Basic physics of ultrasound

Basic Physic of Ultrasound and color doppler at Sonotech, Pakistan

 

Analouge Vs Digital Beam Former

 

 

Analog vs. Digital Beamforming
In analog beamforming (ABF) and digital beamforming (DBF) ultrasound systems, the received pulses reflected from a particular focal point along a beam are stored for each channel, then aligned in time, and coherently summed—this provides spatial processing gain because the noise of the channels is uncorrelated. Images may be formed as either a sequence of analog levels that are delayed with analog delay lines, summed, and converted to digital after summation (ABF)—or digitally by sampling the analog levels as close as possible to the transducer elements, storing them in a memory (FIFO), and then summing them digitally (DBF).

Figures 4 and 5 show basic respective block diagrams of ABF and DBF systems. Both types of systems require perfect channel-to-channel matching. Note that the variable-gain amplifiers (VGAs) are needed in both implementations—and will continue to be in the digital case until ADCs with a large enough dynamic range become available at reasonable cost and low enough power. Note that an ABF imaging system needs only one very high resolution and high speed ADC, but a DBF system requires many high speed, high resolution ADCs. Sometimes a logarithmic amplifier is used in the ABF systems to compress the dynamic range before the ADC.


 

Simplified block diagram of ABF system.

 

 

 

basic physics of ultrasound - Analogue Beamformer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Simplified Block Diagram of Digtial Beamformer

Definition of Ultrasound

 

 

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

Definition of Ultrasound and Color Doppler at Sonotech, Pakistan

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 and Refraction

 

 

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.

 

refraction and reflection of ultrasound and color dopler at sonotech pakistan

 

 
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.
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)
 

 

 

specular and scattered echoes at sonotech pakistan

 

 

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.



 

Sub Beam Former

 

 

A sub-beamforming method and apparatus are applied to a portable, one-dimensional ultrasonic imaging system. The sub-beamforming circuitry may be included in the probe s assembly housing the ultrasonic transducer, thus minimizing the number of signals that are communicated between the probe assembly and the portable processor included in the imaging system. Including the sub-beamformer in the probe assembly also relieves the portable processor of some of the signal processing tasks. The sub-beamforming apparatus may be implemented digitall or analog components.

 

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