Ultrasound 1
Ultrasound involves the transmission of sound waves which are reflected to different extents by tissue boundaries back to the transducer, which is converted into a digital signal and grey scale image. Audible sound waves range from 20 to 20,000Hz. However, ultrasound operates in the range of 2 to 10MHz, deeming it inaudible to the human ear.
Sound must propagate through an elastic medium by compression, high pressure, or rarefaction, low pressure of the particles that comprise that medium. Rarefaction is a reduction in the density of a medium, especially of air or gas.
Differences Between Sound and Electromagnetic Waves
1. Medium Requirement
Sound waves, unlike electromagnetic waves, require a medium through which they propagate. Electromagnetic waves are capable of propagation through a vacuum.
2. Frequency and Penetration
As the frequency of the sound wave increases, penetration decreases. For electromagnetic waves, as the frequency increases, penetration also increases.
3. Direction of Oscillation
Sound waves are longitudinal waves, meaning particles move parallel to the direction of the wave. Electromagnetic waves are transverse waves, meaning particle movement is perpendicular to wave motion.
Frequency Selection
Ultrasound frequency determines wavelength, and thus spatial resolution. A high frequency results in smaller wavelength and high spatial resolution. However, high frequency also decreases penetration. Thus, the frequency must be selected according to the clinical task. For superficial anatomy, a high frequency is selected, as little penetration is required. For deeper anatomy, e.g. abdominal structures, a lower frequency is selected to reach greater penetration.
Characteristics of Ultrasound Waves
1. Wavelength
Wavelength is the distance between two consecutive points of equal distance in the wave, e.g. two rarefactions or two compressions. It is measured in mm or micrometers.
2. Frequency
Frequency is the number of oscillations per second performed by the wave. It is measured in cycles per second or Hz.
3. Period
Period is the time it takes to complete one cycle. It is equal to the inverse of the wave frequency and is measured in seconds.
4. Amplitude
Amplitude measures the energy of the sound wave. It is measured by the maximum change in pressure or particle displacement amplitude.
5. Speed
The speed of the wave is the distance travelled per unit time. It is equal to the product of wavelength and frequency or wavelength divided by period. Ultrasound speed is also given by the square root of the inverse of compressibility by density.
Acoustic Impedance
Acoustic impedance, Z, is the resistance a tissue boundary offers to the transmission of ultrasound waves through it. It depends on the speed (m/s) of the ultrasound and the density (kg/m^3) of the material through which the ultrasound must propagate through. The transfer of energy between two tissues depends on their individual acoustic impedances. The greater the difference in the acoustic impedance between two materials, e.g. muscle and lung, the greater the amount of ultrasound that is reflected. The lesser the difference in the acoustic impedance between two materials, the greater the amount of ultrasound that is transmitted. It is measured in kg/m^2s or rayl.
Ultimately, acoustic impedance gives rise to differences in transmission and reflection in ultrasound imaging, which is the basis for pulse echo ultrasound imaging.
Speed of Ultrasound
The speed of ultrasound is determined by the compressibility of a material, K, and the density of that material, p. It is equal to the square root of the inverse of the product of these two values.
Refraction
Refraction is a change propagation direction of an ultrasound wave when it is non-perpendicular to the tissue boundary. It is determined by the velocity of the ultrasound across the tissue boundary. The angle of transmission is dependent on the incident speed and angle of incidence. For a lung to muscle interface, the incident speed is much higher than the transmission speed, so the angle of incidence will exceed the angle of transmission. If the speed in the incident wave is less than that of the transmission wave, the angle of transmission will exceed the angle of incidence. However, should the speed be equal across the interface, no refraction occurs.
Scattering in Ultrasound Imaging
Scattering in ultrasound is either a result from a boundary or tissue interaction. Boundary interactions are either smooth reflectors, meaning the boundary is smooth, or the reflector is diffuse, meaning the boundary is irregular. This results in reflection of sound in all directions, where these echoes are of weaker amplitude.
Scattering due to tissue interactions, however, is useful in imaging. Small particle reflectors, whose size is less than the wavelength, within a tissue or organ cause diffuse scattering patterns that are characteristic of particle size. This gives rise to specific tissue 'signatures', improving diagnosis.
