Stethoscope, Ultrasound and Sound Characteristics

• Sound in medicine in 2024, presentation by DR. ENTIDHAR ALTAEE

Lecture Topics

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

General Properties of Sound

• Sound waves are patterns of disturbance which create energy which travels away from the sound's source.
• Sound waves transfer energy without transferring matter
• Sound is a mechanical disturbance from a state of equilibrium that propagates through an elastic material.
• In air, sound is defined as a local increase (compression) or decrease (rarefaction) of pressure relative to atmospheric pressure.
• Sound is a vibration in the form of a mechanical wave that propagates through a medium of solid, liquid, or gas.
• Sound travels fastest in solids, slower in liquids, and slowest in gases.
• Sound speed is given by v = fλ
• f = frequency
• λ = wavelength of the sound waves.
• When sound passes from one medium to another, velocity (v) and wavelength (λ) change, but frequency (f) remains the same.

Wave Properties

• Frequency is the number of rarefactions and compressions per unit of time (f=1/T).
• Wavelength is the distance between successive compressions and rarefactions.

Sonic Spectrum

• The sonic spectrum is classified by wave frequency into three ranges: infrasound, audible sound, and ultrasound
• Human ear hears sounds roughly between 20 Hz and 20 KHz.
• Infrasound refers to frequencies less than 20Hz, that is, below normal human hearing range.
• Infrasound is produced by natural phenomena like earthquakes and atmospheric pressure change.

Infrasonic Effects on Human Body

• It can travel long distances without losing energy due to low absorption and long wavelengths.
• Its effects are difficult to minimize given its ability to travel through most media.
• Intense infrasonic noise can cause respiratory impairment, aural pain, fear, visual hallucinations, and chills.
• Infrasonic waves are used in the study of heart mechanical function, as revealed by the seismocardiogram.
• Seismocardiograms measure micro-vibrations that are signals produced by the heart contraction, plus blood ejection into the vascular tree.
• Ultrasound has a frequency range above 20KHz.
• Clinically, ultrasound is used in a number of specialities.
• Ultrasound gives more information than X-rays, and it is less hazardous for the fetus.

Sound Wave Intensity

• The intensity (I) of a sound wave is the energy carried by the wave per unit area per unit time (W/m²).
• Z acoustic impedance of the medium.
• Acoustic impedance (Z) = medium density (ρ) x wave velocity (v)
• Air: ρ = 1.29 kg/m³, v = 3.31 x 10² m/sec, Z = 430 kg/m².sec
• Water: ρ = 1x10³ kg/m³, v = 14.8 x 10² m/sec, Z = 1.48x10⁶ kg/m².sec
• Brain: ρ = 1.02x10³ kg/m³, v = 15.3 x 10² m/sec, Z = 1.56x10⁶ kg/m².sec
• Muscle: ρ = 1.04x10³ kg/m³, v = 15.8 x 10² m/sec, Z = 1.64x10⁶ kg/m².sec
• Bone: ρ = 1.9x10³ kg/m³, v = 40.4 x 10² m/sec, Z = 7.68x10⁶ kg/m².sec

Sound Intensity Level (Ratio)

• The absolute value of sound intensity cannot be measured.
• Instead it can be compared against a reference intensity (I₀).
• Intensity ratio = 10log(I/I₀)
• The intensity of audible sound ranges between Imin = 10⁻¹² W/m² (hearing threshold) and Imax = 1 W/m² (pain threshold).
• Hearing threshold intensity level: 10log(10⁻¹²/10⁻¹²) = 10log1 = 0 dB
• Pain threshold intensity level: 10log(1/10⁻¹²) = 10log10¹² = 120 dB
• Jet plane @30m: Level 140 dB, Intensity W/m² = 100
• Threshold of pain: Level 120 dB, Intensity W/m² = 1
• Loud rock concert: Level 120 dB, Intensity W/m² = 1
• Siren at 30m: Level 100 dB, Intensity W/m² = 1x10⁻²
• Auto interior at 90 km/h: Level 75 dB, Intensity W/m² = 3x10⁻⁵
• Busy street traffic: Level 70 dB, Intensity W/m² = 1x10⁻⁵
• Talk at 50cm: Level 65 dB, Intensity W/m² = 3x10⁻⁶
• Quiet radio: Level 40 dB, Intensity W/m² = 1x10⁻⁸
• Whisper: Level 20 dB, Intensity W/m² = 1x10⁻¹⁰
• Rustle of leaves: Level 10 dB, Intensity W/m² = 1x10⁻¹¹
• Threshold of hearing: Level 0 dB, Intensity W/m² = 1x10⁻¹²

