Why is the intensity of an ultrasound beam generally higher in the focal zone?

The intensity of an ultrasound beam is generally higher in the focal zone due to the phenomenon of beam focusing, which results from having a smaller beam diameter in that region. As the beam is focused, the energy becomes concentrated into a smaller area, thereby increasing the intensity. Focusing occurs as ultrasound waves travel through the medium and meet at a point, which is referred to as the focal point. At this point, the energy is more densely packed than in the surrounding areas where the beam is wider.

Consequently, the smaller beam diameter in the focal zone leads to a higher intensity because intensity is defined as power per unit area. As the area is reduced due to focusing, the same amount of acoustic power is concentrated over a smaller surface area, resulting in a higher intensity measurement.

The other options relate to aspects of ultrasound physics but do not directly explain the increase in intensity at the focal point in the same way. Increased frequency can affect resolution and penetration but isn't the primary reason for higher intensity in the focal zone. Similarly, a longer wavelength is associated with reduced resolution and does not contribute to increased intensity. Lastly, while a higher duty factor can increase average intensity over time, it does not specifically address the reason for the increased intensity in the focal location itself