What happens to the near zone length as the aperture size increases?

The near zone length, also known as the Fresnel zone or near field, is directly affected by the aperture size of the ultrasound transducer. As the aperture size increases, the near zone length also increases. This is due to the relationship defined by the formula for the near zone length, which is calculated as:

Near Zone Length = (Aperture Diameter)² / (4 × Wavelength)

From this equation, it is evident that any increase in the aperture diameter leads to a proportional increase in the near zone length, assuming the wavelength remains constant. A larger aperture allows for better beam focusing and improved spatial resolution within the near field, which is crucial for obtaining clearer images from structures close to the transducer. Thus, an increase in aperture size enhances the focusing capability and extends the distance over which the ultrasound wave remains collimated before it begins to diverge, resulting in an increased near zone length.