Acoustic emission transducers--development of a facility for traceable out-of-plane displacement calibration.

Acoustic emission (AE) is a widely used technique that has been employed for the integrity testing of a range of vessels and structures for many years. The last decade has seen advances in signal processing, such that the reliability of AE technology is now being recognised by a wider range of industries. Furthermore, the need for quality control at the manufacturing stage, and requirements of in-service testing, is encouraging the issue of traceable measurements to be addressed. Currently, no independent calibration service for acoustic emission transducers is available within Europe. The UKs National Physical Laboratory (NPL) is undertaking work to develop a measurement facility for the traceable calibration of AE sensors. Such calibrations can contribute to greater acceptance of AE techniques in general, by meeting quality system and other traceability requirements. In this paper the key issues surrounding the development of such a facility are reviewed, including the need to establish repeatable AE sources, select suitable test blocks and to understand the limitations imposed by AE sensors themselves. To provide an absolute measurement of the displacement on the surface of a test block, laser interferometry is employed. In this way the output voltage of an AE sensor can be directly related to the displacement detected at the block surface. A possible calibration methodology is discussed and preliminary calibration results are presented for a commercially available AE sensor, showing its response to longitudinal wave modes.

The impact of piezoelectric PVDF on medical ultrasound exposure measurements, standards, and regulations.

This paper describes the development of PVDF hydrophones for characterizing medical ultrasound fields. The polymer hydrophone approaches that have resulted from this work are discussed, with emphasis given to the spot-poled membrane design that has become the de facto reference device for these measurements. The various national and international standards and regulations that have followed from the successful use of PVDF hydrophones also are summarized. The works discussed encompass polymer-based hydrophones designed primarily for diagnostic and lithotripsy exposure measurements, both in water and in vivo. The advent of these hydrophones has made possible accurate and reliable measurements of exposure levels encountered in medical ultrasound and, thus, has allowed questions of ultrasound bioeffects and device safety to be addressed in a consistent and scientifically sound manner.

Donor selection for xenotransplantation: detection of Galalpha1-3Gal on different porcine organs.

The expression of the major porcine xenoantigens (Galalpha1-3Gal) in different tissues varies between species. The selection of suitable donors and the interpretation of studies which attempt to prevent hyperacute rejection are dependent on donor expression of Galalpha1-3Gal. Screening of large number of animals to find potential Galalpha1-3Gal negative donors requires a robust, tissue-based and practical method of assessing Galalpha1-3Gal expression. In this study, we have assessed the expression of Galalpha1-3Gal in a variety of pig organs using anti Galalpha1-3Gal antibody. Biopsies of heart, kidney, ear and tail were obtained from 20 outbred pigs. Biopsies were fixed in formalin and stained with a human anti Galalpha1-3Gal antibody obtained from pooled human AB serum passed down a Galalpha1-3Gal immunoadsorbent column. Tissue from all 4 organs from all 20 pigs expressed Galalpha13Gal. This study shows that detection of Galalpha1-3Gal on an ear or tail biopsy is a simple but very reliable method for assessing Galalpha1-3Gal expression on the heart and kidney and facilitates donor selection for xenotransplantation.

Proposed standard thermal test object for medical ultrasound.

A general design for a thermal test object (TTO) is proposed. A number of novel features make the design particularly suitable for use as a standardised device for assessing the heating capability of diagnostic ultrasound beams. To assess performance, soft-tissue TTOs have been made containing thin-film thermocouples sandwiched between discs of tissue-mimicking gel. Installed in an appropriate measurement system, these TTOs exhibit excellent thermal and spatial resolution, allowing the ultrasound beam to be located rapidly and reproducibly. The measured temperature rise after 3 minutes of heating has been compared with theoretical predictions based on measured pressure distributions, and agreement is within 10%. Other studies have shown that soft-tissue- and bone-mimicking TTOs can be used to evaluate a wide range of ultrasound fields and that different physical tissue models can be simulated. It is concluded that this design would be suitable for providing reference assessments of the thermal hazard posed by diagnostic ultrasound under standardised conditions.

The NPL Doppler fetal heart beat detector test facility.