Nature of Sound

• The human ear distinguishes two sound characteristics: loudness and pitch.
• Loudness (or volume) is the degree of sensation produced in the ear, and depends on intensity.
• Pitch refers to the perceived highness or lowness of a sound.

Sound Reflection & Transmission

• When a sound wave is applied perpendicularly to an interface between two media with different acoustic impedances (Z1 and Z2) the wave is either partially passed through or reflected.
• Large differences in Z result in high reflection ratios
• Equations: R = Iref/Iin = ((Z2-Z1)/(Z2+Z1))², T = Itrans/Iin = ((2Z1/Z2)/(Z2+Z1))²
• Z1 is the medium.
• If Z1=Z2, reflection is complete.
• If Z2 < Z1, a sign change indicates a phase change of reflected wave.
• Large ΔZ results in high reflection and low transmission.

Percussion & Stethoscope

• Percussion uses sounds from striking body surface to detect underlining structures.
• The three types of percussion sounds are resonant, hyper-resonant, and dull.
• Stethoscopes are diagnostic instruments that amplify sounds from the heart, lungs or body.
• Modern stethoscopes have a bell which is closed by a thin diaphragm, tubing and earpieces
• The bell serves as impedance matcher between body and air in the tube.
• Frequency of the sounds must resonate in the bell membrane.
• Natural frequency Fres depends on diameter (d) and tension (T) of the diaphragm.
• To select frequency ranges ( low-frequency heart murmurs, high-frequency lung sounds) the bell size and diaphragm tension is chosen.

Ultrasound Waves

• Ultrasound is sound with a frequency of 20kHz to 1GHZ (for medical applications).
• This is above the upper limit of human hearing.
• SONAR is sound navigation and ranging.

Sonography

• SONAR device that uses US waves to generate an image of a particular soft tissue.
• Converts electrical energy to mechanical (ultrasound) energy and vice versa.
• Transducers have different frequencies and foot prints.
• Transducer types: Curvilinear (low f), Phased array (low f), Linear (high f), Hockey stick (high f).

US Generation

• Ultrasound signals are generated and detected by a sensor.
• Transducers are based on the piezoelectric principle.
• AC voltage (electrical energy) across a piezoelectrical crystal produces vibration of the crystal (mechanical energy), resulting in an ultrasound wave.
• Clinically, the piston is represented by the transducer.
• An electric potential difference is applied between the faces of a piezoelectric crystal, which responds by expanding or contracting.
• The push-pull action of the transducer causes regions of compression and rarefaction, passing out of the transducer face into the tissue.

Basic Principle of SONAR

• Medical diagnosis transmits ultrasound pulses through a US transducer in close contact with the skin, using water or jelly to eliminate air. -This creates a good impedance matching between the transducer and skin.
• Backed echoes are detected as a weak signal, then amplified and displayed on an oscilloscope.

US Image Production

• Three concepts that affect US image production: Focal zone, Acoustic impedance, and Refraction.

Focal zone

• Object should be at the focal zone (or near field) of the probe for best US.
• Near field (Fresnel Zone) and Far field (Fraunhofer zone)

Acoustic impedance

• Small Δz leads to low reflection and high transmission.
• Large Δz leads to high reflection and low transmission
• When an ultrasound wave encounters a boundary between two tissues with different Z values, a certain fraction is backscattered towards the transducer.
• Remaining energy is transmitted further into the body
• Liquid (jelly) between skin and transducer helps thick to keep away air bubbles and allow easy passage of ultrasound waves (small Az).

