High ultrasonic power standard by calorimetric method in NMIJ. Takeyoshi Uchida Masahiro Yoshioka

Transcription

1 High ultrasonic power standard by calorimetric method in NMIJ Takeyoshi Uchida Masahiro Yoshioka 1

2 Typical method for measuring ultrasonic power Radiation force balance (RFB) method A method measuring a force generated at ultrasound exposure time Calorimetric method A method measuring temperature change generated at ultrasound exposure time 2

3 NMIJ s ultrasonic power standard Calibration method Calibration range Relative expanded uncertainty Radiation force balance (RFB) 1 mw-500 mw (15 MHz-20 MHz) 1 mw-15 W (0.5 MHz-15 MHz) 5%-12% Calorimetry 15 W-100 W (1 MHz-3 MHz) 9% 3

4 Ultrasonic power standard by RFB method Ultrasonic power: 1 mw~15 W Frequency: 0.5 MHz~20 MHz Relative expanded uncertainty: 5%~12% 4

5 Radiation force balance method A force is generated when ultrasound is irradiated to target. Force is measurd as a change in the weight of targets by using electric balance. Electric balance Receiving target Ultrasound wave Ultrasonic transducer 5

6 天秤本体 Electric balance Radiation force balance device Hanging hook Water vessel Transducer Temperature sensor 6

7 Water vessel Ultrasonic transducer Attachment 7

8 Receiving target Reflecting conical target Absorbing target 8

9 Ultrasonic power standard by Calorimetry Ultrasonic power: 15 W~100 W Frequency: 1 MHz~3 MHz Relative expanded uncertainty: 9% 9

10 Background Ultrasonic power A key quantity related to thermal hazards on patients in medical fields Increased use of high power ultrasound for cancer therapy in medical filed It is requirement ultrasonic power standard in high power range for safety and security of patients. 10

11 Problem of RFB method RFB method can accurately measure ultrasonic power up to about 15 W. RFB method is not applicable to determination of high ultrasonic power Problem of receiving target Absorbing target : Thermal damage Reflecting conical target: Acoustic streaming 11

12 Termal damage of absorbing target Absorbing target thermal damage by high power ultrasound Conical target Absorbing target Thermal damaged at 20 W,5.9MHz (non-focused 20mmf beam) 5 12

13 Acoustic streamng on conical target Conical target Effect of acoustic streaming generated by high power ultrasound Because conical target is pushed laterally by acoustic streaming, the radiation force can not be accurately measured by the electronic balance. Conical target Acoustic streaming Transdcuer 13

14 Power [mw] Example of ultrasonic power measurement by absorbing and conical targets absorbing conical 5.9MHz Absorbing Thermal damage at absorbing target Saturation at conical target Conical Oscillator output voltage [mv] Transducer-Target distance 1mm Upside measurement potential at RFB method 15 W~20 W 14

15 Calorimetric method Requirement to develop as an alternative of RFB method for ultrasonic power in high power range. Development of an accurate measurement technique by calorimetric method Measurement device and calculation method Ultrasonic power of up to 100 W Comparison with RFB method, which is primary standard of NMIJ, between 10 W and 15 W 15

16 Conditions required for claorimetric water vessel a) The total ultrasonic energy should be converted to the temperature rise. b) Irradiated ultrasound should not return to the transducer transmission surface to avoid any change in characteristics of transducer. c) Heat loss should be minimized. 16

17 Calorimteric cylindrical water vessel Temperature sensor Air layer Transducer 17

18 Transducer output [V] Hydrophone output [V] Mesurement of ultrasound pulses in calorimetirc water vessel. 0.8 Transducer Pulse/Echo Propagating ultrasound Hydrophone Input signal Time [s] Ultrasound pules could be measured by hydrophone, periodically. Ultrasound waves do not reenter to the transducer transmission surface. 18

19 Heat-shielding material Polyethylene Reduction in uncertainty of measurement by eliminating effect of outdoor air Insulative case State of measurement 19

