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Noninterpretive Skills in Ultrasound
Bioeffects of Ultrasound
The benefit of diagnostic ultrasound as an imaging modality in clinical practice cannot be overstated. Avoiding the radiation of radiography and computed tomography and the restrictive nature of magnetic resonance imaging coupled with the portability of ultrasound machines make ultrasound an attractive diagnostic tool. Although there are no known deleterious effects in humans when used within normal ranges, knowledge of potential bioeffects of ultrasound is important to avoid improper use, especially with newer techniques and equipment. Imaging professionals should have adequate knowledge of the potential bioeffects and methods to reduce risk to the patient, especially fetal and neonatal patients. This chapter reviews the known and theorized bioeffects of ultrasound and presents current guidelines for safe practice.
Ultrasonic waves deposit energy in the tissues with transmission. Tissues also undergo compression and rarefaction under the mechanical force of ultrasound ( Fig. 21.1 ). Whether or not the energy or waves cause deleterious outcomes to the cells depends on the characteristics of the ultrasound beam and time involved. Data have shown increased caspase levels in cells exposed to ultrasound, intimating an activation of an apoptotic pathway, although this has not been confirmed in the human model in the normal diagnostic range of output. The specific bioeffects of diagnostic ultrasound in the animal model have been examined for many years, through direct evaluation as well as applying results from other modalities.
Dose
Ionizing radiation dosage can be measured and/or inferred and is related to time of exposure, whereas ultrasound dosage is related to indices of pressure and intensity but not associated with time of exposure.
Bioeffects
There are two main bioeffects of ultrasound: thermal effects, resulting in localized tissue heating, and mechanical effects, resulting in tissue distortion and disruption. Another theoretical bioeffect is radiation force, although this is believed to be a minor effect in the output range of diagnostic ultrasound.
Thermal Bioeffects
Sound waves should be viewed as waves with energy. These waves lose amplitude as they pass through tissue by absorption and scatter, which leads to localized tissue heating. This localized increase in heat, if great enough, is presumed to create hydrogen peroxide, a free radical, which can create DNA strand breaks leading to cell death. Mild temperature increases have been shown to accelerate cellular processes without cellular detriment, but moderate increases in temperature can arrest or retard cellular division.
Tissue characteristics play a large role in temperature modulation. In regions of high blood flow, the blood acts as a heat sink and thus temperature increase is attenuated. In regions of poor blood flow, temperature increase is greatest. Due to low perfusion, the globe is particularly sensitive to ultrasound and thus output in ophthalmic ultrasound is limited to 50 mW/cm 2 . Although ophthalmic ultrasound is routinely performed on pediatric and adult patients, ophthalmic ultrasound on a fetus should not be performed unless absolutely necessary.
There are many other modifying factors, including body habitus, perfusion, distance to the transducer, bone, and fluid presence. The first 5 mm of tissue experiences the greatest temperature increase as the sound wave loses energy. This effect is amplified with high-frequency ultrasound, which is common for superficial evaluations.
Different tissues absorb energy at different rates. Soft tissues such as adipose and muscle absorb less heat than bone and cartilage. Bone quickly absorbs energy resulting in heat.
There are many documented effects of temperature increase in fetal animal model studies, including neural tube defects, microphthalmia, cataracts, microcephaly, and functional and behavioral problems. Hyperthermia is also a proven teratogen in animals and is considered teratogenic in humans. None of these congenital abnormalities have been shown to be caused by diagnostic ultrasound.
In the animal model, significant temperature increases of more than 4°C for more than 5 minutes have been shown to cause developmental and congenital abnormalities. This temperature increase is much greater than expected in human diagnostic ultrasound.
There has been no documented adverse effect on an embryo or fetus of an increase of less than 1.5°C in maternal temperature. An increase of 2.5–5.0°C can occur with 1 hour of ultrasound exposure. Effects in nonfetal ultrasound likely require an increase of 18°C for at least 0.1 seconds.
Data from a study on effects of maternal fever on fetal development demonstrate an association of maternal fever exceeding 38.3°C with increased fetal abdominal wall defects. Although maternal fever is a systemic process affecting the entire fetus, it is postulated that an increase in temperature via ultrasound could result in similar effects.
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