Many studies have investigated effects of drug administration and gene transfer to treat tumors in small animal models. Microcirculation plays important roles in the development and growth of tissues; a tumor proliferates by angiogenesis [1], resulting in the formation of a dense microvasculature, while the microvasculature degenerates in locations where necrosis or apoptosis occurs. Hence, the growth of a tumor can be evaluated based on comprehension of the morphology of microcirculation. Enhanced visualizations of small blood vessels enabled by microbubbles or contrast agent have been reported. The maximum intensity persistence (MIP) method, which maintains the maximum brightness when the contrast agent passes through the vessel, is a representative visualization method already incorporated in existing ultrasonic equipment. However, the performance of the MIP method is deteriorated by changes of brightness due to in vivo movement accompanying heartbeat and breathing. Therefore, complicated processes are necessary to eliminate the effects of such movement by monitoring heartbeat with an echocardiogram (ECG) and by detecting breathing with a pressure sensor.

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