Controlling a HIFU-induced cavitation field via duty cycle Caleb H. Farny

1
Controlling a HIFU-induced cavitation field via
duty cycleCaleb H. Farny R. Glynn Holt
Ronald A. RoyDept. of Aerospace and Mechanical
Engineering, Boston UniversityCenSSIS RICC,
October 6-7, 2005Work supported by the US Army
and the Center for Subsurface Sensing and
ImagingSystems via NSF ERC award number
EEC-9986821.
ABSTRACT Cavitation has been implicated in the
lack of control over the shape of thermal lesions
generated by high-intensity focused ultrasound
(HIFU). Employing a single focused, passive
broadband transducer in agar-graphite phantoms,
we have shown a decline in acoustic emissions
from cavitation at the focus, suggesting that
HIFU energy is shielded from the focal region,
possibly by prefocal bubble activity. Our recent
modeling results show, however, that bubble
shielding is not the only mechanism behind such a
change in signal. As the temperature increases
the broadband acoustic emissions from an air
bubble in water decrease, and so a decrease in
signal amplitude from cavitation events should be
expected as heating occurs. In order to evaluate
the relative effects of the temperature and
bubble shielding on the bubble activity, we have
positioned a second passive transducer at various
positions in the prefocal region along the HIFU
axis. Depending on the insonation pressure a
decline in signal from the focus is accompanied
by an eventual increase in prefocal pressure.
The timescale of the focal signal decrease and
prefocal signal increase suggests that both
temperature and bubble shielding effects play a
role in the bubble activity at the focus, and may
provide information on how best to monitor the
cavitation signal and ultimately provide feedback
information necessary to control the HIFU
insonation parameters to avoid bubble shielding.

• EXPERIMENT
• The rapid "inertial" collapse of bubble produces
  broadband emissions (cavitation activity).5
• Cavitation can be detected by passively listening
  to broadband noise emissions.
• Key instrumentation elements
• 1.1 MHz focused HIFU transducer
• Two 15 MHz passive cavitation detectors (PCD)
• One PCD confocal with the HIFU transducer
• One PCD positioned along the prefocal region,
  perpendicular to HIFU axis
• Agar-graphite tissue-mimicking phantom
• Prefocal PCD moved in 1 mm increments between the
  focus and 5 mm in front of focus (in between
  experiments).
• Three peak negative pressures 2, 2.6, 3 MPa.
• Compare cavitation activity at focus with
  activity at prefocal positions as a function of
  time and pressure.

RESULTS 2.6 MPa focal pressure

• There is a rapid decrease in cavitation emissions
  at the focus, 1, 2 mm.
• Amplitude at the focus is higher.
• Cavitation emissions increase over time at 3 and
  4 mm in front of the focus.
• No activity 5 mm prefocal.

EXPERIMENTAL SETUP
Focal region
CONTACT INFORMATION Prof. Ronald A.
Roy Prof. R. Glynn Holt Boston
University Boston University 110 Cummington
Street 110 Cummington Street Boston, MA
02215 Boston, MA 02215 Phone
617-353-4846 Phone [phone]
ronroy_at_bu.edu rgholt_at_bu.edu Grad.
Student Caleb Farny, Boston Univ. (cfarny_at_bu.edu)
RESULTS 3 MPa focal pressure

• There is a rapid decrease in cavitation emissions
  from the focus through 4 mm.
• Cavitation emissions increase over time at 5 mm
  in front of the focus.

STATE OF THE ART/OVERVIEW

• High-intensity focused ultrasound (HIFU) shows
  promise for a variety of therapeutic procedures
  surgery, cancer treatment, hemostasis,
  thrombolysis, etc.1
• Absorption of HIFU pressure waves elevates local
  tissue temperature.2
• Cavitation the growth and violent collapse of
  bubbles due to the acoustic pressure wave has
  both positive and negative effects.
• Bubble effects can disrupt prediction of energy
  deposition from HIFU source.3
• The presence of bubbles is thought to effectively
  reflect the HIFU energy back towards the source,
  creating tadpole-shaped lesions.
• However, cavitation can also greatly enhance
  heating rates.4
• It is important to know when and where the
  shielding is occurring. Decreasing focal
  cavitation activity appears to be a sign of
  bubble shielding.
• How are the bubble expansion and radiated power
  affected by temperature?
• Hypothesis If cavitation emission amplitude
  decrease is due to bubble shielding, the
  cavitation emission amplitude should increase at
  some prefocal location along the HIFU axis.

