Ultrasound Physics Fundamentals

Sound in Medicine 2024

• This lecture covers characteristics of sound waves, reflection and transmission, intensity levels, medical apps of sound, percussion, stethoscopes, Sonar US generation, US generation, production of US images, image quality, US imaging modes, and physiological effects of US.

General Properties of Sound

• A sound wave's pattern of disturbance results as energy travels from the sound's source.
• Waves transfer energy as mechanical disturbance, sound propagates through an elastic material, defined in air as compression (increase) or rarefaction (decrease) of pressure.
• Sound is vibration through a medium as a mechanical wave.
• Sound travels through solids fastest, then liquids, and slowest through gases.
• Sound can transmit through air at 330 m/sec, water at 1480 m/sec, muscle at 1580 m/sec, and bone at 4080 m/sec.
• General sound speed is given by frequency and wavelength.
• The number of compressions and rarefactions per unit of time defines a sound wave's frequency (f=1/T).
• Wavelength is the compression and rarefaction distance in a sound wave.

Sonic Spectrum

• Sonic spectrum classifies into infrasound, audible sound, or ultrasound based on wave frequency.
• The human ear can hear sound in the range of 20 Hz to 20 kHz.
• Infrasound has frequencies below 20 Hz, produced by earthquakes and atmospheric changes.
• Infrasound can travel long distances, is hard to minimize effects, and can cause respiratory and aural impairment, fear, visual hallucinations, and chills.
• Infrasound helps study heart mechanical function via the seismocardiogram.
• Ultrasound has frequencies above 20 kHz, used clinically, providing detailed info with less fetus hazard than X-rays.

Intensity of a Sound Wave

• Sound wave intensity is the energy carried per unit area per unit time (W/m²).
• Acoustic impedance (Z) relates to medium density and sound velocity.
• Air impedance is 430 kg/m²s.
• Water impedance is 1.48x10^6 kg/m²s.
• Brain impedance is 1.56x10^6 kg/m²s.
• Muscle impedance is 1.64x10^6 kg/m²s.
• Bone impedance is 7.68x10^6 kg/m²s.

Sound Intensity Level (Ratio)

• The absolute sound intensity cannot be measured; the measurement is relative to a reference intensity.
• The audible sound intensity ranges from 10^-12 W/m² (Hearing threshold) to 1 W/m² (Pain threshold).

Nature of Sound on Human Hearing

• The ear recognizes sound by loudness (volume) and pitch.
• Volume is sound sensation and depends on intensity levels.
• Pitch is whether a sound is high or sharp.

Sound Reflection and Transmission

• Impeding sound wave interface between media creates reflected and transmitted waves determined by difference in acoustic impedance, therefore:
• The ratio of reflected (Iref) or transmitted (Itran) and incident waves (Iin) is measurable.
• High reflection ratio means a large difference in impedance.
• When Z2 < Z1 or Z1 < Z1 the sign change indicates a phase change of reflected wave.
• Large impedance mismatching creates high reflection and minimal transmission.

Percussion

• Striking body surface produces underlying structure sound.
• Percussion sounds are resonant, hyper-resonant, and dull.

Stethoscope

• Stethoscopes amplify sounds made by the heart, lungs, or other body parts, with a bell, thin diaphragm, tubing, and earpieces.
• The bell serves as an impedance matcher between the body and air within the tube, which needs to resonate the bell membrane sounds at specific frequencies.
• Tune frequency ranges with diaphragm diameter(d) and tension (T).

Ultrasound Waves

• Ultrasound has a frequency of 20kHz to 1GHZ, used for medical applications.
• Infrasound lies beyond human hearing.
• SONAR or SOund NAvigation and Ranging is greater than upper limit of range.

Sonography

• SONAR devices use US waves for soft tissue imaging.
• Transducers convert electrical to mechanical (ultrasound) energy and vice versa, each differing in frequency level and footprint.

US Generation

• The sensor generates and finds the ultrasound signal using piezoelectric principle.
• AC voltage across the crystal produces crystal vibration, generating an ultrasound wave.
• When electrical potential difference is applied to a crystal, the crystal expands/contracts, making the transducer push/pull regions of compression and rarefaction into tissue.

