Acoustics and ultrasound

Sound in Medicine 2024

• A lecture by Dr. Entidhar Altaee

Topics Covered

• Characteristics of sound waves
• Reflection and transmission
• Intensity level ratio
• Applications of sound in medicine
• Percussion and Stethoscope
• Principle of Sonar US generation
• US (Ultrasound) Generation
• Production of US image
• Image quality
• US imaging modes
• Physiological effects of US

General Properties of Sound

• A sound wave is a pattern of disturbance from energy traveling away from a sound source.
• Sound waves transfer energy without transferring matter.
• Sound is a mechanical disturbance propagating through an elastic material medium at a definite velocity.
• In air, sound involves local increases (compression) or decreases (rarefaction) in pressure relative to atmospheric pressure.
• Sound propagates through a medium as a vibration in the form of a mechanical wave, and the medium can be a solid, liquid, or gas.
• Sound travels fastest in solids, slower in liquids, and slowest in gases.
• General sound speed equation: v = λ * f
  • v is the sound speed
  • f is the frequency
  • λ is the wavelength of the sound waves
• The number of rarefactions (low pressure) and compressions (high pressure) per unit time determines the frequency of a sound wave, denoted as f = 1/T.
• The distance between successive compressions and rarefactions defines the wavelength of a sound wave.
• The sonic spectrum is classified into three frequency ranges based on the frequency of the wave, which include infrasound, audible sound, and ultrasound

Sonic Spectrum

• The human ear can hear sounds in the range of roughly 20 Hz to 20 kHz.
• Infrasound refers to sound frequencies below 20 Hz and is produced by natural phenomena like earthquakes and atmospheric pressure changes.
• Infrasound can travel long distances with low absorption and through most media, making its effects hard to minimize.
• Intense infrasonic noise can cause respiratory impairment, aural pain, fear, visual hallucinations, and chills.
• Infrasound is used in the study of heart mechanical function via seismocardiograms that measure micro-vibrations produced by heart contraction and blood ejection.
• Ultrasound has a frequency range above 20 kHz
• Is clinically used in various specialties
• Often provides more information than an X-ray
• Is less hazardous for the fetus

Intensity of a Sound Wave

• A sound wave's intensity (I) is the energy carried per unit area per unit time, measured in W/m².
• Acoustic impedance (Z) relies on medium density and sound velocity, influencing reflection and transmission at tissue interfaces.

Sound Intensity Level (Ratio)

• The absolute value of sound intensity (I) cannot be directly measured; instead, it is compared to a reference intensity (I0).
• Intensity ratio quantifies sound level relative to a reference.
• An audible sound ranges between 10^-12 W/m² as a hearing threshold minimum and 1 W/m² as a pain threshold maximum

Human Hearing and Sound Characteristics

• The human ear distinguishes two main characteristics of sound: loudness and pitch.
• Loudness refers to the intensity of sound waves
• Pitch indicates whether a sound is high (sharp) or low.

Sound Reflection and Transmission

• When a sound wave encounters an interface between two media with differing acoustic impedances (Z1 and Z2), part of the wave passes through, and another reflects
• A large difference in impedance results in a high reflection ratio
• The ratios of reflected (Iref) and transmitted (Itran) waves to incident waves (Iin) can be measured.
• The sign change during reflection indicates a phase change of the reflected wave when Z2 < Z1.
• Significant impedance mismatching results in high reflection and low transmission.

Percussion

• Sounds are produced by striking a body's surface
• Used to detect underlying structures
• Includes resonant, hyper-resonant, and dull percussion sounds

Stethoscope

• Diagnostic tool used to amplify sounds
• Sounds emitting from the heart, lungs, or other sites in the body
• Modern models have a bell, a thin diaphragm, tubing, and earpieces
• Selective pickup of frequency ranges is achieved (low frequency heart sounds, high frequency lung sounds, murmurs)
• This is done by selecting the correct bell size and diaphragm tension
• Bell serves as impedance matcher between the body and air in the tube
• This requires that sound frequencies must resonate in the membrane

Ultrasound Waves

• Ultrasound includes sound frequencies from 20 kHz to 1 GHz for medical uses.
• They are above human's upper limit of hearing.
• SONAR is a Sound Navigation and Ranging system

Sonography

• Sonography is a device that uses US waves to produce an image of soft tissue
• Transducers convert electrical to mechanical energy and vice versa, differing in frequency and footprint.

