Transducer Impulse Response Research Articles

Spatial Response Identification Enables Robust Experimental Ultrasound Computed Tomography.

Ultrasound computed tomography techniques have the potential to provide clinicians with 3-D, quantitative and high-resolution information of both soft and hard tissues such as the breast or the adult human brain. Their practical application requires accurate modeling of the acquisition setup: the spatial location, orientation, and impulse response (IR) of each ultrasound transducer. However, the existing calibration methods fail to accurately characterize these transducers unless their size can be considered negligible when compared with the dominant wavelength, which reduces signal-to-noise ratios below usable levels in the presence of high-contrast tissues such as the skull. In this article, we introduce a methodology that can simultaneously estimate the location, orientation, and IR of the ultrasound transducers in a single calibration. We do this by extending spatial response identification (SRI), an algorithm that we have recently proposed to estimate transducer IRs. Our proposed methodology replaces the transducers in the acquisition device with a surrogate model whose effective response matches the experimental data by fitting a numerical model of wave propagation. This results in a flexible and robust calibration procedure that can accurately predict the behavior of the ultrasound acquisition device without ever having to know where the real transducers are or their individual IR. Experimental results using a ring acquisition system show that SRI produces calibrations of significantly higher quality than standard methodologies across all transducers, both in transmission and in reception. Experimental full-waveform inversion (FWI) reconstructions of a tissue-mimicking phantom demonstrate that SRI generates more accurate reconstructions than those produced with standard calibration techniques.

Superiorized Photo-Acoustic Non-NEgative Reconstruction (SPANNER) for Clinical Photoacoustic Imaging.

Photoacoustic (PA) imaging can revolutionize medical ultrasound by augmenting it with molecular information. However, clinical translation of PA imaging remains a challenge due to the limited viewing angles and imaging depth. Described here is a new robust algorithm called Superiorized Photo-Acoustic Non-NEgative Reconstruction (SPANNER), designed to reconstruct PA images in real-time and to address the artifacts associated with limited viewing angles and imaging depth. The method utilizes precise forward modeling of the PA propagation and reception of signals while accounting for the effects of acoustic absorption, element size, shape, and sensitivity, as well as the transducer's impulse response and directivity pattern. A fast superiorized conjugate gradient algorithm is used for inversion. SPANNER is compared to three reconstruction algorithms: delay-and-sum (DAS), universal back-projection (UBP), and model-based reconstruction (MBR). All four algorithms are applied to both simulations and experimental data acquired from tissue-mimicking phantoms, ex vivo tissue samples, and in vivo imaging of the prostates in patients. Simulations and phantom experiments highlight the ability of SPANNER to improve contrast to background ratio by up to 20 dB compared to all other algorithms, as well as a 3-fold increase in axial resolution compared to DAS and UBP. Applying SPANNER on contrast-enhanced PA images acquired from prostate cancer patients yielded a statistically significant difference before and after contrast agent administration, while the other three image reconstruction methods did not, thus highlighting SPANNER's performance in differentiating intrinsic from extrinsic PA signals and its ability to quantify PA signals from the contrast agent more accurately.

Broad bandwidth air-coupled micromachined ultrasonic transducers for gas sensing

The present work aims to develop ultra-wide bandwidth air-coupled capacitive micromachined ultrasonic transducers (CMUTs) for binary gas mixture analysis. The detection principle is based on time-of-flight (ToF) measurements, in order to monitor gas ultrasound velocity variations. To perform such measurements, CMUTs were especially designed to work out of resonance mode, like a microphone. The chosen membrane size is 32 × 32 µm2 and gap height is 250 nm. The resonance frequency and collapse voltage were found at 8 MHz and 58 V respectively. As mentioned, the CMUTs were exploited in quasi-static operating mode, in a very low frequency band, from 1 MHz to 1.5 MHz frequencies. The transducer impulse response was characterised, and a −6 dB relative fractional frequency bandwidth (FBW) higher than 100% was measured, enabling to use CMUT for the targeted application. Additionally, a measuring cell has been designed to hold the fabricated CMUT emitter and receiver prototypes facing each other. The volume inside the cell was kept lower than 3 mL and the surface of emitter/receiver was 1.6 × 8 mm2. To validate the general principle of the proposed technique, two binary gas mixtures of CO2/N2 and H2/N2, with varying concentrations, have been tested. The results are very promising with a measured limit of detection (LOD) of 0.3% for CO2 in N2 and 0.15% for H2 in N2.

Individual transducer impulse response characterization method to improve image quality of array-based handheld optoacoustic tomography.

