Objective      To study the contribution of cavitation and boiling bubbles to hyperecho in B-mode ultrasound images immediately after in vitro bovine liver tissue exposed to high intensity focused ultrasound (HIFU).       Methods      Forty bovine liver tissue blocks were divided into group A and group B

20 in each group. Group A was exposed to 200 W acoustic power HIFU for 2 s to induce cavitation bubbles in the focal region of HIFU and group B was exposed to 50 W acoustic power HIFU for 80 s to induce boiling bubbles in the focal region of HIFU. Temperatures in the focal region of HIFU were measured using a thermocouple placed in the liver 1 mm beyond to the HIFU and recorded through RS 232 serial port. Signals of acoustic emission from bovine liver tissue were detected with the passive cavitation detection (PCD) system

their frequency spectra were detected with a program written on LabVIEW

and their root mean square (RMS) was calculated after filtration of harmonics at 3-7 MHz. B-mode ultrasound images immediately after  HIFU exposures were recorded and compared with those before HIFU exposures. Livers were cut into sections for gross observation for liver injury.       Results      No hyperecho was observed in B-mode images of in vitro bovine liver tissues 19 times after 200 W acoustic power HIFU  exposure. PCD system showed significant broadband noise in the frequency spectra of signals and increased RMS. The peak temperatures in the focal region of HIFU were 61.10±6.18 ℃. The hyperecho was found in B-mode images of in vitro bovine liver tissues 19 times after 50 W acoustic power HIFU exposure. PCD system showed no broadband noise in the frequency spectra of signals and increased RMS. The peak temperatures in the focal region of HIFU were 93.34±3.37 ℃. Coagulation necrosis occurred in all bovine liver tissues after exposures.       Conclusion      The main reason why hyperecho is found in B-mode ultrasound images of in vitro tissues immediately after exposed to HIFU is the boiling bubbles rather than cavitation bubbles.