[script type="text/javascript" src="p7ap/p7APscripts.js">

Physics

Ultrasound is sound waves with a higher frequency than human hearing. Humans can hear up to around 20kHz, but diagnostic ultrasound usually operates in the range 2-15MHz..

The resolution and depth penetration of ultrasound is dependant upon the frequency and wavelength of the sound wave. A high frequency allows a higher resolution whereas a larger wavelength will allow greater penetration.

However the wavelength and frequency are linked by the velocity of sound through the substrate.

v = λ x ƒ

  • v is the velocity of sound in the medium
  • λ is the wavelength of the sound wave
  • ƒ is the frequency of the sound wave

Therefore ultrasound machines offer a compromise between resolution and penetration.

Some crystals have a structure that when an electrical current is applied to it will cause a change in the crystalline structure producing movement. An alternating current will produce an oscillating change in the crystal structure producing sound/ultrasound waves.
When a force is applied to these crystals an electrical current is produced and can be measured. This means that the same crystals can be used for both producing the ultrasound waves and as a receiving transducer. Each crystal generates a single ultrasound wave, the product of many waves combine to form the ultrasound beam.

The beams are sent out in short bursts, allowing the signal time to travel to the subject and to return to the probe before the next beam is sent out. The duration of each pulse is defined by the Pulse Length (PL) which is the distance traveled by a sound wave from the start of the pulse generation to the end of the pulse.
The image is based on the sound wave to travel from the probe to tissue plane and back to the probe again. The velocity of sound within soft tissues is relatively constant at around 1540m/s. This means that the depth of the reflecting structure can be calculated.
The ultrasound beam is not parallel. It starts at the width of the probe and converges to its narrowest point – the focal zone. After the focal zone the beam spreads again by diffraction.

As the sound waves travel through a medium the energy is absorbed and converted to heat. The amount of energy attenuated is increased by;-

  • increasing acoustic impedance
  • increasing distance from the transducer
  • short wavelengths of sound

Attenuation is minimized in fluids and is maximal in gases, emphasizing the importance of using plenty of acoustic coupling gel

This is the resistance of a tissue to the passage of sound waves. It is dependent upon the density of the medium and the velocity of sound through the medium.


Z = v x p      

  • Z = the acoustic impedance
  • v = the velocity of sound through the medium
  • p = the density of the medium

The ultrasound image is composed of sound waves that have hit an interface between two tissues and been reflected back towards the probe. The amount of reflection is dependant on the differential acoustic impedance of the tissues.

The maximal amount of reflection will occur when the reflecting structure is at 90 degrees to the ultrasound beam. Progressive angulation of the probe will produce a reduced signal. This angle dependant visibility of structures is called Anisotropy.

There are 3 main types of resolution:-

  • Lateral Resolution
  • Axial Resolution
  • Temporal Resolution

Lateral Resolution is the ability to distinguish between objects lying next to each other, equidistant from the US probe. The lateral resolution is dependent upon the beam width and is therefore greatest at the focal zone. The beam width is inversely related to the sound frequency, so a high frequency probe has the greatest lateral resolution.
Axial Resolution is the ability to distinguish between objects at different depths in the same axial plane of the probe. The axial resolution is inversely related to the pulse length. Higher frequencies allow a shorter pulse length and increase the axial resolution.
Temporal Resolution is the ability to distinguish objects at a specific point in time. It is dependent upon the frame rate and by the depth to the object.

The Doppler Effect describes the apparent change in sound frequency as the source moves towards or away from the observer. As the sound source approaches the observer the sound waves arrive at the observer earlier than wound be expected from a stationary source producing an apparent increase in frequency. As the source moves away the sound wave arrive at the observer with an apparent delay producing a reduced frequency.

Doppler can be used to detect the direction of movement by the direction in change of sound frequency (Colour Doppler) and velocity of movement towards or away from the probe, by the magnitude in the change of frequency (Colour Power Doppler)

Note that if flow is perpendicular to the probe then there will be ne doppler shift and there will be no coloured flow on the screen.

(Christian Doppler 1803-1853)

Gain is the amplification of the received signal. Signal amplification will result in a brighter US picture on the screen. However artifacts and noise will also be amplified.

Time Gain Compensation (TGC) is the selective amplification of signals that come from a specific depth from the probe. This allows compensation for signal attenuation and for the gain to be optimised in the area of interest in the picture.

 

 

 

Resources

nysora

neuraxiom

dhmc

university HK

USinRA