On the design of a capillary flow phantom for the evaluation of ultrasound contrast agents at very low flow velocities.

Recently, a new imaging technology has become available that allows the evaluation of tissue perfusion using echo-contrast agents in real-time imaging: power pulse inversion imaging (PPI). Although numerous in vitro phantoms have been designed for different imaging modalities in ultrasound (US), there is a need for a phantom that mimics microcirculation and allows, in particular, the assessment of contrast replenishment kinetics following US-induced destruction of microbubbles using the new method. We, therefore, designed a new capillary flow phantom that takes the requirements of the new US imaging techniques and the physical properties of microbubbles into account and serves flow velocities in the range of microcirculation (1 to 10 mm/s). PPI studies were performed in the newly designed phantom. The contrast agent used was AF0150. We studied homogeneity of contrast distribution within the capillary phantom, constancy of contrast infusion, the dose-effect relationship and, finally, the feasibility of flow assessment using the method of contrast replenishment following US-induced microbubble destruction in a flow velocity range of 2.1 to 9.45 mm/s. Analysis of the replenishment kinetics was performed using the mathematical model f(t) = A(1 - e(-beta t)), with A representing the blood volume and beta the microbubble velocity. The new capillary phantom allowed homogeneous contrast opacification within the perfused capillaries independently of the flow. Constancy of signal intensity was achieved over a time period of almost 2 h, indicating constant contrast delivery. A strong linear correlation between the PPI signal and the contrast dose was found (r = 0.998). Analysis of the replenishment parameters revealed a strong linear relationship between parameter beta and flow (r = 0.994) as well as A * beta and flow (r = 0.984) in the observed flow range. The newly designed perfusion phantom for the evaluation of echo-contrast replenishment kinetics fulfills, at very low flow velocities, important prerequisites such as constancy of contrast delivery, homogeneity of contrast signals, linear dose-effect relation and minimal attenuation. Thus, the new phantom allows standardized analysis of contrast replenishment kinetics using real-time perfusion imaging techniques at flow velocities comparable to those of the microcirculation.