The threshold for generation of lung hemorrhage in adult mice by pulse
d ultrasound has been shown to be approximately 1 MPa at the surface o
f the lung (10-mu s pulse and a carrier frequency of 2 MHz), This inve
stigation used neonatal swine to determine if the findings for mice ca
n be generalized to other species, After exploratory observations, the
inverse sampling method was used in a primary study (22 animals, 88 e
xposure sites) to determine the threshold for lung hemorrhage in neona
tal swine, The primary study was followed by a separate confirmation s
tudy (13 animals, 48 exposure sites), testing the conclusions of the f
irst study and comparing damage at subthreshold levels with sham-expos
ed animals, A separate investigation explored the histological nature
of tissue damage at suprathreshold levels, A 2.3-MHz focused transduce
r (10 mu s at 100-Hz pulse-repetition frequency) was incremented verti
cally for a distance of 2 cm over the chest of the subject for a total
exposure period of 16 min, Animals were euthanized and lungs were sco
red by visual inspection for numbers and areas of gross hemorrhages. T
he threshold level for hemorrhage was approximately 1.5 MPa peak posit
ive pressure in water at the surface of the animal or, at the surface
of the lung, 1.1 MPa peak positive pressure, 1 MPa fundamental pressur
e, 0.9 MPa maximum negative pressure, 25 W cm(-2) pulse average intens
ity or a mechanical index of 0.6, These values are essentially the sam
e as those reported for adult mice.