Objective      To investigate the vascular effect of rat Walker-256 tumor induced by low-pressure-amplitude ultrasound (US) with microbubbles.       Methods      Fifty-two Walker-256 tumors in 26 male SD rats were treated with therapeutic ultrasound at 4 different acoustic pressures: 200 kPa (n=12)

400 kPa (n=10)

600 kPa (n=12)

800 kPa (n=12) or sham exposure (n=6) for 5 min. The duty cycle was set at 1%. For half of the tumors in every treatment group

0.1 mL microbubbles were administered intravenously during US exposure. The tumor blood perfusion was imaged with contrast-enhanced US (CEUS) before and after treatment. The gray scale value (GSV) of tumor CEUS was quantified with Adobe Photoshop. All tumors were harvested for histological examination.       Results      There was no significant difference in GSV before and after treatment in all the groups (P0.05). Plasma extravasation and slight tissue edema were observed in the tumors treated with 600 kPa US

and significant hemorrhage and tissue edema were found in the tumors treated with 800 kPa US

with or without microbubble administration during US exposure.       Conclusion      Low-pressure-amplitude US cannot induce significant changes of blood perfusion in rat Walker-256 tumor

but the tumor microvascular permeability is increased at the acoustic pressure of 600~800 kPa.