Development Of Long Pulse YAG Laser Treatment Machine

Laser has been the focus in the medical filed because of its good haemostatic effect, cohesion, calibration and monochromatic. Laser can accurately resects, vaporizes or coagulates the pathological tissue and make the adjacent normal tissue hardly injured. Laser can directly take an operation through the fiber into human body, it is one of the most important technology of minimally invasive surgical.The traditional continuous YAG laser, the green laser and the holmium laser haven't received satisfactory report in the treatment of benign prostatic hyperplasia yet. This paper improves the continuous YAG laser and develops a pulse YAG laser treatment machine. It is called long pulse YAG laser treatment machine because its pulse width is longer than usual pulse laser.This paper mainly concerns the development of each part of the machine, including laser generator, power supply circuit, control circuit and interactive control interface. In addition to this, the thermal effects of biological tissue irradiated by laser are simulated, analyzed and verified through experiments.The pulse solid laser is composed of active material, pumping source, condenser cavity, resonant cavity and cooling system. The active material is Nd:YAG crystal which has four-level system, photon wavelength is 1064nm launched by Nd:YAG under the condition of stimulated emission. In the resonant cavity, the photons are magnified, oscillated and screened, so the output laser has a high energy. As a pumping source, the role of krypton lamp is to provide sufficient energy keep YAG crystal in the population inversion state. In order to converging more energy to active material, double ellipse which inner surface coated with gold film is adopted. Cooling system is designed to prevent excessive temperature rise and affect laser output.It is desirable for the non-and-mini traumatic laser surgical procedures to remove tissue accurately with the minimal thermal damage to the adjacent non-irradiated tissues. Photo-thermal interactions between laser rays and biological tissues have various thermal effects such as coagulation, vaporization and carbonization. The energy of laser continuously accumulates and diffuses in biological tissues, so that different extent thermal effects and thermal damage of biological tissues have been induced. In this paper, the thermal conduction model and optical parameters of biological tissues are discussed, the relevant thermal effects calculation equations are deduced, the instantaneous temperature distribution of biological tissue irradiated by high power laser is simulated, the thermal damage depth is numerically predicted and the relationship curve between time and the thermal penetration depth is described. The study provides a theoretical guidance to the clinical application of this machine.The control system is based on AT89S52, the parameters inputted through keyboard are converted to corresponding proportion voltage to adjust the output voltage and modulate pulse width after processed by single chip microcomputer. According to the value of input voltage, SG3525 can control the output pulse duty cycle so that the output energy of laser could be precisely controlled. The laser requires the cooling water under a certain temperature, so it is necessary to design a temperature detection circuit. The machine can connect with a computer via RS232 interface, to transfer and save the patient information, view record and playback the operation procedure, or it also can connect with hospital information system to comply with the requirements of modern medical equipment.Various experiments for verification on this machine are still in progress. At the end, the paper gives an overview and puts forward the issues to do in the future.