MRI in stroke and white matter disease Flashcards
is CT or MRI better at diagnosing acute ischemic stroke and what are the advantages and disadvantages of both?
.MRI is more sensitive to these acute ischemic strokes in the brain so its more accurate
CT massively under diagnoses acute stroke, CT is normally first protocol because it is quicker than MRI which takes longer and you need to understand the patients history such as if they have a pacemaker, or nay contraindications of having a MRI scan, this cannot be found out quickly
describe the physics of scanners
The brain is 75% water, it is the hydrogen atoms which have a proton in its core that is generating the signal.
Proton is positively charged particle that spins on its axis.
This creates a magnetic moment. Protons are therefore effectively bar magnets.
Very weak magnetisation but because there are so many magnets per mm3 it ends up being a magnetisation that can be used to generate a signal that can be picked up by the MR.
Protons that are in freely moving water can generate an MR signal, but also lipids with ch2 and ch3- the protons in those can also generate signals.
But majority of the signal comes from the protons in the water
-The MRI machine creates a magnetic field around the patient
-Our body is full of tiny hydrogen atom; the magnetic field makes the protons in these water molecules line up in a certain direction.
-machine then send a pulse of radio waves into your body that knocks the protons out of their alignment, then the pulse stops, protons go back to original alignment, which causes them to send out the radio signals which get detected by the MRI antenna detectors, computing them into an image.
-Providing detailed pictures of the inside of the body.
What does the signal intensity depend on, and what does misalignment of protons do post radiofrequency
and what allows you to identify abnormalities within the brain
the amount of water
generation of MRI image
due to contrasts generated as a result of differences in signal intensity (higher echo time gives greater signal difference and thus greater contrast in comparison to short echo time, allows you to detect abnormalities or pathological changes)
what does TE mean, and what does a long TE mean to the T2W image generated, what’s the difference in relaxation between CSF and grey/white/fat matter for a T2W image?
echo time
longer TE, the stronger the signal (and also depends on T2 relaxation time)
CSF has a long RT so the signal decays slower, whereas grey/white matter has a shorter relaxation time, so the signal decays quickly.
compare the signal decay for tissue water, pure water and bound water? what change is seen in T2W signal as a result of stroke or small vessel disease?
If you look at a cell – there is intracellular water, extracellular water, proteins macromolecules and also water molecules that are tied up with membrane structures.
How does signal change with time – you put radiofrequency pulse to knock that magnetisation out of alignment that generates an MR signal.
Pure water the decay is slow because it has the CSF which has long relaxation time.
Bound water molecules- signal decays very quickly.
Tissues water is average of the different water compartments.
pure water and CSF generate a strong signal because there is not much loss of signal, tissues water decays away giving darker signal and bound water has decayed away totally so you don’t pick up any signal
Stroke or small vessel disease – you get lesions where there is cell membrane damage – change I water content- extracellular water get shucked into the brain through ion channels.
Doesn’t have enough oxygen or glucose to support good balance between intracellular and extracellular spaces.
End up with increase in free water and a decrease in bound water.
There is an average increase in water, so T2 relaxation time goes up so there is an increase in the signal intensity.
Signal – is determined on T2 RT and this is affected by oedema and structural changes in the tissue.
whta determines how quickly the MRI signal decays after a radiofrequency pulse? what does T2 depend on? what factors can reduce T2 signal?
-T2 tissue (tissue relaxation time)
-increases with oedema, demyelination, neurodegeneration, cell death and loss of brain tissue structure ( lots of water influx/ swelling in those regions, increasing echo time and relaxation time therefore reduce signal decay and produces a brighter signal image)
-decreases due to presence of paramagnetic ions, such as Fe from blood breakdown products and Gd from contrast agent
what is known as direction of magnetic field
z axis
what does T1W and T2W differ in terms of signal production?
T2W depends on decay of signal (longer relation time is more favourable to producing bright signal, ie CSF will be bright due to a longer RT, and slower decay)
T1W depends on recovery of signal (shorter recovery ie going back to original alignment of protons produce brighter signal and is more favoured. ie CSF has a slower recovery so signal is slower and appears darker)
so in T1W image what appears brighter in terms of white or grey matter and why? whats the benefit of this type of image
In grey matter, water is intracellular and extracellular, therefore has longer recovery time/ relaxation time, but in white matter, the myelin structures which within T1 relaxation and echo time of lipids are very short therefore strong signal, hence producing brighter image
allow you to see anatomical changes, and abnormalities and grey and white matter differentiation
does the recovery time of edema increase or decrease in the T1W image?
increases
does hemorrhage appear bright or dark in T1W image
appears brighter because a bleed and not more free water
what imaging technique is used to measure white matter lesions and what can white matter lesions be caused by, describe its pathology and what do they increase risk of?
FLAIR T2W
Likely the result of chronic hyperperfusion and more commonly
associated with patients with cardio- and cerebro-vascular disease and risk factors for these
Pathologically-associated with myelin loss,
Clinical studies indicate they’re predictors of increased risk of stroke, dementia (cognitive decline) and mortality in general and the clinical (hospital based) populations
WMD is when they brains white matter is affected leading to pathological changes seen whereby the eyelin sheath covering these nerves which is crutial for speed of electrical transmission, deteriorates, so theere is a loss of myelin sheath. they can be indicated as theses bright spot lesions in flair T2W images.
can be caused due to ageing, high BP, high cholesterol, diabetes, ischemia, that affect blood perfusion/ circulation leading to deacreased blood flow to the brains white matter. can cause symptoms such as cognitive decline, physical impairment in walking and balance.
what is the echo time?
emission of radiowaves from protons once they have been knocked out of alignment.
what is a thrombectomy and what is a ischaemic penumbra
Thrombectomy- is a medical procedure used to remove a blood clot (thrombus) from a blood vessel. It’s often performed to quickly restore blood flow in cases of acute stroke caused by a clot blocking an artery in the brain. During the procedure, a doctor threads a thin tube called a catheter through the blood vessels to the site of the clot. Special tools or devices are then used to capture and remove the clot, or to break it up and suck it out. This helps to re-establish blood flow and minimize damage to the brain or other affected parts of the body.
The ischemic penumbra refers to the area of brain tissue that surrounds the core of a stroke. This area is at risk because it’s not getting enough blood flow to function normally, but it’s not yet dead. With quick treatment, it’s possible to save this part of the brain and reduce the overall damage caused by the stroke. The concept of the ischemic penumbra is crucial in stroke treatment, emphasizing the importance of timely medical intervention to restore blood flow and minimize brain injury.
what are the 3 crucial medical interventions and risk areas.
thrombectomy depending on seveirty and thrombolysis where areas are starting to clot.
ischaemic penumbra- at risk areas
