Sound Beams Chapter 9 - Ultrasound Physics PDF

Sound Beams Chapter 9 Special note This section talks about a beam created by a single disc-shaped, unfocused PZT crystal operating in the continuous wave mode In Diagnostic US, the crystal element is not typically in shape of disc & does not operate in CW.  Sound Beams So...

Sound Beams Chapter 9 Special note This section talks about a beam created by a single disc-shaped, unfocused PZT crystal operating in the continuous wave mode In Diagnostic US, the crystal element is not typically in shape of disc & does not operate in CW.  Sound Beams Sound beams are shaped like an hourglass. Narrow beams create better images As the sound travels, the width of the beam will change. It begins at the same diameter as the transducer, narrows and then widens again. Terms to know Converge – to come together or narrow Diverge – to grow apart or widen  Sound Beam transducer Narrowing/Converging divergence Near Zone Also called Fresnel zone (the S is silent) This is the region from the transducer to the focus. The beam gradually narrows in the near zone For a continuous wave disc shaped crystal the diameter of the sound beam as it leaves the transducer is the same as the diameter of the active element. At the end of the near zone, the beam narrows to ½ of the width of the active element.  Focus Focus or Focal Point-location where the sound beam reaches its minimum diameter Reflections from this zone are more accurate than those from other zones because narrow beams create better images Focal Depth the distance from the transducer face to the focus (also called focal length or near zone length) Determined by characteristics of the crystal Focal Zone region where beam is narrowest and picture is best  focal zone Focus transducer Near zone/ Far zone/ Fresnel zone fraunhofer zone Determining Focal Depth Determined by 2 factors Transducer diameter diameter and focal depth are directly related Frequency of the ultrasound Frequency and focal depth are directly related Beams with deep focus have lower intensity at the focus than beams with a shallow focus (because of attenuation) Deep Focus/Focal Depth Shallow Focus/Focal Depth Larger diameter PZT Small Diameter PZT High Frequency Lower Frequency  Most modern ultrasound machines allow you to alter the focal depth. They use phased array transducers (more on this later) In early development of US, the focal depth was fixed and could not be changed by the sonographer, thus it was determined by the characteristics of the PZT. The Imaging Conundrum… What? High frequency sound How do we over come this creates a deeper conundrum? focus, but we image Manufacturers utilize a very superficially with high frequencies??? tiny diameter crystal in transducers with high frequencies. This can displace the focus and create a shallow focus. The math of the focus…  Anatomy of a Sound Beam – Far Zone Far Zone –(fraunhofer zone or far field) region deeper than the focus. Beam Diverges in the far zone. The beam begins to diverge at the focus and when two near zone lengths, it is the same size as the PZT again. Beyond that point, the beam is wider than the PZT. In this zone the beam diverges  Sound Beam Divergence Describes the spread of the sound beam in the deep far zone Determined by 2 factors: Transducer diameter Frequency of sound In the far field, beam is narrowest with a large diameter, high frequency transducer In the far field, beam is widest with a small diameter low frequency transducer Less Divergence More Divergence Larger diameter transducer Smaller diameter transducer High frequency Low frequency  Large diameter – deep focus, less divergence small diameter – shallow focus, more divergence Summary Large Diameter Crystal Small Diameter Crystal High Frequency Low Frequency Deeper focus Shallow focus Less divergence in the far field More divergence in the far field Spherical Waves (a.k.a Diffraction Pattern, Huygen’s Wavelets) All of the previous information has been regarding a disc shaped crystal When you have a very tiny piece of PZT that is not necessarily a disc shape, the sound beam is shaped like a V. This occurs when the sound source and the wavelength of sound are close to the same size. One Element PZT Huygens's Principle Small sound sources create “v” shaped wavelets. Ultrasound transducers have many tiny elements not one large round element The hourglass shape of our sound beams is produced by interference between these tiny wavelets. Multiple Elements PZT PZT PZT PZT PZT PZT PZT