Quantification in molecular ultrasound imaging using compartmental modeling

The goal of project detailed in this paper was to develop a pharmacokinetic compartmental model for describing the complex activity of targeted microbubble (MB) contrast agents used for molecular ultrasound (US) imaging. This imaging study used renal cell carcinoma (RCC)-bearing mice. Either VEGFRR2-targeted or control MBs were intravenously injected. Each animal was imaged before MB injection and for 5 min after using a low-intensity nonlinear imaging mode implemented on a SONIX RP US system (BK Ultrasound) equipped with a L12-4 transducer. After a 4 h delay to allow MB clearance, a subset of animals that received targeted MBs during the first imaging session were reimaged using control MBs and vice versa. Tumors were manually segmented from the US image sequences and a mean intensity value was computed for each frame. A two compartment kinetic model was developed and fit to time-intensity curve (TIC) data. Input parameters for this model include MB dose, tumor size and vascular volume. Output parameters were then estimated using this model that relate to targeted MB binding and clearance. An estimate of fractional tumor vascularity was computed as the percentage of image pixels above a defined intensity threshold compared to the number of pixels for the tumor space. After euthanasia, immunohistologic analysis was performed on excised tumor tissue. The model was in good agreement with TIC data describing targeted MB flow and binding in the tumor vascularity (R² > 0.89, p < 0.05). Using VEGFR2-targeted MBs, the binding constant was significantly considerably higher than when using control MBs (p = 0.07) In the targeted MB group, regression analysis revealed a positive correlation between the binding constant and VEGFRR2 expression (R² = 0.75) that diminished when using control MBs in the same animals (R² = 0.12). There was no difference in vascular or tumor volume estimates when using either targeted or control MBs (p > 0.77). Overall, the two-compartment model may be useful for describing flow binding kinetics of targeted MBs used during molecular US studies.