Ultrasound Definition

Ultrasound is cyclic mechanical vibration with a frequency greater than the upper limit of human hearing, which is approximately 25 kilohertz (25,000 hertz) in healthy, young adults

Vibration and Wave in physics

• A vibration source in a mechanical medium will cause wave propagation.
• Ultrasound is a mechanical wave, can not exist in vacuum.
• Ultrasound transfer energy within the medium, does not transfer mass.See a particle vibration example
• Longitudinal wave: particle movement in the same direction to the wave propagation. See a one-dimension example
• compression: Ultrasound propagation causes local medium density and pressure varying with time. The density or pressure will increase when it is compressed.
• rarefaction: When low pressure pass through, the local density will drease.
• Transverse wave: particle movement in the direction perpendicular to the wave propagation. See a one-dimension example

Ultrasound Parameters

• Frequency: The number of times a vibrating particle goes through its orginal position within one second
• Propagation speed: The distance the wave peak pass within one second
• Wave length: The distance between the two adjacent wave peaks
• Amplitude: The maximal distance a particle moves away from its original position
• Pressure:
• Power
• Intensity

Medium Acoutic Property

• Air: Only low frequency ultrasound can propagate in air at a speed of 300m/s with a very high attenuation. The acoustic impedance of air near zeros.
• Water: Sound velocity in water is around 1500m/s at room temperature. Water has very little attenuation to ultrasound. The acoustic impedance of water is about 1.5MRayl.
• Soft tissue: Sound velocity in soft tissue is around 1540m/s. The attenuation of soft tissue is around 0.3 dB/cm/MHz. The acoutic impedance of soft tissue is around 1.5MRayl.
• Bone: Sound velocity in bone is much fast that in soft tissue. Bone also has a higher attenuation. Normally, ultrasound beam can not penetrate bone.

Ultrasound Reflection

• Big surface: The surface must be big compared to the ultrasound wavelength when reflection happens.
• Impedance mismatch: The acoustic impedance on both side of the surface must be different.
• Wavefront pattern: The reflected wave from a spherical wave emitted from a point has the same shape as that emitted from its mirrored counterpart.
• Dependence on angle: For plane wave, the reflected wave has the same angle to the surface as that of incident wave.
• A one-dimension example
• Sphereical wave reflected by a plane surface
• Sphereical wave reflected by a tilt plane surface
• Plane wave reflected by a plane surface
• Plane wave reflected by a tilt plane surface

Ultrasound Scattering

• Small particle size: Ultrasound will be scattered when encounters a small particle compared to its wavelength.
• Impedance mismatch: The particle must have a different acoustic impedance from its background.
• Independence of angle: The scattered wave will spherical when the particle size is very small.
• Sphereical wave scattered by a small object
• Plane wave scattered by a small object

Ultrasound Refraction

• Big surface: Ultrasound refraction only happen at big surface compared to its wavelength.
• Velocity mismatch: The acoustic medium at both sides of the surface must have different sound velocity.
• Dependence on angle: The refracted wave obey Snell's Law.
• Plane wave refracted by acoustic medium with faster sound velocity
• Plane wave refracted by acoustic medium with slower sound velocity
• Spherical wave refracted by acoustic medium with faster sound velocity
• Shperical wave refracted by acoustic medium with slower sound velocity

Ultrasound Attenuation

• Causes of ultrasound attenuation:
• Scattering
• Absorption
• Reflection
• Ultrasound attenuation values in type of tissue: Water has minimal attenuation to ultrasound. Blood has an attenuation of 0.03dB/cm/MHz. Soft tissue has a value of 0.3dB/cm/MHz. Air, bone, stone, and metal will normally attenuate all the imaging ultrasound beam energy they encountered.
• Ultrasound attenuation values increase with frequency.
• Effects on image: Far field image will look darker without compensation. Resolution in far field is lower than near field due to the downshift of the center frequency of the echo.

Ultrasound Frequency Range and Application

• HIFU: Depends on application, low than half of the diagnostic frequency.
• Abodminal imaging: 3.5 ~ 5 MHz.
• Cardiovasuclar imaging: 2.5 ~ 3.5MHz.
• Ophthalmology, eye application: 15 ~ 50MHz.
• Dermatology, skin application: 15 ~ 50 MHz.
• Small organ: 5 ~ 12MHz.
• Peripheral vascular: 5 ~ 10 MHz.
• Intravascular (IVUS): 10 ~ 50 MHz.
• Frequency, Penetration, and spatial resolution: Higher frequency gives better resolution, and less penetration. The transmitted power of ultrasound pulse for imaging is regulated by FDA. The highest energy within FDA regulation is always used for the best signal to noise ratio. As long as the signal from the farthest depth has enough SNR, the highest frequency is always the first choice.

