Perfusion and Permeability Imaging Flashcards
What is the basic principle of MR contrast agents?
MR contrast agents are paramagnetic molecules
Presence of the contrast agent alters the magnetic environment and therefore alters the signal
T1 and T2 are both shortened in the presence of gadolinium based contrast agents
T1 weighted images exhibit increased signal
T2 weighted images exhibit decreased signal
What is perfusion imaging?
Perfusion describes the local blood flow through a region of brain tissue
Perfusion imaging uses an injected dye in order to see blood flow through tissues
What 3 techniques are used to derive the perfusion values?
- Dynamic susceptibility contrast
- Dynamic contrast enhanced
- Arterial spin labelling
What are the measurable parameters of interest in perfusion imaging?
CBF - cerebral blood flow
CBV - cerebral blood volume
MTT - mean transit time
What is mean transit time?
How fast the agent is moving around the body
What type of imaging is most commonly used for perfusion imaging, specifically dynamic susceptibility contrast?
Often uses T2* weighted imaging in perfusion imaging as it is more sensitive to intravascular contrast
Generally gradient-echo echo-planar T2* weighted
Describe the methodology of perfusion imaging
- Acquire baseline pre-contrast images
- Inject a contrast agent ‘bolus’
- Observe signal change during first passage of bolus through the brain
- T2* weighted sequence is sensitive to intravascular contrast agent
- Requires high temporal resolution multi-slice imaging
Gadolinium contrast agent (Gd) is injected (usually intravenously) and a time series of fast T2-weighted images is acquired. As Gadolinium passes through the tissues, it induces a reduction of T2 in the nearby water protons; the corresponding decrease in signal intensity observed depends on the local Gd concentration, which may be considered a proxy for perfusion. The acquired time series data are then postprocessed to obtain perfusion maps with different parameters, such as BV (blood volume), BF (blood flow), MTT (mean transit time)
What do the properties of the contrast agent uptake curve tell us?
Perfusion measurements
Area under curve = amount of contrast = CBV
Curve width (FWHM) = first moment = MTT
CBF = CBV/MTT
What is FWHM?
Full width at maximum half
What is a clinical application of MTT perfusion imaging?
Can be used to predict tissue at risk from stroke - if lots of tissue at risk from stroke patient will receive thrombolysis
Why is it important to have both measures of blood flow and mean transit time?
Often, blood flow can over compensate for a previous flow deficit (hyperemia) and therefore mean transit time is important
What is another method of measuring perfusion?
Arterial spin labelling
ASL is an MRI technique that measures perfusion without using an exogenous contrast agent
Instead, blood is magnetically labelled in the. neck as it flows into the brain
The labelled blood acts as an endogenous contrast agent
The difference in signal between labelled and non-labelled images is proportional to the cerebral blood flow
What types of tissue can ASL measure?
Good technique for grey matter (enough signal to measure strokes or tumours reliably)
Lower blood flow in the white matter so harder to image
What is the structure and function of the blood brain barrier?
Endothelial cells form tight junctions creating a blood brain barrier
This barrier allows access to essential molecules, such as glucose, while keeping out toxins
Unfortunately it also severely impedes drug delivery
Breakdown of the blood brain barrier can occur in various disease states e.g., tumours, multiple sclerosis, small vessel disease
What is permeability imaging?
Permeability imaging attempts to quantify the permeability of the blood vessel endothelial wall
In normal subjects blood vessel integrity is maintained and no leakage occurs
Imperfections in the vessel wall can occur in various disease states e.g., tumours, small vessel disease
