Use of the mean transit time of an intravascular contrast agent as arm exchange-insensitive index of myocardial perfusion

A simple two-compartment model was used to study the effects of water excha nge on the signal produced by an inversion recovery prepared rapid gradient -echo sequence during the first passage of a low dose of an intravascular c ontrast agent, water exchange at intermediate rates of exchange (1-10 Hz) b etween the vascular and extravascular spaces caused the form of the signal changes during the first pass to be dependent on both the fractional sizes of the vascular and extravascular compartments and on the exchange rate. Un less the effects of exchange are minimized by using a very short inversion time, parameters such as the peak height and area under the curve will be a ffected by regional and/or pathological variations in the exchange rate and the size of the vascular fraction. The mean transit time (MTT) is, however , less affected by water exchange. Experimental first-pass data produced by intravascular low-dose injections of iron oxide particles were studied in five pigs at 0.5 T. The MTT as derived from the first-pass curves, without deconvolution with the arterial input function, was well correlated with th e myocardial blood now (MBF) as measured using radioactive microspheres (r = 0.70, n = 52, P < 0.01). Other first-pass parameters such as the peak hei ght or area under the curve exhibited either a poorer, or no, correlation w ith the MBF. The data suggest that the MTT of the first pass of an intravas cular contrast agent may be a robust, quantitative method for assessing myo cardial blood now in patients. (C) 1999 Wiley-Liss, Inc.