Citations

A coupled vibration model, which takes into account the elastic arterial wall, the interior blood, and exterior tissue, is presented. The arterial wall is considered as a cylindrical elastic shell and is modeled with the thick shell theory due to the large thickness-to-radius ratios. Both the blood and the tissue are considered as ideal fluids and are modeled with acoustic wave theory. A newly developed method, the wave propagation approach, is extended for the coupled vibration of arterial walls. The theoretic model is evaluated against those available in the literature. The experiments were carried out on a silicone rubber tube in a water tank with an ultrasound stimulated 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 measured fundamental frequency is in good agreement with the theoretical prediction.

Pulse wave velocity (PWV) is widely used for estimating the stiffness of an artery. It is well known that a stiffened artery can be associated with disease and aging. Usually, PWV is measured using the foot-to-foot time-delay method. However, the foot of the pulse wave cannot be accurately determined due to reflected waves. In addition, PWV is an average indicator of artery stiffness between the two measuring points. We propose to generate a wave in the artery wall using localized ultrasound radiation force. The wave velocity can be measured accurately by the phase change versus distance over a few millimeters. A mathematical model is developed by which the real part of the complex Young’s modulus of the artery can be determined from measured wave velocities. The imaginary part of the complex modulus can be assessed by the wave amplitude decay over distance. Experiments were conducted on a pig carotid artery in gelatin. The measured wave velocity is about 3 m/s at 100 Hz and 6.5 m/s at 500 Hz. The real part of complex modulus is estimated to be 300 kPa. The present technique offers a new method for estimating the complex elastic modulus of the artery over a short length.

Characterization of the viscoelastic mechanical properties of skin is important for improving accurate medical examination and diagnosis of disorders involving cutaneous and subcutaneous tissues; more thoroughly understanding skin biophysical and physiological properties; identifying normal and abnormal skin aging and disease processes; developing and applying improved pharmacologic and other therapeutic interventions; as well as cosmetic applications. Several noninvasive methods have been developed for measurement of mechanical properties of skin [1]. However, most methods measure a stiffness parameter but not the material properties of skin. For example, the suction method measures the displacement of skin in response to suction. The measurement is dependent on the aperture and fixation of probes and also difficult to interpret.

KCNJ13 encodes Kir7.1, an inwardly rectifying K+ channel that is expressed in multiple ion-transporting epithelia. A mutation in KCNJ13 resulting in an arginine-to-tryptophan change at residue 162 (R162W) of Kir7.1 was associated with snowflake vitreoretinal degeneration, an inherited autosomal-dominant disease characterized by vitreous degeneration and mild retinal degeneration. We used the Xenopus laevis oocyte expression system to assess the functional properties of the R162W (mutant) Kir7.1 channel and determine how wild-type (WT) Kir7.1 is affected by the presence of the mutant subunit. Recordings obtained via the two-electrode voltage-clamp technique revealed that injection of oocytes with mutant Kir7.1 cRNA resulted in currents and cation selectivity that were indistinguishable from those in water-injected oocytes, suggesting that the mutant protein does not form functional channels in the plasma membrane. Coinjection of oocytes with equal amounts of mutant and WT Kir7.1 cRNAs resulted in inward K+ and Rb+ currents with amplitudes that were ∼17% of those in oocytes injected with WT Kir7.1 cRNA alone, demonstrating a dominant-negative effect of the mutant subunit. Similar to oocytes injected with WT Kir7.1 cRNA alone, coinjected oocytes exhibited inwardly rectifying Rb+ currents that were more than seven times larger than K+ currents, indicating that mutant subunits did not alter Kir7.1 channel selectivity. Immunostaining of Xenopus oocytes or Madin-Darby canine kidney cells expressing mutant or WT Kir7.1 demonstrated distribution of both proteins primarily in the plasma membrane. Our data suggest that the R162W mutation suppresses Kir7.1 channel activity, possibly by negatively impacting gating by membrane phosphadidylinositol 4,5-bisphosphate.

