HIFU on 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 Caleb H. Farny • R. Glynn Holt • Ronald A. Roy Dept. of Aerospace and Mechanical Engineering, Boston University Dept. of Aerospace and Mechanical Engineering, Boston University CenSSIS RICC, October 6-7, 2005 CenSSIS RICC, October 6-7, 2005 Work supported by the US Army and the Center for Subsurface Sensing and Imaging Work supported by the US Army and the Center for Subsurface Sensing and Imaging Systems via NSF ERC award number EEC-9986821. Systems 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. 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. 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. 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. “Cigar- shaped” lesion “Tadpole- shaped” lesion Acrylamide phantom with bovine serum albumin (HIFU source on right) 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: 617-353- 9594 [email protected] rgholt@bu . edu Grad. Student: Caleb Farny, Boston Univ. ([email protected]) RESULTS: 2.6 MPa focal pressure Validating TestBEDs R1 R2 Fundamental Science R3 S1 S4 S5 S2 Bio-Med Enviro-Civil S3 REFERENCES 1. 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) 2. 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) 3. 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) 4. Edson, P., “The role of acoustic cavitation in enhanced ultrasound-induced heating in a tissue-mimicking phantom,” Ph.D. thesis, Boston University (2001) 5. 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. Focal PCD Prefocal PCD 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. Focal region EXPERIMENTAL SETUP 0 20 40 60 80 100 120 20 30 40 50 60 70 80 90 100 Temperature (° 0 50 100 150 200 250 300 MODELING RESULTS: 2.0 MPa RESULTS: 3 MPa focal pressure 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 model 7,8 . 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. 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. 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. 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. 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.
HIFU on
Jan 27, 2016
Acrylamide phantom with bovine serum albumin (HIFU source on right). “Cigar- shaped” lesion. “Tadpole- shaped” lesion. Prefocal PCD. Focal PCD. R2. Fundamental Science. R1. R3. Validating TestBEDs. Bio-Med. Enviro-Civil. HIFU on. S2. S3. S4. S1. S5. - PowerPoint PPT Presentation
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HIFU on
Controlling a HIFU-induced cavitation field via duty cycleControlling a HIFU-induced cavitation field via duty cycleCaleb H. Farny • R. Glynn Holt • Ronald A. RoyCaleb H. Farny • R. Glynn Holt • Ronald A. Roy
Dept. of Aerospace and Mechanical Engineering, Boston UniversityDept. of Aerospace and Mechanical Engineering, Boston UniversityCenSSIS RICC, October 6-7, 2005CenSSIS RICC, October 6-7, 2005
Work supported by the US Army and the Center for Subsurface Sensing and ImagingWork supported by the US Army and the Center for Subsurface Sensing and ImagingSystems via NSF ERC award number EEC-9986821.Systems 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.
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.
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.
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.
“Cigar-shaped”lesion
“Tadpole-shaped”lesion
Acrylamide phantom with bovine serum albumin (HIFU source on right)
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: 617-353-9594 [email protected]@bu.edu
Grad. Student: Caleb Farny, Boston Univ. ([email protected])
RESULTS: 2.6 MPa focal pressure
ValidatingTestBEDsValidatingTestBEDs
R1R1R2R2Fundamental
ScienceFundamentalScience R3
S1 S4 S5S2Bio-Med Enviro-Civil
S3
REFERENCES
1. 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)
2. 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)
3. 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)
4. Edson, P., “The role of acoustic cavitation in enhanced ultrasound-induced heating in a tissue-mimicking phantom,” Ph.D. thesis, Boston University (2001)
5. 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.
FocalPCD
PrefocalPCD
HIFU Transducer ProfileThe focal PCD position is fixed.
The prefocal PCD is moved in between experiments in 1 mm increments along the HIFU axis.
Focal regionEXPERIMENTAL SETUP
0
20
40
60
80
100
120
20 30 40 50 60 70 80 90 100
Temperature (° C)
Radiated power (mW)
0
50
100
150
200
250
300
Expansion ratio
MODELING
RESULTS: 2.0 MPa
RESULTS: 3 MPa focal pressure
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.
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.
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.
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.
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.
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