PHYSICAL PROPERTIES

Definition of Ultrasound (US):

Sound with frequency greater than 20,000 cycles per second or 20kHz.  Audible sound sensed by the human ear are in the range of 20Hz to 20kHz.

Advantages:

Ultrasound can be directed as a beam.
Ultrasound obeys the laws of reflection and refraction.
Ultrasound is reflected by objects of small size.

Disadvantages:

Ultrasound propagates poorly through a gaseous medium.
The amount of ultrasound reflected depends on the acoustic mismatch.

The Four Acoustic Variables:

1. Pressure - the amount of force over a given area.
2. Distance - particle displacement with the wave
3. Temperature -
4. Density

Reflection and Propagation:

Effect of propagation through gaseous zones - poor propagation, inadequate imaging.

Effect of propagation through dense zones - nearly all of the US is reflected. Structures below dense zones are poorly imaged.

Examples of dense materials - bone, calcium, metal.

Material	Speed of Propagation
bone	4080 m/s
blood	1570 m/s
tissue	1540 m/s
fat	1450 m/s
air	330 m/s

© Atlantic Interactive

Definitions:

Cycle - the combination of one rarefaction and one compression equals one cycle.

Amplitude - the maximum displacement of a particle or pressure wave.

Intensity - the amount of force or energy of sound.

Decibel (dB) - a numerical expression of the relative loudness of sound.

Wavelength - the distance between the onset of peak compression or cycle to the next.

Velocity - the velocity is the speed at which sound waves travel through a particular medium. Velocity is equal to the frequency x wavelength.

The velocity of US through human soft tissue is 1540 meters per second.

Frequency - the number of cycles per unit of time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength.

Acoustic Impedance - simply put, acoustic impedance is dependent on the density of the material in which sound is propagated through. The greater the impedance the more dense the material.

Reflection - the portion of a sound that is returned from the boundary of a medium. (echo)  The angle of incidence influences the reflected and refracted waves.

atlantic interactive adapted from weyman

Refraction - the change of sound direction on passing from one medium to another.

Acoustic Mismatch - the boundary between two different media where reflection and refraction occurs.

Attenuation - the decrease in amplitude and intensity as a sound wave travels through a medium.

 

Types of Echoes:

Specular - echoes originating from relatively large, regularly shaped objects with smooth surfaces. These echoes are relatively intense and angle dependent. (i.e. IVS, valves)

Scattered - echoes originating from relatively small, weakly reflective, irregularly shaped objects are less angle dependant and less intense. (ie. blood cells)

Scattering: Reflection and Refraction are affected by the material being imaged.

Frequencies:

Frequencies for adult imaging - 2.0mHz to 3.0mHz.

Frequencies for pediatric imaging - 5.0mHz to 7.5mHz to 12mHz.

Effect of higher frequencies on penetration - the higher the frequency the less penetration, the lower the frequency the greater the penetration.


Artifacts:

Acoustic Shadowing - the loss of information below an object because the greater portion of the sound energy was reflected back by the object. This occurs in objects like prosthetic valves.



Enhancement - the increase in relection amplitude from objects that lie behind a weakly attenuating structure. Enhancement may occur in structures below a cyst.

Reverberation - the unsuitable reflections generated when the sound wave strikes a highly reflective object creating artifacts that degrade the image.  The peak of the sector scan window is usually filled with reverberations due to the initial transmission of sound energy reflecting off of the chest wall and being reflected off the transducer face in a repetitious fashion.  Reverberations may occur in more internal structures like the diaphragm or from dense objects such as a mechanical valve prothesis. Mirroring may occur as sound energy is reflected off dense structures and displayed on the screen as a double image.

Side-Lobe - produced from the side lobes of the ultrasound beam. This artifact appears as false structures in the scan plane.