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Ultrasound Physics

Chapter 10: Sound Beam

QuestionAnswer
What kind of setup is being used to initially describe a sound beam? a single disc-shaped, unfocused PZT operating in CW mode
As sound travels, what changes that affects the shape of a sound beam? the width of the beam
At the starting point, the beam width equals what? transducer diameter
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
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
What is the Focus of a sound beam? location where beam diameter is the narrowest
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
What are the synonyms of Focus? Focal point
What is Focal Length? the distance from the TD to the focus
What determines the focal length? the characteristics of the PZT
What are some synonyms of focal length? focal depth, near zone length (NZL)
What is the Near Zone? region from the TD to the focus
What happens to the sound beam in the near zone? the sound beam gradually narrows
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
At the end of the near zone, the beam narrows to what? 1/2 the width of the active element
What is located at the end of the near zone? the Focus
What are some synonyms of the Near Zone? near field, Fresnel zone
What is the Far Zone? the region starting at the focus and extending deeper
What happens to the sound beam within the far zone? the sound beam diverges
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
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
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
What are some synonyms of the Far Zone? far field, Fraunhofer zone
What is the Focal Zone? the region around the focus where the beam is relatively narrow
Reflections from the focal zone creates images that are what? creates images that are more accurate than those at other depths
Where is the focal zone located? half the focal zone is in the near field and half is in the far field
What is the name of the area where image detail is improved? focal zone
What characteristics of fixed focus TD combine to determine focal depth? Transducer diameter Frequency of the sound
How are transducer diameter and focal depth related? directly related
What happens to to the focus when the transducer diameter is increased? the focus is deeper
What happens to the focal depth when the active element is large? the focus is deeper
When the sound frequency is higher, what happens to the focal depth? the focus is deeper
How are frequency and focal depth related? they are directly related
Why do manufacturers make high frequency TD with very small diameter PZT? to create a shallower focus and a clinically useful TD
Diameter of a sound beam is smallest where? at the end of the near zone
At depths greater than the focus, the sound beam does what? the sound beam diverges
What are the characteristics of TD that combine to determine divergence of the beam in the far field? Transducer diameter Frequency of sound
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
Crystal diameter and beam divergence is related how? inversely related
What diameter of crystals improve lateral resolution in the far zone? Why? large diameter crystals due to less divergence
Sound beams with higher frequency do what in the far field? diverges less in the far field
Frequency and beam divergence are related how? inversely related
What frequency (low or high) improves lateral resolution in the far field? higher frequencies
Sound waves produced by very small sources and diverge into a"V" shape are called what? Spherical Waves
Spherical waves are created how? when the source is about the size of the sound beam's wavelength
Spherical waves are also known as what? diffraction patterns, or Huygen's wavelets
What are Huygen's sources? small sources of sound producing wavelets
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
Large active element is composed of what? composed of millions of tiny distinct sound sources
Each tiny particle of a large active element is a what and creates a what with a V-shape? Huygens' source Huygens' wavelet
The hourglass shape of a sound wave is a result of what? result of constructive and destructive interference of the many Huygens' sound wavelets
What is the ability to distinctly identify 2 structures that are side by side or perpendicular to sound beam's main axis? lateral resolution
What units are used for lateral resolution? mm, cm, any unit of distance
Do smaller or larger numbers indicate more accurate images for lateral resolution? small numbers
Lateral resolution is determined by what? determined by the width of the beam
What type of beam results in better lateral resolution? narrow beam
Beam diameter and lateral resolution varies with changes in what? depth
What does LATA stand for? synonyms of lateral resolution Lateral Angular Transverse Azimuthal
Lateral resolution is good where? within the focal zone
Where is lateral resolution best? at the focus where the sound beam is the narrowest
What is the equation for lateral resolution? lateral resolution(mm) = beam diameter(mm)
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
Which resolution, axial or lateral, is the numeric value lower? axial resolution
Why is LATA resolution not as good as LARRD resolution? because pulses wider than short
What happens when 2 reflectors are closer side by side than the beam width> only 1 reflection will be displayed on the image
What improves both axial and lateral resolution? high frequency TD
Why does a high frequency TD improve axial resolution? due to shorter pulses associated with higher frequency sound
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
What improves lateral resolution by concentrating sound energy into a narrow beam? focusing
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
Which types of focusing can be used with a single element TD? Internal and external focusing
What type of focusing is used in transducers with multiple active elements? Phased array focusing
What are the two focusing techniques? fixed, conventional or mechanical adjustable
What two methods of focusing uses fixed focusing techniques? external and internal focusing
Fixed focusing techniques mean what? that the focal depth cannot be adjusted, and the extent of focusing cannot be changed
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
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
What is the most common form of fixed focusing? Internal focusing
How does electronic focusing-Phased array work? Phased array transducers uses electronics of the U/S system to focus the sound beam
Can sonographers alter the focusing characteristics of electronic focusing - phased array? yes; they have adjustable or multi-focus capabilities; or dynamic or variable focusing
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)
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)
Created by: quietone055 on 2010-03-30



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