• Section 13 Biologic Hazards of Ultrasonography Answers and Feedback

  • Section 13 Biologic Hazards of Ultrasonography Answers and Feedback 

    1. ANSWER: A
    FEEDBACK: The instantaneous intensity (i) is determined from the measured acoustic pressure (p) by using the equation ,  where c is the speed of sound, and ρ is the density of the medium. Intensity parameters, such as (a) spatial peak, temporal average intensity and (b) spatial average, temporal average intensity, are typically derived by averaging the instantaneous intensity with respect to space, time, or both. Frequently, pulsed-wave ultrasound is characterized by a peak negative pressure (expressed in megapascals). The peak negative pressure is also called the peak rarefactional pressure. The correct option is A.

    2. ANSWER: E
    FEEDBACK: Relative measurement of intensity compares the value at one point with a reference intensity and is expressed on a logarithmic scale in units of decibels (dB). The intensity change or level in decibels is

    Level (dB)=10 log  

    where I is the intensity at the point of interest, and Iref is the reference intensity. If the intensity at the point of interest equals one-half the reference intensity, then the level in decibels calculated by 10 log (0.5) is –3 dB. The correct option is E.

    3. ANSWER: C
    FEEDBACK: The “Standard for Real-time Display of Thermal and Mechanical Acoustic Output Indices on Diagnostic Ultrasound Equipment” was adopted by the AIUM and NEMA in 1992, with minor revisions in 1996 and 1998. This benchmark, which has become known as the output display standard (ODS), developed a formalism to calculate in real time the effect of operator-selected parameters on acoustic output. Two acoustic output parameters, the thermal index (TI) and the mechanical index (MI), were devised. The TI, in essence, gives the maximum temperature rise in tissue caused by energy absorption, and the MI describes the likelihood of cavitation based on peak rarefactional pressure and frequency. The correct option is C.

    4. ANSWER: A
    FEEDBACK: The thermal index indicates the maximum temperature rise in tissue based on a homogeneous tissue model and instrument parameters (power, transducer aperture, beam dimensions, and scanning mode). The correct option is A.

    5. ANSWER: E
    FEEDBACK: The heat production rate in a small volume is determined by the absorption coefficient of the tissue and the time-averaged intensity of the ultrasound beam, which depends on the pulse repetition frequency and power. The rate of absorption for most tissues increases linearly with frequency. Variations in the heat production rate occur because of different tissue types and nonuniformity of the ultrasound field. The initial rate of temperature rise cannot be maintained. Heat removal by conduction and perfusion quickly slows the rate of temperature rise. Focused beams create small localized regions of heating. The removal of heat from small volumes is very rapid. Continuous insonation ultimately produces a steady-state condition in which the maximum temperature does not change. Results of experiments quantifying the heating of rat skull bone exposed to a focused ultrasound beam form the basis for thermal models involving the insonation of bone. The correct option is E.

    6. ANSWER: D
    FEEDBACK: Thermal-induced damage is a threshold phenomenon; that is, no biologic effects are observed unless the temperature elevation exceeds a particular value for a minimum time duration. For example, a temperature increase of 2.5°C must be present for 2 hours to cause fetal abnormalities. At higher temperatures, the time necessary to induce damage is shortened dramatically (eg, at 43 °C, the time decreases to 1 minute). The correct option is D.

    7. ANSWER: H
    FEEDBACK: Time-gain compensation (TGC) and gray-scale mapping do not affect acoustic output. TGC is a processing technique during acquisition which applies variable amplification to the detected signal based on time delay since the transmitted pulse. Gray-scale mapping is a post-processing technique which translates echo signal strength into brightness level in the displayed image. The correct option is H.

    8. ANSWER: C
    FEEDBACK: Cavitation is more likely to occur at high pressures and low frequencies. Scientific research has indicated that cavitation-induced effects may be possible at peak pressures and frequencies within the operational range of diagnostic equipment. Specifically, lung and intestinal hemorrhages in mice have been reported at diagnostic output levels. The mechanical index equals the peak rarefactional pressure in tissue divided by the square root of the frequency and is expressed as a dimensionless quantity. For water, cavitation has not been observed if the mechanical index is less than 0.7 over the frequency range 1–10 MHz. The correct option is C.

    9. ANSWER: A
    FEEDBACK: The Bioeffects Committee of the AIUM was established to examine the current knowledge concerning bioeffects and to assess the risk of clinical diagnostic ultrasound. This committee regularly publishes critiques of research reports and issues statements regarding the safety of diagnostic ultra-sound. Its conclusions are acknowledged to be safety guidelines throughout the ultrasound community. The history and rationale for this statement are provided in Bioeffects and Safety of Diagnostic Ultrasound. The correct option is A.

    10. ANSWER: B
    FEEDBACK: Cavitation can be either stable or transient. In stable cavitation, microbubbles already present in the medium expand and contract during each cycle in response to the applied pressure oscillations. The bubbles may also grow as dissolved gas leaves the solution during the negative-pressure phase, a process called rectified diffusion. Each bubble oscillates about the expanding radius for many cycles without collapsing completely. At a characteristic frequency (which is a function of the size of the bubble), the vibration amplitude of neighboring liquid particles is maximized. Transient cavitation is a more violent form of microbubble dynamics in which short-lived bubbles undergo large size changes over a few acoustic cycles before completely collapsing. During the rarefaction phase, bubbles may be formed by dissolved gases leaving the solution, or bubbles of submicron dimensions may already exist in the medium. The correct option is B.

    11. ANSWER: B
    FEEDBACK: Three thermal indices corresponding to soft tissue (TIS), bone (TIB), and cranial bone (TIC) have been developed depending on whether bone is encountered along the path and, if it is, whether bone is located near the transducer or in the interior of the body. TIS applies when the ultrasound beam passes through soft tissue only and bone is not present (examinations of the abdomen and fetus during the first trimester). If bone is encountered near the transducer, then TIC is used (examinations of pediatric and adult head). TIB applies if the ultrasound beam, after passing through soft tissue, impinges on bone near the focal zone (examinations of the fetus during the second and third trimesters). The correct option is B.

    12. ANSWER: D
    FEEDBACK: Epidemiologic studies and surveys of clinical experience have yielded no firm evidence of any adverse effects from in utero ultrasound exposure. An association of diagnostic ultrasound with low birth weight, dyslexia, and delayed speech development has been reported in some studies. However, the majority of epidemiologic studies have been negative for any bioeffect. The correct option is D.

    13. ANSWER: B
    FEEDBACK: Based on bioeffects data and clinical efficacy, as well as psychosocial, economic, and legal/ethical issues, the NIH panel concluded that ultrasound examination in pregnancy should be performed for a specific medical indication. A recommendation for routine screening was not justified by data on clinical efficacy and safety. Specifically, there is not enough evidence that routine screening benefits either the mother or the fetus. There is no evidence showing potential damage to either mother or fetus from routine screening. The correct option is B.

    14. ANSWER: C
    FEEDBACK: The acoustic pressure or temperature rise at the site of interest may be either underestimated or overestimated by the acoustic output parameter, and therefore the knowledgeable user must consider the nature of the clinical situation in order to make an informed judgment. The NCRP recommends the risk-benefit decision if the MI exceeds 0.5 or the TI exceeds 1.0. The correct option is C.Answers Section 13 Biologic Hazards of Ultrasonography

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