Hydrophone Measurements for Biomedical Ultrasound Applications: A Review.

This article presents basic principles of hydrophone measurements, including mechanisms of action for various hydrophone designs, sensitivity and directivity calibration procedures, practical considerations for performing measurements, signal processing methods to correct for both frequency-dependent sensitivity and spatial averaging across the hydrophone sensitive element, uncertainty in hydrophone measurements, special considerations for high-intensity therapeutic ultrasound, and advice for choosing an appropriate hydrophone for a particular measurement task. Recommendations are made for information to be included in hydrophone measurement reporting.

Correction for Spatial Averaging Artifacts in Hydrophone Measurements of High-Intensity Therapeutic Ultrasound: An Inverse Filter Approach.

High-intensity therapeutic ultrasound (HITU) pressure is often measured using a hydrophone. HITU pressure waves typically contain multiple harmonics due to nonlinear propagation. As harmonic frequency increases, harmonic beamwidth decreases. For sufficiently high harmonic frequency, beamwidth may become comparable to the hydrophone effective sensitive element diameter, resulting in signal reduction due to spatial averaging. An analytic formula for a hydrophone spatial averaging filter for beams with Gaussian harmonic radial profiles was tested on HITU pressure signals generated by three transducers (1.45 MHz, F/1; 1.53 MHz, F/1.5; 3.91 MHz, F/1) with focal pressures up to 48 MPa. The HITU signals were measured using fiber-optic and needle hydrophones (nominal geometrical sensitive element diameters: 100 and [Formula: see text]). Harmonic radial profiles were measured with transverse scans in the focal plane using the fiber-optic hydrophone. Harmonic radial profiles were accurately approximated by Gaussian functions with root-mean-square (rms) differences between transverse scans and Gaussian fits less than 9% for frequencies up to approximately 50 MHz. The Gaussian harmonic beamwidth parameter σn varied with harmonic number n according to a power law, σn = σ1/nq where . RMS differences between experimental and theoretical spatial averaging filters were 11% ± 1% (1.45 MHz), 8% ± 1% (1.53 MHz), and 4% ± 1% (3.91 MHz). For the two more highly focused (F/1) transducers, the effect of spatial averaging was to underestimate peak compressional pressure (pcp), peak rarefactional pressure (prp), and pulse intensity integral (pii) by (mean ± standard deviation) (pcp: 4.9% ± 0.5%, prp: 0.4% ± 0.2%, pii: 2.9% ± 1%) and (pcp: 28.3% ± 9.6%, prp: 6% ± 2.4%, pii: 24.3% ± 6.7%) for the 100- and 400- [Formula: see text]-diameter hydrophones, respectively. These errors can be suppressed by the application of the inverse spatial averaging filter.

Directivity and Frequency-Dependent Effective Sensitive Element Size of a Reflectance-Based Fiber-Optic Hydrophone: Predictions From Theoretical Models Compared With Measurements.

The goal of this work was to measure the directivity of a reflectance-based fiber-optic hydrophone at multiple frequencies and to compare it to four theoretical models: rigid baffle (RB), rigid piston (RP), unbaffled (UB), and soft baffle (SB). The fiber had a nominal 105- [Formula: see text] diameter core and a 125- [Formula: see text] overall diameter (core + cladding). Directivity measurements were performed at 2.25, 3.5, 5, 7.5, 10, and 15 MHz from ±90° in two orthogonal planes. Effective hydrophone sensitive element radius was estimated by least-squares fitting the four models to the directivity measurements using the sensitive element radius as an adjustable parameter. Over the range from 2.25 to 15 MHz, the average magnitudes of differences between the effective and nominal sensitive element radii were 59% ± 49% (RB), 10% ± 5% (RP), 46% ± 38% (UB), and 71% ± 19% (SB). Therefore, the directivity of a reflectance-based fiber-optic hydrophone may be best estimated by the RP model.

Challenges and regulatory considerations in the acoustic measurement of high-frequency (>20 MHz) ultrasound.

