MRI of intracerebral Haemorrhage and Stroke Flashcards
What are the appearances of haematomas on MRI principally down to?
Signal generated by the haemoglobin
What happens clot first forms?
Contained within intact red cell membrane and is bound to haemoglobin
What happens within 3 days of haemoglobin?
metabollic processes within RBC and enzyme cascade starts to deteriorate - the haemoglobin becomes oxidised to Met
Where does the advanced blood products occur?
Around periphery of the clot
What is found at the centre of the clot?
Blood products closest to oxygenated haemoglobin
What is the clot?
- Highly proteinous
2. Proteins is very effective at stopping X-rays
What are the signal of T2-w and T1-w for Oxy-Hb?
- T2-w: hyper
2. T1-w: Iso (hyper cf, H2)
What are the signal of T2-w and T1-w for Deoxy-Hb?
- T2-w: Hypo
2. T1-w: Iso
What are the signal of T2-w and T1-w for Met-Hb?
- T2-w: Hypo
2. T1-w: Hyper
What are the signal of T2-w and T1-w for Met-Hb (extracellular)?
- T2-w: Hyper
2. T1-w: Hyper
What are the signal of T2-w and T1-w for Haemoglobin?
- T2-w: Hypo
2. T1-w: Hypo
What happens as proteins get degraded?
clot shrinks and the attenuation values drop and so the clot becomes progressively less bright
Why does Tw-1 appear hyperintense to water and iso-intense to brain parenchyma?
Protein content within brain
Causes T1 shortening effects
What does Met-hb have?
Lots of free electrons and porphyrin ring which contains iron
What happens when there is signal loss?
images look hyperintense
What is the characteristic of T2*?
Centre of the clot is high but the periphery of clot is very low
When does T2* sequence become very dark?
Anything that can cause a susceptible artefact
What is FLAIR at its heart?
Its for parenchymal tissue contrast
T2-weighted
It’s a T2 but the bulk fluid has been suppressed
Why does T2* appear very dark?
Susceptibility signal loss has been amplified on the gradient echo
What is the first blood-sensitive sequence?
Gradient echo
What doesnt have a refocusing pulse?
T2w gradient echo
What is the most useful for acute syndrome?
CT
What is susceptibility weighted imaging?
MRI sequence that is particularly sensitive to compounds which distort the local magnetic field and as such make it useful in detecting blood products, calcium
What are the characteristics of SWI?
- Enhances not only the presence of susceptibility weighted signal loss but also phase changes caused by these little field in homogeneities
- Gradient echo amplified
- Much more sensitive presence of hemisydrin
MRI in Ischaemic Stroke
• Hyperacute • Acute – Stroke workup – Defining final infarct • Longer term – Monitoring eg. Sickle cell, moya moya – Treatment planning eg. Revascularisation, endovascular R
Hyperacute stroke
• Conventional structural MR – Changes of ischaemia – Identifying haemorrhage • Diffusion – Core infarct – Discrimination of new lesions from old • Perfusion – Ischaemic penumbra • MRA and vascular sequences – Vascular stenoses, occlusions and dissections
What are the strucutal sequences?
• T2-weighted
- Using longer TE and TR times
- The contrast and brightness are predominantly determined by T2 properties of tissue
• T1-weighted
- Short TE and TR times
- The contrast and brightness are predominantly determined by T1 properties of tissue
• Flair (fluid-attenuated inversion recovery)
• Gradient echo (T2*)
What are the standard T2 and T1 weighted images sequences depend on?
shifts in water content in order to create a contrast between normal parenchyma and infarcted tissue
T2 weighted
most sensitive and tends to start to become abnormal 4 and ½ hours after onset of ischaemic
4 and 1/2 hours
onset of vasogenic edema and BBB breaks down – infarct, water goes into the infarcted tissue
At 4 and 1/2 hours
about 50% of infarct are visible on T2 FLAIR images
• About 24 hours vast majority are visible
What is Diffusion-weighted imaging?
- Highly cellular tissues or those with cellular swelling exhibit lower diffusion coefficients
- Measure of diffusivity of water through tissue
- T2 weighted sequence
What does DWI determine?
How far the water molecules within the voxel has moved between 2 diffusion gradients
if moved a long way: the voxel will lose a lot of potential signal
What is the amount of signal that the voxel is losing proportional to?
Distance that the water molecules are diffusing
What loses signal?
Tissue with mobile H20
What is restricted diffusion?
Bright • No diffusion no signal loss Bright
• Some diffusion some signal loss Intermediate
• Diffusion signal loss dark
What do normal tissues have?
Natural barriers to diffusion: cell membrane, white matter tracts
What happens within normal tissue?
There is moderate restriction to diffusion
What will normal tissue on DWI image look like?
Grey
What happens if water molecules dont move anymore?
Retain all of that signal
Cytotoxic edema
- Oxygen dependent processes fail, and membrane pump fails, and the cells swells
- The interstitial spaces around them are phased
- Water molecules and tissues like to diffuse between cells
- As swelling occurs, the water molecules diffusivity becomes restricted – sits where there are
Pitfall for T2 shine through
• Final signal on DWI trace dependent on:
- Diffusivity of water
- T2 character of tissue
• High T2 persistent high signal ‘shine through’
• Most pathology has high T2- problem
• Apparent Diffusion Coefficient – ADC map
- an MRI image that more specifically shows diffusion than conventional DWI, by eliminating the T2 weighting that is otherwise inherent to conventional DW
- Measure of diffusivity only
- Restricted diffusion dark
• Bright DWI + Dark ADC = Restricted diffusion
• Bright DWI + Normal/bright ADC = T2 shine through
What is the B value?
Looks at the rate of fall of signal between diffusion gradients
A measure of strong the diffusion gradients are
B value
- B0 – the first image we acquire which is effectively a low resolution T2 map – there is no diffusion gradient applied
- B1000 – the standard image that is produced when 2 diffusion gradients have been applied
What is the slope which is depicted with the ADC map?
- Decline in signal between 2 diffusion gradients creates a slope
- LOW ADC map – dark
- High ADC map – steep diffusion gradient – bright
What is restricted diffusion?
Bright DWI + Dark ADC
What is T2 shine through?
Bright DWI + Normal/bright ADC
What is MR perfusion a measure of?
– MTT- mean transit time
– rCBV- relative cerebral blood volume
– rCBF- relative cerebral blood flow
MRA
• Stroke workup – Intracranial occlusion/ stenosis – Extracranial circulation • Carotid bifurcation stenosis • Vertebral origin stenosis – Dissection- fat sat axial sequences
Impact on MR signal
- Susceptibility
- Proton-electron dipole-dipole interaction
- Protein
Susceptibility
• Para-/ superparamagnetic molecules • Compartmentalised – Local field inhomogeneities – Spin dephasing ® signal loss • Principally ¯T2 (¯signal T2-w) • ++ on gradient echo sequences
Proton-electron dipole-dipole
• H2O protons close to Fe2+/ Fe3+
– Spin down ® spin up (¯T1) [spin-lattice relaxation]
• Principally ¯T1 (¬signal T1-w)
• Only in Hb where Fe accessible (in plane)
Proteins
- Large macromolecules
- Precess close to lamor Hz of H2O protons
- Spin down ® spin up (¯T1)
- Principally ¯T1 (¬signal T1-w)
