Physics of Ultrasound Sesiont 3 (Ultrasound Techniques)

 
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 Ultrasound beam shape and electronic focusing (after Röthlin, Bouillon, and Klotter)
Velocity of sound propagation:
This depends on the density of the medium (approximately 1500–1600m/s in soft tissues and fluids, 331m/s in air, and 3500
m/sin bone).Ultrasound instruments arecalibrated to a mean sound velocity of 1540m/s
Axial resolution:
A sound pulse composed preferably of two (or three) wave lengths is emitted in the longitudinal (axial) direction. The maximum ability to
resolve two separate points in the longitudinal direction is equal to one half the pulse length, or approximately one wavelength. For example, the resolution at an operating frequency of 3.5MHz is approximately equal to 0.5(-1)mm
Lateral resolution: The ultrasound beam initially converges with increasing depth, and then widens out again with decreasing intensity and resolution. The
focal zone (“waist”) of the beam is 3, wavelengths wide and is the area where lateral resolution is highest (Fig.1).
The lateral resolution at a frequency of 3.5 MHz is approximately 2mm, meaning that twoadjacent points can bedistin guished as separate points when they are at least 2mm apart
Focusing: The purpose of beam focusing in sonography is to achieve maximum resolution and improve the ability to recognize fine details :
Technical options:
– Make the transducer face concave to produce a convergent beam (concave mirror effect).
– Use a collecting lens.
Mechanical focusing:
This creates a fixed focal zone that cannot be moved (fixed-focus system), although it can be modified somewhat by scanning
through a fluid offset.
Electronic focusing:
With this option, the focal zone can be set to any desired depth (Fig.1).
For example, the focal zone can be positioned to give a sharp image of the gallbladder, or it can be extended over the full depth of the image field.
Adjusting the focus during an ultrasoundexamination:
This is the hallmark of a proficient examiner. Onefeature ofahigh-qualityultrasound system is that a definite change in resolution is seen as the focal zone is moved
Propagation Characteristics of Sound Waves:
The propagation ofultrasound wavesobeysthelawsofwavephysics. The following terms have been adopted from radiation optics and wave optics.
Reflection: Sound waves are partially reflected and partially transmitted in biological tissues. An image of an organ is generated from the returning echo signals by analyzing the impedance differences at acoustic interfaces.The higher the acoustic impedance, the greater the degreeof reflection, with total reflection occurring at interfaces with a very high impedance mismatch (e.g., between soft tissue and bone, calcium, or air, producing a high amplitude echo). Interfaces with a high acoustic impedance (e.g., gallstones) reflect all of the incident sound and cast an acoustic shadow .
Scattering:
This consists of randomly directed reflections that occur at tissue interfaces and rough surfaces. The echoes generated by scattering centerscontri-
bute significantly to medical imaging (e.g., the imaging of rounded organ contours).
Refraction:
This phenomenon is most pronounced at smooth interfaces with a high acoustic impedance.The sound waves are deflected at an oblique anglerela
tive to the direction of the main beam.
Absorption and attenuation: These describe the “loss” of sound waves due to their spatial distribution in the tissue and the conversion of sound energy to
heat. According to the findings of a WHO commission, the conversion of sound energy to heat is within safe limits at the low energy levels used in diagnostic ultrasound.
Even so, it is prudent to use the lowest possible ultrasound energy when scanning children and pregnant women. Sound waves are also attenuated intissuesasaresultofreflection,scattering,andrefraction.Thisleadstoa significant energy loss,which is off set bya djusting the time gain compensation (TGC) on the scanner.





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