Biophysics Lecture 5: Ultrasound Terminology

Period: The time for a sound wave to complete one cycle, measured in microseconds (µs).
Wavelength: The length of space over which one cycle occurs, equal to the travel distance from the beginning to the end of a cycle.
Frequency: The number of cycles repeated per second, measured in Hertz (Hz).
Acoustic Velocity: The speed at which a sound wave travels through a medium, calculated as frequency multiplied by wavelength.

Density (ρ): The concentration of a medium.
Stiffness (κ): The resistance of a material to compression.
Propagation speed increases with higher stiffness or lower density.
Average propagation speed in soft tissues is approximately 1540 m/s (ranging from 1400 to 1640 m/s).
Ultrasound waves have a self-focusing effect, which is the natural narrowing of the ultrasound beam over a certain travel distance.

Acoustic impedance (z) is the degree of difficulty a sound wave has when passing through a medium.
Calculated as density (ρ) multiplied by acoustic velocity (c), represented as (z = pc).
Impedance increases with higher propagation speed or density.
All materials (tissues) present some impedance to the passage of sound waves

Attenuation Coefficient: A parameter used to estimate the decrease in ultrasound amplitude in certain media.
Attenuation coefficient increases with increasing frequency.
Penetration decreases as frequency increases.

Ultrasound interactions, such as reflection, refraction, scattering, and absorption, are determined by the acoustic properties of matter.
Reflection: Occurs at tissue boundaries with differing acoustic impedance; part of the ultrasound is reflected and part is transmitted.
Refraction: The change in direction of transmitted ultrasound energy due to non-perpendicular incidence.
Scattering: Occurs due to reflection or refraction by small particles, causing the beam to diffuse in multiple directions, creating tissue texture in ultrasound images.
Absorption: The process where acoustic energy is converted into heat energy, effectively losing sound energy.

High-frequency generator: A component that produces the high frequency.
Coaxial cable: Transmits high-frequency AC to the transducer.
Transducer: Contains piezoelectric crystals to produce and detect ultrasonic waves.
A transducer unit has one surface coated in a metal plate, while the other surface is attached to a coaxial cable to transmit high-frequency AC.
The ultrasound instrument comprises a high-frequency generator and a transducer circuit connected via a coaxial cable.

Ultrasound is produced and detected using a transducer, composed of one or more ceramic components with electromechanical properties.
The ceramic elements convert electrical energy into mechanical energy to produce the ultrasound, and vice versa, for detection.

Sound waves travel through tissues and are subject to interactions including:
Reflection: A portion of the wave bounces back off the boundary of two different media.
Scattering: Diffusion of the beam in many directions, creating the image's texture.
Refraction: Change in the wave's direction when passing through tissues with different densities.

Ultrasound imaging (sonography) uses high-frequency sound waves to create real-time images of internal organs and blood flow.
Ultrasound is a non-ionizing imaging technique.
Common procedures include abdominal ultrasound, bone sonometer, breast ultrasound, Doppler fetal heart rate monitoring, Doppler ultrasound, echocardiogram, fetal ultrasound, Ultrasound-guided biopsies, ophthalmic ultrasound and ultrasound-guided needle placement.

Transducer placement: The transducer is placed directly on the skin or inside a body channel.
Gel application: A thin layer of gel is applied to aid in the transmission of ultrasound waves.
Image formation: The image is based on the reflection of the sound waves off of body structures.

Ultrasound image formation depends on the strength of the sound signal and the time it takes for the wave to travel through the body.

Ultrasound is a non-ionizing imaging technique; thus safe, with an established record of use.
Possible biological effects in some circumstances, including slight tissue heating and cavitation (gas pockets).
Long-term consequences of certain effects are still unknown.
Excessive use for non-medical reasons may not be advisable; use restricted to medical indications and medically-indicated examinations for safety.

Patients should discuss the reason for the examination, the potential risks, the obtainable information, and how ultrasound results will be used.
Useful for women of child-bearing age where ionizing radiation procedures aren't ideal.

Ultrasound is the most common method for viewing the fetus during pregnancy.
Routinely used to evaluate fetal health and assess the mother's health.
Provides parents with a way to view and hear the heartbeat, bond with the unborn baby, and share images.

Three-dimensional (3D) and four-dimensional (4D) ultrasound can visualize facial features and other body parts in motion.
Ultrasound is generally safe but risks may increase with extended exposure or use by untrained personnel.