Chapter 10: Sound Beam
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| What kind of setup is being used to initially describe a sound beam? | a single disc-shaped, unfocused PZT operating in CW mode
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| As sound travels, what changes that affects the shape of a sound beam? | the width of the beam
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| At the starting point, the beam width equals what? | transducer diameter
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| What happens to the sound beam's width after leaving the starting point? | the beam narrows progressively to its smallest diameter after which point the sound beam expands and diverges
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| What 5 terms are used to describe shape and regions of a sound beam? | Focus
Near Zone
Focal Length (focal depth, near zone length)
Far Zone
Focal zone
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| What is the Focus of a sound beam? | location where beam diameter is the narrowest
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| With a disc-shaped crystal, the width of the sound beam at the focus is what the width of the beam as it leaves the TD? | 1/2
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| What are the synonyms of Focus? | Focal point
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| What is Focal Length? | the distance from the TD to the focus
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| What determines the focal length? | the characteristics of the PZT
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| What are some synonyms of focal length? | focal depth, near zone length (NZL)
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| What is the Near Zone? | region from the TD to the focus
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| What happens to the sound beam in the near zone? | the sound beam gradually narrows
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| With a disc-shaped crystal, the diameter of the beam as it leaves the TD in the near zone is the same as the diameter of what? | the active element
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| At the end of the near zone, the beam narrows to what? | 1/2 the width of the active element
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| What is located at the end of the near zone? | the Focus
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| What are some synonyms of the Near Zone? | near field, Fresnel zone
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| What is the Far Zone? | the region starting at the focus and extending deeper
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| What happens to the sound beam within the far zone? | the sound beam diverges
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| With a disc-shaped crystal, how wide is the sound beam at the beginning of the far zone? | 1/2 as wide as it is at the TD
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| With a disc-shaped crystal,when the beam is 2 near zone lengths from the TD, the beam is what size? | the sound beam is again the same size as the active element
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| With a disc-shaped crystal, at depths more than 2 near zone lengths, the sound beam is what? | the sound beam is wider than the active element
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| What are some synonyms of the Far Zone? | far field, Fraunhofer zone
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| What is the Focal Zone? | the region around the focus where the beam is relatively narrow
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| Reflections from the focal zone creates images that are what? | creates images that are more accurate than those at other depths
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| Where is the focal zone located? | half the focal zone is in the near field and half is in the far field
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| What is the name of the area where image detail is improved? | focal zone
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| What characteristics of fixed focus TD combine to determine focal depth? | Transducer diameter
Frequency of the sound
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| How are transducer diameter and focal depth related? | directly related
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| What happens to to the focus when the transducer diameter is increased? | the focus is deeper
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| What happens to the focal depth when the active element is large? | the focus is deeper
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| When the sound frequency is higher, what happens to the focal depth? | the focus is deeper
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| How are frequency and focal depth related? | they are directly related
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| Why do manufacturers make high frequency TD with very small diameter PZT? | to create a shallower focus and a clinically useful TD
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| Diameter of a sound beam is smallest where? | at the end of the near zone
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| At depths greater than the focus, the sound beam does what? | the sound beam diverges
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| What are the characteristics of TD that combine to determine divergence of the beam in the far field? | Transducer diameter
Frequency of sound
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| A smaller crystal diameter produce beams that do what in the deep far zone? | produces a sound beam that diverges more in the deep far zone
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| Crystal diameter and beam divergence is related how? | inversely related
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| What diameter of crystals improve lateral resolution in the far zone? Why? | large diameter crystals
due to less divergence
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| Sound beams with higher frequency do what in the far field? | diverges less in the far field
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| Frequency and beam divergence are related how? | inversely related
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| What frequency (low or high) improves lateral resolution in the far field? | higher frequencies
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| Sound waves produced by very small sources and diverge into a"V" shape are called what? | Spherical Waves
