Perfusion Flashcards
What does the presence of contrast agents do?
Alters the magnetic environment and alters the signal
Both T1 and T2 are shorted in the presence of what?
Gandolinium based contrast agents shorten both the T1 and T2 because it is paramagnetic
What do gandolinium based contrast agents do to T1 weighted images?
Images exhibit increased signal because the T1 is shorter so the signal relaxes quicker and there is more signal to image
What do gandolinium based contrast agents do to T2* weighted images?
Images exhibit decreased signal because the T2 is shorted so you get increased dephasing in the XY plane so you lose signal faster
What is MR perfusion Imaging also known as?
Dynamic Susceptibility Contrast MRI
What is perfusion?
Describes the local blood flow through a region of brain tissue- not arteries but how much blood is delivered to tissue, more looking at the capillary level
What are measurable paramaters of interest for perfusion?
CBF (cerebral blood flow)
CBV (cerebral blood volume)
MTT (mean transit time)
How do we acquire a perfusion image?
Acquire baseline pre-contrast images
Inject a contrast agent e.g. ‘bolus’
Observe signal change during first passage of bolus through the brain
T2*-weighted sequence is sensitive to intravascular contrast agent
Why do we use T2* seqeunces to image perfusion?
Because it is more senstive to intravascular contrast and that is what we’re interested in for this methodology
What does perfusion require?
High temporal resolution as we are trying to track the first passage through the brain
From contrast arriving into the brain to leaving is about 15s, so we need to be imaging 1-2s temporal resolution to pick this up and we want to be able to do this multi-slice
How do we measure perfusion ?
Plot the signal of imaging voxels
Area under curve = amount of contrast = CBV
Curve width = first moment = MTT
CBF = CBV/MTT
How can perfusion show stroke?
From day 1, aas with DWI, there is a clear visibility of stroke in the CBF but moreso in the MTT whihc shows a large region of stroke
How can we use perfusion to prevent damage from stroke?
Used to predict tissue at risk- if there is no flow abnormality but a large transit time abnormality, that would indicate tissue at risk- clinically we can try to reperfuse the patient - patient receives thrombolysis
Ischaemic strokes can be treated using injections of a medicine called alteplase, which dissolves blood clots and restores blood flow to the brain- “clot-busting” medicine called thrombolysis.
What is the clinical value of perfusion for stroke?
Clinically- perfusion can help show what is going on e.g. areas at risk, and whether treatment has been effective e.g. has thrombolysis worked to reperfuse
What is hyperemia in stroke?
We can see an area of high blood flow in a stroke region- this occurs commonly in stroke where the vascular system overcompensates in some people for the infarct and this is called hyperemia
What is the clinical value of perfusion for hyperemia?
Helps clinicians see what is occuring in the patients and decide their likely outcome and what treatments are working
How do tumours appear in a perfusion image?
Tumours are highly vascularised and there is a ring enhancement- whereby they grow so quickly they outstrip the blood supply so the middle tends to be dead tissue with ring enhancement that expands out and this is typical of the most aggressive fast growing tumours
How can perfusion be useful clinically for tumours?
Can examine treatment effects e.g. before and after steroids which many tumour patietns reveive as treatment
What is ASL?
arterial spin labelling
What is arterial spin labelling?
Another way to measure perfusion
A new technique that doesn’t need the exogenous contrast agent so instead of having to inject a patient with gandolinium, we actually label the water coming into the brain magnetically
We have a labelling slab in the neck that inverts the spins coming into the neck so water flowing into the brain gets into the labelling slab, it’ll then be inverted and then as that moves into the brain, the water that has been labelled will recover differently from the water already in the brain so we can see it magnetically
Take a set of images with the labelling slice turned off and a set of images with the labelling slice turned on and the difference between those is proprtional to the blood flow
Its a quanitative technique we can measure blood flow per 100mls per minute if we use normalisation
What is a limitation of ASL?
Good technique in the grey matter but in the white matter typically we need 60 control and label pairs because a bit like fMRI the signal change is small so we need to repeat the process many time to get enough signal to noise to see a difference
In the white matter where the blood flow is lower than the grey matter, we still dont really get enough signal change between the control adn label so we dont get really accurate blood flow measurements in the white matter
What does this WM limitation mean?
We need to think about our pathology, if we are looking at a stroke in the deep WM its not a good technique as we would be looking at low blood flow on low blood flow and we wont see much it’ll be noisy
If it was in the GM we would have enough signal to measure things reliably
Can ASL be combined with BOLD?
People have tried to use ASL to quantify the BOLD signal (as the BOLD is not very quantitative) but its not yet been effectively used as its essentially combining two noisy techniques together
