A Comparative Evaluation of Four Acoustic Hydrophones in High Intensity Focused Ultrasound (HIFU) Field Measurements Yunbo Liu and Keith Wear Yunbo.Liu@fda.hhs.gov Food and Drug Administration, Silver Spring, Maryland
HIFU Therapy 0-20 Pressure Distribution 1~5 MHz Transducer Precise focus with high pressure Non-invasive focused ultrasound therapy Thermal ablation and cavitation damages Small focus with high pressures Patient safety & device performance Pre-clinical pressure field testing FDA approvals and clinical trials PII -40-60 -80 0.4 0.2 0 X-axis (mm) Bone Metastases -0.2-0.4-0.4-0.2 0 Z-axis (mm) Uterine Fibroids Insightec Ltd. 0.2 0.4 MRI guidance/thermometry
HIFU Hydrophone Challenges Hydrophone in a strong, nonlinear and focusing HIFU field Hydrophone is a mechanical sensing device to convert acoustic pressure into electrical RF signals. Hydrophone pressure measurement is recommended by IEC ultrasound standards and FDA regulatory guidance. Mechanical/cavitation damages How high a pressure can we measure? Hydrophone sensitivity 1. Bandwidth, frequency/directional response 2. Deconvolution of the nonlinear signals 3. Spatial averaging of the sensing element Measurement Reliability 1. Single hydrophone repeatability 2. Inter-hydrophone comparability Commercial Hydrophones the measurement opportunities are considerably restricted because of the HIFU fields by Bessonova and Wilkens, IEEE-UFFC, 2013
Materials Methods Lateral beam profiles Focal waveforms with increasing pressures 3.3-MHz single element spherical transducer with 2 x10 mm focus 3D scanning water tank with angular controls Four different hydrophones 1. Robustness 2. Sensing element size 3. Fiber Optic vs. Conventional 4. Sensitivity and deconvolution HIFU Transducer Onda Corp., US Piezoceramic needle -- 400 um PVDF Capsule -- 85 um Fiber optic 1 bare tip Fiber optic 2 -- Fabry Perot Sonic Concepts H101 RP Acoustic, GER 100 um 10 um fiber PA Corp., UK
Driving levels Comparison Methods Transducer driving/outputs Driving voltage Acoustic Power (W) Level 1 15 0.25 Level 2 30 3.2 Level 7 115 42 Level 8 130 19.4 Amplitude (V) 0.9 0.7 0.5 0.3 0.1 0.1 0.3 0.5 Hydrophone voltage linear vs nonlinear 51.0000 51.5000 52.0000 52.5000 53.0000 Time(us) Measurement repeatability Needle hydrophone Hydrophone voltage V+ (mv) V- (mv) Vrms (mv) March 2012 test 72.3 64.5 48.6 June 2013 test 70.5 61.3 45.2 June 2014 test 75.2 66.3 50.4 Average 72.7 ± 2.37 64.0 ± 2.53 48.1 ± 2.64 Coefficient of Variation % 3.26% 3.95% 5.49%
Pressure distribution Needle hydrophone Normalized Pressure Normalized Pressure 1.00 0.75 0.50 0.25 0.00 1.0 0.8 0.6 0.4 0.2 0.0 3.00 2.00 1.00 0.00 1.00 2.00 3.00 Same beam distribution with increasing 3.3 MHz HIFU output. Lateral Distance (mm) Level 1 Level 4 Level 8 level 1 Level 4 Level 8 40 45 50 55 60 65 70 75 80 85 Z axis (mm)
Lateral Pressure Distribution Normalized Pressure 1.00 0.75 0.50 0.25 Needle Capsule Fiber Optic-1 Fiber Optic -2 0.00-2.00-1.00 0.00 3.3-MHz 1.00 HIFU 2.00 Lateral distance (mm) Normalized Pressure (db) 0-6 -12-18 -24-30 -36-42 -2.00-1.00 0.00 1.00 2.00 Lateral distance (mm) Spatial Averaging Effect (IEC 62127-1) Hydrophone radius wavelength/2.5 (almost satisfied) Pressure hydrophone radius 1dB decrease from peak (satisfied)
Complex Deconvolution pressure(t) Pressure(f) Hydrophone M(f) voltage(t) Voltage(f) Pressure(f) = Voltage(f) / M(f) IEC 62127-1 Hydrophone deconvolution Frequency Domain Time Domain pressure(t) = -1 [Pressure(f)] = -1 [Voltage(f) / M(f)] Complex operation -----Original Deconvolution
Pressure comparison- waveform Fiber Optics 1 level1 level2 20 level3 level4 16 level5 level6 level7 12 level8 Pressure (MPa) 8 4 0 4 8 48 48.5 49 49.5 50 Time (um) 50.5 51 51.5 52
