Ultrasound and Shock-Wave Therapy


Recommended reading: https://burningscience.wordpress.com/biophysics/detection-and-measurement-of-mechanical-quantities/

Ultrasound Therapy: was introduced in the late 1930s.

A ultrasound therapy device includes two parts:

  • A generator of high-frequency electrical impulses
  • An application head containing an electro-acoustic transducer
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Usually, a good acoustic coupling between the transducer and the body surface is ensured by oil or gel.

For therapeutic purposes, ultrasonic waves with f=0.5MHz to 3MHz are used. The therapeutic effect depends on the intensity(which should range between 0.2W/cm² and 1.5W/cm²).
The area over which the application head has an effect can also range, between 1cm² to 10cm².
The application time is between 5min to 15min, during 5 to 20 therapeutic sessions.

Ultrasound acts over the treated area in three ways:

  • micromassage – mechanical vibrations
  • heating – acoustic energy is transformed into head when tissues absorb it
  • phsical-chemical changes – it can possibly create free-radicals, increasing the local metabolism

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Patients with chronic diseases of articulations, muscles, or nerves are the ones usually advised to undergo ultrasound therapy.

Lithotripsy by Shock Waves: A non-invasive treatment of renal stones

The main principles of lithotripsy are:

  • the generation of shock-waves outside the body
  • focusing the shock-wave into the target area by means of an appropriate coupling
  • positioning the target into focus

A lithotripsy device consists of four main parts:

  1. A shock-wave source
  2. Focusing Device
  3. Coupling Medium
  4. Stone Location System

The shock-wave sources:

  • Point sources – include spark plugs, which induce shock waves by a high voltage discharge, laser-induced shock waves, and microexplosive ignition shock waves.
  • Planar sources – are the electromagnetic shock emitter and the piezoelectric shock wave emitter.

The most frequently used sources of shock waves are spark plugs and piezoelectric shock wave emitters.

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X-rays and Ultrasound imaging are the methods used to determine the location of the stones.

How it works?

The difference in acoustic impedance between the stone and the surrounding water and soft tissues rapidly causes a high-pressure gradient when shock-waves encounter the stone. Mechanical breakdown of the stone takes place as the tensile force exceeds the solid’s comprehensive strength.

The acoustic impedance of water is similar to that of soft tissues, therefore shock-waves can propagate through the body with minimal loss of energy.
The stone is subjected to numerous brief impulses of very high acoustic pressure(>100MPa), until the resulting fragments are deemd small enough to transverse the ureter or common bile duct, or to be dissolved.

The effectiveness of shock wave treatment depends on the shape of the stone, as well as on its composition. Effectiveness ranges between 70% to 95%.

Even though lithotripsy is less harmful than surgical intervention it may still cause haemorrhages in the tissues surround the stone.

Laser Lithotripsy has been used for the fragmentation of stones in the ureters. This method is based on the photoacoustic phenomenon, i.e. acoustic waves are generated by light impulses (1.5μs long).

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Shock Wave Therapy

Devices for non-invasive extracorporal shock wave therapy(ESWT) have a self-adjusting electrode immersed in water contained in the application head. This extends their life by up to 500% . The self-adjusting electrode generates shock waves with variable effective energy, ranging between 1.2mJ to 40mJ. This produces an energetic density of 0.14 to 1.8 mJ/mm², whose penetration in soft tissues is 60mm.

The frequency of the shock waves ranges between 1Hz to 4Hz.

The advantage of this method over lithotripsy is that it does not have any destructive effect since the focus area is by two orders lower than that produced by shock wave lithotripters.

Extracorporal shock wave therapy is used to treat a broad spectrum of diseases or disfunctions such as:

  • tendonitis
  • calcified shoulder joints
  • planar fascitis
  • fixation of hip and knee prosthesis
  • etc.

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