Sound Waves: Understanding Period

Sound Wave Parameters: Period

• The period (T) is determined by the sound source and cannot be altered by the sonographer.
• The period is the time taken for one cycle to occur, and each cycle occurs in 0.2 μs, so the period is 0.2 μs.
• If one cycle takes 0.2 μs to occur, it means that five million cycles occur in 1 second, so the frequency is 5 MHz.

Sound Wave Parameters: Wavelength

• Wavelength (λ) is the length of a cycle in space and is usually measured in meters, millimeters, or any standard unit of length.
• The wavelength cannot be modified by the sonographer and is influenced by factors such as transducer frequency and speed of sound in the medium.
• Typical values for wavelength in soft tissue range from 0.1 to 0.8 mm.
• Wavelength is calculated as Speed divided by Frequency: λ (mm) = c (mm/μs) ÷ f (MHz).

Sound Wave Parameters: Propagation Speed

• Propagation speed (c) refers to the rate at which a sound wave moves through a medium.
• The speed of sound wave propagation varies across different mediums, generally fastest in solids and slowest in gases.
• The average propagation speed of sound in tissues is: Air (330 m/sec), Fat (1450 m/sec), Water (1480 m/sec), Soft tissue (1540 m/sec), and Bone (4100 m/sec).

Sound Wave Parameters: Amplitude

• Amplitude is created by the number of molecules displaced by a vibration and 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 refers to the rate at which work is performed or energy is transferred in ultrasound.
• The standard unit of power is the Watt (W), and power in diagnostic ultrasound is commonly expressed in milliwatts (mW).
• The higher the power, the greater the wave's capacity to perform work of displacing particles.

Sound Wave Parameters: Intensity

• Intensity (I) is the rate at which energy passes through a unit area and is equal to the power in a wave divided by the area over which the power is spread.
• Intensity units include milliwatts per centimeter squared (mW/cm2) and watts per centimeter squared (W/cm2).
• An increase in area decreases intensity because power is less concentrated, while a decrease in area (focusing) increases intensity because power is more concentrated.

Sound Wave Parameters: Pulsed Wave

• A pulse, by definition, must have 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.

Sound Wave Parameters: Pulse Repetition Frequency (PRF) and Pulse Repetition Period (PRP)

• Pulse Repetition Frequency (PRF) refers to the number of sound pulses generated by the transducer per second.
• 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 PRF, expressed in milliseconds or any unit of time: PRP = 1 / PRF.

Sound Wave Parameters: Pulse Duration (PD)

• Pulse duration (PD) is the time that it takes for one pulse to occur and is equal to the period times the number of cycles in the pulse.
• PD decreases if the number of cycles in a pulse is decreased or if the frequency is increased (reducing the period).
• Typical values for pulse duration range from 2 to 30 cycles long.

Sound Wave Parameters: Duty Factor (DF)

• The duty factor is the percentage of time that the ultrasound system transmits sound and 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.

Sound Wave Parameters: Spatial Pulse Length (SPL)

• Spatial pulse length (SPL) is the length of a pulse from front to back and is equal to the length of each cycle times the number of cycles in the pulse.
• SPL determines axial resolution and decreases with increasing frequency.