What do cardiologists know about the safety indexes of echocardiography?

Abstract

Diagnostic ultrasonography (US) is an extensively used imaging method in cardiology and it has been generally known as a safe procedure. Ultrasound (high-frequency sound waves) is a mechanical vibration with frequencies above the human limit of audibility [1]. US exposure causes heating, named as thermal effect. Also, it has some mechanical effects including acoustic cavitation, cell movement in liquid, electrical changes in cell membranes, shrinking and expansion of bubbles in liquid, as well as pressure changes [1]. Acoustic cavitation is defined as the formation of bubbles in a medium exposed to an ultrasonographic field [2]. Cavitation usually occurs when sound energy passes through an area that contains a cavity, such as a gas bubble or other air pocket. Because they contain air bubbles, some adult tissues such as lung and intestine are more vulnerable to this cavitation effect [3]. The bubbles or air pocket can pulsate and resonate due to sound waves in cavitation. Pulsation of the bubbles leads to secondary sound waves moving in all directions. Because these secondary waves also reflect back to the transducer, they can improve US images. Therefore, some artificial bubbles (contrast agents) have been developed for imaging of the circulatory system. When these bubbles contract rhythmically towards the point of collapsing, they can create very high temperatures and pressures for a few tens of nanoseconds [1]. As demonstrated in some animal studies, these high temperatures and pressures can produce free radicals, which can theoretically cause genetic damage [4, 5]. However, very little information is available on the genetic damage in humans; currently there is no evidencebased data that exposure to medical US is capable of inducing genetic defects [6, 7]. In the lung or intestine of laboratory animals containing gas bubbles, these cavitation effects can lead to ruptures in very small blood vessels [8]. The other effects of US not requiring the presence of bubbles are changes in pressure, force, rotation, and streaming. In a liquid media, US causes a sort of stirring action known as acoustic streaming. In theory, this streaming effect could occur in fluid-filled parts of the human body, such as blood vessels, the heart, or bladder [3]. The potential for US to induce acoustic cavitation in a liquid due to an acoustic pressure field is described as the mechanical index (MI) [1]. MI = p/Hf, where p is the derated peak rarefactional pressure in MPa (megapascals) and f is the ultrasonic frequency in MHz. Cavitation is capable of producing damage to cell membranes. Although it is well known that US is a non-ionizing radiation, it can induce apoptosis by cavitation in cancer cell lines [9]. The thermal index (TI) is defined as the level of heating along the US beam. TI = W/Wdeg, where W is the acoustic power produced by the transducer, and Wdeg is the power required to raise tissue in the beam area by 1 C. A TI of 1.0 corresponds to a potential for elevation of 2 C. When US passes through the body, most of the sound energy is converted to heat. It is important to note that heating caused by US is highly localized and limited to the region in interest. The greatest heating is associated with multiple pulsing modes such as Doppler imaging, and high frame rate, high line-density modes [1]. Because the heart is cooled by the high blood flow through its cavities and myocardium, the probability of thermal harm has little relevance [10]. Manufacturers are obliged to provide information on safety indices (e.g., the MI and TI values) but US output energy and its control is mainly based on operators. MI and Y. Koza (&) Department of Cardiology, Ataturk University Faculty of Medicine, Yakutiye, Erzurum 25100, Turkey e-mail: yavuzerkoza@hotmail.com