  A toroidal high-intensity focused ultrasound (HIFU) transducer was used to
expose normal bladder wall tissues non-invasively in vivo in a porcine model in
order to investigate the potential to treat bladder tumors. The transducer was
divided into 32 concentric rings with equal surface areas, operating at 2.5
MHz. Eight animals were split into two groups of 4. In the first group,
post-mortem evaluation was performed immediately after ultrasound exposure. In
the second group, animals survived for up to seven days before post-mortem
evaluation. The ultrasound imaging guided HIFU device was hand-held during the
procedure using optical tracking to ensure correct targeting. One thermal
lesion in each animal was created using a 40 s exposure at 80 acoustic Watts
(free-field) in the trigone region of the bladder wall. The average
($\pm$Standard Deviation) abdominal wall and bladder wall thicknesses were 10.3
$\pm$ 1.4 mm and 1.1 $\pm$ 0.4 mm respectively. The longest and shortest axes
of the HIFU ablations were 7.7 $\pm$ 2.9 mm and 6.0 $\pm$ 1.8 mm, respectively,
resulting in an ablation of the whole thickness of the bladder wall in most
cases. Ablation were performed at an average depth (distance from the skin
surface to the centre of the HIFU lesion) of 42.5 $\pm$ 3.8 mm and extended
throughout the thickness of the bladder. There were two cases of injury to
tissues immediately adjacent to the bladder wall but without signs of
perforation, as confirmed by histological analysis. Non-invasive HIFU ablation
using a hand-held toroidal transducer was successfully performed to destroy
regions of the bladder wall in vivo.