Types of Transducers:

• Curvilinear
• Phased array
• Linear
• Hockey stick

US Generation

• Uses the piezoelectric principle
• An ultrasound signal is created and detected by a sensor.
• Crystals form transducer bases, so that AC voltage across the crystal will produce vibration
• Vibration (mechanical energy) generates ultrasound waves that can go both ways
• To apply this clinically, when there is electric potential differences between the faces of piezoelectric crystals, they will respond by expanding or contracting

Basic Principle of SONAR

• Medical diagnosis uses US pulses that are transmitted into the body via contact between US transducers and skin.
• Gels and water can eliminate air and create correct impedance matching between the skin and transducer
• The back echoes can be amplified and displayed using oscilloscopes

US Image Production

• There are three concepts within affecting US images:
• Focal zones
• Acoustic impedance
• Refraction

Focal Zone

• The object should be at the focal zone/near field
• Plane waves and spherical waves are found in the near and far fields

Acoustic Impedance

• This occurs because AZ-Low reflection + high transmission
• AZ-high reflection and low transmission which is determined as Z=PV(densityxvelocity)
• Boundary happens between two tissues (different values of Z lead to fraction of wave energy backscattered
• The rest gets tranmistted deeper into the body, then it has thick liquid between its tissues
• The fluid will keep the air bubbles off and allow US waves to pass through without difficulty

Refraction

• This is the change of direction in a sound wave as it passes from one tissue to another
• From high to low velocity, or vice versa To minimize, the US transducer should be perpendiculat to the interface between 2 medias

Quality of Ultrasound Imaging

• This relies on the interaction between the acoustic wave and the body tissue
• Interactions include:
• Spatial resolution and attenuation
• Reflection and transmission
• Spatial resolution is limited by wave length of sound
• The smaller the changes, the small the change in space
• Smaller the changes, the better spatial resolution
• Good image relies on this
• Bad image relies on this not being done

Quality of Ultrasound Imaging

• Attenuation occurs through absorption and scattering from other structures
• It is characterized through exponential descrease
• Its reliant on the properties from beams thru tissues
• US reduction increases intensity when it passes through the tissues
• Low frequency means low attenuation
• High attenuation in bad images
• Low attenuation translates to good images

Image Quality

• Choosing ultrasound comes from a compromise between the good resolution and the deep penetration
• Better frequency means better resolution
• But it also means high attenuation and no penetration
• Lower frequency means bad resolution but good penetration
• Its better to use lower attenuation and deep penetration
• Use 3 -5 MHz for large organs
• use 4 -10 MHz for small organs

Reflection

• Perpendicular reflection origninates during echo signals Non perpendicular: causes intesnity loss in the echo
• Smooth Surface → high scattering → good
• Rough surface → high scattering and rough, so it is bad

Types of US Image Modes

• A-Mode (1D): Diagnostic information about the structure's depth, one dimension, sound velocity = 1540 m/sec in average tissue, depth, velocity x time, detects tumours in brain and diseases from they eye.
• B-Mode (2D): The principle remains A-mode when the transducer is moving to create 2D images of the body. Used for internal body structure (size, location etc.)
• M-Mode (2D): USed to study motion as heart and heart valves, combines A and B mode features
• D-Mode (3D+motion)

Physiological effects of Ultrasound in therapy

• Many chemcial and physciological effects happen when ultrasonic waves pass throuhg the body
• May lead to physiological effects thatdepend on the amplitude/ frquency of the sound.
• Low intensity: Low intensity US (~ 0.01 W/cm²) no harmful effects are observed used for diagnostic work (as in the sonar).
• Continues US (~1 W/cm²) → deep heating effect (diathermy) → temperature rise due to the absorption of acoustic energy in the tissue.
• Continues US (1-10 W/cm²) → sound moves through→ region of compression and rarefactions → pressure differences in adjacent regions of tissues (micromassage).
• Continues US (~ 35 W/cm²) →tissue destroying effect → rupture DNA molecules.
• Continues and focused US (~ 103 W/cm²) → selective destroying of deep tissue using a focused ultrasound beam.

Acoustics questions covering stethoscopes to medical ultrasound and infrasound. Questions cover how ultrasound is generated, its use in medical diagnosis, and issues like impedance matching and depth of penetration. Also covered are topics like acoustic impedance and seismocardiograms.