Microcirculatory considerations in NMR flow imaging.

Regional flow estimation can be accomplished either by measuring the concentration of a deposited tracer whose extraction during transit through the organ is 100%, or by observing the time course of concentration of a tracer whose transport passage through the organ is flow-limited. The deposition method is simpler and applies not only to microspheres or other large particles that lodge in the microcirculation, but also to molecular markers which are trapped. It has the advantage of having substantial time in which to observe the local concentrations by external detection. In contrast, flow-limited markers must traverse the tissue without barrier limitation or other diffusional influence on the exchange. When the marker is limited strictly to the intravascular space, the transit time through the organ is short and observations must be made rapidly with a high resolution technique. An indicator which distributes throughout a large volume of distribution within the tissue has a slower washout, providing more time in which to make observations; tracer water is a suitable marker, except perhaps in the brain. For flow measurement by NMR, markers which fulfill these criteria without causing side effects are needed.