Exp tech in Neuro - imaging the brain Flashcards
Types of MR imaging sequences
T2 weighted imaging T1 weighted imaging Diffusion weighted imaging Perfusion weighted imaging MR spectroscopy
Why is MRI important in neuroscience?
Non-invasive, non-ionising radiation.
Can repeatedly scan same individual over time to follow progression of damage or asses drug treatment.
Accessibility to the brain.
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.
Extremely useful in diagnosing a stroke, esp in acute phase where CT is mot sensitive enough.
Image resolution
Resolution of a voxel can be calculated from the matrix size, feild of veiw and slice thickness.
Resolution = (fov/matrix size) x slice thickness
T2 weighted imaging
Assesment of oedema, useful for lesion detection.
Free waterbulk has high T2, appears very bright (ie CSF)
Grey matter appears bright and white matter appears dark.
T2 weighted imaging of a stroke
Breakdown of the BBB occurs around 3-4hrs after stroke, resulting in the movement of water into the extracellular space(vasogenic oedema).
Hyperintensive signal on T2 image is due to the development of vasogemic oedema.
No signal change on T2 images during the acute phase after a stroke due go development of cytotoxic odema.
Increased intracranial pressure compresses the non-Infarcted side of brain causing secondary damage.
Often used to quantify infarct volume.
Infarct volume = sum of hypertensive areas on each slide multiplied y slice thickness.
Multiple sclerosis
Chronic autoimmune inflammatory disease of the CNS
Results in demyelination and injury of axons
T2 weighted MRI shows hyperintense lesions in white matter (demyelination).
T1 weighted imaging
CSF appears dark
Grey matter appears dark and white mater appears bright.
Excellent contrast between grey and white matter
Mainly used for anatomical imaging
Contrast agent uptake is used to assess the integratey of the BBB
Assessment of BBB leakage
Gadolinium contrast agent is too large to cross intact BBB.
Uptake of gadolinium within the brain appears as a bright white signal on T1 weighted images.
Regions of uptake indicate leakage of the BBB
Diffusion weighted imaging
Allows non-invasive measurement of translational motion of water molecules in living tissue.
In biological tissues diffusion is impeded by the presence of cel membranes, axonal fibres etc.
DWI, is one of the earliest detectabke clinical signs of ischemia.
Diffusion is reduced within minutes after the onset of ischemia.
Can detect ischemic damage as early as 5 mins after stroke in experimental models.
Why does diffusion decrease in acute stroke?
CBF deficit leads to enerhy failure. This leads to failure of the Na/K pump, causing jntracelkular accumulation of NA and therefore an influx of water. The cells swell leading to a reduction in intracellular space, which impedes water diffusion causing DWI hyperintensity.
Diffusion tensor imaging (DTI)
Provides directional info about the diffusion of water along white matter tracts.
Used to assess white matter lesions or integrity.
Can map subtle changes in white matter (in ms and epilepsy)
Can be used to assess white matter connectivity (tractography)
Diffusion in no preffered direction is isotopic.
Diffusion in a preferred direction is anisotopic.
Fractional anisotrophy is a measure of directtonality.
Tractigraphy following traumatic brain injury
Fractional anisotrophy is decreased due to axonal degeneration and demyelination.
Can see which tracts propogated into lesioned area.
Perfusion weighted imaging (PWI)
Info about perfusion status of the brain can be obtained (ie bloodflow)
Most commonly via contrast agent injection.
PWI combined with DWI can be used clinically to determine which patients would benefit from thrombolysis.
Functional MRI
Provides info about the activation of different parts of the brain following a stimulus.
FMRI measures heamodynamic response to neural activity and is carried out by a type of imaging called “blood oxygen level dependant imaging” (bold).
Increased neural activity increaees blood flow and therefore increases oxyheamoglobin, this leads to increased T2 and increased MRI signal.
Cell tracking with MRI
Non-invasive imagjng of cell transplants. Celks are labelled with MR contrast agents
Superparamagnetic contrast agents (iron oxides) show hypointensity on T2.
Peremagnetic contrast agents (manganese/gadolinium) show as hyperintensity on T1.