The physical properties of each transducer element play a vital role in the quality of images generated in optoacoustic (photoacoustic) tomography using transducer arrays. Thorough experimental characterization of such systems is often laborious and impractical. A shortcoming of the existing impulse response correction methods, however, is the assumption that all transducers in the array are identical and therefore share one electrical impulse response (EIR). In practice, the EIRs of the transducer elements in the array vary, and the effect of this element-to-element variability on image quality has not been investigated so far, to the best of our knowledge. We hereby propose a robust EIR derivation for individual transducer elements in an array using sparse measurements of the total impulse response (TIR) and by solving the linear system for temporal convolution. Thereafter, we combine a simulated spatial impulse response with the derived individual EIRs to obtain a full characterization of the TIR, which we call individual synthetic TIR. Correcting for individual transducer responses, we demonstrate significant improvement in isotropic resolution, which further enhances the clinical potential of array-based handheld transducers.

Detecting Relative Amplitude of IR Signals with Active Sensors and Its Application to a Positioning System

Nowadays, there is an increasing interest in smart systems, e.g., smart metering or smart spaces, for which active sensing plays an important role. In such systems, the sample or environment to be measured is irradiated with a signal (acoustic, infrared, radio-frequency…) and some of their features are determined from the transmitted or reflected part of the original signal. In this work, infrared (IR) signals are emitted from different sources (four in this case) and received by a unique quadrature angular diversity aperture (QADA) sensor. A code division multiple access (CDMA) technique is used to deal with the simultaneous transmission of all the signals and their separation (depending on the source) at the receiver’s processing stage. Furthermore, the use of correlation techniques allows the receiver to determine the amount of energy received from each transmitter, by quantifying the main correlation peaks. This technique can be used in any system requiring active sensing; in the particular case of the IR positioning system presented here, the relative amplitudes of those peaks are used to determine the central incidence point of the light from each emitter on the QADA. The proposal tackles the typical phenomena, such as distortions caused by the transducer impulse response, the near-far effect in CDMA-based systems, multipath transmissions, the correlation degradation from non-coherent demodulations, etc. Finally, for each emitter, the angle of incidence on the QADA receiver is estimated, assuming that it is on a horizontal plane, although with any rotation on the vertical axis Z. With the estimated angles and the known positions of the LED emitters, the position (x, y, z) of the receiver is determined. The system is validated at different positions in a volume of 3 × 3 × 3.4 m3 obtaining average errors of 7.1, 5.4, and 47.3 cm in the X, Y and Z axes, respectively.

Transducer impulse response correction for a deconvolution derived ultrasound transit time spectrum

Propagation of ultrasound through a complex composite sample may exhibit phase interference between two or more sonic-rays if differences in transit time are less than the pulse length. The transit time spectrum of a test sample, equivalent to its impulse response, was derived through active-set deconvolution of ultrasound signals with, and without, the test sample. The aim of this study was to test the hypothesis that in cases where only the transmit ultrasound transducer’s digitally-coded excitation signal is available, hence not the input ultrasound signal without the test sample, incorporation of the transducer impulse response may increase both accuracy and precision of ultrasound transit time spectroscopy. A digital 1 MHz sinusoid signal was used to create an ultrasound pulse that was propagated through a 5 step-wedge acrylic sample immersed in water. Transit time spectra were obtained through deconvolution utilising an ultrasound input signal, along with a digital input signal, with and without incorporation of the transducer impulse response. Incorporation of the transducer impulse response reduced a quantitative measure of noise-to-signal ratio by a factor of 12.The paper has demonstrated the potential for increased accuracy and precision of transit time spectroscopy when the transducer impulse response is incorporated within active-set deconvolution analysis.

Width-modulated square-wave pulses for ultrasound applications

A method of output pressure control for ultrasound transducers using switched excitation is described. The method generates width-modulated square-wave pulse sequences that are suitable for driving ultrasound transducers using MOSFETs or similar devices. Sequences are encoded using an optimized level-shifted, carrier-comparison, pulse-width modulation (PWM) strategy derived from existing PWM theory, and modified specifically for ultrasound applications. The modifications are: a reduction in carrier frequency so that the smallest number of pulses are generated and minimal switching is necessary; alteration of a linear carrier form to follow a trigonometric relationship in accordance with the expected fundamental output; and application of frequency modulation to the carrier when generating frequency-modulated, amplitude- tapered signals. The PWM method permits control of output pressure for arbitrary waveform sequences at diagnostic frequencies (approximately 5 MHz) when sampled at 100 MHz, and is applicable to pulse shaping and array apodization. Arbitrary waveform generation capability is demonstrated in simulation using convolution with a transducer's impulse response, and experimentally with hydrophone measurement. Benefits in coded imaging are demonstrated when compared with fixed-width square-wave (pseudo-chirp) excitation in coded imaging, including reduction in image artifacts and peak side-lobe levels for two cases, showing 10 and 8 dB reduction in peak side-lobe level experimentally, compared with 11 and 7 dB reduction in simulation. In all cases, the experimental observations correlate strongly with simulated data.

