High Intensity Focused Ultrasound Test & Measurement Techniques

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1 High Intensity Focused Ultrasound Test & Measurement Techniques Shmuel Ben-Ezra June 2011

2 What's HIFU??? 2

3 What's HIFU??? High Intensity Focused Ultrasound Plays an increasing role in medical-therapeutic applications Influence living tissue via thermal/ mechanical/ cavitation effects (tumor ablation) Common frequencies MHz Intensities >1000 W/cm^2 (Pressure ~1 MPa) Continuous wave (CW) or long bursts 3

4 The Challenge of HIFU T&M Cavitation may obscure and scatter the beam, as well as damaging the measuring tool Water conditioning is required (degassing, filter) Streaming due to ultrasound attenuation in water may influence measurement Near Field: pressure p and particle velocity u are not in phase. Non linearity: higher harmonics generation Frequency dependent Attenuation Calibration and hydrophone bandwidth/ flatness What is the relevant parameter? I, p+, p-? 4

5 The Challenge of HIFU T&M Low pressure High pressure p high p low p (t )= p 0 sin (wt ) p high = p low = p 0 2 I = p (t)u(t ) = p 0 /(2ρc) Relations between p+, p-, I are not simple; Heavily depend on medium; The approx. u=p/(ρc) 5 not valid anymore! M. S. Canney et. al.; J. Acoust. Soc. Am. 124(4), (2008)

6 The Challenge of HIFU T&M 6 V. Khokhlova et. al., ISTU-6, Oxford UK (2006)

7 The Status... No applicable standard exists IEC/TR "Requirements for measurement standards for HITU devices" (2010) Some relevant references NPL Report "Requirements for measurement standards in HIFU fields" (2006) FDA Guidance "Information for diagnostic ultrasound systems and transducers" (2008) IEC "Ultrasonics Focusing transducers Definitions and measurement methods..." (2006) IEC "Ultrasonics Hydrophones Part 1: Measurement of medical ultrasonic fields..." (2007) IEC part 2-5 "Particular requirements for ultrasonic physiotherapy equipment" (2005) AIUM/NEMA UD 2 "Acoustic output measurement standard for diagnostic ultrasound..." (2004) 7

8 Outline Hydrophone scan Low pressure hydrophone + extrapolation High pressure hydrophones Radiation Force Balance (RFB, AFB) Schlieren Imaging Discussion 8

9 Hydrophone Scan Hydrophone scan is the gold-standard in diagnostic and low-pressure ultrasound Converts pressure to charge density on the active element voltage on cable ends Calibrated by manufacturer or NPL (UK)/ PTB (DE)/ INRIM (IT)/ UME (TR) Tip: Don't extend the cable (will spoil calibration)! Limited by Manufacturer and calibration to ~600kPa (FDA guidance: for >300kPa degassed water is required) 9

10 Hydrophones Membrane hyd. Needle hyd. ONDA corp. Precision Acoustics (PA) 10

11 Hydrophone Selection Table (by ONDA corp.) D < λ/2.5 HM Series (Membrane) HGL Series (Capsule) HNR Series (Needle) HNA Series (Needle) 200/ /1000 < (±3dB) (±3dB) (±6dB)* 1-10 (±6dB) Nominal Sensitivity** Medium+ Medium Medium Low Ruggedness Low Medium High High Field Disturbance High Low Medium Low Aperture Size (µm) Freq. Res. (MHz) Typical Applications Industrial rugged Standard General Regulatory duty applications, Regulatory test test (CW + pulse), megasonic (pulse), Reference Working cleaning tanks, hydrophone hydrophone lithotriptors Mapping HIFU acoustic fields 11

12 HIFU T&M with Hydrophone Pressure at focus Transducer Performance Evaluation Driving voltage Assume linearity and measure at low pressure Extrapolate to the operational conditions Validate assumption??? 12

HIFU Compatible Hydrophones Rugged design for high-pressure Broadband Good directivity Low sensitivity Muller-Platte 100-100-1 probe http://www.

13 HIFU Compatible Hydrophones Rugged design for high-pressure Broadband Good directivity Low sensitivity Muller-Platte probe Tested at 1.5 MHz, Isppa=1400 W/cm^2 (pressure: Mpa)..." (ONDA) ONDA corp. HNA-400 probe 13

14 The Fiber optic Hydrophone Precision Acoustics (PA) FOPH

15 FOPH Principle of Operation FOPH

16 Radiation Force Balance Transducer (DUT) Driving signal + amplifier Frame Voltage measurement Absorbing target Water vessel Balance 1.23 gram Balance control 16

17 Radiation Force Balance Provides the total time-averaged power Considered as primary physical measurement Compatible with high pressure fields Standard: IEC "Ultrasonics Power measurement Radiation force balances and performance requirements" 17

18 Radiation Force Balance Derivation of basic formula* ρ + ( ρu)=0 eq. continuity, and Euler eq. t The"momentum flux" tensor u 1 +(u )u= p t ρ Π i, k =δ i,k p+ ρ ui u k For absorbing target, plane wave: P absorbed power in target [Watt], c speed of sound [m/sec], F exerted force on target [N] P=c F With c=1500 [m/sec] and 102 [gram/n] we get: P [Watt ]=14.7 F [ gram] 18 * Fluid Mechanics Landau & Lifshitz series vol. 6

19 Radiation Force Balance Correction for focused fields (absorbing target) 14.7 F [ gram] P [Watt ]= (1+cos γ)/ 2 γ =arcsin (a / R) a R With: a the radius of the transducer surface R geometrical focal distance (radius of curvature) 19

20 Radiation Force Balance Precision Acoustics (PA) Reflective target models!

21 Radiation Force Balance Your CUSTOM Design!!! Precision Acoustics (PA) Reflective target models!

22 Schlieren imaging Technique Optical method exploiting the acousto-optic effect Visualizes accumulated pressure (density/ refractive index) gradients in test volume Compatible with high pressure levels On-line, live video stream of the acoustic field Qualitative, characteristic imaging ONDA and some researchers claim for quantitative Schlieren imaging (under restrictive assumptions 22 about the acoustic field)

23 Schlieren imaging Technique Transducer surface Focus 23

24 Schlieren imaging Technique 24

25 Schlieren imaging Technique 25

26 Schlieren imaging Technique ONDA OPTISON ~$180,000.- Edmund Optics NT ~$1,000.26

27 Schlieren imaging Technique Your custom tailored system 27

28 Modality Main Assets Pro's Con's Regular hydrophone Pressure measurement, 3D mapping Calibrated, Common, 'cheap' Low pressure, extrapolated HIFU hydrophone Pressure measurement, 3D mapping Calibrated (?), HIFU compatible Low sensitivity, field disturb, Water cond. Fiber optic hydrophone Press./ velocity measurement, 3D mapping Bandwidth, Calibrated (?), HIFU comp. Low sensitivity, Water cond. Expensive, Tedious work RFB Total timeprimary averaged power measurement, HIFU comp. Photo, movie of On-line, HIFU beam profile comp., no field disturb. Schlieren Spatial averaged power Qualitative (?) 28

29 Thank you! 29

Workshop 2: Acoustic Output Measurements

37 th th UIA Symposium, Washington DC Workshop 2: Acoustic Output Measurements Mark Hodnett Senior Research Scientist Quality of Life Division National Physical Laboratory Teddington Middlesex, UK Workshop