Time Delay Spectrometry Research Articles

Wearable Ultrasound System Using Low-Voltage Time Delay Spectrometry for Dynamic Tissue Imaging.

Wearable ultrasound is emerging as a new paradigm of real-time imaging in freely moving humans and has wide applications from cardiovascular health monitoring to human gesture recognition. However, current wearable ultrasound devices have typically employed pulse-echo imaging which requires high excitation voltages and sampling rates, posing safety risks, and requiring specialized hardware. Our objective was to develop and evaluate a wearable ultrasound system based on time delay spectrometry (TDS) that utilizes low-voltage excitation and significantly simplified instrumentation. We developed a TDS-based ultrasound system that utilizes continuous, frequency-modulated sweeps at low excitation voltages. By mixing the transmit and receive signals, the system digitizes the ultrasound signal at audio frequency (kHz) sampling rates. Wearable ultrasound transducers were developed, and the system was characterized in terms of imaging performance, acoustic output, thermal characteristics, and applications in musculoskeletal imaging. The prototype TDS system is capable of imaging up to 6cm of depth with signal-to-noise ratio of up to 42dB at a spatial resolution of 0.33mm. Acoustic and thermal radiation measurements were within clinically safe limits for continuous ultrasound imaging. We demonstrated the ability to use a 4-channel wearable system for dynamic imaging of muscle activity. We developed a wearable ultrasound imaging system using TDS to mitigate challenges with pulse echo-based wearable ultrasound imaging systems. Our device is capable of high-resolution, dynamic imaging of deep-seated tissue structures and is safe for long-term use. This work paves the way for low-voltage wearable ultrasound imaging devices with significantly reduced hardware complexity.

My contributions to ultrasound metrology

My work in ultrasound metrology has centered around developing pre-clinical testing methods for therapeutic ultrasound devices. These test methods include radiation force balance systems and methods specifically for high-intensity therapeutic ultrasound, time-delay spectrometry methods for measuring hydrophone sensitivity, acoustic characterization of tissue-mimicking material that has been approved as a medical device development tool and a comparison protocol of computational and experimental acoustic characterization for therapeutic ultrasound devices. The purpose of this work at the FDA is to develop bench testing methods to ensure proper review of medical ultrasound devices to evaluate device safety and performance. The test instruments and methods developed have helped promote the development and recognition of voluntary consensus standards, as well as FDA industry guidance, that can provide the basis for least-burdensome data collection requirements by manufacturers and allow the FDA to assess device safety and effectiveness in a more consistent, methodical and scientifically rigorous manner.

Development and characterization of polyurethane-based tissue and blood mimicking materials for high intensity therapeutic ultrasound.

A polyurethane-based tissue mimicking material (TMM) and blood mimicking material (BMM) for the acoustic and thermal characterization of high intensity therapeutic ultrasound (HITU) devices has been developed. Urethane powder and other chemicals were dispersed into either a high temperature hydrogel matrix (gellan gum) or degassed water to form the TMM and BMM, respectively. The ultrasonic properties of both TMM and BMM, including attenuation coefficient, speed of sound, acoustical impedance, and backscatter coefficient, were characterized at room temperature. The thermal conductivity and diffusivity, BMM viscosity, and TMM Young's modulus were also measured. Importantly, the attenuation coefficient has a nearly linear frequency dependence, as is the case for most soft tissues and blood at 37 °C. Their mean values are 0.61f1.2 dB cm-1 (TMM) and 0.2f1.1 dB cm-1 (BMM) based on measurements from 1 to 8 MHz using a time delay spectrometry (TDS) system. Most of the other relevant physical parameters are also close to the reported values of soft tissues and blood. These polyurethane-based TMM and BMM are appropriate for developing standardized dosimetry techniques, validating numerical models, and determining the safety and efficacy of HITU devices.