Shifted-Modified Chebyshev Filters

This paper introduces a new type of filter approximation method that utilizes shifted-modified Chebyshev filters. Construction of the new filters involves the use of shifted-modified Chebyshev polynomials that are formed using the roots of conventional Chebyshev polynomials. The study also includes 2 tables containing the shifted-modified Chebyshev polynomials and the normalized element values for the low-pass prototype filters up to degree 6. The transducer power gain, group delay, and impulse and step responses of the proposed filters are compared with those of the Butterworth and Chebyshev filters, and a sixth-order filter design example is presented to illustrate the implementation of the new method.

Investigation of iterative image reconstruction in three-dimensional optoacoustic tomography

Iterative image reconstruction algorithms for optoacoustic tomography (OAT), also known as photoacoustic tomography, have the ability to improve image quality over analytic algorithms due to their ability to incorporate accurate models of the imaging physics, instrument response and measurement noise. However, to date, there have been few reported attempts to employ advanced iterative image reconstruction algorithms for improving image quality in three-dimensional (3D) OAT. In this work, we implement and investigate two iterative image reconstruction methods for use with a 3D OAT small animal imager: namely a penalized least-squares (PLS) method employing a quadratic smoothness penalty and a PLS method employing a total variation norm penalty. The reconstruction algorithms employ accurate models of the ultrasonic transducer impulse responses. Experimental data sets are employed to compare the performances of the iterative reconstruction algorithms to that of a 3D filtered backprojection (FBP) algorithm. By the use of quantitative measures of image quality, we demonstrate that the iterative reconstruction algorithms can mitigate image artifacts and preserve spatial resolution more effectively than FBP algorithms. These features suggest that the use of advanced image reconstruction algorithms can improve the effectiveness of 3D OAT while reducing the amount of data required for biomedical applications.

Sparse Deconvolution Methods for Ultrasonic NDT

In this work we present two sparse deconvolution methods for nondestructive testing. The first method is a special matching pursuit (MP) algorithm in order to deconvolve the mixed data (signal and noise), and thus to remove the unwanted noise. The second method is based on the approximate Prony method (APM). Both methods employ the sparsity assumption about the measured ultrasonic signal as prior knowledge. The MP algorithm is used to derive a sparse representation of the measured data by a deconvolution and subtraction scheme. An orthogonal variant of the algorithm (OMP) is presented as well. The APM technique also relies on the assumption that the desired signals are sparse linear combinations of (reflections of) the transmitted pulse. For blind deconvolution, where the transducer impulse response is unknown, we offer a general Gaussian echo model whose parameters can be iteratively adjusted to the real measurements. Several test results show that the methods work well even for high noise levels. Further, an outlook for possible applications of these deconvolution methods is given.

Modeling transducer impulse responses for predicting calibrated pressure pulses with the ultrasound simulation program Field II

Field II is a simulation software capable of predicting the field pressure in front of transducers having any complicated geometry. A calibrated prediction with this program is, however, dependent on an exact voltage-to-surface acceleration impulse response of the transducer. Such impulse response is not calculated by Field II. This work investigates the usability of combining a one-dimensional multilayer transducer modeling principle with the Field II software. Multilayer here refers to a transducer composed of several material layers. Measurements of pressure and current from Pz27 piezoceramic disks as well as pressure and intensity measurements in front of a 128 element commercial convex medical transducer are compared to the simulations. Results show that the models can predict the pressure from the piezoceramic disks with a root mean square (rms) error of 11.2% to 36.2% with a 2 dB amplitude decrease. The current through the external driving circuits are predicted within 8.6% to 36% rms error. Prediction errors of 30% and in the range of 5.8%-19.9% for the pressure and the intensity, respectively, are found when simulating the commercial transducer. It is concluded that the multilayer transducer model and the Field II software in combination give good agreement with measurements.

