In the Fraunhofer zone, how does the beam behave?

In the Fraunhofer zone, also known as the far field, the behavior of the ultrasound beam is characterized by its divergence. As the beam propagates through this zone, it begins to spread out, causing an increase in the beam's width. This is due to the wavefronts emanating from the transducer becoming increasingly planar as they travel away from the source. When ultrasound waves move away from the transducer and enter the Fraunhofer zone, they exhibit a predictable pattern of spreading, which is essential for understanding how ultrasound images are formed as the energy interacts with tissues.

The other options do not accurately describe the behavior of the beam in the Fraunhofer zone. While the intensity of the beam is measurable, it does not remain the greatest in this region nor is it inconsistent; rather, it can be more uniform but decreases as it spreads out. Similarly, the beam is not conical in shape in this zone; instead, it aligns more towards planar wavefronts as the phase differences become less significant at greater distances from the source.