Sound Wave Period and Frequency

Sound Wave Parameters: Period

• The period (T) is determined by the sound source and cannot be altered by the sonographer.
• Each cycle occurs in a specific time, and the period is the time for one cycle to occur.
• If one cycle takes 0.2 μs to occur, the frequency is 5 MHz.

Sound Wave Parameters: Wavelength

• Wavelength (λ) is the length of a cycle in space.
• Units for wavelength are measured in meters, millimeters, or any standard unit of length.
• Typical values in soft tissue range from 0.1 to 0.8 mm.
• The wavelength cannot be modified by the sonographer.
• Wavelength is calculated as Speed divided by Frequency (λ = c / f).

Sound Wave Parameters: Propagation Speed

• Propagation speed (c) refers to the rate at which a sound wave moves through a medium.
• Within a specific medium, sound waves travel at a consistent speed, regardless of their frequency.
• The speed of sound wave propagation varies across different mediums.
• The average propagation speed of sound in tissues:
  • Air: 330 m/sec
  • Fat: 1450 m/sec
  • Water: 1480 m/sec
  • Soft tissue: 1540 m/sec
  • Bone: 4100 m/sec

Sound Wave Parameters: Amplitude

• Amplitude is created by the number of molecules displaced by a vibration.
• Amplitude is indicative of the strength or intensity of a sound wave.
• Amplitude is typically measured in units of pressure, such as Mega Pascals (MPa).

Sound Wave Parameters: Power

• Power is the rate at which work is performed or energy is transferred.
• In ultrasound, power refers to the generation of ultrasound waves by the transducer and their propagation through tissues.
• The standard unit of power is the Watt (W).
• Power in diagnostic ultrasound is commonly expressed in milliwatts (mW).

Sound Wave Parameters: Intensity

• Intensity (I) is the rate at which energy passes through a unit area.
• Intensity is equal to the power in a wave divided by the area (A) over which the power is spread.
• Intensity units include milliwatts per centimeter squared (mW/cm²) and watts per centimeter squared (W/cm²).

Pulsed Wave

• A pulse has a distinct beginning and end.
• Pulsed ultrasound comprises two main components: The Cycle (the "on" or "transmit" time) and The Dead Time (the "off" or "receive" time).
• Pulsed transducers are designed to generate multiple, sequential, short pulses, allowing for the simultaneous use of the same crystal or group of crystals for both sound transmission and echo reception.

Pulsed Repetition Frequency (PRF)

• Pulse Repetition Frequency (PRF) refers to the number of sound pulses generated by the transducer per second.
• The determination of PRF is attributed to the sound source and can be adjusted by the sonographer.
• There is an inverse relationship between imaging depth and PRF, meaning as imaging depth increases, PRF decreases.

Pulse Repetition Period (PRP)

• Pulse-repetition period (PRP) refers to the time from the beginning of one pulse to the beginning of the next one.
• PRP is the reciprocal of Pulse Repetition Frequency (PRF), expressed in milliseconds or any unit of time (PRP = 1 / PRF).
• The determination of PRP is influenced by the sound source, and it can be adjusted by the operator.

Pulse Duration (PD)

• Pulse duration (PD) is the time that it takes for one pulse to occur.
• PD is equal to the period (the time for one cycle) times the number of cycles in the pulse (n) and is expressed in microseconds (PD = n × T).
• Sonographic pulses are typically two or three cycles long, while Doppler pulses are typically 5 to 30 cycles long.

Duty Factor (DF)

• The duty factor is the percentage of time that the ultrasound system transmits sound.
• DF is the fraction of the PRP that the sound is on.
• Typical DFs for sonography are in the range of 0.1% to 1.0%, and for Doppler ultrasound, the range is 0.5% to 5.0%.
• The sonographer can adjust the duty factor when changing imaging depth.

Spatial Pulse Length (SPL)

• SPL is the length of a pulse from front to back.
• SPL is equal to the length of each cycle times the number of cycles in the pulse (SPL = n × wavelength).
• SPL determines axial resolution.
• Because wavelength decreases with increasing frequency, SPL decreases with increasing frequency.