Cardiovascular disease (CVD) has been the number one killer in the United States. It has long been recognized that a high percentage of all cardiovascular disease is associated with the hardening of arteries or arteriosclerosis. Several imaging techniques, including x-ray angiography, ultrasound, magnetic resonance imaging (MRI), etc., are available for imaging atherosclerotic arteries. Most imaging techniques measure luminal diameter, wall thickness and plaque volume. However, no current imaging methods can measure elasticity of the arterial wall. Pulse wave velocity (PWV) is a simple and robust method to measure arterial stiffness. However, PWV can not evaluate the local arterial stiffness. We have developed a novel ultrasound method for estimating local elasticity of arteries by the wave speed dispersion. In this paper, we present a method to measure the wave speed dispersion by resonance. Theory was developed to calculate the wave speed in cylindrical tubes. Experiments were done on a rubber tube. The resonance of the tube was generated by an air-coupled ultrasound transducer. The resonance frequency was measured by a laser vibrometer. Theoretical and experimental wave dispersions were compared. This method may be useful in characterizing the elastic properties of arterial samples where the wave speed dispersion can not be easily measured.

Pulse wave velocity (PWV) is widely used for estimating the stiffness of an artery. PWV is measured by the time of travel of the “foot” of the pressure wave over a known distance. This technique has a low time resolution and is an average measurement of artery stiffness between the two measuring positions. In this paper an arterial wave is generated non-invasively in the vessel wall by the radiation force of ultrasound. The wave velocity in the vessel is measured non-invasively by ultrasound with high time resolution over a short distance of a few millimeters.

The mechanical response of tissues to external forces has gained considerable interest in medical diagnosis. Typically, a force is needed to produce displacement in the tissue for estimation of the tissue’s elasticity. One approach is to apply an external force on the body’s surface. Another emerging technique is to generate a localized force inside the tissue with ultrasound. Shear waves can be used in bulky tissues for estimating the tissue’s elasticity. The bending wave can be generated and measured in vessels for noninvasive estimation of a vessel’s elasticity. We have found that the Rayleigh surface wave can be generated on the tissue’s surface, [X. Zhang et al., Proc. International Congress on Ultrasonics, Vienna, April 9–13, 2007], [X. Zhang et al., IEEE Trans. Medical Imaging, 26, 843–852 (2007)]. In our method, a localized radiation force of ultrasound is remotely and noninvasively applied inside the tissue. This force can generate the shear wave as well as compression wave inside the tissue. However, only the surface wave can exist on the body’ surface. The surface wave can be used to estimate and image the stiffness of tissue. Some results of using the surface wave are presented.

A new method for imaging and detecting modal shapes of vessels is introduced. Theory is developed that predicts the measured velocity is proportional to the value of the mode shape at the focal point of the ultrasound beam. Experimental a cylindrical gel phantom of large radius. This model simulates approximately a large artery and the surrounding body. The fundamental frequency was measured 83 Hz for the tube-phantom system. At this frequency the ultrasound transducer was scanned across the vessel plane with velocity measurement at one single point on the vessel and on the phantom by laser. The images obtained show clearly the interior tube and the modal shape of the tube.

Vibro-acoustography is a new noncontact imaging method based on the radiation force of ultrasound. We extend this new technique for tissue characterization of arterial vessels by vibration techniques. In this paper a theoretical model for vibration analysis of arterial vessel with tissue is developed. Experimental studies were carried out on a silicone rubber tube embedded in a cylindrical gelatin phantom of larger radius, which simulates a large artery and the tissue body. The fundamental mode is well excited by the radiation force of ultrasound. The fundamental frequency was measured 81.8 Hz for a tube-phantom structure that is quite close to our theoretical prediction of 83.3 Hz.

Considerable interest in the elastic properties of soft tissue has increased in medicine due to their clinical relevance for monitoring various diseases as well as a biomarker for cancer. A recent emerging field is the use of acoustic radiation force for imaging and estimating the elastic properties of tissue. Newly developed imaging modalities include shear wave elasticity imaging, supersonic shear wave imaging, acoustic radiation force impulse imaging, and vibro-acoustography. In these new methods, the shear wave is used to estimate the elasticity of tissue. We have developed a novel surface wave method for noninvasively estimating the elasticity of tissue [X. Zhang et al., J. Acoust. Soc. Am., 122, 2522-2525 (2007)]. In this method, a localized ultrasound radiation force is remotely and noninvasively applied inside the tissue. The surface wave speed is used to estimate the elasticity of tissue. This method has been further developed and applied to estimate the viscoelasticity of animal myocardium tissue. In this abstract, we report our recent results of in vitro studies on animal myocardium tissue. Both the surface wave and shear wave are measured and analyzed. Noninvasive characterization of viscoelastic properties of myocardium tissue may be a very important tool for assessing heart function.