Thermal Effect in Ultra Sound - Coursework Example

Thermal effect in ultrasound This research paper discusses thermal effect in ultrasound. Ultrasound is used for medical diagnostics and attracts attention of medicals and scientists. On the one hand, ultrasound diagnostics is a favorable method of human body’s examination and diagnostics. On the other hand, thermal effect occurring in the process of ultrasound diagnostics can be hazardous for human health. The majority of the studies are focused on positive effects of ultrasound diagnostics. Nevertheless it is evident that it is necessary to pay attention to drawbacks of ultrasound diagnostics as well. The main issues covered in the studies on thermal effect in ultrasound concern therapeutic effect of this kind of treatment; an impact influencing on human body etc. The following issues are covered in the research paper: the role of thermal index in clinical imaging; the discussion of the most risky possibility of thermal effects in ultrasound; physical principles of thermal effects; the likelihood of thermal effects occurring in normal ultrasound. Physical principles of thermal effects Discussion of thermal effect requires consideration of the fact that thermal diagnostics is preferable while it doesn’t hurt human body (Liu 2001, 1). Moreover, its effectiveness has been proven by numerous experiments. Especially, the high-resolution thermography of the skin surface is valuable nowadays. Dynamic imaging is the most optimal way to improve thermal diagnostics. The following figure presents signal flow chart of dynamic imaging. In order to discuss the basic physical properties of thermal effects, it is necessary to give a brief overview of ultrasound induced temperature rise. It changes with regard to tissue properties such as absorption coefficient, density and others; depends on pulse repetition frequency and configuration of scanning (Smith 2001, 1427). A unique property of ultrasound induced temperature rise is its relation to the center or its focal nature, which prevents from systemic heat dissipating thermoregulatory mechanisms’ triggering. Physical properties of thermal effects are the following: thermal conductivity of tissue and blood perfusion of tissue (Smith 2001, 1429). A poor vascularization of tissues and essential heat conduct of tissue lead to the temperature rise. Tissues which are closer to bone have greater susceptibility to heat increase via conduction. Thermal effects in ultrasound are measured in accordance with the following parameters: a form of the wave, which can be pulsed or continuous; normal and spatial intensity (W/cm2), time of exposure, the cycle of operation, rate of occurrence. Thermal effects are more often caused by continuous waves. Thermal effects in normal ultrasound US or ultrasound has been acknowledged by clinical medicine as a proper means of clinical diagnostics. Nevertheless, there are some drawbacks in using ultrasound. A growing number of patients subjected to ultrasound witnesses that it is necessary to pay a proper attention to the symptoms of the patient before subject him to ultrasound diagnostics. Ultrasound safety is of high relevance to medicine nowadays. The fact that ultrasound inevitably causes thermal effects, which may be hazardous to sensitive organs and the embryo/fetus, should be properly considered by the scientists and the heat produced should be decreased to the greatest possible extent (EFSUMB 2008, 1) Thermal effects occurrence is evident in bones and soft tissues which are the closest to the bones. In case an increased mineralization of fetal bones occurs, there is “a greater possibility of heating sensitive tissues such as brain and spinal cord” (Leighton 2001, 1). Taking into account the fact that the ultrasound waves are absorbed, energy produced by them is turns out to be heat. The highest level of conversion is high in tissue with a high absorption coefficient (e.g. bone) and is low in tissue with little absorption (e.g. amniotic fluid). Thermal characteristics of the tissue influence on temperature rise as well as the ultrasound intensity, the period and the volume of tissue to be scanned should be considered in the process of ultrasound diagnostics. An operator deals with numerous indices such as power output, mode, depth of scanning, focus’ exactness etc while scanning. Therefore it is rather hard to exactly define temperature rise in tissues. A face of the transducer is also heated while scanning. Thus it is evident that thermal effects occurrence in normal ultrasound is evident. Thermal effects in normal ultrasound turned this medical sphere into an effective and perspective one. Thus there are a lot of therapeutic effects such as: “…reduced scar tissue and adhesions, reduced pain, increased metabolism, and increased healing” (Leighton 2001, 1). In order these thermal effects would occur, ultrasound is conducted transcutaneously and continuously with the help of coupling medium. In spite of thermal effects occurrence in normal ultrasound, a positive moment is that therapeutic thermal ultrasound is directly connected with the coupling medium’s ability of the sound energy transfer to the tissues to be treated. In such a way, tissue temperature is changed. Deep-tissue temperature is used by researches as a determinant factor of ultrasound study. Thermal index useful in clinical imaging The definition of the thermal index (TI) can be the following: “…an on-screen guide to the user of the potential for tissue heating” (Andreassi 2004, 1). In order to explain what role TI plays in clinical imaging, it is relevant to refer to the Mechanical index (MI) which signifies possibility and magnitude of nonthermal effects’ occurrence. Therefore it is relevant to speak not only about TI importance in clinical imaging, but also about MI importance. There is a necessity to check both of these indices in order to achieve complete