Physical Principles of Laser Ablation

Abstract

Laser ablation (LA) is a percutaneous tumor ablation technique that utilizes laser light delivered interstitially into the biological tissue to provoke a local hyperthermia according to a planned action. The laser light is coherent and monochromatic, it can be very collimated and focused and delivered though optical fibers with little loss of energy from the source to the target. The nature of the effects of the interaction of the laser light with the tissues depends on many factors, among which the most relevant are the laser wavelength, laser power, exposure time, pulse duration and repetition frequency in case of pulsed emission, the beam characteristics, the optical characteristics of the applicator, and physical properties of the tissue. Inside the biological tissue, light can be reflected, transmitted, scattered and absorbed. Only absorbed energy can produce biological effects while the other above-mentioned phenomena could affect the shape, the extension and the position of the warmed up volume. During the ablation process, coagulation becomes appreciable in the range of temperatures between 54 and 60 {\textdegree}C, depending on the heating rate. Above 60 {\textdegree}C, both the denaturation of larger structural proteins and cellular components accelerate, leading to widespread coagulation and rapid cell death in a duration of less than one second. Currently, most LA procedures use Nd:YAG ($λ$ = 1064 nm) or semiconductor diode lasers ($λ$ = 800--980 nm) operating in the range of 2--40 W. Laser fibers can be multiple and placed into the tissue and can be activated simultaneously to rapidly treat a large volume of tissue if the laser equipment has several laser sources inside. The cooled catheters are now a new technology, a progress for ablative techniques. These cooled systems allow avoiding a too rapid dehydration, reducing carbonization and then sublimation of the tissue which is a limiting factor in the efficiency of the ablation process in terms of the transfer of energy to the tissue itself. The most used guidance systems for positioning the applicator in the portion of tissue to be ablated is ultrasonic imaging; the least used is the systems using CT imaging, while the systems using Magnetic Resonance imaging are very interesting, but also they are very expensive, cumbersome and not so comfortable for the patient. They, however, allow to control in real-time of all the ablation phases from planning to final assessment of the ablative process.