Pressure (Mpa) 3 2 1 0-1 -2 Pressure comparison Output level 1 Capsule Needle FO-1 FO-2 0 0.1 0.2 0.3 0.4 Time (us) Pressure (Mpa) Pressure (Mpa) 9 7 5 3 1-1 -3-5 -7 22 17 12 7 2-3 -8-13 Output level 4 0 0.1 0.2 0.3 Time (μs) Needle FO-1 Capsule Needle FO-1 0.00000 0.10000 0.20000 0.30000 Output level 8 Time (us)
Pressure comparison 3.3 MHz outputs P+, Compressional pressure (MPa) P-, Rarefactional pressure (MPa) Capsule Needle FO-1 FO-2 Variation % Capsule Needle FO-1 FO-2 Variation % Level 1 2.02 1.95 1.46 1.92 12.00% 1.89 1.88 1.42 1.64 11.32% Level 2 4.38 4.03 2.63 4.02 17.78% 3.70 3.55 2.40 3.17 15.67% Level 3 6.24 6.30 4.45 6.04 13.23% 4.08 5.00 3.49 4.70 13.47% Level 4 9.62 8.17 6.15 -- 17.86% 6.26 4.64 4.37 -- 16.45% Level 5 13.42 11.47 8.67 -- 17.43% 6.66 6.12 5.10 -- 10.88% Level 6 -- 14.87 11.53 -- 12.65% -- 7.19 5.85 -- 10.30% Level 7 -- 18.73 16.14 -- 7.43% -- 8.86 6.68 -- 14.04% Level 8 -- 21.52 20.65 -- 2.08% -- 8.57 7.20 -- 8.68% Medium nonlinearity was presented at high output levels. Similar measurement variation across most output levels. Rarefactional pressure P- varied more than the compressional pressure P+ (15% vs 7%).
Nonlinear Propagation Parameters (local distortion parameter) Zp is the axial distance of the point of interest to the source aperture plane; Pm is the mean-peak-cycle acoustic pressure β is the nonlinearity parameter (β = 3,5 for water at 20 C); ω is the angular frequency; Fa is the local area factor (beam shape). * IEC 62556 (2014): Specification and measurement of field parameters for high intensity therapeutic ultrasound (HITU) transducers and systems Nonlinear Propagation 3.3 MHz outputs Capsule Needle FO-1 FO-2 Level 1 0.2391 0.2632 0.1763 0.2177 Level 2 0.4936 0.5208 0.3078 0.4387 Level 3 0.6308 0.7756 0.4852 0.6563 Level 4 0.9707 0.8794 0.6423 -- Level 5 1.2274 1.2074 0.8414 -- Level 6 -- 1.5137 1.0618 -- Level 7 -- 1.8935 1.3944 -- Level 8 -- 2.0650 1.7015 -- www.wikibooks.org
Pressure (Mpa) 22 17 12 7 2-3 -8-13 0.00 0.10 0.20 0.30 F F T Output level 8 Time (us) Needle FO-1 How nonlinear? Nonlinearity Needle Hydrophone: σ q = 2.0 FO-1 Hydrophone: σ q = 1.7 According to IEC 62556 (HITU Measurement Standard), σ m < 0.5 is considered quasi-linear condition and applicable to this standard. O FO-1 ----- Needle O FO-1 ----- Needle 10
Spatial Averaging Correction IEC-62556 Specification and measurement of field parameters for high intensity therapeutic ultrasound (HITU) transducers and systems When the hydrophone is translated from the position of maximum received signal in any direction normal to the beam axis by an amount equal to the effective hydrophone radius element, the decrease in signal should be less than 1 db. If this is not the case, corrections for spatial averaging should be made. See Annex E of IEC 62127-1:2007 The correction factor, Ksa, at the center of the beam is: Ksa = (3 β)/2=1.125 Where β = 0.75 (signal at one hydrophone radius from the axis)/(signal on axis) 3.3 MHz P+ (MPa) P_ (MPa) Needle Original Corrected Original Corrected Level 1 1.95 2.198 1.88 2.110 Level 2 4.03 4.534 3.55 3.990 Level 3 6.30 7.075 5.00 5.617 Level 4 8.17 9.182 4.64 5.210 Level 5 11.47 12.888 6.12 6.872 Level 6 14.87 16.700 7.19 8.072 Level 7 18.73 21.035 8.86 9.952 Level 8 21.52 24.171 8.57 9.623
HIFU Hydrophone Test Summary HIFU beam profile and pressure amplitude comparison by 4 medical ultrasound hydrophones Good agreement for P+ and P- among hydrophones Hydrophone calibration accuracy spatial averaging signal deconvolution and nonlinearity electronics/system/operator repeatability IEC 62556 (New HITU hydrophone standard) does not cover high amplitude focused field measurement. Higher frequency measurement is underway. The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the FDA.