Use of MRI in neurological disease Flashcards
What generates MRI signals?
Nuclei with a positive charge.
Which atoms most commonly generate MRI signals?
Hydrogen
How is the magnetic charge generated by a hydrogen atom?
By the proton of the hydrogen spinning.
What happens to protons in an MRI scanner?
Spins align with the magnetic field.
Spins adopt either parallel low energy state or anti-parallel high energy state.
Since protons have a magnetic movement they are reactive to external magnetic fields.
How does an MRI scanner work?
A radiofrequency (RF) pulse is applied which causes the magnetisation of the protons to change direction.
As the RF pulse is switched off the spins relax back to their natural alignment.
As the protons relax they induce a voltage which can be recorded by a receiver coil.
The rate at which they relax is dependent on the environment in which the protons are bound, e.g. water, fat etc.
Why is MRI important in neuroscience?
Non-invasive
Can repeatedly scan the same individual over time to follow progression of damage or assess drug treatment.
Accessibility of the brain.
It can characterise and discriminate among tissues using their physical and biochemical properties.
Gives good contrast between grey and white matter making it useful in diagnosing nerve fibre disorders (e.g. MS)
Extremely useful in diagnosing stroke, especially during the acute phase where CT scanning is not as sensitive.
In an ischaemic stroke what is the difference between the ischaemic core and the ischaemic penumbra?
Core: irreversibly damaged, cells are destined to die.
Penumbra: tissue does not function normally but is still viable and may recover if blood flow is restored or drugs given to support survival.
What is the only licensed treatment for ischaemic stroke?
Recombinant tissue plasminogen activator (rt-PA)
What can diffusion weighted MRI (DWI) measure?
The translational motion of water molecules in living tissue.
Why does diffusion decrease in acute stroke?
CBF deficit Energy failure Failure of Na/K pump Intracellular accumulation of Na Influx of water Cell swelling Reduction in extracellular space Water diffusion impaired DWI hyperintensity.
How many hours after a stroke does break down of the blood brain barrier occur?
3-4 hours.
What does perfusion MRI (PI) measure?
Blood flow
What is the most common method of perfusion imaging?
Contrast agent injection.
What can PI combined with DWI be used to determine?
Which patients will benefit from thrombolysis.
What does diffusion tensor imaging (DTI) give information about?
The diffusion of water along white matter tracts.
It’s used to assess white matter lesions or white matter integrity.
Can map subtle changes in the white matter such as in multiple sclerosis or epilepsy.
Can be used to measure white matter connectivity (tractography).
What is the DTI signal dependent on?
The distance and direction the average water molecule in a voxel moves.
In DTI what is diffusion in no preferred direction called?
Isotropic.
In DTI what is diffusion in a preferred direction called?
Anisotropic.
What does fractional anisotropy measure?
Directionality.
In grey matter is diffusion more isotropic or anisotropic?
Isotropic
In white matter is diffusion more isotropic or anisotropic?
Anisotropic (prefers movement along fibers).
What is multiple sclerosis?
Chronic autoimmune, inflammatory disease of the CNS.
Results in demyelination and injury of the axons.
Relapsing-remitting MS present in >80% of patients with a female:male dominance of 2:1.
Immune reaction against autoantigens particularly myelin proteins.
Results in leakage of BBB
How is MS diagnosed?
Clinical findings - at least 2 different neurological episodes in the disease course.
MRI - at least 2 different lesions in the white matter of CNS.
CSF - chronic inflammation of the CNS.
Which different MRI types would be used to detect MS?
T2 weighted MRI: Hyperintense lesions in white matter (demyelination)
T1 weighted MRI: In the acute inflammatory phase contrast enhanced MRI can be use to detect disruption of the blood brain barrier.
DTI: can be used for assessing WM demyelination.
Magnetisation transfer imaging (MTI): sensitive to myelin contact.
Molecular MRI: combining Ab’s with MRI contrast agents
By combining different MRI techniques (multiparametric imaging) a greater insight into the tissue pathology can be obtained.
How does magnetisation transfer imaging (MTI) work?
Proton MRI detects signal from mobile protons while less mobile protons are not detected as freely.
Signal dependent on the transfer of magnetisation from protons bound in macromolecules to protons in the cytosol.
Decrease in magnetisation transfer ratio (MTR) correlates well with demyelination.
What is Alzheimer’s disease characterised by?
Brain shrinkage, loss of neurons in hippocampus and basal forebrain.
What kind of MRI can be used to detect amyloid plaques?
T1/T2 weighed imaging.
What does functional MRI provide information about?
Provides information about activation of different parts of the brain following a stimulus (e.g. physical sensation, sight, sound).
Have you done the further reading?
Weber, R. Ramos-Cabrer P, Hoehn M. Present status of magnetic resonance imaging and spectroscopy in animal stroke models. J Cereb Blood Flow Metab. 2006 May; 26(5):591-604
Dijkhuizen RM, Nicolay K. Magnetic resonance imaging in experimental models of brain disorders. J Cereb Blood Flow Metab. 2003 Dec;23(12):1383-402
