Cellular-interstitial water exchange and its effect on the determination of contrast agent concentration in vivo: dynamic contrast-enhanced MRI of human internal obturator muscle.

The purpose of this study was to assess the effects of cellular-interstitial water exchange on estimates of tracer kinetics parameters obtained using rapid dynamic contrast-enhanced (DCE) MRI. Data from the internal obturator muscle of six patients were examined using three models of water exchange: no exchange (NX), fast exchange limit (FXL), and intermediate rate (shutter-speed [SS]). In combination with additional multiple flip angle (FA) data, a full two-pool exchange model was also used. The results obtained using the NX model (transfer constant, K(trans) = 0.049 +/- 0.027 min(-1), apparent interstitial volume, v(e) = 0.14 +/- 0.04) were marginally higher than those obtained using the FXL model (K(trans) = 0.045 +/- 0.025 min(-1), v(e) = 0.13 +/- 0.04), but the error bars overlapped in two-thirds of these parameter estimate pairs. Estimates of K(trans) and v(e) obtained using the SS model exceeded those obtained using the NX model in half the patients, and many estimates, including all those of intracellular residence time of water, t(i), were imprecise. Results obtained using the full two-pool model fell between those obtained using FXL and NX models, and estimates of t(i) were also imprecise. The results suggest that data obtained using clinically relevant DCE-MRI are exchange-insensitive and unsuitable for the assessment of cellular-interstitial water exchange.