Biological Effects of Diagnostic Medical Ultrasound Biophysical research and ultrasoundbiological effect studies show that ultrasound can produce changes in livingsystems 4. The biological effects of ultrasound depend on the total energyapplied to a given tissue. Therefore, duration variation of exposure to waveemission, frequency and intensity of theultrasound beam, pulsedor continuous emission modality and acousticpower, may lead to significant biological effects, that are commonlydivided in thermal and non-thermal effects 5.The thermal mechanism refers to heatingof tissues due to absorption of acoustic energy in tissues and its transformation into heat. In contrast to the non-thermal effects of ultrasound heat is considered as a potential teratogenic factor.
Risk levels due to heatingof tissues are 39.5 °C (103.1 °F) for embryonic tissues and 41.0°C (105.
8 °F) for adult tissues. Factors that improveheating include higher ultrasound intensity, longer exposure time, higherfrequency, higher absorption, higher thermal conductivity, and lower perfusion3. As much as 70% of the total temperature increase associatedwith ultrasound occurs within the first minuteof exposure 1, but temperature continuesto rise as exposure time is prolonged. Minimizing the exposure time isprobably the single most important factor for ensuringpatient safety fromthermal injury 2. Whenever ultrasonic energy is propagatedinto anattenuating material such as tissue, the amplitude of the wave decreases with distance.
This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of the wave energy that is converted into heat, andscattering can be thought of as that portion which changes direction. Since themedium can absorb energy to produce heat, a temperature rise may occur as longas the rate at which heat is produced id greater than the rate at which the heat is removed 4 .Thenon-thermal mechanism involves the mechanical phenomena of ultrasonic action, mainly cavitation_l3l Cavitation, in broad sense, refers to ultrasonical induced activity occurringin liquid or liquidlike solid materialthat contains bubbles or pockets containing gas or vapor. These bubblesoriginate within materials at locations termed “nucleation sites,”the exact nature and source of whichare not well understood in a complexmedium such as tissue.
Cavitationcan affect a biologicalsystem by virtue of temperature rise, a mechanical stress, and/or free radicalproduction. Even so, this is traditionally referred to as a nonthermalmechanisml4l.This mechanism is responsible for the possible mechanical damage to biologicalstructures. actors that improve cavitation include higher negativeamplitude of acousticpressure, lower frequency, longer duration of acoustic impulses, higher repetitionfrequency, and lower viscosity. In general, biological risks depend on physicalcharacteristics of the ultrasound wave (mode, intensity, and frequency) andsensitivity of the tissue examined to ultrasonic action (size, structure, andattenuation). For the assessment of possible biological risks there existthree main approaches. The first one is theoretical approach which is based onproduction of simplified models of biological systems and calculation ofphysical parameters responsible for biological effects.
Second, experimentalapproach, comprises investigation of the experimental influence of biomedicalultrasound on different levels of biological organization (biomolecules, cellsand tissues, and whole organisms). And the third one is epidemiological approachthat comprises retrospective and prospectivestudies of ultrasound diagnostic exposures on human population, especiallyduring pregnancy. This approachhas majorimportance for safety assessment3.The biologicaleffects discussed in this paper do not appear to represent a hazard if you know the limits on the acousticoutput and transfers the responsibility for a safe examination from the manufacturer to the examiner. Ipersonally feel all right about allowing myself to be scanned in studyingpurposes in the lab as well as scanning other students. References 1. 2 Doody C.
,Porter H., Duck F.A., Humphrey V.F. (1999).
In vitro heating of human fetalvertebra by pulsed diagnostic ultrasound. Ultrasound Med Biol, 25: 1289-94.2. 3 Deane C., Lees C. (2000). Doppler obstetric ultrasound:A graphical display of temporal changes in safety indices.
Ultrasound ObstetGynecol, 15: 418-23.3. Hlinomozova Z, Hrazdira I.
(2005). ALARA- Principle andsafety problems of diagnostic ultrasound. Scripta Medica (BRNO), 78(6),341-3464. William D. O’Brien (1986).
Biological effects ofultrasound: rational for the measurement of selected ultrasonic outputquantities. FuburaPublishing Company, 3(3),165-179.5. http://www.wikiecho.org/wiki/Biologic_Effects_of_Ultrasound_and_Safety