Unit 9

Beam Anatomy

  1. Near Zone

  • Also known as near field or fresnel zone

  • Starts at the transducer ends at the focus

  • The width of the near field can get is equal to the diameter of the element

  • Diameter of crystal = aperture (D)

Example: aperture = 5mm

  • The widest the near zone will be is 5mm

  1. Focal length

  • Also known as near zone length on focal depth

  • Distance from the transducer to the focal point

  • Near zone length is directly related to both frequency and diameter

  • Near Zone Formula :

  1. Focus

  • Also known as focal point, end of near zone, beginning of far zone, and middle of focal zone

  • Most narrow part of the beam

  • The focus is ½ of the diameter

  • Example diameter = 10, focus =5 mm

  1. Far zone

  • Also known as far field or Fraunhofer zone

  • Starts at the transducer away from the focus

  • Narrowest point is focus

  • At 2 near zone length, the far field diameter = the element diameter

  • Formula:

    1. Focal Zone

  • Extends equally to the far zone and near zone with the focus point in the middle

Practice

12MHz transducer and 8 mm wide element

  1. What is the width of the beam as it exits the transducer?

  • 8 mm because equals the element diameter

  1. At what depth is the focus?

  1. How wide is the beam at the focus?

  • 4 because it is ½ of the element

  1. At what depth in the far field does the beam diameter equal the 8 mm?

  • 128x128= 256 mm

  1. If the focal zone starts at 9.8 cm into the beam where does it end

  1. What diameter is the beam at 30 cm?

>8mm

  • frequency and focal depth are DIRECTLY related

  • Diameter and focal depth are DIRECTLY related

Beam Divergence

  • diameter and divergence are INVERSELY related

  • Frequency and divergence are INVERSELY related

  • Lateral resolution is dependent on the width of the beam

- the focal zone should be placed at area of interest or below because that is the narrowest place of the beam and will provide superior detail

  • diverging far fields = poor image detail.

  • High frequency transducers = less divergence which results in better detail

  • Frequency and beam divergence are inversely related

•Low frequency= shallow focus and more divergence

  • Less divergence wide diameter = deep focus and less divergence

  • Numerator in a equation = directly related

  • Denominator in a equation= inversely related