R. R. Kinnick

Noninvasive ultrasound image guided surface wave method for measuring the wave speed and estimating the elasticity of lungs: A feasibility study.

Lung diseases, such as acute respiratory distress syndrome (ARDS) and idiopathic pulmonary fibrosis (IPF), are closely associated with altered lung elastic properties. Pulmonary function testing and imaging are routinely performed for evaluating lung diseases. However, lung compliance, a measure of lung elastic properties, is rarely used in clinic, because it is invasive and provides only a global and arguably biased estimate of lung elastic properties. Current ultrasound methods also cannot be used for imaging lungs because ultrasound cannot penetrate the lung tissue. In this paper, an ultrasound image guided and surface wave based method is proposed to measure regional lung surface wave speed and estimate lung elasticity noninvasively. The method described here was not explored before to the best knowledge of the authors. Experiments in an ex vivo pig lung and an in vivo human lung pilot study are reported. The surface wave speed is measured to be 1.83±0.02m/s at 100Hz by ultrasound for the ex vivo pig lung at 3mmHg pressure, which is validated by an optical measurement. An in vivo human lung pilot experiment measures the surface wave speed to be 2.41±0.33m/s for the 100Hz sinusoidal wave at total lung capacity (TLC) and 0.99±0.09m/s at functional residual capacity (FRC). These values of wave speed fall well within the range of available literature.

An error analysis of Helmholtz inversion for incompressible shear, vibration elastography with application to filter-design for tissue characterization

For over fifteen years there has been significant effort in elastography, which describes the general area of imaging material mechanical properties. Shear vibration elastography uses dynamic tissue displacements to infer material properties from the physics of motion. The method can be used with both magnetic resonance and ultrasound data, which can both be modeled with the time-harmonic, Helmholtz equation if the material is linear, isotropic, incompressible, and piecewise-homogeneous. In this work, we develop a unified perspective on direct Helmholtz inversion. Using the fundamental theorem of statistics and a Gaussian noise model, we present a closed form for the joint conditional probability distribution of the real and imaginary parts of the squared wavenumber given the data and an arbitrary set of weights. An approximate distribution can be used in the case of high SNR which allows a figure-of-merit to be established to objectively compare inversion approaches. Adaptively choosing the inversion weights for each subregion as the smoothed and windowed conjugate of the data results in a narrow conditional probability distribution function and, consequently, high-quality estimates of complex shear modulus. To test the results, we used experimental ultrasound data-collected using a focused 5 MHz transducer with a pulse-repetition frequency of 4 kHz in a block of 15% bovine gel. The gel was harmonically compressed using a signal containing equal amplitudes at frequencies of 200, 300, 400 and 500 Hz. Noise on the measured displacement was estimated from the magnitude of the complex (baseband) correlation function and used with the conditional probability distribution function to report error bars on single-region estimates of complex shear modulus, wave-speed and attenuation.

Noncontact ultrasound stimulated optical vibrometry study of coupled vibration of arterial tubes in fluids

Coupled vibration of arterial tubes is analyzed with the wave propagation approach and first-order shear deformation theory. Both the interior and exterior fluids are considered as compressible so that acoustic waves can be generated and propagated in the fluids. Results obtained using the theory have been evaluated against those available in the literature and the agreement has been found to be good. The theory can be used for future research on the vibration and acoustics of arterial walls. Vibration experiments were carried out on a silicone rubber tube in a water tank with a novel ultrasound stimulated optical vibrometry system. This system uses the radiation force of ultrasound to vibrate the tube at low frequency and records the resulting response by a laser vibrometer. Both the excitation and measurement are remote and noncontact. The silicone rubber tube was chosen because it has mechanical properties close to those of arteries. The fundamental frequency is well excited by the radiation force and measured with the laser. The measured fundamental frequency is in good agreement with the present theory.

Skin viscoelasticity with surface wave method

Characterization of the viscoelastic mechanical properties of skin is important for improving accurate medical examination and diagnosis of disorders involving cutaneous and subcutaneous tissues. Several noninvasive methods have been developed for measurement of mechanical properties of skin. However, most methods measure a stiffness parameter but not the material properties of skin. A novel surface wave method has been developed by us for non-invasively estimating the elasticity of tissue. In this paper, we have extended this method for measuring viscoelastic material properties of human skin. A small force is generated by a mechanical shaker on the skin. The surface vibration of skin is measured using a laser vibrometer. Viscoelasticity of human skin at six anatomic sites are measured.