A miniature ultrasound M-Mode imaging system based on time delay spectrometry

For wearable ultrasound applications, power and safety are a concern. We have developed an imaging method based on time-delay spectrometry (TDS) that employs a low-voltage, wideband chirp to establish a relationship between reflector depth and frequency by mixing the transmit and receive signals. We will show a low power, 4 channel ultrasound M-Mode system capable of tracking in vivo muscle interfaces in the forearm. The battery powered imaging system consists of power regulation subsystem, a RF chirp subsystem, and four channels consisting of a transmit RF amplifier, a mixing unit, and a receive amplifier which are sampled at 40 KSPS by a processor with a four channel ADC. The processor performs an FFT on each demodulated return to get 4 scanlines that are tracked over time to produce M-Mode images that are sent to a host computer. The wearable system can provide M-Modes images of up to 4 cm in forearm at a framerate of 50 Hz for ∼7 h with a total power consumption of 1.92 W. We demonstrate the ability of a miniaturized TDS ultrasound imaging system to collect in-vivo M-Modes that can be used for long term monitoring. The system can be used for wearable applications due to its low power consumption. [United States Department of Defense, Congressionally Directed Medical Research Program, award number W81XWH-16-1-0722 (PI: S Sikdar). National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health under Award No. U01EB027601.]

Time delay spectrometry methods for broadband characterization of plastics and tissue-mimicking materials for high intensity therapeutic ultrasound

Because of the nonlinear nature of High Intensity Therapeutic Ultrasound (HITU), tissues and tissue-mimicking materials must be characterized over a broad frequency band. Time Delay Spectrometry (TDS) measures the frequency response of a system or sample by applying a swept frequency source and using a tracking receiver to select only those through-transmission signals that have the desired time delay. Due to the greater time-bandwidth product, TDS provides considerable signal-to-noise improvement over Fourier transform techniques. This system has been used previously to provide broadband measurements of complex hydrophone sensitivity. We have further refined our TDS system to provide frequency-dependent attenuation and phase velocity characterization of materials over the range from 1 MHz to 19 MHz. We have compared the results to predicted dispersion curves based on the Kramers-Kronig relations. The measurements of low-density polyethylene (LDPE) and a HITU-compatible tissue-mimicking material agree to within 1% for both TDS and PR acquisition methods, but the TDS measurements provide superior bandwidth over the PR. Measured phase velocity agrees to within 1% with Kramers-Kronig-based dispersion curves from 1 MHz to 19 MHz.

Ultrasonic interrogation of tissues and tissue-mimicking materials with the aid of wideband hydrophone-based transducer characterization

Ultrasonic measurements can provide useful information to characterize biologic tissues and tissue-mimicking materials. Quantitative accuracy may be improved by first characterizing spatial and frequency dependence of ultrasound transducer beams, which may be measured with hydrophones in a water tank. Accuracy of estimates of acoustic output detected by hydrophones may be enhanced by deconvolving hydrophone sensitivity from hydrophone voltage output. Full deconvolution requires knowledge of magnitude and phase of hydrophone sensitivity, which may be measured over frequency bands of 1–40 MHz using time delay spectrometry. Time-delay spectrometry may also be used to provide wideband through-transmission measurements of attenuation and sound speed in tissues and tissue-mimicking materials. Much information regarding tissue composition and structure may be obtained from (1) through-transmission mode measurements of attenuation, sound speed, and dispersion, (2) pulse-echo mode measurements of backscatter, and ...

Correction for frequency-dependent hydrophone response to nonlinear pressure waves using complex deconvolution and rarefactional filtering: application with fiber optic hydrophones.

