ARDMS (American Registry for Diagnostic Medical Sonography) Practice Test

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If the diameter of the PZT crystal is 13 mm, which sentence describes the sound wave's diameter in the Fresnel zone?

The sound beam will be half of the diameter of the crystal in the Fresnel zone.
13 mm
26 mm
The sound wave narrows as it leaves the transducer.

The correct answer is: The sound wave narrows as it leaves the transducer.

In ultrasound physics, the Fresnel zone is the region in which the sound waves are converging and diverging. When sound waves leave a transducer, they spread out due to diffraction, causing the beam diameter to be larger than the crystal diameter in the far field. Therefore, as the sound wave travels away from the transducer, it does not maintain the same diameter as the PZT crystal. Instead, it narrows and spreads out, resulting in a larger diameter in the Fresnel zone compared to the crystal size. Hence, the correct answer is D: "The sound wave narrows as it leaves the transducer." Options A, B, and C are incorrect because they do not accurately describe the behavior of sound waves in the Fresnel zone.