Ultrasound

=Ultrasound Physics = toc
 * Ultrasonography is an essential tool in anesthesiology and a basic understanding of the basic physics of this modality is paramount.
 * ==Definition ==
 * Sound is a mechanical wave, which requires a medium in which to travel
 * The wavelength is the distance traveled during one cycle, the frequency of the wave is measured in cycles per second or Hertz




 * Ultrasound uses sound waves.
 * For humans audible sound ranges between 20 Hz and 20,000 Hz (20 kHz). The hearing range of other species can be much higher than 20 kHz and is inaudible for us. These higher wave frequencies are referred to as ‘ultrasound’.
 * Thus ‘Ultrasound’ is high frequency mechanical vibrations or pressure waves above a frequency the human ear can hear. Medical Ultrasound 2 000 000Hz – 16 000 000Hz

Pulse Echo Principle

 * The production of ultrasound waves is based on the so-called ‘pulse echo' principle. The source of the ultrasound wave is the piezoelectric crystal, which is placed in the transducer. This crystal has the ability to transform an electrical current into mechanical pressure waves (ultrasound waves) and vice versa.
 * The crystal converts electrical energy into a mechanical ultrasound wave




 * Reflected echoes return to the scanhead where the piezoelectric elements convert the ultrasound wave back into an electrical signal
 * The electrical signal is then processed by the ultrasound system




 * Once the ultrasound wave is generated and travels through the medium, the crystal switches from ‘sending’ into ‘listening’ mode and awaits returning ultrasound echoes. Approximately 99% of the time is spent 'listening'. This cycle is repeated several million times per second. This principle is called 'pulse echo' principle
 * Pulses transmitted into patient and give rise to echoes when they encounter interfaces/reflectors.
 * These interfaces/reflectors are caused by variations in the "acoustic impedence" between different tissues.
 * <span style="font-family: Arial,Helvetica,sans-serif;">Echo signals are amplified electronically and displayed on a monitor using shades of grey (from black to white), stronger reflectors
 * <span style="font-family: Arial,Helvetica,sans-serif;"> brighter shades of grey and appear white in an image. Those with no echoes will appear black.
 * <span style="font-family: Arial,Helvetica,sans-serif;">Returning sound waves are converted into images on the ultrasound monitor.The thickness of the crystal determines the frequency of
 * <span style="font-family: Arial,Helvetica,sans-serif;"> the scanhead




 * <span style="font-family: Arial,Helvetica,sans-serif;">The frequency also affects the QUALITY of the ultrasound image

↑ frequency - ↑ resolution - ↓ penetration ↓ frequency - ↓ resolution - ↑ penetration

<span style="font-family: Arial,Helvetica,sans-serif;">Interactions of ultrasound with soft tissues
<span style="font-family: Arial,Helvetica,sans-serif;">When an ultrasound wave passes through tissues ...
 * <span style="font-family: Arial,Helvetica,sans-serif;">Propagation velocity
 * <span style="font-family: Arial,Helvetica,sans-serif;">Increased by increasing stiffness
 * <span style="font-family: Arial,Helvetica,sans-serif;">Reduced by increasing density
 * <span style="font-family: Arial,Helvetica,sans-serif;">Bone: 4,080 m/sec
 * <span style="font-family: Arial,Helvetica,sans-serif;">Air: 330 m/sec
 * <span style="font-family: Arial,Helvetica,sans-serif;">Soft Tissue Average: 1,540 m/sec
 * <span style="font-family: Arial,Helvetica,sans-serif;">Acoustic impedance (AI) is dependent of the density of the material in which sound is propagated
 * <span style="font-family: Arial,Helvetica,sans-serif;">the greater the impedance the denser the material.
 * <span style="font-family: Arial,Helvetica,sans-serif;">Reflections comes from the interface of different AI’s
 * <span style="font-family: Arial,Helvetica,sans-serif;">Greater D of the AI
 * <span style="font-family: Arial,Helvetica,sans-serif;">more signal reflected
 * <span style="font-family: Arial,Helvetica,sans-serif;">• Greater the AI, greater the returned signal


 * <span style="font-family: Arial,Helvetica,sans-serif;">Attenuation
 * <span style="font-family: Arial,Helvetica,sans-serif;">Reduction in amplitude and intensity of wave
 * <span style="font-family: Arial,Helvetica,sans-serif;">When traveling through a medium the sound waves' intensity and amplitude decreases. This is called 'attenuation' and is the reason why echoes from deeper structures are weaker than echoes from superficial areas. Hence - the deeper the wave travels in the body, the weaker it becomes predominantly via 3 processes: reflection, absorption, refraction.

Air (lung)> bone > muscle > soft tissue >blood > water <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: center;">


 * <span style="font-family: Arial,Helvetica,sans-serif;">The major cause of attenuation in soft tissue is absorption, which is the conversion of acoustic energy into heat. Other mechanisms are reflection (some of the wave bounces back towards the source as an echo, refraction (change in direction & velocity of wave) and scatter (Scatter occurs when ultrasound waves encounter a medium with a nonhomogeneous surface. A small portion of the sound wave is scattered in random directions while most of the original wave continues to travel in its original path.)

<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: center;">

**//<span style="font-family: Arial,Helvetica,sans-serif; font-size: 150%;"> Finally: //** <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: center;"> <span style="display: block; font-family: Arial,Helvetica,sans-serif; font-size: 120%; text-align: center;"> AND YOU THOUGHT ULTRASONOGRAPHY WAS NOT FUN!!!!

<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: center;"> * [|http://www.lsora.co.uk/AssetLibrary/Presentations/Physics.ppt#374,1,Slide] ​
 * 1) http://www.sonoguide.com/physics.html

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