Refraction

• Refraction refers to the change in direction of the sound wave as it passes from one tissue to another with higher or lower sound velocity.
• Artifacts (errors) result in US images due to change in US wave path.
• The US transducer should be perpendicular to the interface between the two media to minimize refraction.

Quality of Ultrasound Imaging

• Image quality is determined by the interaction of the acoustic wave with body tissue and includes spatial resolution, attenuation, and reflection and transmission.
• It is limited by wavelength of sound and uses equation: Δs ≈ λ = v/f
• Spatial resolution and image: Small Δs and short wavelength = good resolution and image; Large Δs and long wavelength = bad spatial resolution and image

Attenuation

• Attenuation of the ultrasound beam occurs as it propagates through tissue.
• It is the sum of absorption and scattering from small structures that result in an exponential decrease in pressure and intensity of the beam over distance (z).
• It represents the reduction in intensity (I) of US as it passes through tissue, given as equation: μ = μabs + μscat
• Low frequency yields low attenuation and a good image.
• High frequency results in high attenuation and bad image.

Image Quality

• Choice of ultrasound is a compromise between good resolution and deep penetration.
• High frequency provides good resolution, high attenuation, and no deep penetration.
• Low frequency provides bad resolution, low attenuation, and deep penetration.
• Use 3-5MHz for large organs and 4-10MHz for small organs.

Reflection Characteristics

• Perpendicular reflection originates the echo signal.
• While non-perpendicular reflection causes an intensity loss in echo signal.
• Smooth surface leads to low scattering which creates a "good" image
• Rough surface leads to high scattering which creates a "bad" image

US Image Modes: A-Mode (1D)

• It is used to obtain diagnostic information about the depth of structure in an image with 1-dimention.
• US waves are sent into the body, and the time required to receive the reflected sound (echoes) from the interface between the tissues is measured.
• Depth of the interface recorded is proportional to the time it takes for the echo to return (Depth= Velocity x time).
• A sound velocity of 1540 m/sec in average soft tissue means the echo takes 13 µsec at a depth of one cm.
• A-mode is used to detect brain tumors and eye diseases.
• A-Mode Applications: Echo encephalography - For brain tumors, pulses of ultrasound are sent into the skull above the ear and can be used to compare differences between the left and right side of the skull. A shift of >3mm for adults and >2mm shift for children is abnormal.
• Opthalmology Applications for the eye includes ultrasound frequencies up to 20MHZ. This can be used to produce high resolution since bone absorption is insignificant in the eye because of its small size.

US Image Modes: B-Mode (2D)

• Used to obtain 2D images of the body with the same principle as A-mode, but the transducer is moving.
• A storage oscilloscope is used to form the image.
• Provides information about the internal structure such as size, location, and change over time of the eye, liver, breast, heart, and fetus.

US Image Modes: M-Mode (2D+motion)

• M-mode is used to study motion like that of heart and heart valves (image w/ 2D + motion).
• M-mode combines the features of A-mode and B-mode.
• Transducer is held still like it is for A-mode
• Echoes appear as dots like with B-mode.
• Used for diagnostic information about the heart (mitral valve) and detection of pericardial effusion.

US Image Modes: D-Mode (3D+motion, or 4D)

• This mode takes 3-dimensional US images and adds the element of time to the process.

Physiological Effects of Ultrasound

• Ultrasonic waves passing through the body can create a variety of physiological and chemical effects relating to the magnitude frequency and amplitude of the sound.
• Low Intensity (~0.01 W/cm²) shows no harmful effects and is used in diagnostic work such as sonar.
• Continuous US is used for deep heating effects and temperature increases due to the absorption from acoustic energy (diathermy).
• US (1-10 w/cm²) is used to move sound through regions of compression to apply pressure to tissues like a micromassage.
• US (35 w/cm²) is responsible for destroying tissues and rupturing DNA.
• Continuous and focused US at 10³ w/cm² can be used selectively destroy deep tissue.

Acoustic principles in medical instruments, ultrasound, and seismocardiography are explained. The function of stethoscope components, effects of acoustic impedance, the use of coupling mediums, and the piezoelectric principle are explained. Characteristics of sound perceptible to the human ear are also discussed.