Ultrasound Physics and Instrumentation Part 1

The Evolution of Ultrasound Technology

• The evolution of ultrasound technology began with World War I's naval warfare, which spurred the advancement of SONAR technology for military purposes.
• 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 1912, Langevin created an early ultrasound device using piezoelectric principles.
• Diagnostic uses for ultrasound started to emerge in the 1930s, marking a new era in medical imaging.
• Institutions worldwide developed pulsed ultrasound technology in the 1950s, leading to 'B Mode' imaging.
• The real-time B-scan ultrasound was developed and introduced in obstetric imaging in the 1980s.
• Ultrasound technology expanded with the advent of three-dimensional (3D) and four-dimensional (4D) imaging in the 1990s.

What is Sound?

• Sound is an energy form generated through vibration, transmitting energy from one location to another through a mechanical or longitudinal wave.
• Sound waves require a medium (solid, liquid, or gas) and 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, where the vibrating source of the sound wave moves away from the molecules, causing them to become less densely packed.

Wave Formation and Classification

• A wave is a disturbance or fluctuation that transfers energy from one location to another within a medium.
• Mechanical waves are longitudinal waves, where the displacement of the medium is in the same direction as the direction of the wave's propagation.
• Examples of mechanical waves include ocean waves, sound waves, and seismic waves.
• Electromagnetic waves (transverse wave) do not require a medium and can propagate through a vacuum, including examples such as radio waves, X-rays, and light.

Longitudinal vs. Transverse Wave

• Longitudinal waves: particle displacement occurs parallel to the wave's direction of energy movement.
• Transverse waves: particle displacement occurs 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, and intensity.

Sound Wave Parameters: Frequency

• Frequency (f) measures the occurrence rate of an event, referring to the number of complete cycles of pressure variation in one second.
• Units of frequency are measured in hertz (Hz), kilohertz (kHz), and megahertz (MHz).
• Typical frequency values in medical ultrasound range from 2 to 15 MHz.
• Frequency is determined by the sound source, and 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.