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 'Absorption' 
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AbsorptionMRI Resource Directory:<br> - UltraSound Physics -
 
Absorption is the transfer of energy from the ultrasound beam to the tissue. Absorption of acoustic energy increases the temperature of the tissue. This phenomenon, known as thermal radiation, has been used with some limited success to treat cancerous lesions in the breast and prostate gland. The absorption is proportional to the frequency.
See also Absorbed Dose, Thermal Effect, Thermotherapy.
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 Further Reading:
  Basics:
Physics Tutorial: Ultrasound PhysicsOpen this link in a new window
   by www.physics247.com    
  News & More:
ultrasound characteristics of breast cancerOpen this link in a new window
   by rad.usuhs.mil    
Ultrasound in diagnostic and therapyOpen this link in a new window
   by www.anst.uu.se    
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Absorbed DoseMRI Resource Directory:<br> - UltraSound Physics -
 
In physics, the absorbed dose is the ultrasonic power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg). The absorption increases with ultrasound intensity and frequency.
The thermal index describes the potential for heating of the patient's tissue due to the application of energy.
See also Thermal Effect, Ultrasound Safety, Ultrasound Regulations.
Radiology-tip.comRadiation Absorbed Dose
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Radiology-tip.comAbsorbed Dose
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 Further Reading:
  Basics:
Physics Tutorial: Ultrasound PhysicsOpen this link in a new window
   by www.physics247.com    
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Acoustic LensMRI Resource Directory:<br> - UltraSound Physics -
 
The acoustic lens is placed at the time the transducer is manufactured and cannot be changed. The acoustic lens is generally focused in the mid field rather than the near or far fields. The exact focal length varies with transducer frequency, but is generally in the range of 4-6 cm for a 5 MHz curved linear probe and 7-9 cm for a 3.5 MHz curved transducer.
Placing the elevation plane (z-plane) focal zone of the acoustic lens in the very near or far field would improve the beam width at precisely those depths. However, this would degrade the beam width to a much greater and unacceptable degree at all other depths.
There are some chemicals in ultrasound couplants that can degrade the acoustic lens, destroy bonding, or change the acoustic properties of the lens. Problematic chemicals include mineral oil, silicone oil, alcohol, surfactants, and fragrances. Fragrance can affect the transducer's acoustic lens or face material by absorption over time into elastomer and plastic materials, thus changing the material's weight, size, density, and acoustic impedance. Surfactants can degrade the bond between the lens and the piezoelectric elements and contribute to the accelerated degeneration of the lens.
See also Retrolenticular Afterglow.
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 Further Reading:
  Basics:
INTRODUCTIONOpen this link in a new window
   by www.fink.com    
  News & More:
Sound-field modification with acoustic lenses for high-intensity focused ultrasound therapyOpen this link in a new window
   by www.ncbi.nlm.nih.gov    
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AttenuationMRI Resource Directory:<br> - UltraSound Physics -
 
Attenuation is the reduction of power, for example due to the passage through a medium or electrical component. In ultrasound imaging, attenuation means the decrease in amplitude and intensity as a sound wave travels through a medium. In ultrasound attenuation is often characterized as the half-value layer, or the half-power distance. These terms refer to the distance that ultrasound will travel in a particular tissue before its energy is attenuated to half its original value.

Attenuation originates through:
point divergence of the wavefront;
point absorption of wave energy;
point elastic reflection of wave energy;
point elastic scattering of wave energy.
A thick muscled chest wall will offer a significant obstacle to the transmission of ultrasound. Non-muscle tissue such as fat does not attenuate acoustic energy as much. The half-value layer for bone is still less than muscle, that's why bone is such a barrier to ultrasound.
See also Attenuation Coefficient, and Derated Quantity.
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 Further Reading:
  Basics:
An Introduction to UltrasoundOpen this link in a new window
   by www.cis.rit.edu    
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    
  News & More:
ACOUSTIC CARACTERIZATION OF ULTRASOUND CONTRAST AGENTSOpen this link in a new window
2007   by etd.adm.unipi.it    
Ultrasound artifactsOpen this link in a new window
   by rad.usuhs.mil    
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Axial ResolutionMRI Resource Directory:<br> - Image Quality -
 
Axial resolution is the minimum separation between two interfaces located in a direction parallel to the beam (objects above and below each other) so that they can be imaged as two different interfaces. The axial space resolution directly relates with the wave frequency, but higher frequencies have lower penetration into tissues.
The axial resolution is inversely proportional to the frequency of the transducer depending on the size of the patient. The higher the frequency the lower the axial resolution in large patients. This state results from the rapid absorption of the ultrasound energy with lower penetration. Lower frequencies are utilized to increase depth of penetration.
See also Damping.
Radiology-tip.comSpatial Resolution,  Frequency Encoding
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 Further Reading:
  Basics:
Ultrasound Physics Main differences between Ultrasound and X-rays, Velocity of sound in some Biological MaterialsOpen this link in a new window
   by www.drgdiaz.com    
Physics, instrumentation and basic technique.Open this link in a new window
   by dynamicultrasound.org    
  News & More:
Ultrasound and k-space K-space transforms of elementary geometriesOpen this link in a new window
   by dukemil.egr.duke.edu    
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