The acoustic impedances of air and bone are radically different from other tissues. As a consequence, interfaces between soft tissue and air (or bone) reflect most of the sound energy striking the interface. The practical implication is that ultrasound cannot be used to image structures located deep to bone or air interfaces.
Soft tissue and blood have slightly different acoustic impedances, which explains why the needle becomes more visible as it enters the blood vessel
Transmission
When the sound wave propagates in a medium without striking any interfaces, it passes through the medium without any reflection, scattering or refraction.
In a homogeneous medium the transmission is only reduced by absorption. Even in a heterogeneous medium some of the sound wave is usually transmitted when it strikes an interface.
Penetration
The ability of a sound wave to penetrate through tissue depends on the attenuation. This loss of penetration capacity is proportional to frequency.
Penetration expresses how deep the ultrasound wave can penetrate down into the tissue.
As a sound wave propagates through a medium, the sound wave loses energy proportional to distance travelled from the source of sound
This energy loss or weakening of the sound wave amplitude is called attenuation of sound. It is mainly due to absorption but also to reflection and dissipation at tissue interfaces
Attenuation of a sound wave is proportional to the frequency of the sound wave and differs among body tissues
Absorption is the major cause of energy loss (attenuation) of ultrasound in biological tissue and is due to friction which converts kinetic energy to heat energy (thermal relaxation).
Absorption depends on the tissue type (e.g. the absorption is high in bone and low in fluids) and on the frequency of the sound wave. High frequency causes more absorption.
Absorption accounts for 80% of the attenuation of sound in soft tissues.
Interaction of sound wave and a medium:
When a sound wave propagates in a medium, the wave and the medium interact in a way that changes the energy and direction of the sound wave.
The categories of interaction are:
– attenuation
– absorption
– penetration
– transmission
– acoustic impedance
– reflection (specular and diffuse)
– refraction
– diffraction
The magnitude of the attenuation is expressed as the attenuation coefficient (AC). High AC (e.g. bone) means that the tissue attenuates the sound wave strongly. AC also varies with the sound wave frequency.
Examples:
When your neighbour downstairs plays music on his stereo, you may hear the low frequency bass but not the high frequency rhythm guitar.
Bone cannot be penetrated by ultrasound due to the high AC for this tissue. This is one of the reasons why structures situated deep to a bone cannot be imaged with ultrasound.
Amplitude, wave length, period, frequency, velocity
Waves can be described mathematically with a location-amplitude-coordinate system and a time-amplitude-coordinate system with a sinus curve.
The basic parameters of a harmonic wave are:
Human speech and ultrasound are examples of sound wave transmission
When people speak, air is the medium that carries the sound waves of speech from one person to another
When medical ultrasound is used to visualise a blood vessel, the soft tissues of the patient is the medium
When the molecules of the medium vibrate, they transmit the sound wave. The vibrations that transmit sound are not the result of an entire volume moving back and forth at once. Instead, the vibrations occur among the individual molecules of the substance, and the vibrations are transferred from one molecule to the next and thereby move through the medium in waves
The result is that regions of the medium become alternately more dense (compressions or condensations) and less dense (rarefactions). The individual particles only move directly toward or directly away from the vibration source to create compression or rarefaction. This means that sound waves are longitudinal waves
A sound wave is a mechanical wave
Mechanical waves require a medium in order to transport their energy from one location to another. They cannot propagate in vacuum. Mechanical waves can be longitudinal or transverse.
A sound wave is a longitudinal wave
In a longitudinal wave the oscillating disturbance is parallel to the direction of travel. Sound waves are always longitudinal waves: The air molecules vibrate in the same direction as the sound wave travels and form a series of compressions (high pressure) and rarefactions (low pressure), where the molecules are squeezed together and pulled apart respectively.
Understanding the basic concepts of wave and sound physics is necessary in order to learn how ultrasound works.
