Medical student

What is a medium?

A medium is the substance or material that carries the wave

The medium carries the wave from one point in the medium to another point

The medium is not the wave and it doesn’t create the wave; it merely carries or transports the wave from its source to other locations

The medium is made of particles, which are temporarily displaced and then return to the point of origin without transport of particles

A sound wave is transmitted by the vibration of the molecules in a medium. The medium can be air, water, wood, or any other material

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When a stone is dropped into a pond, the water molecules are disturbed
The molecules start oscillating around their original position – they vibrate

Introduction

Clinical ultrasound has evolved into a pervasive and universal tool facilitating increased success, quality, safety, and cost-effectiveness of all kinds of clinical diagnostic and interventional procedures

Ultrasound should be applied wherever the diagnostic and interventional clinical procedures can be improved for the benefit of the patients

Securing proficiency of clinical ultrasound application mandates educational resources, training, and certification developed by and anchored in recognized institutions and societies

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One of the two discoverers of the piezoelectric effect Pierre Curie with his famous wife Marie in their laboratory in Paris

Absorption

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.

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Absorption is not a safety problem. The heat energy is relatively low and dissipates in the tissue
Absorption is only a concern in ophthalmological and obstetrical sonography
The power output from the ultrasound transducer is kept as low as required to generate adequate clinical images

Attenuation

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 scattering at tissue interfaces

Attenuation of a sound wave is proportional to the frequency of the sound wave and differs among body tissues

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The figure shows that low frequencies are less attenuated than higher frequencies. This means that lower frequencies can penetrate deeper into e.g. soft tissues