## Chapter 27. Physics of Ultrasound

Ultrasound is defined as sound that has a frequency greater than ___ cycles per second.

A. 1000

B. 10,000

C. 20,000

D. 5000

C is correct. Ultrasound is high-frequency sound and refers to mechanical vibrations above 20 kHz. Ultrasound frequencies commonly used for medical diagnosis are between 2 and 15 MHz.

A, B, and D are incorrect. These frequencies are not related to the definition of ultrasound.

What type of ultrasound image mode results in a two-dimensional image of an area that is also scanned by a linear array of 100–300 piezoelectric elements?

A. A-mode

B. B-mode

C. Doppler mode

D. M-mode

B is correct. This produces a two-dimensional image by scanning 100–300 piezoelectric elements as opposed to only one element that occurs in a mode. In B-mode the horizontal and vertical directions represent real distances in tissue, whereas the intensity of the grayscale indicates echo strength.

A is incorrect. A-mode is when the transducer sends a single pulse of ultrasound into the medium and creates only a one-dimensional image.

C is incorrect. Color Doppler produces a color-coded map of Doppler shifts superimposed onto a B-mode ultrasound image. Blood flow direction depends on whether the motion is toward or away from the transducer.

D is incorrect. This occurs when a single beam in an ultrasound scan can be used to produce a picture with a motion signal, where movement of a structure such as a heart valve can be depicted in a wavelike manner.

Which of the following is increased by an increase in either the propagation speed or density of the medium?

A. Frequency

B. Axial resolution

C. Acoustic velocity

D. Acoustic impedance

D is correct. Acoustic impedance is the degree of difficulty demonstrated by a sound wave being transmitted through a medium; it is equal to density ρ multiplied by acoustic velocity c (z = ρc). It increases if the propagation speed or the density of the medium is increased.

A is incorrect. Frequency is the number of cycles repeated per second and measured in hertz (Hz).

B is incorrect. Axial resolution is the minimum ...

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