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 'Huygens Principle' 
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Huygens PrincipleMRI Resource Directory:<br> - UltraSound Physics -
 
Huygens principle states that an expanding sphere of waves behaves as if each point on the wave front were a new source of radiation of the same frequency and phase. The principle explains how a flat ultrasound transducer can transmit a narrow ultrasound beam, which in the near field is confined to the dimensions of the transducer surface.
Spherical wavelets are emitted from numerous point sources on the transducer surface. They interfere to form a narrow, slightly converging beam of ultrasound in the near field. The wavefronts in the beam are nearly parallel. A precondition for this interference is that the transducer surface is much larger than the ultrasound wavelength.
See also Interference Artifact.
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 Further Reading:
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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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Pelvic - Pregnancy - Thyroid - Online Books - Societies - Ultrasound Guided Interventions
 
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    
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Ultrasonic Testing Using Phased ArraysOpen this link in a new window
   by www.ndt.net    
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Endoscopic - Portals - Preferential Sites - Breast - General - UltraSound Technician and Technologist Career
 
FrequencyMRI Resource Directory:<br> - UltraSound Physics -
 
(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
The effect of different frequencies on tissue penetration:
The higher the frequency the less the penetration, the lower the frequency the greater the penetration. As frequency increases, resolution improves but the imaging depth or penetration decreases. The lower the axial resolution, the more detail can be seen.
Usual frequencies for pediatric ultrasound: 5.0mHz to 7.5mHz and 10mHz.
Usual frequencies for adult ultrasound: 2.0mHz to 3.0mHz.
See also Doppler Interrogation Frequency, Multi-frequency Probe, and Huygens Principle.
Radiology-tip.comFrequency
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Radiology-tip.comFrequency
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 Further Reading:
  Basics:
An Introduction to UltrasoundOpen this link in a new window
   by www.cis.rit.edu    
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Normal Ultrasound of the SpineOpen this link in a new window
   by rad.usuhs.mil    
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Safety - Universities - Societies - Equipment and Parts - RIS - Education
 
Sound BeamMRI Resource Directory:<br> - UltraSound Physics -
 
(short for ultrasound beam) The sound beam is the confined, directional beam of ultrasound traveling as a longitudinal wave from the transducer face into the propagation medium. The near field and the far field are two separate regions along the beam. Sound beams are either steered mechanically or electrically. Both rapidly sweep sound waves through tissues.
See also Sheer Wave, Beam Vessel Angle, Beam Steering, and Huygens Principle.
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  News & More:
Ultrasonic Testing Using Phased ArraysOpen this link in a new window
   by www.ndt.net    
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UltraSound Technician and Technologist Career - UltraSound Technician and Technologist Jobs - Liver - Directories - Preferential Sites - 4d UltraSound
 
TransducerInfoSheet: Probes/Transducers
Intro,
Probes, 
TransducersMRI Resource Directory:<br> - Probes Transducers -
 
A transducer is a device, usually electrical or electronic, that converts one type of energy to another. Most transducers are either sensors or actuators. A transducer (also called probe) is a main part of the ultrasound machine. The transducer sends ultrasound waves into the body and receives the echoes produced by the waves when it is placed on or over the body part being imaged.
Ultrasound transducers are made from crystals with piezoelectric properties. This material vibrates at a resonant frequency, when an alternating electric current is applied. The vibration is transmitted into the tissue in short bursts. The speed of transmission within most soft tissues is 1540 m/s, producing a transit time of 6.5 ms/cm. Because the velocity of ultrasound waves is constant, the time taken for the wave to return to the transducer can be used to determine the depth of the object causing the reflection.
The waves will be reflected when they encounter a boundary between two tissues of different density (e.g. soft tissue and bone) and return to the transducer. Conversely, the crystals emit electrical currents when sound or pressure waves hit them (piezoelectric effect). The same crystals can be used to send and receive sound waves; the probe then acts as a receiver, converting mechanical energy back into an electric signal which is used to display an image. A sound absorbing substance eliminates back reflections from the probe itself, and an acoustic lens focuses the emitted sound waves. Then, the received signal gets processed by software to an image which is displayed at a monitor.
Transducer heads may contain one or more crystal elements. In multi-element probes, each crystal has its own circuit. The advantage is that the ultrasound beam can be controlled by changing the timing in which each element gets pulsed. Especially for cardiac ultrasound it is important to steer the beam.
Usually, several different transducer types are available to select the appropriate one for optimal imaging. Probes are formed in many shapes and sizes. The shape of the probe determines its field of view.
Transducers are described in megahertz (MHz) indicating their sound wave frequency. The frequency of emitted sound waves determines how deep the sound beam penetrates and the resolution of the image. Most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency, but multi-frequency probes are also available.
See also Blanking Distance, Damping, Maximum Response Axis, Omnidirectional, and Huygens Principle.
Radiology-tip.comTransmitter
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 Further Reading:
  Basics:
Testing Diagnostic Ultrasound Probes For Bad CrystalsOpen this link in a new window
Saturday, 19 September 1998   by www.pamia.org    
  News & More:
Ultrasound beamforming and image formation(.pdf)Open this link in a new window
2007   by dukemil.bme.duke.edu    
Transmission Line Matrix (TLM) modelling of medical ultrasound(.pdf)Open this link in a new window
   by www.era.lib.ed.ac.uk    
US Resources  
Musculoskeletal and Joint - Contrast Agents - Fetal - Breast - Services and Supplies - UltraSound Technician and Technologist Career
 
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