Transcript Controlling a HIFU-induced cavitation field via duty cycle

Controlling a HIFU-induced cavitation field via duty cycle
Caleb H. Farny • R. Glynn Holt • Ronald A. Roy
Dept. of Aerospace and Mechanical Engineering, Boston University
CenSSIS RICC, October 6-7, 2005
Work supported by the US Army and the Center for Subsurface Sensing and Imaging
Systems via NSF ERC award number EEC-9986821.
ABSTRACT
EXPERIMENT
Cavitation has been implicated in the lack of control over the shape of thermal lesions generated by highintensity focused ultrasound (HIFU). Employing a single focused, passive broadband transducer in agargraphite 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.
 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.
CONTACT INFORMATION:
Prof. Ronald A. Roy
Boston University
110 Cummington Street
Boston, MA 02215
Phone: 617-353-4846
[email protected]
Prof. R. Glynn Holt
Boston University
110 Cummington Street
Boston, MA 02215
Phone: 617-353-9594
[email protected]
Fundamental
Science
R1
Validating
TestBEDs
S1
 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
R2
R3
RESULTS: 3 MPa focal pressure
Bio-Med
Grad. Student: Caleb Farny, Boston Univ. ([email protected])
Focal region
RESULTS: 2.6 MPa focal pressure
S2 S3
Enviro-Civil
S4
S5
 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.
Acrylamide phantom with bovine serum albumin (HIFU source on right)
“Cigarshaped”
lesion
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.
“Tadpoleshaped”
lesion
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.
 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.
Prefocal Focal
PCD
PCD
MODELING
Expansion ratio
Radiated power (mW)
 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.
120
300
 Neglect evaporation and condensation effects.
 The bubble was modeled as an air bubble in
100
250
water, where the initial bubble size was chosen
80
200
from the size which gave the maximum power
deposition at each temperature.
60
150
 The expansion ratio and radiated power both
decreased as the temperature increased.
40
100
 Vapor pressure increases with temperature,
20
50
reducing the bubble expansion.
 The increased vapor pressure will also
0
0
cushion the inertial forcing upon collapse,
20
30
40
50
60
70
80
90 100
Temperature (¡ C)
decreasing the radiated power.
FUTURE WORK
RESULTS: 2.0 MPa
 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.
REFERENCES
 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.
1.
2.
3.
4.
HIFU on
5.
6.
7.
8.
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Watkin, N.A. et al., “The intensity dependence of the site of maximal energy deposition in focused ultrasound surgery,” Ult.
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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.
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