This article examines the challenges associated with making acoustic output measurements at high ultrasound frequencies (>20 MHz) in the context of regulatory considerations contained in the US Food and Drug Administration industry guidance document for diagnostic ultrasound devices. Error sources in the acoustic measurement, including hydrophone calibration and spatial averaging, nonlinear distortion, and mechanical alignment, are evaluated, and the limitations of currently available acoustic measurement instruments are discussed. An uncertainty analysis of acoustic intensity and power measurements is presented, and an example uncertainty calculation is done on a hypothetical 30-MHz high-frequency ultrasound system. This analysis concludes that the estimated measurement uncertainty of the acoustic intensity is +73%/-86%, and the uncertainty in the mechanical index is +37%/-43%. These values exceed the respective levels in the Food and Drug Administration guidance document of 30% and 15%, respectively, which are more representative of the measurement uncertainty associated with characterizing lower-frequency ultrasound systems. Recommendations made for minimizing the measurement uncertainty include implementing a mechanical positioning system that has sufficient repeatability and precision, reconstructing the time-pressure waveform via deconvolution using the hydrophone frequency response, and correcting for hydrophone spatial averaging.

Variability in effective radiating area at 1 MHz affects ultrasound treatment intensity.

BACKGROUND AND PURPOSE: Previous research has indicated that not all ultrasound transducers heat at equal rates; however, the cause of this disparity is unclear. Variability in spatial average intensity (SAI) has been implicated in this disparity at 3 MHz. This variability has not been explored at 1 MHz. METHODS: Sixty-six 5-cm(2) ultrasound transducers were purchased from 6 different manufacturers. Transducers were calibrated and assessed for effective radiating area (ERA), total output power, and SAI using standardized measurement techniques. RESULTS: Total output power values fell within US Food and Drug Administration guidelines, but there were large variations in ERA. The resulting SAI values showed large deviations (-43% to +61%) from the digitally displayed value. Intra-manufacturer SAI values varied up to 53%. DISCUSSION AND CONCLUSION: Spatial average intensity can vary largely from the values displayed on these ultrasound generators; in a calibrated cohort, this difference is primarily attributable to differences in measured ERA. Patterns of SAI variability within the manufacturer at 1 MHz do not follow previous reports of variability at 3 MHz.

The role of quantitative Schlieren assessment of physiotherapy ultrasound fields in describing variations between tissue heating rates of different transducers.

Differences in tissue heating rates between ultrasound transducers have been well documented; however, comparative analysis between ultrasound fields to determine why tissue heating rates may differ is lacking. We selected three transducers from the same manufacturer with similar effective radiating area, output power, effective intensity and beam nonuniformity ratio [as defined by the FDA, 21 CFR Chap. 1, part 1,050 (10)], but markedly different Schlieren images. Each transducer was utilized to heat tissue with a standardized ultrasound application to determine whether Schlieren analysis may be useful in understanding variability in tissue heating rates. Thermocouples were inserted into the left triceps surae of 12 volunteers at a depth of 1.5 cm below one half the measured skin fold thickness (estimated average depth of the thermocouple was 1.99 +/- 0.27 cm). Each subject received one treatment from each transducer in a single session (n = 3); 3 MHz at 1.2 W/cm(2) for 8 min with a 100% duty cycle. Each transducer increased the IM temperature over time (p < 0.0001). IM temperatures were not significantly different between transducers from time zero to the fourth minute of treatment. After the fourth min, transducers B and C generated significantly higher tissue temperatures (p < 0.01). Transducer A, B and C increased IM temperature from 34.9 +/- 0.5 to 41.2 +/- 1.3 degrees C, 34.9 +/- 0.6 to 42.5 +/- 1.4 degrees C and 34.9 +/- 0.5 to 42.7 +/- 1.7 degrees C, respectively. Interestingly, transducer C emitted 22% lower output power but heated 24% higher than transducer A and our Schlieren images demonstrate that transducers B and C produced a more concentrated field compared with transducer A. The data we present here supports the general contention that a more concentrated field will heat to a higher temperature than a more disperse field, however, technical challenges in estimating output power, ERA and Schlieren analysis remain an issue.

Variability in effective radiating area and output power of new ultrasound transducers at 3 MHz.

