Ultrasound Physics

• The lecture covers sound in medicine, discussing sound wave characteristics, reflection, transmission, intensity, and medical applications.
• A key focus is on ultrasound (US) in medicine, covering generation, image production, imaging modes, and physiological effects.

General Properties of Sound

• Sound is a disturbance pattern caused by energy traveling from a source, transferring energy without transferring matter.
• Sound is a mechanical disturbance propagating through an elastic material medium at a specific velocity.
• In air, sound is defined by local increases aka compressions or decreases aka rarefactions in pressure relative to atmospheric pressure.
• Sound travels fastest in solids, slower in liquids, and slowest in gases.
• The speed of sound is given by a formula including frequency and wavelength

Wave Properties

• Frequency is the number of rarefactions and compressions per unit time, denoted as f=1/T, where T is time.
• Wavelength is the distance between successive compression and rarefaction.

Sonic Spectrum

• The sonic spectrum is classified by frequency into infrasound, audible sound, and ultrasound.
• The human ear hears sounds roughly from 20 Hz to 20 kHz.
• Infrasound refers to frequencies below 20Hz, often from natural phenomena.
• Intense infrasonic noise produces symptoms like respiratory issues, aural pain, fear, visual hallucinations and chills.
• Infrasound is used in the study of heart function via seismocardiograms, measuring micro-vibrations.
• Ultrasound has a frequency range above 20kHz and has many clinical applications often providing more information than X-rays, and is safer for fetuses.

Intensity and Loudness

• The intensity 'I' of a sound wave is the energy carried per unit area per unit time, expressed in W/m².
• Acoustic impedance 'Z' is a property of the medium.
• The absolute sound intensity cannot be measured directly and is compared to a reference intensity (I₀).
• Intensity is compared to a reference intensity (I₀).
• The audible sound intensity ranges from 10⁻¹² W/m² (hearing threshold) to 1 W/m² (pain threshold).
• Two characteristics of sound include loudness, which depends on intensity, and pitch, which is determined by frequency.

Reflection and Transmission

• Sound waves applied perpendicularly to the interface between two media with differing acoustic impedances result in reflection and transmission.
• The ratio of reflected or transmitted waves to incident waves can be measured.
• A large difference in impedance 'Z' leads to a high reflection ratio.
• A sign change indicates a phase change in the reflected wave.
• A large impedance mismatch results in high reflection and low transmission.

Medical Applications

Percussion

• Percussion involves striking a body surface to detect underlying structures
• Types of sounds from percussion include resonant, hyper-resonant, and dull.

Stethoscope

• Stethoscopes amplify body sounds from the heart, lungs, or other body sites.
• Modern stethoscopes consist of a bell, a thin diaphragm, tubing, and earpieces.
• The frequency of sounds must resonate in the bell membrane.
• To selectively pick up certain frequency ranges the appropriate bell size and diaphragm tension must be chosen.

Ultrasound (US)

• Diagnostic ultrasound uses frequencies from 20kHz to 1GHz.
• SONAR (Sound Navigation and Ranging) is a device using US waves to generate images of soft tissue.
• Transducers convert electrical energy to ultrasound energy and vice versa, and vary by frequency and footprint.

Ultrasound Generation

• Ultrasound signals are generated and detected by a sensor using the piezoelectric principle.
• The piezoelectric principle involves using crystals that vibrate in response to alternating current (AC) voltage, producing ultrasound waves.
• In clinical applications, applying an electric potential difference to a piezoelectric crystal causes it to expand or contract, creating compression and rarefaction regions.

Basic Principles of SONAR

• In medical diagnosis, US pulses transmit into the body using water or jelly paste to eliminate air for impedance matching.
• Backed echoes are detected as weak signals, amplified, and shown on an oscilloscope.

US Image Production

• Three concepts affect US image production: focal zone, acoustic impedance, and refraction.
• For the best US image, the object should be at the focal zone (near field).
• When an ultrasound wave goes through different tissues a fraction of the sound wave is backscattered towards the transducer while the rest is transmitted.

Acoustic Impendance

• The liquid aka jelly between the transducer and the patients skin helps keep away any air bubbles and allows the passage of ultrasound waves.
• Small changes in impedance result in low reflection and high transmission while large changes results in high reflection and low transmission.
• Refraction involves changes in direction, and minimizing it requires transducers positioned perpendicular to the interface between two media.

Quality of Ultrasound Imaging

• Ultrasound imaging quality depends on the interaction these interactions includes spatial resolution, attenuation and reflection, and transmission.
• Spatial resolution is defined by the wavelength of sound.

Attenuation

• Attenuation of ultrasound beams propagating through tissue, resulting in decreased intensity due to absorption and scattering.
• The material/tissue is relevant to the images.
• Higher frequency attenuates better in the tissue resulting in a better image.
• Higher attenuation and decreased penetration gives a better resolution.

Image Quality

• Is determined by a compromise between good resolution and deep penetration.
• The choice of the ultrasound is determined by a compromise between good resolution and deep penetration.
• Smooth vs Rough surfaces matter for images.
• Perpendicular reflection relates to echo while non-perpendicular reflection relates to intensity in the signal.

Types of US Image Modes

A-Mode (1D)

• It is used to obtain diagnostic information about the depth of structure (image with 1-dimention).
• It sends US waves into the body and measures the required to receive the reflected sound.
• The depth of the inteface recorded is proportional to the time it takes for the echo to return.

A-Mode applications

• Used to diagnostic brain tumors and in ophthalmology and biometry.
• For ophthalmology the frequency is upto 20 MHz.

B-Mode (2D)

• B-Mode provides 2D images of the body.
• It's used to view internal structure of the body and changes over time of the liver, breast, heart, and fetus.

M-Mode (2D + motion)

• M-Mode is used to study motion such as of the heart and heart valves.

D-Mode (3D + motion; or 4D)

• D-Mode take images with 3-dimensions and adds the process of time.

Physiological Effects

• Various physiological and chemical effects occur when ultrasonic waves pass through the body, and they can cause physiological effects.
• The magnitude of physiological effects depends on amplitude and frequency.
• Different effects with varying usage.
  • Low intensity US (~ 0.01 W/cm²) relate to no harmful effects and diagnostic work.
  • Continues US (~1 W/cm²) relates to a deep heating effect and increased temperature.
  • Continues US (1-10 W/cm²) relates to movements and pressure regions in tissues aka micro-massages.
  • Continues US (~ 35 W/cm²) relates to destroying tissue as in the rupture of the DNA.
  • Continues and focused US (~ 103 W/cm²) relates to selectively destroying deep tissue by using a focused beam.

Questions covering stethoscope mechanics, piezoelectric transducers, acoustic impedance matching, ultrasound frequencies, energy conversion, sound propagation speed, the relationship between period and frequency of a wave, infrasonic noise, and calculations involving wave speed, frequency, and wavelength.