Basic principles and safety of diagnostic ultrasound in obstetrics and gynecology
- Thomas D Shipp, MD, RDMS
Thomas D Shipp, MD, RDMS
- Associate Professor of Obstetrics, Gynecology, and Reproductive Biology
- Harvard Medical School
The term "ultrasound" refers to sound waves of a frequency greater than that which the human ear can appreciate, namely frequencies greater than 20,000 cycles per second. For diagnostic ultrasound imaging in obstetrics and gynecology, frequencies of 2 to 12 million cycles per second are used. Ultrasound imaging has been used for medical purposes for several decades and is safe when properly performed .
Understanding the physical principles underlying ultrasound technology can help the sonographer optimize image quality, and thus improve diagnostic capabilities. This information is also vital for maintaining the safety of this technology for the woman and, during obstetrical examinations, her fetus.
Sound waves are a type of mechanical vibration. They are described in terms of their frequency, which is the number of repetitions (ie, cycles) per second. The unit for measuring frequency is the Hertz (Hz). Other characteristics of sound waves are wavelength, the distance between excitations; the amplitude of excitation, measured in decibels (dB); and the period, the time it takes for one cycle to occur, measured in seconds.
The speed with which an acoustic wave moves through a medium is dependent upon the density and resistance of the medium. Media that are dense will transmit a mechanical wave with greater speed than those that are less dense. As an example, the acoustic speed of a mechanical wave through air is 331 meters/second; through water, it is 1495 meters/second; through soft tissue, it is 1540 meters/second; and through bone, it is 4080 meters/second .
Frequency and wavelength are mathematically related to the velocity of the ultrasound beam within the tissue as indicated by the following equation:
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- SOUND WAVES
- INTERACTION OF ULTRASOUND WAVES WITH TISSUES
- Compact, portable ultrasound systems
- ENERGY OUTPUT MEASUREMENT
- External fetal heart rate monitors
- Theoretical concerns about thermal effects, cavitation, and vibration
- Effects in humans
- Statements of major organizations
- - American Institute of Ultrasound in Medicine
- GUIDELINES FOR CLINICAL USE
- B-mode and M-mode
- NONMEDICAL USE
- TECHNIQUES AND INDICATIONS
- SUMMARY AND RECOMMENDATIONS