Introduction

Unlike diffusion imaging, which is unique to MRI, imaging of cerebral blood flow is possible with a number of tomographic techniques including PET, SPECT, and X-ray CT. Relative to these other approaches, blood flow imaging with MRI can be advantageous because of its speed, spatial resolution, lack of ionizing radiation, and MRI's ability to obtain other, spatially registered images such as diffusion, MR angiography, or even MR spectroscopy within the same study.

Imaging of hemodynamic parameters such as time to peak, mean transit time, relative blood flow, and relative blood volume is possible with the bolus injection of a contrast agent. This dynamic susceptibility contrast (DSC) approach yields a large signal change, which permits the sensitive characterization of the passage of the bolus through the vasculature. It is particularly good at delineating very delayed or extended transit of the bolus through a region of tissue and has thus been widely applied to the study of stroke in animals and humans.

While clearly a powerful technique, hemodynamic MRI with contrast injection is not without weaknesses. First, it requires a well-timed i.v. injection of a contrast agent. The injection increases the cost of the study significantly, requires good venous access, preferably a power injector, and limits the short term repeatability of the study. Second, because it is an intravascular tracer, quantification of perfusion can be confused by pathologic conditions such as delayed flow or collateral flow through multiple paths.