Perfusion Variables

Perfusion Variables. 

From the drop in MR signal observed during a contrast perfusion study, it is possible to calculate the changing concentration of gadolinium within a voxel (proportional to 1/T2*). This knowledge can be used to calculate a number of physiological variables of interest using tracer kinetic theory.

 The most commonly used variable in gadolinium perfusion imaging is the relative mean cerebral blood volume (rCBV). This value is calculated easily from a graph of gadolinium concentration as a function of time, as the area under the curve. 

Other variables that can be calculated include

 the time from injection to peak signal drop (time to peak, TTP) and

 the time for contrast to pass through a voxel (relative mean transit time, rMTT). These nonquantative variables cannot provide absolute measures of blood flow, like xenon or perfusion CT. 

A separate measure of blood velocity is required to obtain an accurate measurement of CBF. Several techniques have been suggested,[19] but whether the information gained from CBF will be superior to rCBV is uncertain.

 Imaging Cerebral Ischemia.

 Focal hypoperfusion from arterial thrombo embolism decreases both CBV and CBF and increases mean transit time. It remains unclear which of these variables will prove most useful in clinical practice. Early results have suggested that a low rCBV may most accurately predict eventual infarct volume, while infarct growth may be best demonstrated by the difference between low CBF on perfusion imaging and diffusion restriction on diffusion-weighted imaging.