Continuous-wave ultrasound reflectometry for surface roughness imaging applications

BACKGROUND Measurement of surface roughness irregularities that result from various sources such as manufacturing processes, surface damage, and corrosion, is an important indicator of product quality for many nondestructive testing (NDT) industries. Many techniques exist, however because of their qualitative, time-consuming and direct-contact modes, it is of some importance to work out new experimental methods and efficient tools for quantitative estimation of surface roughness. OBJECTIVE AND METHOD Here we present continuous-wave ultrasound reflectometry (CWUR) as a novel nondestructive modality for imaging and measuring surface roughness in a non-contact mode. In CWUR, voltage variations due to phase shifts in the reflected ultrasound waves are recorded and processed to form an image of surface roughness. RESULTS An acrylic test block with surface irregularities ranging from 4.22 microm to 19.05 microm as measured by a coordinate measuring machine (CMM), is scanned by an ultrasound transducer having a diameter of 45 mm, a focal distance of 70 mm, and a central frequency of 3 MHz. It is shown that CWUR technique gives very good agreement with the results obtained through CMM inasmuch as the maximum average percent error is around 11.5%. CONCLUSION Images obtained here demonstrate that CWUR may be used as a powerful non-contact and quantitative tool for nondestructive inspection and imaging of surface irregularities at the micron-size level with an average error of less than 11.5%.

Tomographic Schlieren imaging for measurement of beam pressure and intensity

The visualization of ultrasonic fields via acousto-optic interaction is an old technique. Shadowgraph and schlieren imaging produce data representing a line integral related to pressure and time-average intensity, respectively. These “projections” can be used in computed tomography. We have compared the reconstructed pressure distribution in a plane obtained via tomographic inversion with those obtained by mechanically scanning a 0.5 mm calibrated hydrophone through the same plane. Schlieren methods result in the reconstruction of a time average intensity approximation. Shadowgraph methods reconstruct pressure at a given point in time. The advantage of the tomographic methods is that they can be done quickly. A fully automated system could produce a three-dimensional image of an ultrasound beam in a few minutes

Viscoelasticity of lung tissue with surface wave method

Considerable interest in the elastic properties of soft tissue has developed in medicine due to their clinical relevance for monitoring the progression of various diseases as well as a biomarker for cancer. Most newly developed imaging modalities use shear wave to estimate the elasticity of tissue. A novel surface wave method has been developed by us for non-invasively estimating the elasticity of tissue. In this paper, we have extended this method for measuring viscoelastic material properties of pig lungs. A small force is generated by a mechanical shaker on the lung. The surface wave on the lung is measured by laser. The viscoelasticity of a 14 Kg pig lung is measured for different pressures.

Imaging Prostate Brachytherapy Seeds with Pulse-Echo Ultrasound and Vibro-Acoustography

Treatment of early stage prostate cancer by transperineal interstitial permanent prostate brachytherapy (TIPPB) is a common and minimally invasive procedure. It is estimated that in 1999 over 30,000 men in the U.S. were treated with this modality1. The American Brachytherapy Society (ABS) provided recommendations in 1999 for the practice of TIPPB and includes the use of trans-rectal ultrasound (TRUS) usually operated at frequencies from 5.0 to 7.5 MHz to guide needle and seed placement. Indeed, improvements in prostate brachytherapy technique using an ultrasound-guided closed transperineal approach as compared to the open retropubic free hand approach3 have facilitated better seed placement3 and outcome4. However, the ABS recommends further that methods of real-time intra-operative radiation dosimetry be developed. In order to achieve this, immediate determination of seed location during is necessary.

Quantitative surface wave method for measuring local viscoelasticity of lungs

Certain lung diseases, particularly idiopathic pulmonary fibrosis, are associated with altered lung mechanical properties. While adequate techniques exist to measure the lung functions, current methods only provide global measurements of lung stiffness. We have developed a novel surface wave method for noninvasively measuring the viscoelasticity of lungs. In this paper, three types of experiments were carried out on an ex vivo pig lung. The lung was pressurized through a connecting tube to its trachea. The first experiment measured the pressure-volume relation of the lung during the inspiration and expiration. The second experiment measured the surface wave speed of the lung at different pressures. The surface wave was generated by an electromechanical shaker, and the surface wave propagation on the lung surface was measured by a laser vibrometer. The third experiment measured the surface wave speed with an ultrasound linear array transducer.

P2E-1 In Vivo Breast Vibro-Acoustography: Recent Results and New Challenges

Vibro-acoustography is an imaging modality that has emerged in recent years. This method is based on low-frequency harmonic vibrations induced in the object by the radiation force of ultrasound. The sound produced due to object vibration is received by an audio hydrophone and the information is mapped into an image. This paper describes application of vibro-acoustography for in vivo breast imaging. Recently, we have developed a vibro-acoustography system for in vivo breast imaging and have tested it on a number of volunteers. These results demonstrate that vibro-acoustography has suitable resolution, contrast, and signal-to-noise ratio such that soft tissue structures, cysts, and a variety of breast abnormalities and masses can be delineated in the image. Lack of image speckle in vibro-acoustography allows one to detect small microcalcifications in the image. The results have been verified using X-ray mammography. The encouraging results from in vivo experiments suggest that further development of vibro-acoustography technology may lead to a new clinical tool for breast imaging applications