An in vitro study of low-frequency ultrasound transmission through a musculoskeletal fragment of the thoracic cage

Abstract

Multifunctional capacity of ultrasonic technology and effects of ultrasound (US) on biological objects (cells, tissues, and organs) are widely used for diagnosis, therapy, and surgery of various diseases. Safety of US applications to medicine is an important factor.The effects of low intensity (<0.5 W/cm 2 ) low fre quency (50 300 kHz) US on isolated functionally active rat heart were considered in [1 3]. It was found that US induced changes in the contractile activity of myocardi um and arrhythmogenic effects, the extent of the effects being dependent on the US intensity and frequency. Similar effects of US of higher frequency were reported in earlier works [4,5]. A conclusion on possible therapeutic effect of US in cardiology was drawn in [1,3]. The car dioprotective effect of US in myocardium ischemia (sim ilar to ischemic preconditioning) was suggested in [8].However, in contrast to laboratory experiments with isolated heart, actual medical practice raises the problem of US transmission from the ultrasonic generator to the heart. US transmission through the thoracic cage wall is accompanied by its attenuation. The problem of the US effect on thoracic cage organs (ribs, intercostal tissues) is very important from the practical viewpoint. The effects of noninvasive therapeutic passage of US through the heart require quantitative experimental evaluation.The concept of US effect on biological tissues has been put forward in medical diagnosis (echocardiogra phy). The physico geometrical conditions of US use in diagnosis and therapy differ from one another. This is due to different US frequency ranges inherent in these tech nologies. This difference brings about different dimen sions of US generators used for diagnosis and therapy and different structural scales (intercostal distance) in the tho racic cage. Brief explanation is given below. The main goal of echocardiography is to obtain sufficient image resolu tion. At a first approximation, the US image resolution is determined by the length of the US half wave or US fre quency. The higher is US frequency, the larger is the atten uation decrement in biological tissue. Therefore, the depth of penetration of high frequency US is smaller. Given the compromise conditions, many US apparatuses and modes of echocardiography employ US frequencies within the range 1.5 7.5 MHz. This frequency range cor responds to the wavelength range 0.2 1 mm. To form the US beam, the characteristic aperture size of the generator should be at least one order of magnitude larger than the radiation wavelength. Thus, generators with aperture size 1 2 cm can be used for echocardiography. Such generators during a procedure can be positioned at the intercostal space, avoiding thereby the screening effect of ribs (Fig. 1a). In this case, the US passage through the thoracic cage during echocardiography is reduced to US passage through intercostal tissues [6,7]. The absence of acoustic barriers in the intercostal space makes the US beam pass ing through the intercostal windows uniform (Fig. 1a).Di...