HIFU Transducer Profile
The focal PCD position is fixed. The prefocal PCD
is moved in between experiments in 1 mm
increments along the HIFU axis.

• CONCLUSIONS
• Higher temperatures limit bubble expansion
• Combination of reduced expansion and increased
  vapor pressure reduce the radiated power upon
  collapse.
• The bubble emissions should be expected to
  decrease as a function of temperature.
• Bubbles become irrelevant heating sources as the
  temperature increases
• Should the bubble contribution guide the desired
  sustained temperature to enhance HIFU efficiency?
• Cavitation emissions increase over time
  prefocally as the cavitation emissions at the
  focus decrease
• Evidence of bubble shielding.
• Decreased focal cavitation emissions appear to be
  a combination of both temperature and bubble
  shielding effects.
• Positioning of the prefocal PCD should provide
  spatial extent of cavitation field.
• FUTURE WORK
• Signal detected from PCD is a measurement of the
  power radiated from inertial bubble collapses
• Calibrate the PCD for sound power measurement
  near the cavitation pressure threshold, relate
  measurement to bubble heating model.

MODELING

• Local absorption of sound emitted from collapsed
  bubble is a source of heating.
• PCD can detect the sound emitted from the
  collapsed bubble, but bubble dynamics will change
  with temperature.
• The bubble dynamics were evaluated using the
  Prosperetti, Crum Commander model7,8.
• Obtain size of the bubble and radiated power as a
  function of time.
• The effects of temperature on the sound speed,
  vapor pressure, density, thermal conductivity,
  viscosity and surface tension were included in
  the model.

RESULTS 2.0 MPa

• Neglect evaporation and condensation effects.
• The bubble was modeled as an air bubble in water,
  where the initial bubble size was chosen from the
  size which gave the maximum power deposition at
  each temperature.
• The expansion ratio and radiated power both
  decreased as the temperature increased.
• Vapor pressure increases with temperature,
  reducing the bubble expansion.
• The increased vapor pressure will also cushion
  the inertial forcing upon collapse, decreasing
  the radiated power.

• REFERENCES
• Wu, F. et al., Extracorporeal focused ultrasound
  surgery for treatment of human solid carcinomas
  Early Chinese clinical experience, Ult. Med.
  Biol., 30 245-260 (2004)
• Fry, W.J., Fry, R.B., Determination of absolute
  sound levels and acoustic absorption coefficients
  by thermocouple probes-Theory, J. Acoust. Soc.
  Am., 26 294-310 (1954)
• Watkin, N.A. et al., The intensity dependence of
  the site of maximal energy deposition in focused
  ultrasound surgery, Ult. Med. Biol. 22 483-491
  (1996)
• Edson, P., The role of acoustic cavitation in
  enhanced ultrasound-induced heating in a
  tissue-mimicking phantom, Ph.D. thesis, Boston
  University (2001)
• Leighton, T.G., The Acoustic Bubble, Academic
  Press, San Diego, CA (1994).
• C. R. Thomas, et al., Dynamics and control of
  cavitation during HIFU application, ARLO, 6
  182-187 (2005)
• Prosperetti A., Crum L.A., Commander K.W.,
  Nonlinear bubble dynamics, J. Acoust. Soc. Am.,
  82 502-514, 1988.
• Kamath V., Prosperetti A., Numerical integration
  methods in gas-bubble dynamics, J. Acoust. Soc.
  Am., 84 1538-1548, 1989.

• There is a rapid decrease in cavitation emissions
  at the focus and 1 mm.
• Cavitation emissions increase over time at 2 and
  3 mm in front of the focus.
• Very little activity 4 mm prefocal.