • T2* can be recorded using GRE sequences with a low flip angle, long TE, and long TR
  • Mostly used to detect iron in its various forms. Haemorrhages cause an influx of hemoglobin and hemosiderin into the lesion.
  • The iron in haemoglobin and haemosiderin is paramagnetic and thus causes an susceptibility difference in the damaged region, which shows up as dark regions in T2* scans.
  • Blood looks black and Hemosiderosis can also be detected by T2* scans for the same reason.
  • Calcification shows up bright in a T2* MRI due to its higher diamagnetism than the surrounding tissue.

FLAIR

  • Fluid attenuated inverse recovery.
  • FLAIR scans are T2 scans with the free water signal nulled.
  • CSF is now dark. Useful to see oedema and periventricular lesions which appear bright.
  • An acute ischaemic stroke will be bright

Gradient echo (GRE)

  • Excellent at identifying areas of blood and haemosiderin deposition such as in macrophages around an old bleed.
  • Also useful in identifying microbleeds. Sensitivity for blood approaches CT within first 24 hours of a bleed.
  • After several days GRE is the more sensitive modality.

Diffusion Weighted Imaging

  • Identifies areas where Brownian motion is restricted due to cytotoxic cell death. Useful for identifying early Ischaemic stroke.
  • This is explained by loss of ATP causes of ion exchange pumps.
  • Water from the extracellular space enters into the intracellular compartment (cytotoxic oedema) and produces a typical bright spot on DWI.
  • These changes are seen even within 30 minutes. A new stroke is bright but an old stroke will have low signal intensity on DWI.
  • The DWI is initially bright white and then gradually fades after 10-15 days when the lesion will be best seen on T2 and FLAIR.
  • The ADC often shows the inverse and is black.
  • The signal intensity of acute stroke on DW images increases during the 1st week after symptom onset and decreases thereafter.
  • The ADC map shows a similar findings.
Restricted diffusion: Bright DWI and Dark ADC
  • Infarction
  • Abscess/Cerebritis
  • Lymphoma
  • Creutzfeldt-Jakob disease

Left MCA infarctDWI of an Anterior cerebral artery infarct

Apparent Diffusion Coefficient: (ADC map)

Used with DWI. Ischaemic lesions appear dark.If bright this may suggests the DWI increased signal changes are due to T2 shine through and old i.e. false positive.

Gadolinium Enhancement

Identifies pathology in which there is 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 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

    MRI Imaging

    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
    Cardioembolic stroke
    • Typically large vessel stroke or strokes
    • Multiple lesions : Anterior and posterior circulation and bilateral
    • Strokes of different age
    Lacunar Stroke
    • 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
    Carotid Dissection
    • 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 sagittal sinus
    CADASIL
    • 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.
    MELAS syndrome
    • 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

    Hypertensive haemorrhage
    • 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 haemorrhages

    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

    Communicating Hydrocephalus

    • All ventricles are modestly dilated.
    • Prominent Subarachnoid spaces and basal cisterns
  • Last updated: 18/11/2018

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