20 Air-backing ultrasonic transducer The transducer has low heat generation owing to low elastic and dielectric losses. PZT diameter: 20 mm Frequency: 1 MHz Outside diameter of transducer: 40 mm PZT (Fuji ceramics,c-213) 20mmf Ultrasonic transdcuer Ultrasound transducer cross-section view 20

21 Temperature (deg) New calorimetric method by NMIJ The method obtains ultrasonic power from water temperature before and after ultrasound irradiation for eliminating the effect of viscous heating T off T on t b t x t a Ultrasound ON Temperature rise T=T off- T on Ultrasound OFF Time (s) Example of temporal change in water temperature in calorimetric vessel. P T = C p t M x C p : Specific heat capacity M: Water mass T: Temperature rise t x : Ultrasound exposre The difference between the two extrapolated temperatures is defined as T 21

22 Effect of acoustic cavitation bubbles It is necessary to investigate the effect on dissolved oxygen (DO) level of water for precise measurement Experimental condition Frequency: 1 MHz, 2MHz, 3MHz Saturated water: DO level of 8mg/l Degassed water: DO level of 2mg/l Compared range: 10 W-15 W Initial temperature: 22.8 ±0.3 Experimentals are performed under the same conditions expect for DO level of water 22

23 Ultrasonic power (W) Ultrasonic power (W) Ultrasonic power (W) Saturated Degassed Applied voltage (V) Saturated Degassed Applied voltage (V) Saturated Degassed 1 MHz 2 MHz 3 MHz Applied voltage (V) There were significant different between ultrasonic power at all frequencies. Higher ultrasonic power was obtained with saturated water rather than with degassed water. Differences between two samples increased with voltage applied to transducer and decreased with ultrasound frequency. Difference of two samples might be due to acoustic cavitation 23

24 Sonochemiluminescence (SCL) We investigated the occurrence of acoustic cavitation bubbles using SCL. SCL is chemical reaction between luminol anion and reactive oxygen species generated by acoustic cavitation bubbles. SCL is blue glow. It can also be used for the qualitative assessment of amount generated cavitation by measuring SCL intensity. 24

25 Relative brightness value (%) Relative brightness value (%) Relative brightness value (%) Saturated Degassed Applied voltage (V) Saturated Degassed Applied voltage (V) Saturated Degaased 1 MHz 2 MHz 3 MHz Applied voltage (V) Given the positive correlation between SCL intensity and ultrasonic power measured by calorimetry It is quite likely that acoustic cavitation bubbles accounts for the difference in measured ultrasonic power between two sample water. We infer that the effect is due to specific heat capacity of cavitation bubbles It is important to use degassed water for calorimetry. 25

26 Ultrasonic power (W) Ultrasonic power measurement by calorimetric method Calorimetric RFB Applied voltage (V) Calorimetric measurement system constructed by NMIJ could meaure ultrasonic power of up to 100 W. 26

27 Ultrasonic power (W) Comparison of calorimetric method with 15 RFB method Calorimetric RFB Applied voltage (V) The values obtained by the new calorimetric method are within 5% of those obtained by the RFB method. 27

28 Uncertainty budget Uncertainty factor Type 15 W 20 W 30 W 40 W 50 W 60 W 70 W 80 W 90 W 100 W Mass of water V Uncertainty on read of measuring cylinder B Density of waterρ Uncertainty on measuring cylinder B Remaining quality of measuring cylinder B Change of water density by temperature change B Specific heat capacity of water C p Change of specific heat capacity by temperature change B Uncertainty on water temperature measurement Effect on dissolved oxygen level B Uncertainty on thermometer B Variation of extrapolation B Uncertainty on ununiformity of temperature dispersion in water vessel B Uncertainty on outflow and inflow of heat Uncertainty on correction of outflow of heat A Inflow of heat B Measurement time t Uncertainty on read of measurement time B Measurement system Repetition of measurement A Combined standard uncertainty u Expanded uncertainty U(k=2)

29 Summary We explain about radiation force balance method and calorimetry in NMIJ. We created a stable and accurate method for the establishment of a high ultrasonic power standard. We plan to extend ultrasonic power range up to 200 W. 29