diagnostic value. Moreover, it is necessary to conduct diagnostics taking into account ALARA principle and thus keeping a time of examination as short as possible. Especially it is necessary to mention that in case low values are impossible, a time of examination should be decreased. Moreover, it is necessary to mention that the thermal index for bone depends on factors of tissue attenuation and shows values for bone heating. A study by Horder et al. "In Vivo heating of the Guinea-Pig Fetal Brain by Pulsed ultrasound and estimates of thermal index" (1998, 1467) pays a special attention to the importance of thermal index in clinical imaging. This study is based on temperature measurement in the brain in live near-term fetal guinea pigs at the time of pigs’ exposure to a constant beam of pulsed ultrasound. In the result of the study it was concluded that TIB didn’t fix the measured temperature increase properly. The scientists suggest involvement of the cranial thermal index. This kind of index enables more appropriate and exact measurement of the temperature rise at a biologically important point of interest at the brain–bone interface. Clinical ultrasound scenario with the highest risk of thermal effects In order to discuss the highest risk of thermal effects in ultrasound scenario, it is relevant to mention about ultrasound contrast agents (UCA). In case of their occurrence, cavitation and microstreaming can be caused and MI value can be increased. With regard to the experiment conducted at small animals, there is data indicating that microvascular damage may occur. In such a way, health damages such as clinical implications in the brain, the eye and the neonate may occur. Therefore the main attention should be paid to the MI and TI values, which should be kept in the lowest level (Nyborg 2000, 911). Moreover, it is necessary to focus attention on patients with acute coronary syndrome or clinically unstable ischaemic heart disease. UCA usage among these patients should be conducted 24 hours before extra-corporeal shock wave therapy. As far as it can be seen, there is a great risk of thermal effects if UCA are used after extra-corporeal shock wave therapy. This figure shows us a scenario of the highest risk of thermal effects’ occurrence (Leighton 2001, 36). This figure presents a simultaneous occurrence of both self-focusing and self-defocusing caused by both temperature and bubble effects. On the example of transducer emission into hot water it was shown that there is a four ray’s propagation away from the transducer. Temperature increases in the sector where the water is closer to the transducer axis. The transducer is very hot and the water is heated up to 474 1C. This sector also signifies self-focusing occurrence because of the sound speed decrease. In the uppermost ray and its closest wave front the temperature is higher than in the region further from the axis. Consequently, it is possible to claim that the wave front turns to the transducer axis. The same process happens to all other rays thus causing self-focusing. Further rays enter the region where de-focusing happens. Then the rays reach an area of bubbly water and this may cause either focusing or defocusing. Therefore it is necessary to pay a great attention to intense sound fields; e.g. the focus could become closer to the transducer, generating a bubble cloud enclosed in tissue in other areas. For example, temperature changes in tissue near the transducer with the increase of the temperature may occur. In such a way this study gives an example of potentially hazardous ultrasound scenario causing the highest temperature increase. For example, temperature changes in tissue near the transducer with the increase of the temperature may occur. Figure 12 (Leighton 2001, 37) shows the influence of temperature rise in pure water on the sound speed. In such a way this study gives an example of potentially hazardous ultrasound scenario causing the highest temperature increase. Conclusion In the result of the reviewed sources on thermal effects in ultrasound further conclusion can be made: thermal effects are a favorable condition for thermal diagnostics, which is preferable nowadays due to its effectiveness and safety for human body. On the basis of physical properties of thermal effects, it is possible to conclude that influential factors of thermal effects are: thermal conductivity of tissue and blood perfusion of tissue. Temperature rise occurs in case of a poor vascularization of tissues and essential heat conduct. Furthermore, thermal effects occurring in the process of ultrasound diagnostics may be hazardous to sensitive organs and the embryo/fetus. Therefore a problem of thermal effects occurrence in ultrasound should be properly considered by the scientists and heat produced should be decreased to the greatest possible extent. Works cited 1. Andreassi, M. G. 2004. The biological effects of diagnostic cardiac imaging on chronically exposed physicians: the importance of being non-ionizing. Cardiovascular Ultrasound 2 (25): 1-12. 2. EFSUMB Clinical Safety Statement for Diagnostic Ultrasound. 2008: 1-2. 3. Horder, M. et al. 1998. In vivo heating of the guinea-pig fetal brain by pulsed ultrasound and estimates of thermal index. Ultrasound in Med. & Biol. 24 (9): 1467–1474. 4. Leighton, T. G. 2007. What is ultrasound? Progress in Biophysics and Molecular Biology 93: 3–83. 5. Liu, J. 2001. Temperature sensor array system for thermal diagnostics on human disease. Proceedings – 23rd Annual Conference – IEEE/EMBS: 1-5. 6. Nyborg, W. 2000. Biological effects of ultrasound: development of safety guidelines. Ultrasound in Med. & Biol. 26 (6): 911–964. 7. Smith, N. et al. 2001. Thermal effects of focused ultrasound energy on bone tissue. Ultrasound in Med. & Biol. 27 (10): 1427–1433. 8. Srbely, J. et al. The biophysical effects of ultrasound. A review of the current literature: 1-23. Read More