Keith A Wear

Keith A Wear
U.S. Food and Drug Administration | FDA · Center for Devices and Radiological Health

Ph.D.

About

238
Publications
16,976
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
5,171
Citations
Introduction
Keith Wear has a Ph.D. in Applied Physics from Stanford University. He is a Fellow of the American Institute of Ultrasound in Medicine, the Acoustical Society of America, the American Institute of Medical and Biological Engineering, and the IEEE. He has served as Associate Editor in Chief and Associate Editor of IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. He is an Associate Editor of the Journal of the Acoustical Society of America and Ultrasonic Imaging.
Additional affiliations
January 1989 - present
U.S. Food and Drug Administration
January 1986 - December 1989
Education
April 1983 - October 1986
Stanford University
Field of study
  • Applied Physics
September 1980 - April 1982
Stanford University
Field of study
  • Applied Physics
September 1975 - June 1980
University of California, San Diego
Field of study
  • Applied Physics

Publications

Publications (238)
Article
A 2012 review of therapeutic ultrasound was published to educate researchers and physicians on potential applications and concerns for unintended bioeffects (doi: 10.7863/jum.2012.31.4.623 ). This review serves as an update to the parent article, highlighting advances in therapeutic ultrasound over the past 12 years. In addition to general mechanis...
Article
Acoustic pressure measurements with hydrophones are critical to characterize safety and effectiveness of diagnostic and therapeutic medical ultrasound devices. This tutorial will cover (1) fundamental interactions between ultrasound and biologic tissues that determine thresholds for safety and effectiveness, (2) underlying physics of various hydrop...
Article
Full-text available
Accurate and continuous monitoring of cerebral blood flow is valuable for clinical neurocritical care and fundamental neurovascular research. Transcranial Doppler (TCD) ultrasonography is a widely used non-invasive method for evaluating cerebral blood flow¹, but the conventional rigid design severely limits the measurement accuracy of the complex t...
Article
Complete characterization of high intensity focused ultrasound (HIFU) systems includes measurements of transmitted pressure at the focal plane with a hydrophone. However, highly focused, nonlinear HIFU beams can result in considerable spatial averaging artifacts due to finite area of the hydrophone active element. Spatiotemporal deconvolution (STD)...
Article
Full-text available
Critical skeletal sites for osteoporotic fractures include femur and vertebrae. However, many ultrasound devices designed for the management of osteoporosis instead target the calcaneus, which is far more accessible to ultrasound than femur or vertebrae. Calcaneus-based devices have been shown to be very effective for predicting osteoporotic fractu...
Article
Full-text available
Wearable ultrasound has the potential to become a disruptive technology enabling new applications not only in traditional clinical settings, but also in settings where ultrasound is not currently used. Understanding the basic engineering principles and limitations of wearable ultrasound is critical for clinicians, scientists, and engineers to advan...
Article
Full-text available
Recent reports have raised concerns of potential racial disparities in performance of optical oximetry technologies. To investigate how variable epidermal melanin content affects performance of photoacoustic imaging (PAI) devices, we developed plastisol phantoms combining swappable skin-mimicking layers with a breast phantom containing either India...
Article
In 1988, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control (TUFFC) hosted a Special Issue on Ultrasound Exposimetry (vol. 35, no. 2). This issue is now recognized as seminal; it laid the groundwork for the development of numerous domestic and international standards that are still widely used for calibration and regulatory cha...
Article
Nonalcoholic fatty liver disease (NAFLD) is believed to affect one-third of American adults. Noninvasive methods that enable detection and monitoring of NAFLD have the potential for great public health benefits. Because of its low cost, portability, and noninvasiveness, US is an attractive alternative to both biopsy and MRI in the assessment of liv...
Article
Full-text available
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 frequency-dependent sensitivity and spatial averaging across the hydropho...
Article
Frequency-dependent effective sensitive element radius $\boldsymbol{a}_{\text {eff}}{({f})}$ is a key parameter for elucidating physical mechanisms of hydrophone operation. In addition, it is essential to know $\boldsymbol{a}_{\text {eff}}{({f})}$ to correct for hydrophone output voltage reduction due to spatial averaging across the hydrophone...
Article
Excessive liver fat (steatosis) is now the most common cause of chronic liver disease worldwide and is an independent risk factor for cirrhosis and associated complications. Accurate and clinically useful diagnosis, risk stratification, prognostication, and therapy monitoring require accurate and reliable biomarker measurement at acceptable cost. T...
Article
Full-text available
Standardized phantoms and test methods are needed to accelerate clinical translation of emerging photoacoustic imaging (PAI) devices. Evaluating object detectability in PAI is challenging due to variations in target morphology and artifacts including boundary buildup. Here we introduce breast fat and parenchyma tissue-mimicking materials based on e...
