# Ultrasound Beam Dynamics

**Near zone, far zone, diameter, frequency and beam width. **

**Equation and relationships. **

The **focus **is the narrowest point of the beam. This is where you will have the greatest lateral resolution. Intensity will be highest here (not accounting for attenuation). Remember that intensity is power distributed over an area. Because the focus will have the smallest area of any portion of the beam the intensity will be the highest there.

The beam diameter will be around half of the diameter of the crystal at the focus. It will diverge and become the thickness of the crystal at 2 times the NZL.

**NZL = D²/ 4****λ** near zone length = transducer diameter (mm)² / 4 x wavelength(mm)

**NZL = D²f / 4****c** near zone length = transducer diameter (mm)² x frequency / 4 x propagation speed

Near zone length is inversely related to wavelength, if the wavelength goes up then the near zone length will go down. It is directly related to the frequency. Increase the frequency and you increase the near zone length.

* **You may wonder why then if you have a high frequency and a long near zone length with a deep focus why then are high frequency transducers only used for shallow objects. Remember the attenuation coefficient. Attenuation = Frequency / 2. If you increase the frequency you increase the attenuation. *

#### Franco

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