Nonlinear acoustic signals contain significant energy at many harmonic frequencies. For many applications, the sensitivity (frequency response) of a hydrophone will not be uniform over such a broad spectrum. In a continuation of a previous investigation involving deconvolution methodology, deconvolution (implemented in the frequency domain as an inverse filter computed from frequency-dependent hydrophone sensitivity) was investigated for improvement of accuracy and precision of nonlinear acoustic output measurements. Timedelay spectrometry was used to measure complex sensitivities for 6 fiber-optic hydrophones. The hydrophones were then used to measure a pressure wave with rich harmonic content. Spectral asymmetry between compressional and rarefactional segments was exploited to design filters used in conjunction with deconvolution. Complex deconvolution reduced mean bias (for 6 fiber-optic hydrophones) from 163% to 24% for peak compressional pressure (p+), from 113% to 15% for peak rarefactional pressure (p-), and from 126% to 29% for pulse intensity integral (PII). Complex deconvolution reduced mean coefficient of variation (COV) (for 6 fiber optic hydrophones) from 18% to 11% (p+), 53% to 11% (p-), and 20% to 16% (PII). Deconvolution based on sensitivity magnitude or the minimum phase model also resulted in significant reductions in mean bias and COV of acoustic output parameters but was less effective than direct complex deconvolution for p+ and p-. Therefore, deconvolution with appropriate filtering facilitates reliable nonlinear acoustic output measurements using hydrophones with frequency-dependent sensitivity.

Determination of ultrasound hydrophone phase from Fourier-Hilbert transformed 1 to 40 MHz time delay spectrometry amplitude

A Fourier-Hilbert transform (FHT) algorithm procedure was developed to obtain the complex characteristics of different hydrophones which included the most widely used membrane and capsule types. The theory presented was experimentally verified using magnitude response of the hydrophone probes determined with the time-delay-spectrometry (TDS) technique. The TDS setup implemented was similar to one used by Wear et al. in 2011, with the frequency range being extended to 1 to 40 MHz to ensure consistency with recently published international standards covering the properties, calibration, and performance evaluation of ultrasound hydrophones. The theoretical and experimental limitations of FHT were examined and revealed that extrapolations of the available logarithmic (volts per pascal magnitude) frequency response were needed to carry out acceptable FHT calculations of the phase response. The complex-valued analytic signal Hilbert transform (HT) algorithm was less practical in developing an efficient minimum phase calculation protocol than the equivalent real-valued discrete convolution HT algorithm. Examples of measured transfer function and FHT calculated phase response spectra of representative membrane and capsule ultrasound hydrophones are presented and are shown to obey the simple rule for minimum phase presented by Heyser in 1969.

Improved measurement of acoustic output using complex deconvolution of hydrophone sensitivity

The traditional method for calculating acoustic pressure amplitude is to divide a hydrophone output voltage measurement by the hydrophone sensitivity at the acoustic working frequency, but this approach neglects frequency dependence of hydrophone sensitivity. Another method is to perform a complex deconvolution between the hydrophone output waveform and the hydrophone impulse response (the inverse Fourier transform of the sensitivity). In this paper, the effects of deconvolution on measurements of peak compressional pressure (p+), peak rarefactional pressure (p_), and pulse intensity integral (PII) are studied. Time-delay spectrometry (TDS) was used to measure complex sensitivities from 1 to 40 MHz for 8 hydrophones used in medical ultrasound exposimetry. These included polyvinylidene fluoride (PVDF) spot-poled membrane, needle, capsule, and fiber-optic designs. Subsequently, the 8 hydrophones were used to measure a 4-cycle, 3 MHz pressure waveform mimicking a pulsed Doppler waveform. Acoustic parameters were measured for the 8 hydrophones using the traditional approach and deconvolution. Average measurements (across all 8 hydrophones) of acoustic parameters from deconvolved waveforms were 4.8 MPa (p+), 2.4 MPa (p_), and 0.21 mJ/cm(2) (PII). Compared with the traditional method, deconvolution reduced the coefficient of variation (ratio of standard deviation to mean across all 8 hydrophones) from 29% to 8% (p+), 39% to 13% (p_), and 58% to 10% (PII).

Assessing the sound directivity of ducts based on time delay spectrometry

The sound directivity factor is an important parameter for describing the spatial distribution of acoustic energy radiated by duct terminations. Nevertheless, the available standard techniques for measuring the directivity factor rely on the free-field condition, which implies on the availability of a non-reflective environment, such as an anechoic chamber. In this paper, a technique known as time delay spectrometry is used to determine the directivity factor and the sound reflection coefficient of an open unflanged pipe in a normal reflective environment. The results show a good agreement between the experimental data and the theory. Details of the experimental procedures, as well as the discussion on the advantages and shortcomings of the method are presented in this letter.

