Sound Waves: Frequency Parameters

Ultrasound Physics and Instrumentation

• Decrease in area (focusing) increases intensity because power is more concentrated.
• Ultrasound pulse is weakened (reduction of amplitude) as it travels through a medium, known as attenuation.
• Amplitude is the maximum amount of variation that occurs in an acoustic variable (pressure, in this case).
• Intensity is the power in a sound wave divided by the area over which the power is spread (the beam area).

Pulsed Wave Ultrasound

• A pulse 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.
• Pulsed wave transducers emit ultrasound waves that span a variety of frequencies, referred to as the frequency bandwidth.
• Pulsed wave transducers are responsible for generating all types of ultrasound diagnostic images, including both real-time and static.

Duty Factor (DF)

• Duty factor (DF) is the percentage of time that the ultrasound system transmits sound.
• DF is calculated by dividing the pulse duration by the pulse repetition period.
• Typical DFs for sonography are in the range of 0.1% to 1.0%, and for Doppler ultrasound, 0.5% to 5.0%.
• The sonographer can adjust the DF by changing the 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 determines axial resolution.
• SPL decreases with increasing frequency because wavelength decreases with increasing frequency.

Ultrasound Interaction with Tissue

• Attenuation refers to the progressive reduction in amplitude or intensity of ultrasound waves as they travel through a medium.

Pulse Repetition Frequency (PRF)

• PRF refers to the number of sound pulses generated by the transducer per second.
• There is an inverse relationship between imaging depth and PRF, meaning as imaging depth increases, PRF decreases.
• The sonographer can adjust PRF, and the adjustment is particularly relevant to achieve optimal imaging depth.

Pulse Repetition Period (PRP)

• 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.
• The determination of PRP is influenced by the sound source, and it can be adjusted by the operator.

Pulse Duration (PD)

• 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.
• Sonographic pulses are typically two or three cycles long, and Doppler pulses are typically 5 to 30 cycles long.
• PD decreases if the number of cycles in a pulse is decreased or if the frequency is increased (reducing the period).

Sound Wave Parameters: Frequency

• Frequency is the number of complete variations (cycles) that an acoustic variable (pressure, in this case) goes through in 1 second.
• Units of frequency: Measured in hertz (Hz), kilohertz (kHz), and megahertz (MHz), where one hertz is equivalent to one cycle per second, one kilohertz equals 1,000 Hz, and one megahertz is 1,000,000 Hz.
• Frequency is determined by the sound source.
• Frequency-period relationship: The product of frequency and period equals 1 second.

Resonance Frequency in Ultrasound Transducers

• The resonance frequency of an ultrasound transducer is primarily determined by its piezoelectric crystals.
• Thinner crystals in the transducer vibrate at higher frequencies compared to thicker crystals.
• Frequency plays a crucial role in determining the resolution and penetration of sonographic images.
• Frequency is adjustable based on the transducer and sonographic instrument used.

The Evolution of Ultrasound Technology

• 1794: Spallanzani's exploration led to the discovery of sound beyond the audible spectrum.
• 1880: The Curie brothers, Pierre and Jacques, identified the piezoelectric effect, foundational for later ultrasound technology.
• 1912: Utilizing piezoelectric principles, Langevin created an early ultrasound device.
• 1917: World War I's naval warfare spurred the advancement of SONAR technology.
• 1930s: Diagnostic uses for ultrasound started to emerge, marking a new era in medical imaging.
• 1942: Dussik published the pioneering study on the ultrasound examination of the brain.
• Late 1940s: The medical industry began experimenting with ultrasound for medical purposes.
• Institutions worldwide developed pulsed ultrasound technology, leading to 'B Mode' imaging.
• The real-time B-scan ultrasound was developed and introduced in obstetric imaging.
• Ultrasound technology expanded with the advent of three-dimensional (3D) and four-dimensional (4D) imaging.