basics - Rolle + Rolle

Lasers are sources of light with very special properties, whose practical combination enables medical applications that outperform other instruments which are based on purely mechanical or electromagnetic effect principles. The radiation created by lasers is coherent, i.e. all waive trains are in precise phase with one another in terms of time and space and it is strongly collimated, which means that the ray bundles are arranged nearly in parallel, making their diameter also increase only minimally over a large distance. Due to these two properties, bundles, highly energetic radiation can be created, which can be pointed directly at the target tissue or transferred via fibre or mirroring alarm systems in endoscopy (stomach and intestines, bladder, bronchia, neck, nose and ears, etc.) or microscopes.  

As equally important as the two qualities described is the third property, the monochromasy, i.e. all wave trains have the same wave length, frequency and energy. For the medical application practice, this property is of huge significance, as this opens up the opportunity to select a very specific wave length and to merely use the interaction with a target tissue  that is highly specific for it.  For the effects achieved on the target tissue, the output densities and the impact time are in turn largely responsible. Due to the large variation range of output densities and impact time, very different tissue effects can be achieved that range from low-energy biostimulation through to high-energy photodisruption in kidney stone destruction for instance.  

For surgical laser applications, the photothermal laser effects with output densities of 1 to 100 kW/cm² are of particularly great interest, as in permanent shift operation (continuous radiation) the simultaneous transection, coagulation and welding or sealing of highly differentiated tissue (liver, lungs, kidneys, etc.) can be expected from different  temperature levels. 

For the surgical practice, it is therefore necessary to generate laser radiation with output densities that can create in the centre of the radiated tissue temperatures from 100° to 300° Celsius, which result in an explosion-like vaporisation of the tissue and thereby form the section crater. On the basis of tissue-specific heat conducting, in parallel, lateral penetration penetration into the tissue cools down the temperatures within a matter of seconds from 100° to approx. 40°. Below these temperatures, tissue rich in vessels of so-called parenchymatose organs, such as the liver, lungs, spleen kidneys and the prostate, shrinks and coagulates. These organs are to be looked at in greater detail below.   

If the histomorphology of these organs is taken into account, the tissue section affected by these temperatures and therefore also the penetration depth of the laser ray should be 2  mm to 4 mm inasmuch possible so that the blood, lymphoid and gall vessels shbould be affected by the shrinking process and safely coagulated.  As already explained, the selection of a certain laser wave length for the tissue effects required as mandatory in the surgical application are of essential significance, as the  interaction with the target tissue is exclusively wave length dependent. Responsible for the interaction of a tissue with a special laser wave length are the individual tissue components (proteins, haemoglobin, water content) and their percentage volume share. Parenchumatose organs, such as the lung, liver, kidneys and prostate, have very unified values for the protein and haemoglobin content, the average water content of 80% is also high. Only the tissue density fluctuates considerably, whereas the lungs have oly 1/5 of the density of liver tissue due to the air content of the alveoles.