The Chrome Thin Coating Characteristics Using Ultrasonic Testing

According to the different thickness of the thin coatings, the echo signal of ultrasonic testing was extracted. The sound velocity in the coating medium was obtained by solving the mathematical model for transfer function and the time delay spectrometry method, and the coating thickness was calculated, the comparison error of the calculated thickness and the actual thickness, combined with the actual thickness of the coating. The results showed that, two kinds of testing methods are able to detect thin coating thickness.

A hybrid method for calibrating acoustic arrays

A method to calibrate the elements of large arrays devoted to underwater applications is presented. The goal is to measure the sensitivity and directivity of the elements over their full bandwidth. The main constraint comes from the bounded geometry of the experimental setups that limits the duration of the time windows available for analyzing the received signals. Using a short wideband pulse is detrimental to obtaining high signal-to-noise ratios. A classical method for handling this problem is time-delay spectrometry (TDS), which is based on the transmission of a linear frequency- modulated signal combined with a sliding frequency filter. An alternative, hybrid method based on the transmission of a sequence of time-frequency-limited signals is proposed. This hybrid method is shown to provide the same spectral density as TDS in the frequency scanning, but the filtering process is quite different. The transmitted signals are designed to take advantage of the coherent sums of the received signals to track the time of flight of the direct paths between the source and the elements. In addition, a fitting process based on the calibration geometry of data acquisition enables the boundaries of the interference-free time windows to be precisely delineated. An example of the application is described.

Time-delay spectrometry measurement of magnitude and phase of hydrophone response

A method based on time-delay spectrometry (TDS) was developed for measuring both magnitude and phase response of a hydrophone. The method was tested on several types of hydrophones used in medical ultrasound exposimetry over the range from 5 to 18 MHz. These included polyvinylidene fluoride (PVDF) spot-poled membrane, needle, and capsule designs. One needle hydrophone was designed for high-intensity focused ultrasound (HIFU) applications. The average reproducibility (after repositioning the hydrophone) of the phase measurement was 2.4°. The minimum-phase model, which implies that the phase response is equal to the inverse Hilbert transform of the natural logarithm of the magnitude response, was tested with TDS hydrophone data. Direct TDS-based measurements of hydrophone phase responses agreed well with calculations based on the minimum-phase model, with rms differences of 1.76° (PVDF spot-poled membrane hydrophone), 3.10° (PVDF capsule hydrophone), 3.43° (PVDF needle hydrophone), and 3.36° (ceramic needle hydrophone) over the range from 5 to 18 MHz. Therefore, phase responses for several types of hydrophones may be inferred from measurements of their magnitude responses. Calculation of phase response based on magnitude response using the minimumphase model is a relatively simple and practical alternative to direct measurement of phase.

Two approaches to the continuous-radiation field calibration of a hydrophone in a nonanechoic water tank

A comparative analysis is carried out of a time-delay spectrometry method and a proposed method for the sliding complex weighted averaging of the frequency dependence of the transfer impedance of a radiator and receiver, which provide different approaches to the free-field continuous-radiation calibration of hydrophones in a nonanechoic water tank. The advantages of the second method are demonstrated.

The use of coded signals in the measurement of diffusing surfaces.

Following the introduction of quantifiable number theoretic, reflection phase grating diffusing surfaces by Schroeder [J. Acoust. Soc. Am. 57, 149–151 (1975)], several approaches have been used to measure and characterize them. In the early 1980s, time delay spectrometry, developed by Heyser, was used to measure the temporal, spatial, and spectral responses of diffusing surfaces. Once maximum length sequences were introduced as an excitation signal, they were adopted in the late 1980s. These explorations led to the creation of a measurement standard for the diffusion coefficient, which is the circular autocorrelation of the third octave polar responses. The limitations of periodic number theoretic surfaces, namely, bandwidth, grating lobes, and flat plate effects, were addressed by computer optimization, which combined the power of boundary element and multi‐dimensional minimization techniques, and modulation. Both one‐dimensional (phase variation in one direction) and two‐dimensional (phase variation in two orthogonal directions) surfaces were characterized. The data collection and reduction procedures will be reviewed with several examples of diffusing surfaces. A summary of this research can be found in the work of Cox and D’Antonio [Acoustic Absorbers and Diffusers: Theory, Design and Application (Spon, 2004)].

