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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.
-How much farther will a 5 MHz signal penetrate compared to a 10 MHz signal when the initial intensity of each signal decreases by a factor of 100?

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.
-How much farther will a 5 MHz signal penetrate compared to a 10 MHz signal when the initial intensity of each signal decreases by a factor of 100?

Quizplus · Accepted Answer

11ecba2d_12eb_06d5_a3bf_43ede6685618_MD0008_00 Intensity is the amount of power (energy per unit time) delivered per unit area. In Figure 1, the relative intensity of ultrasound signals is graphed using the decibel scale. The decibel scale relates the perceived loudness of a sound to its actual intensity. The perception of sound is approximately logarithmic: a sound that is 10 times more intense is twice as loud. The ratio of the attenuated signal intensity I to the source intensity I_S in decibels (dB) is given by: 11ecba2d_12eb_06d6_a3bf_c96d8ac9bb18_MD0008_00 A simple relationship can be used to relate dB changes to relative intensities: a 10-fold increase in intensity is a change of +10 dB; similarly, a 10-fold decrease in intensity is a change of −10 dB. Therefore, a 100-fold decrease in intensity corresponds to a change of (−10) + (−10) = −20 dB. Alternatively, this relationship can be resolved mathematically. From Figure 1, the 10 MHz and 5 MHz signals are attenuated by 20 dB at a depth of 2 cm and 4 cm, respectively. The difference between the two depths is: 11ecba2d_12eb_06d7_a3bf_173c41f19cdb_MD0008_00 Educational objective: The decibel (dB) scale is logarithmic and relates the perceived loudness of a sound to its actual intensity. For each 10-fold decrease, sound intensity decreases by 10 dB. Therefore, a 100-fold decrease in sound intensity corresponds to a decrease by 20 dB.

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