Principle of SONAR

• Ultrasound pulses pass into the body with US transducer when contacting the layer of skin.
• Use of water or jelly eliminates air for correct impedance matching is required for scanning with SONAR.
• Backed echoes are detected as weak signal which is amplified and displayed on oscilloscope.

US Image Production

• Three concepts affect Ultrasound image production, which include:
• Focal Zone
• Acoustic Impedance
• Refraction

Focal Zone

• The object needs to be at the focal zone (or near field) of the probe for best US image.

Acoustic Impedance

• Ultrasound wave hitting boundary with different Z values, a bit of the wave energy gets backscattered towards the transducer.
• Remaining wave energy transmits through deeper boundary into body.
• Liquid (jelly) is utilized between skin and transducer to allow for passage of the ultrasound waves with thick liquid.

Refraction

• Refraction includes a change in sound wave direction between tissues of varying velocity.
• Refraction can also create the potential (errors) in image as a result of the US change in wave path.
• To minimize refraction the US transducer should be perpendicular to interface between the 2 mediums.

Quality of Ultrasound imaging

• Determined by interaction with tissue, quality includes spatial resolution, attenuation, reflection, and transmission.
• The rule is: limited as wavelength of sound.
• With spatial resolution:
• Small As is Good
• Large As is Bad
• The ultrasound beam attenuation is the sum of absorption from small structures as it moves through tissue.
• Attenuation includes decreased pressure/intensity of beam function of propagation distance.
• Decreasing intensity relates to US wave over distance; lower frequencies and lower attenuation leads to better image.

Image Quality

• Ultrasound selection is determined by good resolution and deep penetration.
• Higher frequencies yields:
• Good resolution
• High attenuation
• No deep penetration
• Lower frequency yields:
• Bad resolution
• Low attenuation
• Deep penetration
• US frequency depends on well enough resolution and necessary penetration depth.

Reflection

• Perpendicular reflection starts the echo signal, while non-perpendicular creates intensity loss.

Types of US Image Modes

• A-Mode (1D) finds depth of tissues structures (image with 1-dimension).
• The US waves pass through the body, which measures receipt of wave.
• Echoes take 13 µsec at a depth of 1 cm.
• A-mode is used to detect brain tumors and eye diseases.
• :Applications of A- mode scan
• Echo encephalography helps the detection of brain tumors by using pulses in the skull regions with displayed echoes.
• If the shift > 3 mm for an adult, is abnormal.
• If the shift > 2 mm for a child, is abnormal.

Opthalmology Type US Image Modes

• Application can be split into two divisions: diagnosis and biometry for measurement purposes.
• Ultrasound with frequencies up to 20 MHz is utilized for high resolution. -There is no bone to interfere with the energy, absorption is insignificant because eye is smaller.
• B-Mode (2D) obtains 2D images, the same as A-mode, but more dynamic with better imaging.
• Provides information on size & structure of body.
• M-Mode (2D + motion) studies motion of organs with features of both the A and B mode.
• The detection and use of pericardial/mitral valve function in the heart has been made in the M-mode.
• D-Mode (3D + motion; or 4D) produces 3-dimensional views of internal structures with motion.

Effects of ultrasound in therapy

• Ultrasonic can cause physiological effects in body, relating amplitude of audio wave.
• Low intensity ultrasound (~ .01) has no harmful effects, used for diagnostics.
• Continuous ultrasound (~1) has heating properties which rise temperature via acoustic energy.
• Continues ultrasound (~1-10) allows for micromassage, with compression/rarefaction.
• Continues ultrasound (~35) can have DNA rupture.
• Continued focus (~10) destroys deep tissue using a focused ultrasound beam.

Examine key principles of ultrasound physics, including frequency effects, acoustic impedance, and wave properties. Understand how these concepts relate to diagnostic applications and signal transmission. Topics include diagnostic ultrasound, sonic spectrum, and infrasound.