Broadband PVDF Membrane Hydrophone for Comparisons of Hydrophone Calibration Methods up to 140 MHz

A PVDF membrane hydrophone has been constructed in particular for comparisons of broadband ultrasound hydrophone calibration methods and of the results obtained by different laboratories. Intercomparisons have to accompany the efforts currently undertaken to enhance the calibration frequency ranges and to implement the extension from the determination of amplitude-only to complex-valued calibration data. It can be expected that such hydrophone data will be used much more frequently in the future for exposure measurements on medical ultrasound equipment, in particular for the detection of nonlinearly distorted waveforms. The hydrophone design chosen has a foil thickness of 9 microm and an electrode diameter of 210 microm. A broadband differential preamplifier (-3 dB roll-off frequency: 95 MHz) is integrated to achieve a high signal-to-noise ratio over a broad frequency range (e.g., 26 dB-30 dB in the range 50 MHz to 140 MHz for measurements of nonlinearly distorted pulses). The hydrophone response was characterized by means of a primary interferometric calibration technique, by substitution calibration using time-delay spectrometry, and by complex broadband pulse calibration using nonlinear sound propagation. The results show a flat frequency response up to 40 MHz (maximum variations below +/-0.6 dB) and a thickness mode resonance at about 105 MHz. They indicate a useable bandwidth up to 140 MHz. The effective diameter as derived from directional response measurements is 240 microm at frequencies beyond 15 MHz.

An ultrasonic time-delay spectrometry system employing digital processing

Time-delay spectrometry (TDS) is a swept-frequency technique that has proven useful in several ultrasonic applications. Commercial TDS systems are available, but only in the audio frequency range. Several ultrasonic research TDS systems have been constructed, and they have been used effectively for substitution calibration of hydrophones and for measurement of attenuation and sound velocity in materials. Unfortunately these systems depend on features of commercial equipment no longer manufactured, so a new system has been designed using modern equipment and straightforward signal processing. This system requires a frequency source with a reasonably linear sweep of frequency versus time, audio frequency filters, a standard double-balanced mixer, a power splitter, a waveform digitizer capable of handling audio frequency signals, and a personal computer. An optional implementation that shifts the signal to a lower frequency for more convenient digitization and easier velocity measurements additionally requires an audio frequency oscillator and an audio-range analog multiplier. The processing steps are performed with standard signal processing software. To demonstrate the operation of the system, substitution calibration measurements of hydrophones as well as attenuation measurements on a tissue mimicking material were obtained and compared to a custom TDS system previously described by the authors. The data from these two TDS systems agree to within +/- 0.5 dB in the 1-10 MHz frequency range used. Higher frequency source transducers could be used to extend this range.

An investigation of Columbia College Chicago reverberation chamber

The Audio Arts and Acoustics department at Columbia College in Chicago acquired a new building in 2003. The facility, a former bank, contained an old steel and concrete vault that was converted to a reverberation chamber. The acoustic properties of the space, including reverberation time, modal density, and early reflection maps were analyzed and compared to a computer model. Reflectograms were predicted at various locations and compared to test data acquired with Time Delay Spectrometry (TDS). Polar Energy Time (PET) techniques were also used to identify the location of a single 4×4 sample of foam absorber and the results of the test were compared to the predicted value from the computer model. The results of the tests show that, under its current configuration, the room is usable as a reverberation chamber down to 300 Hz, and that the computer model was able to accurately predict the results from the PET methodology.

Method of compact archiving of signals in time delay spectrometry

Method of compact archiving of signals in time delay spectrometry

Method of compact archiving of signals in time delay spectrometry

Method of compact archiving of signals in time delay spectrometry