The main advantage of ultrasound is that certain structures can be observed without using radiation. However, ultrasound is energy and there are ultrasoundsafety regulations, because two bioeffects of ultrasound are heat and cavitation. Ultrasound is a mechanical energy in which a pressure wave travels through tissue. Reflection and scattering back to the transducer are used to form the image. As sound energy is transmitted through the tissue, some energy is reflected and some power is absorbed.
Possible physical effects with ultrasound:
Thermal effects of ultrasound, because tissues or water absorb the ultrasound energy with increase in temperature.
Cavitation is the formation, growth, and dynamic behavior of gas bubbles (e.g. microbubbles used as contrast agents) at high negative pressure. This dissolved gases come out of solution due to local heat caused by sound energy. This has been determined harmful at the level of the medical usage.
The ultrasoundsafety is based on two indices, the mechanical index (MI) and the thermal index (TI). The WFUMB guidelines state that ultrasound that produces temperature rises of less than 1.5°C may be used without reservation. They also state that ultrasonic exposure causing temperature rises of greater than 4°C for over 5 min should be considered potentially hazardous. This leaves a wide range of temperature increases which are within the capability of diagnostic ultrasound equipment to produce and for which no time limits are recommended. However, it has not been determined that medical ultrasound causes any adverse reaction or deleterious effect.
The American Institute of Ultrasound in Medicine states that as of 1982, no independently confirmed significant biologic effects had been observed in mammalian tissue below (medical usage) 100mW/cm2.
See also Ultrasound Regulations and Ultrasound Radiation Force.
The FDAultrasound regulations allow an eight-fold increase in ultrasoundintensity to be used in fetalultrasound examinations. They place considerably responsibility on the user to understand the output measurements, the mechanical index (MI), the thermal index (TI) and to use them in their scanning. The primary safety concern in prenatal diagnostic imaging is temperature rise. It is known that hyperthermia is teratogenic. The efforts of investigators have concentrated on defining the temperature increases and exposure times which may give rise to biological effects and on determining the ultrasound levels which might, in turn, lead to those temperature rises.
In fetalultrasound, the highest temperature increase would be expected to occur at bone and the thermal index with bone at/near the focus (TIB) would give the ‘worst case’ conditions. The mechanical index and thermal index must be displayed if the ultrasound system is capable of exceeding an index of 1. The displayed indices are based on the manufacturer’s experimental and modeled data. However, an independent study has demonstrated significant discrepancies over declared spatial peak time averaged intensity (I-SPTA) output of up to 400%.
See also ALARA Principle, Pregnancy Ultrasound and Doppler Fluximetry in Pregnancy.
As low as reasonably achievable (ALARA) is in ultrasound imaging (ultrasonography) as well as in other medical imaging modalities (MRI, X-RAY, etc.) the guiding principle to keep patient exposure as low as possible for the diagnostic result.
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