Article
MicroCT-based finite element models were used to compute power law relations for uniaxial compressive yield stress versus bone volume fraction for 78 cores of human trabecular bone from five anatomic sites. The leading coefficient of the power law for calcaneus differed from those for most of the other sites (p < 0.05). However, after normalizing b...
Chapter
Two theoretical models for ultrasonic scattering from cancellous bone have been extensively validated in human cancellous bone in vitro. Many metrics have been devised to assess scattering in vivo. In the diagnostic frequency range (<1 MHz), multiple scattering is much weaker than single scattering. However, evidence for multiple scattering has bee...
Article
This article reports experimental validation for spatiotemporal deconvolution methods and simple empirical formulas to correct pressure and beamwidth measurements for spatial averaging across a hydrophone sensitive element. The method was validated using linear and nonlinear beams transmitted by seven single-element spherically focusing transducers...
Article
This article reports spatiotemporal deconvolution methods and simple empirical formulas to correct pressure and beamwidth measurements for spatial averaging across a hydrophone sensitive element. Readers who are uninterested in hydrophone theory may proceed directly to Appendix A for an easy method to estimate spatial-averaging correction factors...
Article
Full-text available
Phantom-based performance test methods are critically needed to support development and clinical translation of emerging photoacoustic microscopy (PAM) devices. While phantoms have been recently developed for macroscopic photoacoustic imaging systems, there is an unmet need for well-characterized tissue-mimicking materials (TMMs) and phantoms suita...
Article
Histotripsy is a therapeutic ultrasound technology to liquefy tissue into acellular debris using sequences of high-power focused ultrasound pulses. Research on histotripsy has been rapidly growing in the past decade; newer applications are being proposed and evaluated for clinical use. In contrast to conventional high-intensity focused ultrasound (...
Article
Full-text available
Magnetic resonance‐guided focused ultrasound (MRgFUS) is a completely non‐invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report add...
Article
Full-text available
This article explores the relative roles of absorption and scattering of ultrasound in cancellous-bone-mimicking phantoms.
Article
Full-text available
This review article describes the physics of interaction between ultrasound and cancellous bone.
Article
Full-text available
This article describes a clinical trial that investigates the role of phase cancellation in measurements of attenuation in 73 women.
Article
Full-text available
This article gives theory for predicting speed of sound in cancellous bone as a function of porosity.
Article
Full-text available
This paper reviews theory, measurements, and computer simulations of scattering from cancellous bone reported by many laboratories. Three theoretical models (Binary Mixture, Faran Cylinder, and Weak Scattering) for scattering from cancellous bone have demonstrated some consistency with measurements of backscatter. Backscatter is moderately correlat...
Article
Full-text available
Acoustic pressure can be measured with a hydrophone. Hydrophone measurements can underestimate incident acoustic pressure due to spatial averaging effects across the hydrophone sensitive element. The spatial averaging filter for a nonlinear focused beam is a low-pass filter that decreases monotonically from 1 to 0 as frequency increases from 0 to i...
Article
Full-text available
This article describes how to increase accuracy of acoustic pressure measurements for high intensity therapeutic ultrasound systems.
Article
Full-text available
This article describes measurements and theory for Directivity and Frequency-Dependent Effective Sensitive Element Size of Membrane Hydrophones
Article
Full-text available
This article describes a method for measuring cortical thickness, which is related to osteoporotic fracture risk.
Article
Full-text available
This article describes a model for ultrasonic backscattering from cancellous bone.
Article
Full-text available
This articles investigates the effect of phase cancellation on estimates of attenuation and backscatter in human calcaneus in vitro.
Article
Full-text available
This paper investigates sources of error on speed of sound measurements in human calcaneus in vitro.
Article
Full-text available
This article describes Directivity and Frequency-Dependent Effective Sensitive Element Size of a Reflectance-Based Fiber Optic Hydrophone. These hydrophones are often used for characterization of high intensity therapeutic ultrasound systems.
Article
Full-text available
This article provides advice on how to choose the right needle hydrophone for a measurement. It describes the effects of frequency-dependent sensitivity and spatial averaging across the hydrophone sensitive element.
Article
Full-text available
This article shows signal processing steps to increase accuracy of pressure measurements performed with fiber optic hydrophones.
Article
Full-text available
Needle and fiber optic hydrophones have frequency-dependent sensitivity, which can result in substantial distortion of nonlinear or broadband pressure pulses. A rigid cylinder model for needle and fiber optic hydrophones was used to predict this distortion. The model was compared with measurements of complex sensitivity for a fiber optic hydrophone...
Article
Full-text available
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 MHz to 18 MHz. These included PVDF spot-poled membrane, needle, and capsule designs. One needle hydrophone w...
