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Ultrasound Physics
Chapter 10: Sound Beam
| Question | Answer |
|---|---|
| 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
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