Introduction - Terminology
Absorption - the loss of ultrasound energy as a result of its conversion to another form of energy such as heat or intracellular mechanical vibration
Acoustic impedance - the resistance to sound transmission through a medium
Acoustic intensity - the concentration of energy in a sound beam; the amount of acoustic power transmitted per unit area
Acoustic power - the amount of acoustic energy generated per unit time
Amplitude - the strength of a sound signal
Artifacts - display distortions, additions or errors that can adversely affect ultrasound image acquisition or interpretation
Attenuation - the loss of ultrasound energy when the wave travels deep into the tissues due to absorption, reflection and scattering of sound energy
Axial resolution - the ability to distinguish two structures as separate when the structures are lying close to each other along the same axis as the ultrasound path
Cycle - the combination of one rarefaction and one compression equals one cycle
Diffuse reflection - the reflection that comes off a reflector with an irregular surface
Doppler effect - a change in the frequency of sound as a result of motion between the sound source and the receiver; a positive shift occurs when the source and receiver are approaching each other and a negative shift occurs when they are moving away from each other
Dynamic range - the ratio of the maximum level of a given parameter to its minimum level; in ultrasound, the dynamic range defines a range of echo intensities that are displayed as a gradient of grey values (minimum value in black and maximum value in white pixels in the final image)
Echogenicity - the degree of brightness of a structure displayed on ultrasound; this is influenced by the amount of beam returning to the transducer (reflection) after encountering the target structure.
Frequency - the number of cycles per second; frequency is the inverse of wavelength; the higher the frequency, the shorter the wavelength.
Hyperechoic - the image characteristic of a structure that is highly reflective resulting in a brighter displayed image compared to the surrounding structures; bone and pleura are examples of hyperechoic structures.
Hypoechoic - the image characteristic of a structure that is less reflective than the surrounding structure resulting in a darker displayed image compared to the surrounding structures; fluid filled structures e.g., vessels and cyst are hypoechoic.
Interface - the boundary between two tissue media with different acoustic impedances.
Lateral resolution - the ability of the system to distinguish two structures as separate when the structures are lying side by side.
Longitudinal wave - movement of particles in the same direction as the direction of the wave propagation.
Period - the amount of time required to complete one cycle.
Pulse repetition frequency (PRF) - the number of pulses occurring in a given time interval; for example, 1 Hz (Hertz) is one cycle per second, 10 Hz is 10 cycles per second; a lower PRF is required for unambiguous discrimination of structures at deeper imaging depths.
Pulse repetition period - time from the start of one pulse to the start of the next pulse.
Pulse duration - the time measured from the start of one pulse to the end of the same pulse.
Rayleigh scattering - scattering of the wave in all directions when the reflector is much smaller than the ultrasound wavelength
Reflection - mirror-like redirection and return of a propagating sound wave towards the transducer that follows a standard law of reflection; for example, specular reflection results in the reflected angle being equal to the incident angle of the energy propagation.
Refraction - a change in the direction of wave propagation when traveling from one medium to another with different propagation speeds according to the Snell's Law of refraction.
Resolution - the ability to distinguish between two structures that lie close to one another.
Scattering - a process by which the ultrasound is forced to deviate from a straight-line reflection and trajectory due to small, localized non-uniformities in the tissue.
Specular reflection - the reflection that comes off a smooth reflector (e.g., a mirror).
Transverse wave - movement of particles perpendicular to the direction of the wave propagation.
Velocity - the sound speed and direction of motion.
Wavelength - the distance traveled between two consecutive peaks or troughs of a wave