Classification of Sound Waves

• Sound waves are classified based on their frequency.

Sound Wave Classifications

• Infrasound waves have a frequency less than 20 Hz and are imperceptible to humans.
• Audible sounds range from 20 Hz to 20,000 Hz.
• Ultrasound refers to any sound wave frequency exceeding the upper limit of human hearing.
• Diagnostic Ultrasound typically uses frequencies from 1 MHz to 20 MHz.

Frequency in Diagnostic Ultrasound

• Higher frequencies yield greater resolution in ultrasound imaging.
• Higher frequencies enable the detection of smaller objects.
• Higher frequencies cause the beam to become more collimated and directional.

Resolution

• Resolution is the ability to distinguish two distinct objects as separate entities.
• Good resolution results in a clear image where two objects appear as two distinct objects.
• Poor resolution produces a blurred image where two objects may appear as one.

Advantages of Ultrasound Imaging

• The transducer both sends and receives reflected signals from the tissue.
• Ultrasound produces high-resolution images comparable to X-rays.
• Ultrasound provides real-time imaging.
• Ultrasound scanning is noninvasive, requiring no needles or injections.
• Ultrasound is widely accessible, user-friendly, and cost-effective compared to other imaging modalities.
• Ultrasound imaging is safe and does not use radiation.
• Ultrasound scanning provides clear images of soft tissues, which are not well-visualized on X-rays.
• Ultrasound is valuable in cardiology, obstetrics, gynecology, surgery, pediatrics, radiology, and neurology.

Acoustic Impedance (Z)

• Acoustic impedance results from the density of a material and the speed of sound within that material.
• The mathematical formula for acoustic impedance is Z = ρV, where ρ is density (kg/m³) and V is velocity (m/s).
• Acoustic impedance is measured in kg/m².s.
• Higher density and higher velocity of sound in a medium lead to greater acoustic impedance.

Attenuation

• Attenuation is the loss of intensity of the ultrasound wave as it travels through a medium.
• Attenuation is caused by scattering and absorption of the incident beam.
• Absorption involves energy transfer to tissues, leading to heating or mechanical disruption, decreasing the detectable sound intensity.
• As the ultrasound wave travels through a medium, molecules oscillate, depleting energy from the wave.
• Attenuation in biological tissues can occur via absorption, refraction, diffraction, scattering, or reflection.
• The ultrasound transducer can only detect sound waves returned to the crystal.
• Since most soft tissues transmit ultrasound at similar velocities, refraction is usually a minor concern.
• Organs may appear displaced or have an incorrect shape due to refraction.
• Attenuation in soft tissue is highly dependent on the ultrasonic frequency and is nearly proportional to it.
• Absorption, the conversion of acoustic energy into heat, is a major source of sound wave attenuation in soft tissue.
• This results in a weaker ultrasound signal over distance.
• Deeper structures are more difficult to visualize as the signal attenuates.
• Time Gain Compensation (TGC), or gain, is used to compensate for signal loss.
• Increasing TGC or gain brightens deeper structures to improve visibility.

Reflection

• When an ultrasound beam encounters an interface between two materials with different acoustic impedances, part of the energy is reflected.
• The amplitude of the reflected wave is determined by the difference in acoustic impedance between the two materials.
• At most soft tissue-soft tissue interfaces in the body, the ultrasound transmits to the interface because of the small reflection coefficient.
• Difficulty in transmitting ultrasound beyond any tissue to air interface because the beam is completely reflected.
• Complete reflections at air interfaces necessitate a coupling medium (gel) between the transducer and the patient's tissue.
• Soft tissue-to-bone interfaces strongly reflect ultrasound beams.
• Differences in acoustic impedance causing reflections can arise from changes in speed of sound, density, or both.

Scattering

• When a sound wave encounters a very irregular surface or small particles (size approximately equal to or smaller than the wavelength), scattering occurs.
• Scattering involves the reflection of sound energy in multiple directions instead of just one direction.
• Examples of small scatterers include red blood cells in blood vessels and heterogeneous tissues like the liver.

Frequency and its Relationship to Scattering

• At low frequencies (1-5 MHz), the wavelength is relatively long.
• Large tissue boundaries appear smooth or mirror-like reflectors.
• Reflection is directional and allows for clearer images when studying large boundaries.
• At high frequencies (5-15 MHz), the wavelength becomes shorter.
• The boundaries become less regular and more irregular, increasing non-specular reflection.
• A larger portion of the wave is reflected in multiple directions.
• The technique is useful for studying fine structures such as capillaries or lymph nodes.
• Specular reflection happens when the wavelength is much greater than the surface roughness.
• Non-specular reflection happens when the wavelength is smaller than the surface roughness.