Ultrasound Imaging: Wavelength and Sound Wave Parameters

Ultrasound Image Quality

• Shorter-wavelength sound waves have superior spatial resolution but less penetration.
• Wavelength is a crucial parameter that influences the diagnostic quality of ultrasound images.

Sound Wave Parameters

• Propagation speed (c) refers to the rate at which a sound wave moves through a medium.
• Within a specific medium, sound waves travel at a consistent speed, regardless of their frequency.
• The speed of sound wave propagation varies across different mediums.

Propagation Speed

• Sound waves carry mechanical energy that facilitates the displacement of particles within the medium.
• The higher the power, the greater the wave's capacity to perform this work of displacing particles.

Power

• The standard unit of power is the Watt (W).
• Power in diagnostic ultrasound is commonly expressed in milliwatts (mW).

Intensity

• Intensity (I) is the rate at which energy passes through a unit area.
• Intensity units include milliwatts per centimeter squared (mW/cm2) and watts per centimeter squared (W/cm2).
• An increase in area decreases intensity because power is less concentrated.
• A decrease in area (focusing) increases intensity because power is more concentrated.

Attenuation

• Attenuation is the weakening of an ultrasound pulse as it travels through a medium.
• The sonographer can change PRF, and the adjustment is particularly relevant to achieve optimal imaging depth.

Pulse Repetition Period (PRP)

• Pulse-repetition period (PRP) refers to the time from the beginning of one pulse to the beginning of the next one.
• PRP decreases while PRF increases because, when more pulses occur in a second, the time between them decreases.
• PRP is the reciprocal of PRF, expressed in milliseconds or any unit of time.
• The determination of PRP is influenced by the sound source, and it can be adjusted by the operator.

Acoustic Impedance

• Formula: Z = ρ x C, where ρ is measured in kg/m³ and C in m/s.
• Units for Z are expressed in rayls (1 rayl = 1 kg/(m²·s)).
• The transmission and reflection of ultrasound at tissue interfaces are influenced by acoustic impedance disparities.
• Similar impedance values across tissues result in greater transmission; substantial differences cause increased reflection.

Incident Sound in Ultrasound

• Types of incidence: Normal (perpendicular) and oblique.
• Normal incidence occurs when the ultrasound path is perpendicular to the boundary.
• Oblique incidence occurs when the ultrasound beam strikes the interface between two media at an angle other than 90°.

Factors Contributing to Attenuation

• Absorption: conversion of sound energy into heat within a medium.
• Factors affecting absorption: relaxation time, frequency.
• Reflection: redirection of part of the ultrasound wave back towards the source.
• Factors influencing reflection: acoustic impedance difference between the two media, boundary size and smoothness.
• Types of ultrasound reflection: specular and non-specular.