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| Spherical waves are created how? | when the source is about the size of the sound beam's wavelength
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| Spherical waves are also known as what? | diffraction patterns, or Huygen's wavelets
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| What are Huygen's sources? | small sources of sound producing wavelets
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| What explains the inconsistency between the large PZT crystal producing an hourglass-shaped beam and the small sound source creating a V-shaped beam? | Huygens' principle
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| Large active element is composed of what? | composed of millions of tiny distinct sound sources
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| Each tiny particle of a large active element is a what and creates a what with a V-shape? | Huygens' source
Huygens' wavelet
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| The hourglass shape of a sound wave is a result of what? | result of constructive and destructive interference of the many Huygens' sound wavelets
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| What is the ability to distinctly identify 2 structures that are side by side or perpendicular to sound beam's main axis? | lateral resolution
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| What units are used for lateral resolution? | mm, cm, any unit of distance
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| Do smaller or larger numbers indicate more accurate images for lateral resolution? | small numbers
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| Lateral resolution is determined by what? | determined by the width of the beam
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| What type of beam results in better lateral resolution? | narrow beam
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| Beam diameter and lateral resolution varies with changes in what? | depth
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| What does LATA stand for? | synonyms of lateral resolution
Lateral
Angular
Transverse
Azimuthal
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| Lateral resolution is good where? | within the focal zone
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| Where is lateral resolution best? | at the focus where the sound beam is the narrowest
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| What is the equation for lateral resolution? | lateral resolution(mm) = beam diameter(mm)
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| Which type of resolution is superior in clinical imaging and why? | axial resolution is better than lateral resolution- U/S pulses are shorter than wide
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| Which resolution, axial or lateral, is the numeric value lower? | axial resolution
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| Why is LATA resolution not as good as LARRD resolution? | because pulses wider than short
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| What happens when 2 reflectors are closer side by side than the beam width> | only 1 reflection will be displayed on the image
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| What improves both axial and lateral resolution? | high frequency TD
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| Why does a high frequency TD improve axial resolution? | due to shorter pulses associated with higher frequency sound
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| Why does a high frequency TD improve lateral resolution? | because high frequency pulses diverge less in the far field; and
high frequency sound beams are narrower than low frequency
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| What improves lateral resolution by concentrating sound energy into a narrow beam? | focusing
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| What are the three methods of focusing? | External focusing - with a lens
Internal focusing - with a curved active element
Phased array focusing - done by electronics of U/S system
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| Which types of focusing can be used with a single element TD? | Internal and external focusing
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| What type of focusing is used in transducers with multiple active elements? | Phased array focusing
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| What are the two focusing techniques? | fixed, conventional or mechanical
adjustable
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| What two methods of focusing uses fixed focusing techniques? | external and internal focusing
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| Fixed focusing techniques mean what? | that the focal depth cannot be adjusted, and
the extent of focusing cannot be changed
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| How does external focusing work? | lens placed in front of the PZT material
fixed focus with acoustic lens
Arc of lens becomes prominent-degree of focusing increases-beam narrow in focal zone
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| How does internal focusing work? | Uses curved PZT element to concentrate sound energy
forms narrower sound beam
As curvature becomes more pronounced-degree of focusing increases
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| What is the most common form of fixed focusing? | Internal focusing
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| How does electronic focusing-Phased array work? | Phased array transducers uses electronics of the U/S system to focus the sound beam
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| Can sonographers alter the focusing characteristics of electronic focusing - phased array? | yes; they have adjustable or multi-focus capabilities; or
dynamic or variable focusing
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| What are the first 2 of the four distinct alterations of the sound beam when its focused? | Beam diameter in the near field and focal zone narrows ("waist" narrows);
Focus is moved closer to the TD (NZL is reduced or focal is shallower)
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| What are the last 2 of the four distinct alterations of the sound beam when it is focused? | Beam diameter in the far zone increases (more divergence) which improves lateral resolution in near and focal zone and degrades it beyond the focal zone;
Focal zone is smaller (region on each side of the focus)
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