A summary of a report from the National Council on Radiation Protection and Measurements (NCRPM) published in 2002 states that ultrasound radiation produces both thermal and mechanical effects in living tissue. While ultrasound imaging is considered one of the safest forms of medical imaging, the long-term effects of exposure have yet to be determined, and the Food and Drug Administration (FDA) advises its use only when medically necessary.
Basics of Diagnostic Ultrasound Physics
Diagnostic ultrasound, or diagnostic sonography, involves the application of high-frequency sound waves to allow for the viewing of body tissues and structures in real time. Images can depict blood flow and/or the movement of internal organs. A handheld transducer, which acts much like a loudspeaker, is placed against the skin and slowly moved over the region to be imaged. The sound waves put out by the transducer bounce off the body structures. These reflected sound waves, or echoes, are then translated into images based on frequency and strength of the signal along with the time it takes to return from the internal structures to the transducer.
Types of Diagnostic Ultrasound
Diagnostic ultrasound procedures include Doppler fetal heart rate monitors, bone sonography for detecting osteoporosis and echocardiograms to view the heart. Ultrasound also guides biopsy procedures to obtain tissue samples.
Effects on Living Tissue
When ultrasound waves enter the body, tissues are heated slightly. In some cases, ultrasound has been known to produce pockets of gas in tissue or body fluids, a phenomenon known as cavitation. In addition to the heating or thermal effects, ultrasound also produces nonthermal or mechanical effects such as audible sounds, the movement of cells in liquid, pressure changes and electrical changes in cell membranes.
Non-Ionizing Vs. Ionizing Radiation
As discussed previously, ultrasound waves put out enough energy to cause the basic constituents of matter or living tissue to move around resulting in both thermal and nonthermal effects. The energy produced is not enough, however, to break chemical bonds holding that matter or tissue together. This puts ultrasound in the category of non-ionizing radiation. Other examples of non-ionizing radiation are visible light, radiowaves and microwave radiation. The more damaging type of radiation to living tissue is known as "ionizing radiation." Gamma and X-rays fall into this category. These types of waves are high enough in frequency and energy to break chemical bonds and, therefore, cause lasting damage and/or fundamental changes to living tissue.
Considerations
Because ultrasound is a non-ionizing radiation, it doesn't have the same sort of risks as other types of ionizing radiation like X-ray; however, there is some concern regarding fetal exposure to ultrasound, which is why both international and national organizations, like the FDA, have advised that ultrasound only be used when medically necessary. Nonmedical use of ultrasound, such as fetal keepsake videos, is discouraged. The NCRPM advises that more research is required to determine the long-term effects of ultrasound.
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