Waves: Formation and Classification

Wave Formation

• All matter, including air, comprises molecules interconnected through elastic intermolecular forces.
• Mechanical waves require a physical medium and can be classified into:
  • Ocean waves
  • Sound waves
  • Seismic waves
• Electromagnetic waves do not require a medium and can propagate through a vacuum, including:
  • Radio waves
  • X-rays
  • Light

Longitudinal vs. Transverse Waves

• Longitudinal waves have particle displacement parallel to the wave's direction of energy movement.
• Transverse waves have particle displacement perpendicular to the wave's direction of propagation.

Understanding Parameters in Acoustics

• Parameters can exhibit direct or inverse proportional relationships.
• Key parameters of sound waves include:
  • Frequency
  • Period
  • Wavelength
  • Propagation speed
  • Amplitude
  • Power
  • Intensity

Sound Wave Parameters: Frequency

• Frequency measures the occurrence rate of an event, in sound it refers to the number of complete cycles of pressure variation in one second.
• Units of Frequency: Measured in hertz (Hz), kilohertz (kHz), and megahertz (MHz).
• Typical frequency values in medical ultrasound: 2-15 MHz.

Frequency-Period Relationship

• The product of frequency and period equals 1 second.

Wavelength

• Wavelength is a crucial parameter that influences the diagnostic quality of ultrasound images.
• Shorter-wavelength sound waves have superior spatial resolution but less penetration.

Sound Wave Parameters: Propagation Speed

• Propagation speed 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.

Sound Wave Parameters: Amplitude and Intensity

• 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 has a distinct beginning and end.
• Pulsed ultrasound comprises two main components: the cycle (or "on" or "transmit" time) and the dead time (or "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

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

Evolution of Ultrasound Technology

• The evolution of ultrasound technology began with the discovery of sound beyond the audible spectrum, which led to the development of SONAR technology during World War I.
• In 1794, Spallanzani explored the discovery of sound beyond the audible spectrum.
• The Curie brothers, Pierre and Jacques, identified the piezoelectric effect in 1880, which is foundational for later ultrasound technology.
• In the late 1940s, the medical industry began experimenting with ultrasound for medical purposes, marking a new era in medical imaging.

Ultrasound Technology Advancements

• Institutions worldwide developed pulsed ultrasound technology, leading to 'B-Mode' imaging.
• The real-time B-scan ultrasound was developed and introduced in obstetric imaging in the 1980s.
• In 1994, three-dimensional (3D) and four-dimensional (4D) ultrasound imaging emerged.
• Clinical adoption of ultrasound commenced in Glasgow, paving the way for broader medical applications.

Sound Waves

• Sound is an energy form generated through vibration, transmitting energy from one location to another.
• Sound waves are characterized by alternating compressions and rarefactions.
• Compressions signify an increase in pressure or density, while rarefactions occur during the troughs of the sound wave.

Wave Formation

• When a vibration occurs, it disrupts the particles within a medium.
• A pulsed transducer emits ultrasound waves that span a variety of frequencies, referred to as the 'frequency bandwidth.'

Pulsed Wave Transducers

• Pulsed wave transducers are responsible for generating all types of ultrasound diagnostic images, including real-time and static images.
• A pulse has a distinct beginning and end, comprising the cycle and dead time.

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.

Sound Wave Parameters

Period

• The period is determined by the sound source and cannot be altered by the sonographer.
• The period is represented by the symbol (T).

Wavelength

• Wavelength (λ) is the length of a cycle in space, measured in meters, millimeters, or any standard unit of length.
• The wavelength cannot be modified by the sonographer.

Power

• Power is the rate at which energy is transferred, measured in Watts (W) or milliwatts (mW).
• One milliwatt equates to one-thousandth of a Watt.

Intensity

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

Attenuation

• Attenuation is the weakening of an ultrasound pulse as it travels through a medium, resulting in a reduction of amplitude.

Evolution of Ultrasound Technology

• World War I's naval warfare spurred the advancement of SONAR technology, which led to the development of ultrasound technology for medical purposes.
• 1794: Lazzaro Spallanzani discovered sound beyond the audible spectrum.
• 1880: Pierre and Jacques Curie brothers identified the piezoelectric effect, which is foundational for later ultrasound technology.
• 1912: Dussik published a pioneering study on the ultrasound examination of the brain.
• 1917: The medical industry began experimenting with ultrasound for medical purposes.
• Late 1940s: Diagnostic uses for ultrasound started to emerge, marking a new era in medical imaging.
• 1950: Clinical adoption of ultrasound commenced in Glasgow, paving the way for broader medical applications.
• 1956: Institutions worldwide developed pulsed ultrasound technology, leading to 'B Mode' imaging.
• 1965: Advancements made real-time ultrasound imaging feasible.
• 1980s: Three-dimensional (3D) and four-dimensional (4D) ultrasound technology expanded.
• 1990s: Steven Kapral and his team pioneered the use of B-mode ultrasound for brachial plexus blockade procedures.
• 1994: The real-time B-scan ultrasound was developed and introduced in obstetric imaging.

Characteristics of Sound

• Sound is an energy form generated through vibration, a mechanical action that transmits energy from one location to another.
• Sound displays as a mechanical or longitudinal wave, requiring a medium—solid, liquid, or gas.
• Sound waves are characterized by alternating compressions and rarefactions.
• Compressions signify an increase in pressure or density, while rarefactions occur during the troughs of the sound wave.

Wave Formation

• When a vibration occurs, it disrupts the particles within a medium, creating a wave.
• Pulsed transducers emit ultrasound waves that span a variety of frequencies, referred to as the 'frequency bandwidth.
• Pulsed wave transducers generate all types of ultrasound diagnostic images, including real-time and static images.

Ultrasound Types

• Continuous wave (CW) ultrasound is predominantly employed in echocardiography for acquiring CW Doppler information.
• Continuous wave sound is incapable of creating anatomic images.