notes 3. Ultrasound Physics Notes

Decibel Unit

• Decibel (dB) unit is used to measure the intensity (strength), amplitude, and power of an ultrasound wave.
• Using decibels allows sonographers to compare the intensity or amplitude of two signals.

Power and Intensity

• Power: The rate at which energy is transmitted.
  • Power is the rate of energy flow over the entire beam of sound.
  • Measured in watts (W) or milliwatts (mW).
• Intensity: The power-per-unit area.
  • Intensity is the rate of energy flow across a defined area of the beam.
  • Measured in watts per square meter (W/m^2) or milliwatts per square centimeter (mW/cm^2).
• Power and intensity are directly related: If the power is doubled, the intensity also doubles.

Frequency

• Sound is characterized according to its frequency.
• Frequency: Describes the number of oscillations per second performed by particles of the medium in which the wave is propagating.
  • 1 oscillation/sec = 1 cycle/sec = 1 hertz (1 Hz)
  • 1000 oscillations/sec = 1 kilocycle/sec = 1 kilohertz (1 kHz)
  • 1,000,000 oscillations/sec = 1 megacycle/sec = 1 megahertz (1 MHz)
• Higher frequency = deeper penetration.

Frequency and Wavelength

• Wavelength is inversely related to frequency.
  • Higher Frequency: Shorter wavelength.
  • Lower Frequency: Longer wavelength.

Propagation of Sound Through Tissue: Reflection

• Reflection: Occurs whenever the pulse encounters an interface between tissues with different acoustic impedances.
• Acoustic impedance: The measure of a material's resistance to the propagation of sound.
• When a sound wave strikes an interface between two objects with different acoustic impedances, some of the energy is transmitted across the interface, and some is reflected.

Propagation of Sound Through Tissue: Reflection Strength

• Strength of reflection depends on:
  • Difference in acoustic impedance between tissues.
  • Interface size, surface characteristics, and orientation to the transmitted sound pulse.
• The greater the acoustic mismatch, the greater the backscatter, or reflection.

Propagation of Sound Through Tissue: Scattering

• Specular reflectors: Large, smooth interfaces.
  • When aligned perpendicular to the direction of the transmitted pulse, sound is reflected directly back to the active crystal elements in the transducer, and a strong signal is produced.
  • When not oriented perpendicular to sound, a weaker signal is produced.
• Scattering: The redirection of sound in multiple directions, which produces a weak signal. This occurs when the pulse encounters a small acoustic interface or a large interface that is rough (nonspecular reflector).

Propagation of Sound Through Tissue: Refraction

• Refraction: Change in the direction of sound.
• Refraction occurs when sound encounters the interface between two tissues that transmit sound at different speeds.
• Sound frequency remains constant, but the wavelength changes to accommodate the differences in the speed of sound.
• The result of this change in wavelength is a redirection of the sound pulse as it passes through the interface.

Propagation of Sound Through Tissue: Absorption

• Absorption: The loss of sound energy, secondary to its conversion to thermal energy.
• Absorption is greater in soft tissues than in fluid; it is greater in bone than in soft tissues.
• Absorption is the major cause of acoustic shadowing.

Instrumentation: Piezoelectric Crystals

• When a ceramic crystal is electronically stimulated, it deforms and vibrates and produces the sound pulses used in diagnostic sonography.

Instrumentation: Piezoelectric Effect

• Piezoelectric Effect:
  • When a sound wave is applied perpendicular to the surface of a ceramic crystal, an electric charge is created.
• Reverse Piezoelectric Effect:
  • If a piezoelectric element is exposed to an electric shock, it will begin to vibrate and transmit a sound wave.

Instrumentation

• Pulse duration: The amount of time the piezoelectric element vibrates after electrical stimulation.
• Each pulse consists of a band of frequencies called bandwidth.
• Center frequency
  • Produced by the transducer
  • Resonant frequency of the crystal element
  • Depends on the thickness of the crystal

Instrumentation: Echoes and Image Formation

• Echoes that return to the transducer distort the crystal elements and generate an electric pulse that is processed into the image.
• Higher-amplitude echoes produce greater crystal deformation and generate larger electronic voltage, which is displayed as a brighter pixel.
• These two-dimensional images are known as B-mode, or brightness mode, images.

Image Resolution

• Resolution of an imaging process distinguishes the adjacent structures in an object.
• Important measure of image quality.
• Determined by the size and configuration of the transmitted sound pulse.
• Always considered in three dimensions: axial, lateral, and azimuthal.

Image Resolution: Axial

• Axial resolution describes the ability to resolve objects that are located at different depths along the direction of the sound pulse within the imaging plane.
• Axial resolution depends on the direction of the sound pulse, which, in turn, depends on the wavelength.
• Higher frequency probes produce shorter pulses and better axial resolution but with less penetration.
• Axial resolution is the minimum distance between two structures positioned along the axis of the beam where both structures can be visualized as separate objects.

Image Resolution: Lateral

• Lateral resolution: The ability to resolve objects within the imaging plane located side by side at the same depth from the transducer.
• Beam width determines lateral resolution.
• Can be varied by adjusting the focal zone of the transducer (point at which the beam is narrowest).
• If two reflectors are closer together than the diameter or width of the transducer, they will not be resolved.

Image Resolution: Azimuthal

• Azimuthal (elevation) resolution: The ability to resolve objects the same distance from the transducer but located perpendicular to the plane of imaging.
• Azimuthal resolution is also related to the thickness of the tomographic slice.
• Slice thickness is usually determined by the shape of crystal elements or characteristics of fixed acoustic lenses.
• Slice thickness is the thickness of the section in the patient that contributes to the echo signals on an image.