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Beam PatternMRI Resource Directory:<br> - UltraSound Physics -
 
The beam pattern shows the relative amplitude of the acoustic pressure as a function of direction relative to the transducer. Beam patterns are three-dimensional. The transmit and receive beam patterns are basically the same.
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Beam SteeringMRI Resource Directory:<br> - Image Quality -
 
Beam steering is a method of steering the main lobe of a transducer to a certain direction. Array transducers have the ability to be steered as well as focused. Like focusing, the beam is directed by sequentially stimulation of each element. This feature creates the sector scan by rapidly steering the beam from left to right to give the two dimensional cross sectional image.
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 Further Reading:
  Basics:
Transmission Line Matrix (TLM) modelling of medical ultrasound(.pdf)Open this link in a new window
   by www.era.lib.ed.ac.uk    
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Ultrasound compounding with automatic attenuation compensation using paired angle scans(.pdf)Open this link in a new window
June 2006   by mi.eng.cam.ac.uk    
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Beam WidthMRI Resource Directory:<br> - UltraSound Physics -
 
The width of the main beam lobe of the transducer is described in degrees. The beam width is usually defined as the width between the half power point and the -3dB point.
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Physics, instrumentation and basic technique.Open this link in a new window
   by dynamicultrasound.org    
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Beam Width ArtifactInfoSheet: - Artifacts - 
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ReductionMRI Resource Directory:<br> - Artifacts -
 
The dimension of the ultrasound beam and the transducer array are the origin of the beam width artifact or volume averaging artifact. When the ultrasound beam is wider than the diameter of the lesion being scanned, normal tissues which lie immediately adjacent to the lesion arc included within the beam width, and their echotexture is averaged in with that of the lesion.
Thus, what appears to be the echogenicity of the lesion is really that of the lesion plus the averaged normal tissues. Because of volume averaging, cystic lesions may falsely appear to be solid, and some subtle solid lesions may become impossible to distinguish from surrounding normal tissue and, therefore, not identified at all.
See also Ultrasound Picture and Vector Array Transducer.
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BeamformingMRI Resource Directory:<br> - Artifacts -
 
The wider the ultrasound beam, the more severe the problem with volume averaging and the beam-width artifact, to avoid this, the ultrasound beam can be shaped with lenses.
Different possibilities to focus the beam:
point Mechanical focusing is performed by placing an acoustic lens on the surface of the transducer or using a transducer with a concave face.
point Electronic focusing uses multiple phased array (annular or linear) elements, sequentially fired to focus the beam.
Conventional multi-element transducers are electronically focused in order to minimize beam width. This transducer type can be focused electronically only along the long axis of the probe where there are multiple elements, along the short axis (elevation axis) are conventional transducers only one element wide. Electronic focusing in any axis requires multiple transducer elements arrayed along that axis. Short axis focusing of conventional multi-element transducers requires an acoustic lens which has a fixed focal length.
For operation at frequencies at or even above 10 MHz, quantization noise reduces contrast resolution. Digital beamforming gives better control over time delay quantization errors. In digital beamformers the delay accuracy is improved, thus allowing higher frequency operation. In analog beamformers, delay accuracy is in the order of 20 ns.
Phased beamformers are suitable to handle linear phased arrays and are used for sector formats such as required in cardiography to improve image quality. Beamforming in ultrasound instruments for medical imaging uses analog delay lines. The signal from each individual element is delayed in order to steer the beam in the desired direction and focuses the beam.
The receive beamformer tracks the depth and focuses the receive beam as the depth increases for each transmitted pulse. The receive aperture increase with depth. The lateral resolution is constant with depth, and decreases the sensitivity to aberrations in the imaged tissue. A requirement for dynamic control of the used elements is given. Since often a weighting function (apodization) is used for side lobe reduction, the element weights also have to be dynamically updated with depth.
See also Huygens Principle.
Radiology-tip.comBeam
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 Further Reading:
  Basics:
Ultrasound beamforming and image formation(.pdf)Open this link in a new window
2007   by dukemil.bme.duke.edu    
  News & More:
Ultrasonic Testing Using Phased ArraysOpen this link in a new window
   by www.ndt.net    
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Journals - Pelvic - Calculation - Artifacts - Endoscopic - Vaginal
 
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