Calibration of Field II using a convex ultrasound transducer

Field II is an ultrasound simulation program capable of simulating the pressure scattering from inhomogeneous tissue. The simulations are based on a convolution between spatial impulse responses from the field in front of the transducer and the volt-to-surface acceleration impulse response of the transducer. For such simulations to reflect actual measured intensities and pressure levels, the transducer impulse response is to be known. This work presents results of combining a modified form of a 1D linear transducer model originally suggested by Willatzen together with the Field II program to calibrate the pressure and intensity simulations of a 128 element convex medical transducer with elevation focus at 70 mm. The simulations are compared to pressure measurements from a transducer driven at 4.0 MHz using a research scanner with a commercial transducer amplifier from BK Medical (Herlev, Denmark). As input waveform to the Field II model we measured the output voltage of the research amplifier. The peak voltage was limited to 31 V to avoid too high non-linear effects. We measured the hydrophone output from three transducer front elements by averaging 40 shoot sequences on each element using a remotely controlled Agilent MSO6014A oscilloscope. The pressure along the center line in a distance of 33 mm, 72 mm, and 112 mm from each element was measured as well as the intensity in the elevation plane in each depth.Results show that the 1D modeling of the linear transducer impulse response predicts the pulse waveforms satisfying. We show a root mean square error of approximately 26% on the pressure prediction and a root mean square error on the intensity prediction from 5.8–16.2% from 72 mm to 112 mm, and an average of 15.4% at 33 mm, when a 1.7 dB amplitude correction is made.

Use of modulated excitation signals in medical ultrasound. Part II: design and performance for medical imaging applications

In the first paper, the superiority of linear FM signals was shown in terms of signal-to-noise ratio and robustness to tissue attenuation. This second paper in the series of three papers on the application of coded excitation signals in medical ultrasound presents design methods of linear FM signals and mismatched filters, in order to meet the higher demands on resolution in ultrasound imaging. It is shown that for the small time-bandwidth (TB) products available in ultrasound, the rectangular spectrum approximation is not valid, which reduces the effectiveness of weighting. Additionally, the distant range sidelobes are associated with the ripples of the spectrum amplitude and, thus, cannot be removed by weighting. Ripple reduction is achieved through amplitude or phase predistortion of the transmitted signals. Mismatched filters are designed to efficiently use the available bandwidth and at the same time to be insensitive to the transducer's impulse response. With these techniques, temporal sidelobes are kept below 60 to 100 dB, image contrast is improved by reducing the energy within the sidelobe region, and axial resolution is preserved. The method is evaluated first for resolution performance and axial sidelobes through simulations with the program Field II. A coded excitation ultrasound imaging system based on a commercial scanner and a 4 MHz probe driven by coded sequences is presented and used for the clinical evaluation of the coded excitation/compression scheme. The clinical images show a significant improvement in penetration depth and contrast, while they preserve both axial and lateral resolution. At the maximum acquisition depth of 15 cm, there is an improvement of more than 10 dB in the signal-to-noise ratio of the images. The paper also presents acquired images, using complementary Golay codes, that show the deleterious effects of attenuation on binary codes when processed with a matched filter, also confirmed by presented simulated images.

Waveform mapping of piezoelectric transducer impulse responses for multi-transducer pattern recognition-based ultrasonic NDE systems : Bartos, A.L.; Gill, T.J.; Lyon, T. Materials Science Forum, Vols. 210–213, Part 2, pp. 863–870 (1996). (Proceedings of the 7th International Symposium on Nondestructive Characterization of Materials, Prague (Czech Republic), 19–22 Jun. 1995. Edited by A.L. Bartos, R.E. Green and C.O. Ruud. Transtec Publications (1996))

Waveform mapping of piezoelectric transducer impulse responses for multi-transducer pattern recognition-based ultrasonic NDE systems : Bartos, A.L.; Gill, T.J.; Lyon, T. Materials Science Forum, Vols. 210–213, Part 2, pp. 863–870 (1996). (Proceedings of the 7th International Symposium on Nondestructive Characterization of Materials, Prague (Czech Republic), 19–22 Jun. 1995. Edited by A.L. Bartos, R.E. Green and C.O. Ruud. Transtec Publications (1996))

Sensitivity of a spherically focused transducer to focal length and aperture

A method is presented to calculate the impulse respone of a spherically focused transducer which generates a signal and then receives the reflection of the signal from a rigid surface. Results of calculations are shown in order to illustrate the effects of varying the focal length and the aperture. These results suggest a method for empirical determination of effective values of the governing parameters of the transducer impulse response.

Acoustic transducer using phase shift interference

A transducer device includes delay sections for creating a phase differential of acoustic waves. The delay sections are spaced apart by sections having an absence of delay. In a preferred embodiment, the phase differential is 180°, so that constructive and destructive interference of pressure waves function occurs to reduce the ringdown time of the transducer device. In the preferred embodiment, the array of delay sections is at the back surface of a piezoelectric element. However, delay sections may be at the front, radiating surface of the piezoelectric element for control of the shape of emitted pulses. Vectorial summation of wave energy cancels unwanted energy that is present as a result of reverberations within the transducer device. Alternative delay structures or multiple delay sections can be used to control the transducer impulse response.

