Physics of Ultrasound Bioeffects
William P. Mulvoy III
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1. A higher mechanical index (MI) increases the likelihood that cavitation will occur. A higher MI is most likely to occur in which of the following tissues?
C. Adipose tissue
1. Correct Answer: A. Lung
Rationale: Microbubbles get bigger during the rarefactions caused by ultrasound waves. When the bubbles expand, it causes a shearing effect and the bubbles may burst, creating cavitation. The MI gives the operator information about the magnitude of energy administered to the patient’s tissues during an ultrasound evaluation. The MI is a direct measure of acoustic power per unit time, thus as the number for MI increases, the acoustic power delivered to the surrounding tissues also increases, and cavitation becomes more likely. The organs most sensitive to cavitation are those that are air-filled, such as the lung or the intestine. Thus, the correct answer is the lung, as the other three answers do not have air, which directly relates to the potential for cavitation to occur. MI is a unitless number defined as the peak negative pressure divided by the square root of the frequency of the ultrasound wave:
MI = Peak Rarefaction (Negative) Pressure (MPa)/[check mark] frequency (MHz)
The Food and Drug Administration (FDA) considers 1.9 the maximum allowable MI for diagnostic imaging to prevent the biologic effects of cavitation.
1. Apfel RE, Holland CK. Gauging the likelihood of cavitation from short pulse, low duty cycle diagnostic ultrasound. Ultrasound Med Biol. 1991;17:179.
2. Şen T, Tüfekçioğlu O, Koza Y. Mechanical index. Anatol J Cardiol. 2015;15(4):334-336.
2. Thermal bioeffects are directly responsible for the temperature elevation in the surrounding tissues caused by absorption and scattering of the ultrasound beam. Which intensity description is associated with thermal bioeffects on the surrounding tissues?
A. Spatial peak, temporal peak
B. Spatial peak, temporal average
C. Spatial peak, pulse average
D. Spatial peak, pulse peak
2. Correct Answer: B. Spatial peak, temporal average (SPTA)
Rationale: SPTA is directly related to tissue heating. SPTA limits are 100 mW/cm2 for unfocused ultrasound and 1 W/cm2 for focused ultrasound. The current FDA output limits for diagnostic ultrasound are an SPTA intensity of less than 720 mW/cm2. The output depends on the power, pulse repetition frequency, and the scanner operation mode (i.e., M-mode, pulsed-wave, or continuous-wave Doppler). The SPTA is the highest intensity measured at any point in the ultrasound beam averaged over the pulse repetition period, which directly relates to the biothermal effects on the surrounding tissues. The maximum allowable increase in tissue temperature approved by the FDA is 2 °C. Temperature elevation in fetal soft tissue is potentially more harmful than adult tissue, and this has been determined safe at this level. An elevation of 4 °C for 5 minutes or more will cause thermal bioeffects to the surrounding tissue and fetus, as this has been well documented in laboratory studies.
1. Nelson TR, Fowlkes JB, Abramowicz JS, Church CC. Ultrasound biosafety considerations for the practicing sonographer and sonologist. J Ultrasound Med. 2009;28(2):139-150.
3. The biologic effects of ultrasound are directly proportional to the intensity of the ultrasound beam. Which of the following imaging modalities produces the highest energy output?
A. Color Doppler
B. Continuous-wave Doppler
D. Pulsed-wave Doppler
3. Correct Answer: D. Pulsed-wave Doppler
Rationale: Examination exposure time produces the greatest bioeffect on tissues and therefore lengthy examinations should be avoided whenever possible. The lowest output intensity is with grayscale imaging, while the highest output is used with pulsed-wave Doppler signals. When comparing different Doppler output intensities, pulsed-wave Doppler spends about 8% to 10% of the time transmitting compared to grayscale imaging, which transmits only 1% to 2% of the time. This four- to fivefold increase in transmission time directly results in an intensity that is 5-10 times higher, with potentially more bioeffects. Also, the beam is held in a relatively constant position at the point of interest (sampling volume), which may induce a further increase in temporal average intensity. Hence, the intensity of pulsed-wave Doppler is greater than continuous-wave Doppler or color Doppler. Color Doppler is essentially small multiple pulsed-wave Dopplers, which has all the advantages of depth acuity but lacks the ability to measure very high velocities. Even though color Doppler is multiple small pulsed-wave Dopplers, it is still of lower energy when directly compared to pulsed-wave Doppler.
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