Passage
Ultrasound is a technique that uses the propagation properties of high-frequency sound waves to construct images of internal organs or to measure blood flow velocity. The shape and position of internal organs are resolved by measuring the return time of reflected ultrasound waves. Similarly, blood flow velocity is resolved by measuring the Doppler shift of reflected ultrasound waves. An ultrasound device consists of an array of piezoelectric crystals and an acoustic lens. The piezoelectric crystals generate sound when an alternating voltage is applied, and the acoustic lens increases the transmission efficiency from the device to the body by reducing sound reflection at the skin.Ultrasound is performed using frequencies in the range of 2-15 MHz. The lower ultrasound frequencies are used for deep abdomen and obstetric/gynecological imaging due to their greater penetrating ability. The higher ultrasound frequencies have less penetrating ability but provide higher resolution, and therefore are used for blood flow measurements and the imaging of more superficial structures. Detailed ultrasound penetration data is shown in Figure 1.
Figure 1 Penetration in soft tissue for three different ultrasound frequenciesSound waves propagate through soft tissue at an average speed of 1,500 m/s and through blood at 1,570 m/s.
-Blood flows toward and later passes by a doppler ultrasound probe that is fixed in place. Relative to the probe, the velocity of blood changes from negative to positive. Which of the following best illustrates the relationship between blood velocity v_B and the frequency shift of returning waveforms Δf?

Question

Passage
Ultrasound is a technique that uses the propagation properties of high-frequency sound waves to construct images of internal organs or to measure blood flow velocity. The shape and position of internal organs are resolved by measuring the return time of reflected ultrasound waves. Similarly, blood flow velocity is resolved by measuring the Doppler shift of reflected ultrasound waves. An ultrasound device consists of an array of piezoelectric crystals and an acoustic lens. The piezoelectric crystals generate sound when an alternating voltage is applied, and the acoustic lens increases the transmission efficiency from the device to the body by reducing sound reflection at the skin.Ultrasound is performed using frequencies in the range of 2-15 MHz. The lower ultrasound frequencies are used for deep abdomen and obstetric/gynecological imaging due to their greater penetrating ability. The higher ultrasound frequencies have less penetrating ability but provide higher resolution, and therefore are used for blood flow measurements and the imaging of more superficial structures. Detailed ultrasound penetration data is shown in Figure 1.

Figure 1 Penetration in soft tissue for three different ultrasound frequenciesSound waves propagate through soft tissue at an average speed of 1,500 m/s and through blood at 1,570 m/s.
-Blood flows toward and later passes by a doppler ultrasound probe that is fixed in place. Relative to the probe, the velocity of blood changes from negative to positive. Which of the following best illustrates the relationship between blood velocity v_B and the frequency shift of returning waveforms Δf?

Quizplus · Accepted Answer

11ecba2d_12eb_7c0c_a3bf_85f32f6faa65_MD0008_00 With relative motion between a source and an observer, the observed waveform differs from the original, emitted waveform in terms of both frequency and wavelength. This phenomenon is known as the Doppler effect, and the frequency shift is proportional to the relative velocity of the source. According to the passage, blood flow velocity is determined by the magnitude of the Doppler shift (Δf) detected by the ultrasound device (ie, the observer). Because the ultrasound waves are first reflected off the blood before detection by the probe occurs, the flowing blood acts as the source of sound. When blood is flowing toward the device, flow velocity (v_B) is negative (ie, subtracted from waveform velocity) and the frequency shift is positive (ie, observed frequency > original frequency). A positive frequency shift occurs because each successive wave is reflected closer to the device. Conversely, when blood is flowing away from the device, flow velocity is positive (ie, added to waveform velocity) and the frequency shift is negative (ie, observed frequency < original frequency). A negative frequency shift occurs because each successive wave is reflected farther from the device. (Choice A) A positive frequency shift means that observed waveform frequency increases. An increase in received waveforms would only occur as blood flows toward the ultrasound device (ie, negative velocity). (Choice B) A negative frequency shift means that observed waveform frequency decreases, which would only occur as blood flows away from the ultrasound device (ie, positive velocity). (Choice D) The plot must cross the x-axis because the frequency shift will change from positive to negative as blood transitions from approaching the probe to moving away from the probe. Educational objective: The Doppler effect occurs when the observed frequency and wavelength of a sound are shifted from those of the original due to relative motion between the source and the observer. The frequency shift is positive when the source velocity is negative (moving closer) and negative when the source velocity is positive (moving away).

Passage Ultrasound Is a Technique That Uses the Propagation Properties of of High-Frequency