chapter 9: sound beams

what 5 terms describe the shape and regions of a sound beam?

• focus

• near zone

• focal length or near zone length

• far zone

• focal zone

focus/focal point (end of the near zone, beginning of the far zone, middle of the focal zone)

the location where the beam is the narrowest

at the focus the width of the sound beam is:

½ the width of the beam as it leaves the transducer

near zone/near field/Fresnel zone

• the region from the transducer to the focus

• the beam gradually narrows/converges within the near zone

for a CW, disc-shaped crystal, the diameter of the sound beam as it leaves the transducer is:

the same as the diameter of the active element

focal length/near zone length

the distance from the transducer to the focus

far zone/far field, Fraunhofer zone

• the region that starts at the focus and extends deeper

• the beam diverges/spreads out

at the beginning of the far zone, the beam is:

½ as wide as it is at the transducer

when the beam is two near zone lengths from the transducer, the beam is:

the same size as the active element

at depths more than two near zone lengths, the beam is:

wider than the active element

focal zone

• a region on either side of the focal point where the beam is relatively narrow, resulting in superior image detail

• half in near field and half in far field

phased array

adjustable focus systems

with a fixed focus transducer, what 2 factors combine to determine the focal depth?

• transducer diameter

• frequency of the sound

how are transducer diameter and focal depth related?

directly

how are frequency and focal depth realted?

directly

deep focus

• larger diameter PZT

• higher frequency

beam divergence

the gradual spread of the ultrasound beam in the far field

what 2 factors combine to determine beam divergence?

• transducer diameter

• frequency of the sound

how are crystal diameter and beam divergence related?

inversely

do larger or smaller diameter crystals improve lateral resolution in the far field?

larger diameter crystals improve lateral resolution in the far field

how are frequency and beam divergence related?

inversely

does higher or lower frequency sound improve lateral resolution in the far field?

higher frequency sound improves lateral resolution in the far field

more divergence

• smaller diameter

• lower frequency

Huygens wavelets/spherical waves/diffraction patterns

• V-shaped waves produced by tiny pieces of PZT

• created when the source is about the size of the sounds wavelength

Huygens sources

the small sources of sound that produce Huygens wavelets

Huygens Principle

• states that a large active element may be thought of as millions of tiny, distinct sound sources and each of these tiny particles is a Huygens source and creates a Huygens wavelet with a V-shape

• explains the shape of an imaging transducers emitted sound beam based upon in-phase and out-of-phase wavelets interfering with each other

the hourglass shape produced by a large crystal is the result of:

interference of the many Hyugens sound wavelets emitted from these numerous sound sources