Image Characteristics Related Terminology

• Echogenic: Scatter or reflect strong echo in general, like bone, stone, or air.
• Anechoic: No echo area, like water or blood pool.
• Hyperechoic: Generate stronger or increased echo than surronding area, like a solid mass in soft tissue.
• Hypoechoic: Opposite to hperechoic, Generate weaker or decreased echo than surronding area, like a lipid pool or cyst.
• Isoechoic: Generate echo with normal amplitude, like normal soft tissue.
• Homogeneity: Parameters like acoustic impedance, geometry texture structure uniform in the area, such as healthy liver tissue.
• Heterogeneity: Contrary to homogeneity, contains dissimilar elements, like kidey or breast.

Piezoelectric Effect

• Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential in response to applied mechanical stress.
• Curie temperature
• Kt
• Common piezoelectric materials

Transducer Construction and Characteristics

• Thickness resonance mode
• bandwidth and Q
• Damping
• Matching layer

Ultrasound Beamformation

• interference phenomena
• Huygen's principle
• Aperture size and wavelength
• Beam field from a piston aperture
• Near field and farfield
• Beamwidth
• Natural focus

Ultrasound Imaging Resolution

• Axial resolution
• Lateral
• Point spread function
• Lateral resolution from a focused aperture
• Slice Thickness Resolution (Elevational Resolution)

Ultrasound Array Transducer

• Linear array
• Curved array
• Annualar array
• Circular array
• 1.25D 1.5D 1.75D array
• 2D array

Ultrasound Beam Focus

• Method
• Focal zone characteristics
• Focal distance (length)
• Beam width
• Maximal Intensity
• Beam field from a piston aperture
• Near field and farfield
• Beamwidth
• Natural focus

Ultrasound Beam Steering

• Mechanical
• Phase array
• Apodization
• Dynamic Aperture
• Dynamic focus

Ultrasound Pulse Echo Method

• Range equation
• Pulse duration
• Pulse repetition frequency,period
• Pulse spatial shape
• Duty factor

Ultrasound Transmitter

• Pulser
• Linear

Ultrasound Receiver

• Protection circuit, TR switch
• Impedance control
• Gain

Time Gain Compensation (TGC)

• Attenuation and beam spread
• Effect on Image
• Method

Envelope Detection

• Demodulation
• Rectification
• Rejection

Dynamic range compression

• Dynamic range from ultrasound signal
• Dynamic range of display equipment
• Method

Ultrasound Image Mode

• A-Mode
• B-Mode
• M-Mode
• C-Mode
• BD-Mode

Ultrasound Image Frame Rate

• Image depth and PRF
• Frame rate, Number of lines per frame, and depth

Preprocessing and Postprocessing

• Preprocessing
• Postprocessing
• Freeze frame
• Black/white inversion
• Contrast variation
• Frame averaging
• Edge enhancement

Ultrasound Spectrum Doppler

• Doppler effect
• Continue wave ( CW )Doppler
• Pulsed wave ( PW )Doppler
• Aliasing
• Range ambiguity
• Frequency and flow speed
• Narrow band width transducer
• receiver
• Demodulater
• Wall filter
• Demodulater

Ultrasound Color Doppler Flow Imaging

• Color Flow Map
• Transmit method
• Autocorrelation
• Flow direction
• Average velocity
• Velocity variance
• Time domain process
• Color maps
• Hue
• Saturation
• Luminace

Ultrasound Power (Energy)Doppler Flow Imaging

• Method
• Application
• Advantages and limitations

Ultrasound Ultrasound Image Artifacts

• Definition
• Resolution related
• Speckle
• Propagation related
• Mirror image
• reverberation
• Comet-tail
• Ring-down
• Side lobe
• Attenuation related
• Shadowing
• Enhancement
• Focal Enhancement or Focal Bandin
• Doppler related
• Aliasing
• Incident Beam Angle
• Clutter
• Ghosting or Flash

Acoustic Output

• Pressure
• Unit
• Peak pressure
• hydrophone
• Power
• Intensity
• SATA
• SPTA
• SPPA
• SPTP
• Cavitation
• Mechanical index
• Thermal index
• TIS
• TIB
• TIC

Guidelines and Regulations

• American Institute of Ultrasound in Medicine (AIUM) Statements
• National Electrical Manufacturers Association (NEMA)
• Food and Drug Administration (FDA)