Article
Full-text available
Directivity is a hydrophone specification that describes response as a function of angle of incidence. The goal of this study was to compare, in the context of needle hydrophones, the commonly-used rigid baffle (RB) model for hydrophone directivity to three alternative models: soft baffle (SB), unbaffled (UB), and rigid piston (RP). Directivity mea...
Article
Full-text available
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 wavef...
Article
Full-text available
As photoacoustic imaging (PAI) begins to mature and undergo clinical translation, there is a need for well-validated, standardized performance test methods to support device development, quality control, and regulatory evaluation. Despite recent progress, current PAI phantoms may not adequately replicate tissue light and sound transport over the fu...
Article
Full-text available
Objectives To quantify the bias of shear wave speed (SWS) measurements between different commercial ultrasonic shear elasticity systems and a magnetic resonance elastography (MRE) system in elastic and viscoelastic phantoms. Methods Two elastic phantoms, representing healthy through fibrotic liver, were measured with 5 different ultrasound platfor...
Article
This article reports underestimation of mechanical index (MI) and nonscanned thermal index for bone near focus (TIB) due to hydrophone spatial averaging effects that occur during acoustic output measurements for clinical linear and phased arrays. TIB is the appropriate version of thermal index (TI) for fetal imaging after ten weeks from the last me...
Article
This article reports experimental validation of a method for correcting underestimates of peak compressional pressure (pc), peak rarefactional pressure (pr), and pulse intensity integral (pii) due to hydrophone spatial averaging effects that occur during output measurement of clinical linear and phased arrays. Pressure parameters (pc, pr, and pii),...
Article
Two scientists from the US Food and Drug Administration comment on limitations of acoustic safety indexes that can arise from spatial averaging effects of hydrophones that are used to measure acoustic output.
Article
No PDF available ABSTRACT This paper reports an investigation of an inverse-filter method to correct for experimental underestimation of pressure due to spatial averaging across a hydrophone sensitive element. The spatial averaging filter (SAF) depends on hydrophone type (membrane, needle, or fiber-optic), hydrophone geometrical sensitive element d...
Conference Paper
This paper investigates experimental underestimation of pressure measurements due to spatial averaging across a hydrophone sensitive element. Empirical relationships are measured to enable correction for hydrophone spatial averaging errors in peak compressional pressure (p c ), peak rarefactional pressure (p r ), and pulse intensity integral (pii)....
Conference Paper
This paper reports underestimation of peak compressional pressure (p c), peak rarefactional pressure (p r ), and pulse intensity integral (pii) due to hydrophone spatial averaging of acoustic radiation force impulse (ARFI) beams generated by clinical linear and phased arrays. Although a method exists for correcting for hydrophone spatial averaging...
Article
This article reports an investigation of an inverse-filter method to correct for experimental underestimation of pressure due to spatial averaging across a hydrophone sensitive element. The spatial averaging filter (SAF) depends on hydrophone type (membrane, needle, or fiber-optic), hydrophone geometrical sensitive element diameter, transducer driv...
Article
Full-text available
Multispectral photoacoustic imaging (MPAI) is a promising emerging diagnostic technology, but fluence artifacts can degrade device performance. Our goal was to develop well-validated phantom-based test methods for evaluating and comparing MPAI fluence correction algorithms, including a heuristic diffusion approximation, Monte Carlo simulations, and...
Article
As photoacoustic imaging (PAI) technology matures, computational modeling will increasingly represent a critical tool for facilitating clinical translation through predictive simulation of real-world performance under a wide range of device and biological conditions. While modeling currently offers a rapid, inexpensive tool for device development a...
Article
Ultrasound is now a clinically-accepted modality in the management of osteoporosis. The most common commercial clinical devices assess fracture risk from measurements of attenuation and sound speed in cancellous bone. This review discusses fundamental mechanisms underlying the interaction between ultrasound and cancellous bone. Because of its two-p...
Article
It is important to know hydrophone frequency-dependent effective sensitive element size in order to account for spatial averaging artifacts in acoustic output measurements. Frequency-dependent effective sensitive element size may be obtained from hydrophone directivity measurements. Directivity was measured at 1, 2, 3, 4, 6, 8, and 10 MHz from ±60°...
Article
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 sensit...
Article
Plasmonic nanoparticles (PNPs) continue to see increasing use in biophotonics for a variety of applications, including cancer detection and treatment. Several PNP-based approaches involve the generation of highly transient nanobubbles due to pulsed laser-induced vaporization and cavitation. While much effort has been devoted to elucidating the mech...
Article
Full-text available
Innovative biophotonic modalities such as photoacoustic imaging (PAI) have the potential to provide enhanced sensitivity and molecule-specific detection when used with nanoparticles. However, high peak irradiance levels generated by pulsed lasers can lead to modification of plasmonic nanoparticles. Thus, there is an outstanding need to develop prac...