CONTEXT: Spatial average intensity (SAI) is often used by clinicians to gauge therapeutic ultrasound dosage, yet SAI measures are not directly regulated by US Food and Drug Administration (FDA) standards. Current FDA guidelines permit a possible 50% to 150% minimum to maximum range of SAI values, potentially contributing to variability in clinical outcomes. OBJECTIVE: To measure clinical values that describe ultrasound transducers and to determine the degree of intramanufacturer and intermanufacturer variability in effective radiating area, power, and SAI when the transducer is functioning at 3 MHz. DESIGN: A descriptive and interferential approach was taken to this quasi-experimental design. SETTING: Measurement laboratory. PATIENTS OR OTHER PARTICIPANTS: Sixty-six 5-cm(2) ultrasound transducers were purchased from 6 different manufacturers. INTERVENTION(S): All transducers were calibrated and then assessed using standardized measurement techniques; SAI was normalized to account for variability in effective radiating area, resulting in an nSAI. MAIN OUTCOME MEASURE(S): Effective radiating area, power, and nSAI. RESULTS: All manufacturers with the exception of Omnisound (P = .534) showed a difference between the reported and measured effective radiating area values (P < .001). All transducers were within FDA guidelines for power (+/-20%). Chattanooga (0.85 +/- 0.05 W/cm(2)) had a lower nSAI (P < .05) than all other manufacturers functioning at 3 MHz. Intramanufacturer variability in SAI ranged from 16% to 35%, and intermanufacturer variability ranged from 22% to 61%. CONCLUSIONS: Clinicians should consider treatment values of each individual transducer, regardless of the manufacturer. In addition, clinicians should scrutinize the power calibration and recalibration record of the transducer and adjust clinical settings as needed for the desired level of heating. Our data may aid in explaining the reported heating differences among transducers from different manufacturers. Stricter FDA standards regarding effective radiating area and total power are needed, and standards regulating SAI should be established.

Analysis of effective radiating area, power, intensity, and field characteristics of ultrasound transducers.

OBJECTIVE: To characterize the ultrasound fields produced by a cohort of transducers from a single manufacturer via hydrophone and Schlieren technology. DESIGN: Descriptive study. SETTING: Measurement laboratory. PARTICIPANTS: Seven same-model ultrasound transducers from a single manufacturer. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Effective radiating area (ERA), total power, spatial average intensity (SAI), beam nonuniformity ratio (BNR), and Schlieren beam widths at 1.0 and 3.3 MHz. RESULTS: Values for ERA (1.0 MHz range, 3.62-4.38 cm(2); 3.3 MHz range, 3.74-4.76 cm(2)), total power (1.0 MHz range, 5.0-5.6 W; 3.3 MHz range, 4.7-5.7 W), SAI (1.0 MHz range, 1.2-1.4 W/cm(2); 3.3 MHz range, 1.0-1.5 W/cm(2)), and BNR (1.0 MHz range, 2.79-5.85; 3.3 MHz range, 2.51-4.56) fell within manufacturer's specifications and U.S. Food and Drug Administration (FDA) regulations. Schlieren analysis showed significantly larger beam widths at 3.3 MHz compared with 1.0 MHz and a large degree of variability in the ultrasound fields generated by the different transducers. There were no significant correlations between beam widths and ERA values. CONCLUSIONS: ERA and total power values in a test cohort exist within a range that met FDA regulations. Individual variability in ERA and total power resulted in 50% variability in SAI. This variability may help explain previous reports of heating differences between transducers.

Schlieren metrology for high frequency medical ultrasound.

The increased use of medical ultrasound above 40 MHz poses the challenge of measuring beam features that may be less than 40 microm. We have successfully used the optical Schlieren technique for transducers operating as high as 110 MHz. After a brief discussion of the technique, results are presented, including comparisons to state-of-the-art hydrophones and wire targets.

Imaging of high-intensity focused ultrasound-induced lesions in soft biological tissue using thermoacoustic tomography.

An imaging technology, thermoacoustic tomograpy (TAT), was applied to the visualization of high-intensity focused ultrasound (HIFU)-induced lesions. A single, spherically focused ultrasonic transducer, operating at a central frequency of approximately 4 MHz, was used to generate a HIFU field in fresh porcine muscle. Microwave pulses from a 3-GHz microwave generator were then employed to generate thermoacoustic sources in this tissue sample. The thermoacoustic signals were detected by an unfocused ultrasonic transducer that was scanned around the sample. To emphasize the boundaries between the lesion and its surrounding tissue, a local-tomography-type reconstruction method was applied to reconstruct the TAT images of the lesions. Good contrast was obtained between the lesion and the tissue surrounding it. Gross pathologic photographs of the tissue samples confirmed the TAT images. This work demonstrates that TAT may potentially be used to image HIFU-induced lesions in biological tissues.