Chapter 27. Physics of Ultrasound
Ultrasound is defined as sound that has a frequency greater than ___ cycles per second.
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?
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?
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 ...