Sound Intensity
Sound intensity is detected on the returning echo of an ultrasound. Intensity is the power dissipated per unit area, however, ultrasound gives intensities that range over many powers of 10, often exponentially. Thus, we express the relative intensity in decibels (dB), given by:
Piezoelectric Materials
Piezoelectric materials are made of lead-zirconate-titanate, and convert electrical energy into mechanical energy, and vice versa. They are the functional component of an ultrasound transducer. They consist of electrical dipoles, which are molecular entities composed of positive and negative charges which have an overall neutral charge.
When in receive mode, the piezoelectric material converts mechanical into electric energy. A mechanical pressure disturbs the alignment of the dipoles, resulting in an imbalance of charge. A voltage is created across the surface.
When in transmit mode, the piezoelectric material converts electrical energy into mechanical energy. An external voltage is applied to the crystal, which causes the dipoles to realign themselves and compress or expand from equilibrium. This deformation of the crystal creates mechanical energy in the form of an ultrasound.
Transducer Components
Sound must propagate through an elastic medium by compression, high pressure, or rarefaction, low pressure of the particles that comprise that medium. Rarefaction is a reduction in the density of a medium, especially of air or gas.
Differences Between Sound and Electromagnetic Waves
1. Medium Requirement
Sound waves, unlike electromagnetic waves, require a medium through which they propagate. Electromagnetic waves are capable of propagation through a vacuum.
2. Frequency and Penetration
As the frequency of the sound wave increases, penetration decreases. For electromagnetic waves, as the frequency increases, penetration also increases.
3. Direction of Oscillation
Sound waves are longitudinal waves, meaning particles move parallel to the direction of the wave. Electromagnetic waves are transverse waves, meaning particle movement is perpendicular to wave motion.
Frequency Selection
Ultrasound frequency determines wavelength, and thus spatial resolution. A high frequency results in smaller wavelength and high spatial resolution. However, high frequency also decreases penetration. Thus, the frequency must be selected according to the clinical task. For superficial anatomy, a high frequency is selected, as little penetration is required. For deeper anatomy, e.g. abdominal structures, a lower frequency is selected to reach greater penetration.
Characteristics of Ultrasound Waves
1. Wavelength
Wavelength is the distance between two consecutive points of equal distance in the wave, e.g. two rarefactions or two compressions. It is measured in mm or micrometers.
2. Frequency
Frequency is the number of oscillations per second performed by the wave. It is measured in cycles per second or Hz.
3. Period
Period is the time it takes to complete one cycle. It is equal to the inverse of the wave frequency and is measured in seconds.
4. Amplitude
Amplitude measures the energy of the sound wave. It is measured by the maximum change in pressure or particle displacement amplitude.
5. Speed
The speed of the wave is the distance travelled per unit time. It is equal to the product of wavelength and frequency or wavelength divided by period. Ultrasound speed is also given by the square root of the inverse of compressibility by density.
Acoustic Impedance
Acoustic impedance, Z, is the resistance a tissue boundary offers to the transmission of ultrasound waves through it. It depends on the speed (m/s) of the ultrasound and the density (kg/m^3) of the material through which the ultrasound must propagate through. The transfer of energy between two tissues depends on their individual acoustic impedances. The greater the difference in the acoustic impedance between two materials, e.g. muscle and lung, the greater the amount of ultrasound that is reflected. The lesser the difference in the acoustic impedance between two materials, the greater the amount of ultrasound that is transmitted. It is measured in kg/m^2s or rayl.
Ultimately, acoustic impedance gives rise to differences in transmission and reflection in ultrasound imaging, which is the basis for pulse echo ultrasound imaging.
Speed of Ultrasound
The speed of ultrasound is determined by the compressibility of a material, K, and the density of that material, p. It is equal to the square root of the inverse of the product of these two values.
- Highly compressible materials give rise to a low ultrasound speed.
- Less compressible materials give rise to a high ultrasound speed.
- Dense materials give rise to a low ultrasound speed.
- Less dense materials give rise to a high ultrasound speed.
No one factor of compressibility or density is truly a determinant for speed, but it is the combination and ratio of these two factors.
- The speed of air is 330m/s
- The speed of bone is 4080m/s
Refraction
Refraction is a change propagation direction of an ultrasound wave when it is non-perpendicular to the tissue boundary. It is determined by the velocity of the ultrasound across the tissue boundary. The angle of transmission is dependent on the incident speed and angle of incidence. For a lung to muscle interface, the incident speed is much higher than the transmission speed, so the angle of incidence will exceed the angle of transmission. If the speed in the incident wave is less than that of the transmission wave, the angle of transmission will exceed the angle of incidence. However, should the speed be equal across the interface, no refraction occurs.
