Vibroacoustography imaging of kidney stones in vitro

Vibroacoustography (VA) is an ultrasound-based modality sensitive to stiffness and free from speckle and possesses some advantages over conventional ultrasound imaging in terms of image quality. The primary objective here is to show its feasibility in detecting/imaging kidney stones (KSs) in vitro . In VA, two intersecting ultrasound beams driven at two different frequencies f (1) and f (2), respectively, are focused within a freshly excised porcine kidney attached to a solid frame with elastic rubber bands, while the amplitude of the acoustic emission pressure field produced at the difference frequency Δf = | f(1) - f(2) | is detected by a low-frequency hydrophone. The received low-frequency signal is bandpass filtered and amplified, then digitized by a 14-bits/sample digitizer. The data are then recorded on a computer and processed numerically to construct the images. 2-D magnitude VA images are obtained at different depths within the kidney before and after stone implantation, showing kidney features and stones shapes. Experiments conducted in a water tank on a chalk sphere as well as a series of excised kidneys in which stones are artificially embedded show that all the implanted stones are detected at all chosen depths, when compared with an X-ray fluoroscopy taken to be the reference image. The resulting VA images, obtained from a nonionizing type of radiation (i.e., ultrasound waves) as compared to fluoroscopy, are speckle free unlike conventional ultrasound images. The results presented in this preliminary feasibility study show that VA allows imaging KSs in vitro, and provide the impetus to further develop and investigate VA imaging in a clinical setting for in vivo applications.