Medical Applications of Ultrasound

Nick V. Solokhin    solokhin@comcast.net

Our customers have business in medical and cosmetic applications of ultrasound. It provided some experience in this field. When customer applies to us, his company has already clear idea about structure of ultrasonic device that should be built. It is natural human feature. Now we suggest some recommendations that could help people to make right choice. Our recommendations include cosmetic applications of ultrasound – as well.


     1. HIFU transducers for surgical applications
     2. Increasing of medicinal activity of drugs by ultrasound
     3. Local heating just under the skin
     4. Mist medicinal therapy

 

  1. HIFU transducers in surgical applications


HIFU = High Intensity Focused Ultrasound. Basic idea of realization – for HIFU – is shown below.


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Fig. 1 HIFU realization: power ultrasonic transducer with concave (spherical) radiating surface. Diameter of the radiating area should be > 10 wave-lengths. Lesion is formed at the focal spot – in a region with max intensity of ultrasonic flow. Diameter of the ultrasonic flow at the focal spot is ~ one wave-length.


Biological tissues have property of attenuation – for ultrasound. This attenuation converts part of ultrasonic energy into heating. The higher intensity of ultrasonic flow – the stronger heating. If local temperature in biological tissue exceeds + 70 0C – it leads to fast decomposition of albumin with generation of cloud of micro-bubbles with gases N2 and CO2. This area with decomposed albumin (killed tissue) has special name: “lesion”. Formation of lesion starts at the focal spot – it is formation of lesion nucleus. Further treatment generates growing of this nucleus back to the transducer. You can find more information in our section: “Lesion formation in HIFU”.


Actually, the lesion is a local area that is already not alive. If it needs to destroy cancer tumor, it may be filled with lesions – one to one. Ultrasonic treatment of human tumor includes formation of 50 – 500 lesions. Real dimensions of one lesion are close to dimensions of rice grain. Human body can dissolve formed lesions for 1 – 2 month – it means: dissolving of killed tissue of cancer tumor.


Same technique of lesion formation is applied to reduce fat tissue. Human body dissolves these lesions for the same time – 1 – 2 month. Unfortunately, human body will restore lost fat tissue after few months, if patient will not change his style of food eating.
Typical work frequency for lesion formation in surgical application is ~ 3 MHz. Lower frequency provides weaker heating, higher frequency provides too high energy losses over the way from HIFU transducer to an area of treatment.
Lesion formation can be done at small depth in biological tissue – just under the skin. It is closer to cosmetic applications. In this way we can use higher work frequency – up to 6 – 7 MHz. Increased work frequency reduces dimensions of the lesion.


HIFU transducers are applied in medical and biological researches as well. Each HIFU transducer should have test certificate with work frequency, electric impedance, thermal effect in meat (for 5 sec), focal distance, diameter of ultrasonic beam at the focal spot.
There is a lot of new information about “shock ultrasonic waves” – power nonlinear ultrasonic waves. Terminology “shock waves” appeared to describe strong single pulse of pressure from explosion. This terminology was extended – not fully accurate – to power nonlinear ultrasonic waves. Some properties of such waves are not clear or just were not investigated. Hence, we recommend be careful at attempts to build new medical application with such ultrasonic waves.

 

  2. Increasing of medicinal activity of drugs by ultrasound


Ultrasonic waves are ordered oscillations of particles in a medium. If doctor entered some medicine into human body, activity/efficiency of this medicine may be increased by oscillation of medicine particles and particles of the tissues.


Examples


2.1. Some patients have blood clots in his brains. Standard way – entering some
       medicine (into blood) to dissolve such clots. Natural side effect – partial dissolving of
       blood vessels – will be observed in whole patient body. This method can be modified
       by reducing of medicine dose and local increasing of its efficiency – in the area with
       blood clots. This local increasing of efficiency can be obtained by local ultrasonic
       treatment. Such treatment can be done at frequency ~ 100 kHz. Ultrasound provides big
       displacements of particles in blood and it is higher audible frequencies for the patient.
       Ultrasonic treatment should follow to all FDA requirements and do not exceed them.

2.2 Some medicines work better at moderate local heating of treatment area. If this thermal
      treatment should be done at some depth under the skin – it provides big advantage to
      focused ultrasonic treatment in respect to standard heating frough the skin. Required
      heating in this case is, usually, ~ 10 0C. Best work frequency depends on depth of
      treatment area under the skin. If depth is just few mm – we can use work frequency 5 – 7
      MHz. If depth of treatment exceeds 1 cm – there should be frequency ~ 3 MHz.

2.3 Mist therapy of skin or open wounds. Ultrasonic transducer transforms liquid medicine
        into cloud of mist and pushes this cloud to the spot of treatment by radiation pressure of
        ultrasonic waves. More ever, ultrasonic waves provide good penetration of particles of
        this mist into human tissue. This penetration is much better than in case of mechanical
        splashing of liquid medicine. This ultrasonic application is considered separately –
        below. This application applis low ultrasonic frequencies.

 

3. Local heating just under the skin


This ultrasonic treatment can be applied, for example, to melt plugs in tear channels of human eyes. Required heating + 10 0C – should be obtained at small depth ~ 2 mm. Ultrasonic transducer should have such structure, that allows to perform 2 different kinds of treatment:


- thermal treatment of some area with tear channels – this treatment converts plugs into more
  soft state
- pressing out softened plugs from the tear channels – this action should be done just after
  heating, with minimal time interval. Actually, transducer should have good mechanical
  strength to be used as mechanical stick to press treated spot mechanically (manually)

 

4. Mist medicinal therapy


Basic idea of this ultrasonic application: treatment of biological tissues with medicine, but without direct mechanical contact. Ultrasonic transducer transforms liquid medicine into cloud of mist. Such transformation may be called “splashing”, but droplets in ultrasonically induced mist are much smaller (in many times) than at mechanical splashing.

The transducer, generating the mist, radiates ultrasound into air. This ultrasound makes static radiation pressure on the mist and pushes it to the place of treatment. The ultrasound forces small particles in the mist to oscillate with frequency of ultrasound. These oscillations give to droplets higher ability to penetrate into the tissue.

Hence, ultrasonic transducer performs 3 different functions:

- transformation of liquid medicine into mist with very small droplets

- pushing this mist to the spot of treatment (by ultrasonic waves)

- strong oscillations of the droplets in the mist – it provides good penetration of these droplets
  into the treating tissue.