Scattering in Ultrasound Imaging
Scattering in ultrasound is either a result from a boundary or tissue interaction. Boundary interactions are either smooth reflectors, meaning the boundary is smooth, or the reflector is diffuse, meaning the boundary is irregular. This results in reflection of sound in all directions, where these echoes are of weaker amplitude.
Scattering due to tissue interactions, however, is useful in imaging. Small particle reflectors, whose size is less than the wavelength, within a tissue or organ cause diffuse scattering patterns that are characteristic of particle size. This gives rise to specific tissue 'signatures', improving diagnosis.
Sound Intensity
Sound intensity is detected on the returning echo of an ultrasound. Intensity is the power dissipated per unit area, however, ultrasound gives intensities that range over many powers of 10, often exponentially. Thus, we express the relative intensity in decibels (dB), given by:
dB = 10log(I2/I1)
The half value thickness is the thickness of a medium required to decrease the intensity of the ultrasound to half its original value. This is equal to a decrease of 3dB.
The tenth value thickness is the thickness of a medium required to decrease the intensity of the ultrasound to one tenth of its original value. This is equal to a decrease of 10dB. Thus, 90% of the incident pulse energy must be absorbed, if only 10% of the original intensity is returning in the echo.
Five Interactions with Matter
- Reflection, which arises from differences in tissues' acoustic impedance values
- Rarefaction, which arises from propagation direction changes when non-perpendicular to a tissue interface
- Scattering, which arises from small particle reflectors creating tissue signatures
- Attenuation, which arises from a loss in intensity due to scattering and absorption
- Absorption, which is where energy is converted into heat which is bad for the patient and the image
Attenuation
Attenuation of ultrasound intensity results from absorption of the intensity by the patient (in the form of heat) and scattering by small particle reflectors. Attenuation is a loss in acoustic energy with the distance travelled. The attenuation coefficient is expressed in the unit of dB/cm, i.e. the relative intensity loss per cm.
Ultrasound attenuation is proportional to frequency. Thus, as frequency increases, attenuation also increases. This means as frequency increases, penetration decreases and so does the half-value thickness. For soft tissue, attenuation of 0.5dB/cm per MHz.
Piezoelectric Materials
Piezoelectric materials are made of lead-zirconate-titanate, and convert electrical energy into mechanical energy, and vice versa. They are the functional component of an ultrasound transducer. They consist of electrical dipoles, which are molecular entities composed of positive and negative charges which have an overall neutral charge.
When in receive mode, the piezoelectric material converts mechanical into electric energy. A mechanical pressure disturbs the alignment of the dipoles, resulting in an imbalance of charge. A voltage is created across the surface.
When in transmit mode, the piezoelectric material converts electrical energy into mechanical energy. An external voltage is applied to the crystal, which causes the dipoles to realign themselves and compress or expand from equilibrium. This deformation of the crystal creates mechanical energy in the form of an ultrasound.
Transducer Components
- Piezoelectric Material
- Matching Layer
- Backing (Damping) Block
The matching layer compensates for the difference in acoustic impedance between the transducer and patient by having a Z value which is intermediate to the two materials.
The damping block is located on the back of the piezoelectric material and functions to absorb the backward directed ultrasound energy, and attenuates stray ultrasound signals from the housing.
Linear Array Transducer
- 256-512 transducer elements
- Elements fired simultaneously to create a wider aperture, a useful beam shape and an effective transducer width
- The ultrasound beam is emitted 90 degrees to the transducer
- Echoes are detected in all elements
- 'A' mode acquisition is done by firing another group of transducer elements, displayed by one or two elements
- Linear arrays produce a rectangular field of view, while curvilinear arrays produce a trapezoidal field of view
- Linear arrays are rarely used except for 'air mode acquisition' of the eyes
Phased Array Transducer
- Fewer elements than linear array transducers, 64-128 elements
- Transducer elements are fired nearly simultaneously to produce a single US beam
- Time delays can be used in the activation of discrete elements, resulting in phase shifts in the emitted US pulses across the face of the transducer
- Time delays result in wave interferences which can be used to steer and focus the beam electronically, without physically moving the transducer
- Echoes are detected in all elements
- Detection algorithms synthesise the data to form an image
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