In vivo shock wave measurements
Measurements of
lithotripsy shock waves inside pigs were made. A small PVDF hydrophone
was built (see Fig 1) by stretching a 25-µm PVDF membrane
over a small plastic ring 20 mm in diameter. Silicone rubber was
used to electrically insulate the PVDF from the internal fluids
of the pig. The hydrophone was placed on either the anterior or
posterior surface of the kidney of a young pig via a subcostal
flank incision, see Figure 2. The incision was surgically closed
and the pig was placed in a Dornier HM3 lithotripter.
Alignment of the hydrophone with the shock wave blastpath is
important for successful measurements to be made. We achieved
alignment by means of the fluorographic localisation system that
is used with the Dornier HM3 for the placement of the kidney stone
in clinical treatment.
Figure 3 shows
a tyical waveforms measured in vitro and in vivo the pig using
the miniature hydrophone. The parameters given are peak positive
pressure (p+), peak negative pressure (p-), 10 to 90 percent rise
time (rt), and the duration of the positive cycle (T). The in
vivo waveforms were of similar shape to the in vitro waveforms.
However the amplitude in vivo was approximately 30% less at the
focus of the lithotripter. The rise time was increased from around
20 ns in vitro (which is hydrophone limited) to about 100 ns in
vivo. The measurements of the negative pressure are corrupted
by waves diffracted from the outer edge if the hydrophone; an
unavoidable consequence when using a small hydrophones. The reduced
pressure and increased rise time are thought to be due to three
factors: 1/ the higher absorption in tissue, 2/ degradation of
the focus by tissue inhomogenities and 3/ excess absorption due
to nonlinear distortion.
Reference: R. O. Cleveland, D. A. Lifshitz, B. A. Connors,
A. P. Evan, L. R. Willis, and L. A. Crum, "In vivo pressure
measurement of lithotripsy shock waves," Ultrasound Med.
Biol. 24: 293--306 (1998).
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