There are many thousands of Doppler fetal heart beat detectors in medical use and many different detector manufacturers but, until recently, there has been no well-defined quantitative method for measuring the sensitivity of these detectors and, therefore, no way of directly comparing their technical performance under standardised test conditions. At NPL, we have developed a reference test facility for measuring detector sensitivity to meet the needs of manufacturers, and to comply with the requirements of an international standard (IEC 1995) that defines methods of measurement of the sensitivity of fetal heart beat detectors. The test facility has primarily been developed for detectors operating at a transmitted frequency of 2 MHz and with Doppler shifts of up to 1 kHz. The detectors are tested by directing the ultrasound beam at a small moving target being driven at a constant velocity, and then monitoring the output signal from the detector, which will be at the Doppler shift frequency. To determine the sensitivity, attenuators are inserted into the beam until the output signal is reduced to 6 dB above the noise level. The sensitivity is calculated by adding the final signal level above the noise to the total insertion loss of the attenuators in the ultrasound path and the reflection loss of the target. A crucial aspect of this calculation is the knowledge of the target strengths and characteristics. This has already been extensively studied (Preston and Bond 1997) over the frequency range of interest. The NPL test facility developed for undertaking the sensitivity measurements is described, including an assessment of the uncertainties in such a measurement and solutions to problems encountered along the way.

An experimental study of the reflection from spherical and flat ended cylindrical targets suitable for fetal Doppler performance assessment.

The performance of small-diameter targets suitable for use as oscillating targets for testing the sensitivity of Doppler fetal heartbeat detectors has been systematically studied. Experimental results are presented in the 1.6-3.0 MHz frequency range for the plane-wave reflection loss for a total of 16 targets: spherical balls made of stainless steel; hemispherical-ended rods made of PTFE; and flat-ended rods made of stainless steel, PTFE, polycarbonate, and tungsten carbide. Results show that the fine-structure variation of reflection loss with frequency is greatest in the case of spherical ball targets and least for flat-ended targets. It has been shown that, providing care is taken during manufacture, the reflection loss from a flat-ended target can be predicted using a simple theory based on a plane disc reflector. Tungsten carbide targets consisting of a long rod with a diameter of 1.6 mm tapered down to a cylindrical flat end with a diameter of 0.4, 0.5, and 0.6 mm have been shown to provide reflection losses of between 60 and 40 dB, and to have a smooth variation of reflection loss with frequency. They can also be manufactured in a form that allows no significant interference from the supporting structure and, therefore, are ideal targets to meet the requirements of International Electrotechnical Commission 1266:1995.

Perfusion corrections for ultrasonic heating in nonperfused media.

The linear bio-heat transfer equation proposed by Pennes is widely used to predict temperature rise in perfused media. In this article, a mathematical relationship between the predicted temperature increase under perfused and nonperfused conditions is derived based on an analysis of the Pennes equation. The perfused temperature at time t and position r, Tperf(r,t) can be calculated from the unperfused temperature history Tunperf(r,t) and the time constant for perfusion, tau: [formula: see text] When the full nonperfused temperature history is not available, a simpler approximate method of estimating the perfused temperature is also suggested, requiring only a knowledge of the time constant for perfusion. Results are given showing the effects of perfusion after different exposure times for a range of beamwidths and perfusion time constants.

The decision to enter a medical specialty: timing and stability.

The timing and stability of the decision to enter a medical specialty were examined for one class of medical students. Students were asked to predict specialty choices for themselves on six occasions from orientation day in year 1 to January of the senior year. Choices were compared to actual National Residency Matching Programme results. Forty-five per cent predicted their ultimate specialty choice at orientation, and 69% predicted their ultimate choice by the end of the second year. Specialty choices are made early, and are more stable and accurate than the previous literature has suggested. Variations in timing among the specialties are described, and implications for medical education are discussed.

Prediction of in situ exposure to ultrasound: an acoustical attenuation method.

The prediction of the acoustic levels occurring in a patient during ultrasound examination is important for the assessment of equipment safety. While considerable effort has been devoted to theoretical methods of predicting exposure levels, there is a need to develop simple experimental methods which are universally applicable to the wide range of ultrasonic fields generated by medical ultrasonic equipment. This article outlines a number of methods that have been proposed and explores in detail a new experimental method based on the use of acoustical attenuators, made of low-density polyethylene, which are placed in the ultrasound beam between the transducer and the measuring hydrophone. Measurements of important acoustical quantities have been made using a measurement system based on a multielement hydrophone and comparisons made between this new method and other methods such as electrical attenuation and linear derating theory. The acoustical attenuation technique has been systematically studied, and results are compared with reference levels obtained using tissue-mimicking material. A procedure involving making measurements with the attenuator placed first at the face of the transducer and then at the hydrophone has been developed. By taking the mean value of these two measurements, it has been shown that this new method can simulate within +/- 10% the acoustic levels obtained using the reference. It is concluded that it could be the basis of a universally applicable method.

Prediction of in situ exposure to ultrasound: a proposed standard experimental method.