Article
Full-text available
Multispectral photoacoustic oximetry imaging (MPOI) is an emerging hybrid modality that enables the spatial mapping of blood oxygen saturation (SO2) to depths of several centimeters. To facilitate MPOI device development and clinical translation, well-validated performance test methods and improved quantitative understanding of physical processes a...
Article
Acoustic pressure can be measured with a hydrophone. Hydrophone measurements can underestimate incident acoustic pressure due to spatial averaging effects across the hydrophone sensitive element. The spatial averaging filter for a nonlinear focused beam is a low-pass filter that decreases monotonically from 1 to 0 as frequency increases from 0 to i...
Article
The spatiotemporal transfer function for a needle or reflectance-based fiber-optic hydrophone is modeled as separable into the product of two filters corresponding to frequency-dependent sensitivity and spatial averaging. The separable hydrophone transfer function model is verified numerically by comparison to a more general rigid piston spatiotemp...
Article
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- $\mu \text{m}$ diameter core and a 125- $\mu \text{m}$ overall diameter (c...
Article
Porous media can support two longitudinal waves, which often overlap in time and frequency domains. Each wave has its own attenuation coefficient and phase velocity. These properties are related to volume fraction of solid phase, tortuosity, viscous characteristic length, and elasticity. Therefore, knowledge of individual wave properties is useful...
Article
Full-text available
A rigid piston model has been developed to predict needle hydrophone transfer functions. The model predicts that “effective” sensitive element diameter depends on the frequency and can exceed geometrical sensitive element diameter by a factor of up to 2.25. The model predicts harmonic distortion for hydrophone measurements of focused pressure waves...
Article
Full-text available
Directivity is a hydrophone specification that describes response as a function of angle of incidence. The goal of this study was to compare, in the context of needle hydrophones, the commonly used rigid baffle model for hydrophone directivity to three alternative models: soft baffle, unbaffled (UB), and rigid piston (RP). Directivity measurements...
Article
Full-text available
Time delay spectrometry (TDS) is extended for broadband characterization of plastics (low-density polyethylene, LDPE) and tissue-mimicking material (TMM). The results suggest that TDS and the conventional broadband pulse method give comparable measurements for frequency-dependent attenuation coefficient and phase velocity near the center frequency,...
Conference Paper
A blood flow phantom with tunable oxygen saturation (SO2) was developed as an oximetry performance test method for rapidly emerging photoacoustic imaging systems. Results highlight the importance of fluence corrections in improving SO2 measurement accuracy.
Conference Paper
Nanorod reshaping during photoacoustic imaging (PAI) may severely impact performance. Nanorod damage thresholds were studied with electron microscopy and spectrophotometry, and the effect of laser exposure on PAI was studied in a turbid phantom.
Article
Full-text available
Needle and fiber optic hydrophones have frequency-dependent sensitivity, which can result in substantial distortion of nonlinear or broadband pressure pulses. A rigid cylinder model for needle and fiber optic hydrophones was used to predict this distortion. The model was compared with measurements of complex sensitivity for a fiber optic hydrophone...
Article
Photoacoustic tomography (PAT) is emerging as a potentially important aid for breast cancer detection. Well-validated tissue-simulating phantoms are needed for objective, quantitative, and physically realistic testing for system development. Prior reported PAT phantoms with homogenous structures do not incorporate the irregular layered structure of...
Article
Bone sonometry is a rapidly evolving diagnostic technology for managing osteoporosis. Bone sonometers that measure ultrasound propagation through calcaneus or along long bones (parallel to the long axis) have been used for many years. However, the Food and Drug Administration recently cleared two new, innovative designs that measure ultrasound prop...
Article
Reliable acoustic characterization is fundamental for patient safety and clinical efficacy during high intensity therapeutic ultrasound (HITU) treatment. Technical challenges, such as measurement variation and signal analysis, still exist for HITU exposimetry using ultrasound hydrophones. In this work, four hydrophones were compared for pressure me...
Article
As photoacoustic imaging (PAI) technologies advance and applications arise, there is increasing need for standardized approaches to provide objective, quantitative performance assessment at various stages of the product development and clinical translation process. We have developed a set of performance test methods for PAI systems based on breast-...
Article
Full-text available
Reliable acoustic characterization is fundamental for patient safety and clinical efficacy during high-intensity therapeutic ultrasound (HITU) treatment. Technical challenges, such as measurement variation and signal analysis, still exist for HITU exposimetry using ultrasound hydrophones. In this work, four hydrophones were compared for pressure me...
Article
Full-text available
Clinical bone sonometers applied at the calcaneus measure broadband ultrasound attenuation and speed of sound. However, the relation of ultrasound measurements to bone strength is not well-characterized. Addressing this issue, we assessed the extent to which ultrasonic measurements convey in vitro mechanical properties in 25 human calcaneal cancell...