Waveform Mapping of Piezoelectric Transducer Impulse Responses for Multi-Transducer Pattern Recognition-Based Ultrasonic NDE Systems

Waveform Mapping of Piezoelectric Transducer Impulse Responses for Multi-Transducer Pattern Recognition-Based Ultrasonic NDE Systems

Computation of the impulse diffraction of any obstacle by impulse ray modeling—Prediction of the signal distortions

In this article, a numerical method of modeling the output signal of a transducer after reflection of the pulsed radiated wave on a reflector of arbitrary geometry is presented. This method, based on impulse ray modeling, corresponds to a discrete approach of the Kirchhoff‐radiation integral: It leads to a numerical evaluation of the diffraction effects. The overall response can be written as a time convolution of the transducer impulse response with the radiation coupling function related to the reflector geometry. This linear system approach permits an easy explanation of the signal distortion due to the reflector geometry. After a presentation of the basic principles of the method and their justification, the model is applied to several geometrical configurations: tilted plane, cylindrical, and spherical reflectors of infinite acoustical impedance. The method is extended to model a focused transducer and a solid layer reflector of finite acoustical impedance.

Time deconvolution of diffraction effects—Application to calibration and prediction of transducer waveforms

The concept of a radiation coupling function is used to write a transducer response that includes the diffraction losses. This concept leads to easy interpretations of experimental observations. In a second part a method is proposed for removing the diffraction effects from the observed transducer response. A linear system approach is taken to define the transducer output signal in terms of successive convolutions. The proposed method, based on a numerical deconvolution of the radiation filter, leads to an absolute calibration of the transducer impulse response. Deconvolved waveforms are presented for circular and annular arrays. Once the intrinsic transducer response is known, a direct convolution enables the prediction of the output signal for any distance transducer/reflector. Comparisons between predicted and observed transient waveforms are given for circular and annular arrays.

Practical aspects of ultrasound dosimetry: Hydrophone performance

Abstract Wideband piezoelectric polymer (PVDF) hydrophones are now available with well documented performance characteristics; these probes are especially well suited for the measurement and comprehensive characterisation of both pulsed and continuous wave acoustic fields generated by ultrasound diagnostic devices. The hydrophones feature flat (to within ± 1.5 dB) frequency response, voltage (pressure) sensitivity uniform well beyond 15 MHz, well-behaved, predictable directivity patterns and excellent linearity in the pressure response up to 10 8 Pa (kidney stone crushers). These characteristics make the polymer probes uniquely applicable to measurements of the acoustic output parameters of ultrasound devices, including peak instantaneous pressure amplitude, as specified in the AIUM/NEMA Standard, IEC Draft document and FDA 510(K) requirements. The hydrophones are absolutely calibrated in terms of the end-of-cable voltage sensitivity (in units V/Pa or their derivatives) with a specified terminating load. Most common designs of ultrasonic polymer probes are the needle and membrane type which when properly designed provide reference hydrophones of good spatial and temporal resolution. Absolute, (end-of-cable) voltage sensitivity calibration has been carried out in the range of intensities from 10 mW/cm 2 to 100 W/cm 2 (SPTP) using both the conventional two transducer reciprocity calibration technique and the Planar Scanning Technique. Calibration techniques indicate that overall uncertainty of typically ±1.5 dB can be obtained. The area where uncertainties mostly occur are in plane wave approximation due to transmitter and receiver size and excitation frequency, and phase variation. Combining one of the absolute calibration techniques (e.g. reciprocity and a swept frequency measurement procedure) allows hydrophones to be calibrated as a continuous function of frequency. Moreover both amplitude and phase response may be determined. The spectral analysis technique, combined with swept frequency system (TDS), constitutes a useful tool for assessment of both the amplitude only and the complex frequency response of acoustic radiators. Transducer impulse response can be determined, based on far field measurements. Transducer artifacts can be removed by inverse filtering (deconvolution) procedures for quantitative ultrasound measurements. The effect of spatial averaging was determined experimentally and it was found that it is not significant at frequencies up to 5 MHz, providing 0.5–0.6 mm diameter probes (the smallest currently available) are used. For frequencies above 5 MHz the experimental data indicate that in the “worst case” the estimated “true” spatial peak value in the measured waveform exceeded the measured value by a factor of 2, depending on frequency and the diameter of the polymer probe used. Work is in progress to estimate more accurately the effect of spatial averaging due to the finite aperture of the hydrophone probes.