This article presents results for the prediction of in situ exposure levels using a new experimental technique based on the use of acoustical attenuators. These are made of low-density polyethylene and are placed in the ultrasound beam between the transducer face and the measuring hydrophone. The method has been tested for a number of different ultrasonic fields including those generated by diagnostic medical ultrasonic equipment. Using a tissue-mimicking material as a reference, it is shown that it is possible to use this new method to simulate acoustic pressure levels to within +/- 10% and acoustic intensity and power levels within +/- 20%. The method is proposed as the basis for a standard test method with wide applicability.

Calibration of medical ultrasonic equipment-procedures and accuracy assessment.

A beam-plotting facility has been developed to provide a reference measurement system for determining the acoustic output of medical ultrasonic equipment. It consists of two coordinate-positioning systems controlled by stepping motors and a minicomputer. One system is used for holding and manipulating an ultrasonic transducer and the other for a hydrophone. A membrane hydrophone made from polyvinylidene fluoride of 9-mum thickness with an active element of 0.5-mm diameter is used for most measurements. The hydrophone is connected to an amplifier and digitizer, also controlled by the minicomputer, and the whole system has a measurement bandwidth of 75 MHz (-3 dB). A detailed description of this system is given together with a full assessment of measurement uncertainties and the methods used to correct for the effects of nonlinear distortion and spatial averaging. Typical overall uncertainties (95% confidence) for the determination of the peak-positive acoustic pressure, peak-negative acoustic pressure, spatial-peak pulse-average intensity and spatial-peak temporal-average intensity are +/-13%, +/-8%, +/-17%, and +/-23%, respectively.

The NPL ultrasound beam calibrator.

The calibrator consists of a multielement membrane hydrophone made from polyvinylidene fluoride which is mounted in a versatile test-tank and linked to a fast data acquisition and presentation system. Acoustic output parameters of both simple medical ultrasonic systems and complex automatic scanners operating in imaging modes can be determined in a straightforward manner. Designed as a comprehensive measurement system, with the calibration traceable to United Kingdom national standards and the software configured to conform with written standards, it provides a convenient and rapid method of determining the acoustic output for both manufacturers and users of medical ultrasonic equipment. A detailed description of all aspects of the beam calibrator is given. The computer operating system is described, followed by an analysis of the postprocessing calculations which are applied to the acquired waveform data for the determination of acoustical parameters. Finally, a full assessment of the sources of uncertainty in the measurements is given, together with a comparison of results obtained using a reference measurement system.

Interlaboratory comparison of hydrophone calibrations.

The results of an interlaboratory comparison of hydrophone calibration techniques in the frequency range 1-10 MHz are reported. Two membrane hydrophones were calculated to six laboratories, and each laboratory determined the end-of-cable loaded sensitivities using their normal calibration methods; these included optical interferometry, planar scanning, reciprocity combined with time-delay spectrometry, and suspended-sphere radiometry. After converting the results to end-of-cable open-circuit sensitivities, in most cases agreement between the various values was within +/-10% at all frequencies.

Pressure waveforms generated by a Dornier extra-corporeal shock-wave lithotripter.

Pressure waveforms in the acoustic field generated by a Dornier (HM3) shock-wave lithotripter have been measured using a bilaminar shielded PVDF membrane hydrophone in water. Using these waveforms, values of the peak-positive (p+) and peak-negative pressure (p-) at various positions in the field have been estimated. At the focus, p+ is 38.6 MPa (standard deviation = 9.0 MPa) and p- is 10.1 MPa (standard deviation = 1.0 MPa) at 20 kV discharge potential and an electrode separation in the range 1.3 to 2.4 mm. The peak-positive pressure is found to fall to 50% (-6 dB level) at about 60 mm either side of the focus on the major axis of the reflector and on a 10 mm radius circle around the focus in the focal plane. A shot-to-shot variation of +/- 25% in p+ is attributed to the inherent variability of the electrical discharge which may result in changes in the exact position and strength of the acoustic field. The results reported are considered to be more accurate than those of previous measurements due to the relatively flat frequency response of this type of hydrophone.

A comparison of the AIUM/NEMA, IEC and FDA (1980) definitions of various acoustic output parameters for ultrasonic transducers.

Comparison is made between the values obtained for the acoustic output from different types of medical ultrasonic equipment when determined according to the definitions given in different documents. In particular, the American Institute of Ultrasound in Medicine with the National Electrical Manufacturers Association Safety Standard for Diagnostic Ultrasound Equipment, the U.S. Food and Drug Administration Diagnostic Ultrasound Reporting Guide (1980) and the proposed International Electrotechnical Commission standard are compared for pulse-echo and Doppler equipment. Differences of up to 40% in the spatial-average temporal-average and spatial-peak pulse-average intensities are shown to be a result of the different definitions of beam area and pulse duration.