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Definition
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Definition
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Definition
Shaped like an hourglass
Beam starts same size as the crystal -> narrows like a funnel to smallest diameter -> widens again
Narrow beams create better images |
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Definition
Focus
Near zone
Focal length aka near zone length
Far zone
Focal zone
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Definition
| Location where the sound beam reaches minimum diameter |
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Term
| Focal Depth/Length aka Near Zone Length |
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Definition
The distance from the transducer face to the focus
Deep focus = lower intensity at focus
Shallow focus = higher intensity at focus
Determined by:
- Transducer diameter (directly related)
- Frequency of ultrasound (directly related)
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Term
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Definition
Region where beam is narrowest and picture is best
Diameter = diameter of crystal ÷ 2 |
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Term
| Near Zone aka Fresnel Zone |
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Definition
Region between transducer and focus
Beam converges here |
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Term
| Far Zone aka Fraunhofer Zone |
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Definition
Region deeper than the focus
Beam diverges here |
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Term
| Calculating Focal Depth in Soft Tissue |
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Definition
FD = [diameter (mm)*frequency (MHz)] ÷ 6
OR
FD = [diameter (mm)]2 ÷ 4*wavelength
To overcome dilemma of higher frequency sound creating deeper focus in superficial imaging, manufacturers make very small diameter, high frequency crystals |
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Term
| Sound Beam Divergence ie Far Field Beam |
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Definition
Determined by:
Transducer diameter (inversely related)
Frequency of sound (inversely related)
Larger diameter and higher frequency improve lateral resolution in the far field
sin divergence θ = 1.85 ÷ [diameter (mm)*frequency (MHz)]
OR
sin divergence θ = 1.2*wavelength ÷ diameter |
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| High Frequency Pulsed Transducers Summary |
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Definition
Less divergence in far field
Narrower beams
Improve axial resolution across the whole image
Improve lateral resolution in far field only |
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Term
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Definition
| The ability to distinguish two structures when the structures are parallel to the sound beam |
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Definition
| The ability to distinguish two structures when the structures are perpendicular to the sound beam |
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Term
| Spherical Waves aka Diffraction Patterns aka Huygen's Wavelets |
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Definition
A tiny piece of PZT that is not necessarily a disc shape produces V-shaped sound beams
Occurs when the sound source and the wavelength of sound are close to the same size |
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Term
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Definition
Small sound sources create V-shaped wavelets
Occurs when sound source and wavelength of sound are close to the same size
In-phase and out-of-phase interference between wavelets creates hourglass shape of sound beams in imaging transducer |
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