Propagation velocity and attenuation of a shear wave pulse measured by ultrasound detection in agarose and polyacrylamide gels

Polyacrylamide hydrogel phantoms for performance evaluation of multispectral photoacoustic imaging systems

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 full range of optical wavelengths and acoustic frequencies employed by reported PAI devices. Here we introduce polyacrylamide (PAA) hydrogel as a candidate material for fabricating stable phantoms with well-characterized optical and acoustic properties that are biologically relevant over a broad range of system design parameters. We evaluated suitability of PAA phantoms for conducting image quality assessment of three PAI systems with substantially different operating parameters including two commercial systems and a custom system. Imaging results indicated that appropriately tuned PAA phantoms are useful tools for assessing and comparing PAI system image quality. These phantoms may also facilitate future standardization of performance test methodology.

Response of adherent cells to mechanical perturbations of the surrounding matrix

We present a generic and unified theory to explain how cells respond to perturbations of their mechanical environment such as the presence of neighboring cells, slowly applied stretch, or gradients of matrix rigidity. Motivated by experiments, we calculate the local balance of forces that give rise to a tendency for the cell to locally move or reorient, with a focus on the contribution of feedback and homeostasis to cell contractility (manifested by a fixed displacement, strain or stress) that acts on the adhesions at the cell boundary. These forces can be either reinforced or diminished by elastic stresses due to mechanical perturbations of the matrix. Our model predicts these changes and how their balance with local protrusive forces that act on the cell's leading edge either increase or decrease the tendency of the cell to locally move (toward neighboring cells or rigidity gradients) or reorient (in the direction of slowly applied stretch or rigidity gradients).

Propagation characteristics of photomechanical waves and their application to gene delivery into deep tissue

Targeted gene transfection can be achieved by the use of photomechanical waves (PMWs) generated by irradiating a solid material with high-power nanosecond laser pulses. To examine the treatable tissue depth, we investigated propagation characteristics of PMWs and depth-dependent properties of gene transfection with different laser fluences and spot diameters. Pressure characteristics of PMWs were measured at different propagation distances using tissue phantoms and their propagation was imaged by shadowgraphing. Phantoms with various thicknesses were placed on rat dorsal skin that had been injected with plasmid DNA coding for a reporter gene and three pulses of PMWs were applied from the top of each phantom. Significant gene expression was observed in the skin even under a 15-mm-thick tissue phantom and the depth-dependent relationships between PMW parameters and gene expression level were revealed. The data obtained will be useful for determining appropriate laser parameters for PMW-based gene transfer into deep-located tissue.

Medical ultrasound: imaging of soft tissue strain and elasticity

After X-radiography, ultrasound is now the most common of all the medical imaging technologies. For millennia, manual palpation has been used to assist in diagnosis, but it is subjective and restricted to larger and more superficial structures. Following an introduction to the subject of elasticity, the elasticity of biological soft tissues is discussed and published data are presented. The basic physical principles of pulse-echo and Doppler ultrasonic techniques are explained. The history of ultrasonic imaging of soft tissue strain and elasticity is summarized, together with a brief critique of previously published reviews. The relevant techniques-low-frequency vibration, step, freehand and physiological displacement, and radiation force (displacement, impulse, shear wave and acoustic emission)-are described. Tissue-mimicking materials are indispensible for the assessment of these techniques and their characteristics are reported. Emerging clinical applications in breast disease, cardiology, dermatology, gastroenterology, gynaecology, minimally invasive surgery, musculoskeletal studies, radiotherapy, tissue engineering, urology and vascular disease are critically discussed. It is concluded that ultrasonic imaging of soft tissue strain and elasticity is now sufficiently well developed to have clinical utility. The potential for further research is examined and it is anticipated that the technology will become a powerful mainstream investigative tool.

Shear elasticity estimation from surface wave: the time reversal approach

In this work the shear elasticity of soft solids is measured from the surface wave speed estimation. An external source creates mechanical waves which are detected using acoustic sensors. The surface wave speed estimation is extracted from the complex reverberated elastic field through a time-reversal analysis. Measurements in a hard and a soft gelatin-based phantom are validated by independent transient elastography estimations. In contrast with other elasticity assessment methods, one advantage of the present approach is its low sound technology cost. Experiments performed in cheese and soft phantoms allows one to envision applications in the food industry and medicine.