Perfusion MRI provides a relative and/or absolute measurement of the parameters of cerebral microvascularisation: regional blood volume, mean transit time, regional blood flow.
It relies on the use of a tracer that may be exogenous (contrast agent: Gadolinium) or endogenous (spin labeling)
The first pass technique exploits the magnetic susceptibility effect of the Gadolinium chelates, either with T2 weighting or with T2*weighting, in SE-EPI or GE-EPI sequences. The quality of the injection and the timing of acquisition are of prime importance in obtaining a good examination. The drop in the signal during the first pass of the product allows perfusion parameters to be extracted after post-processing.
In spin labeling technique, it is the saturated blood just upstream of the slice of interest which serves as a tracer. This saturation provokes a variation in the signal received in relation to an acquisition without prior saturation. An estimate of the local hemodynamic parameters can be deduced by comparing these two signals. Different labeling techniques are possible and imaging is carried out in echo planar sequence. It is restricted to the exploration of a limited region of the brain and produces only a weak signal-to-noise ratio.
The main applications of first pass perfusion MRI are vascular pathologies (ischemic strokes, vasospasm) and tumoral pathologies. Progress in sequences and the very high fields should lead to improvements in spin labeling technique.
Summary