Lasers in urology

natural light are coherence, collimation and monochroIntroduction maticity. Coherence means that all the photons are in phase with one another, collimation that they travel Albert Einstein (1879–1955) can be considered as the ‘father’ of laser (light amplification by stimulated emisparallel to each other with no divergence and monochromaticity that they all have the same wavelength. sion of radiation) technology, with his theory on the photoelectric eCect, photons and stimulated emission. A laser system consists of three main parts: the substrate (medium) that is stimulated and produces the Einstein showed that light does not consist of continuous waves or small particles, but actually it exists as ‘packets’ photons, the excitation source that provides the energy for stimulation and a resonator system that produces of energy, termed photons [1]. The first laser in the visible spectrum was constructed in 1960 by Maiman the collimated beam of light. The substrate determines the wavelength of the laser light and can be solid, liquid [2]. It consisted of a synthetic ruby crystal surrounded by a helicoidal flash tube. For improved performance, or gas. With these features, laser light can deliver focused both ends of the crystal were coated with silver, creating an optical resonator. energy over long distances, provided that the medium through which is transmitted does not absorb this light. Almost immediately after the presentation of the first laser, many laboratories, trying to find commercial appliThe energy generated by a laser is not high, but with proper adjustments, the tissue eCect can be significant. cations, studied other substrates such as the rare earths (Nd, Pr, Tm, Ho, Er, Yb, Gd), complex molecules such The power density is termed irradiance and represents the key element of energy delivery to tissues (the as yttrium–aluminium–garnet (YAG), glass and CaF 2 . This period of laser research can summarized by the irradiance is the power per surface are, i.e. W/cm2). The total amount of energy applied to a tissue is described words of H. Stine, ‘Initially, the laser was an invention looking for an application’. as the fluence, and is the irradiance multiplied by the time of exposure, i.e. power×time/surface area, In urology, Parsons in 1966 experimentally used a pulsed-ruby laser in the canine bladder. Pioneer appliexpressed in W.s/cm2 or J/cm2 . cations of the laser in urology included the use of a CO 2 laser for condylomata in 1980, photodynamic therapy Types of laser (PDT) for bladder carcinoma in 1982, the pulsed-dye laser for urolithiasis in 1987 and the Nd:YAG laser for Laser-tissue interactions prostatic carcinoma in 1984 [3]. The modern era of Urology is among the medical specialities that apply lasers in urology begins with the use of side-firing fibres many diCerent types of laser systems (Table 1). These and their application in the treatment of BPH in 1990. systems diCer in the particular wavelength used, the power and the mode of emission (pulsed or continuous) Laser physics [5–8]. The interaction of tissues with laser energy is determined by several physical principles [4]: Absorption, The basic principle of laser technology is the stimulated essential for the conversion of light to thermal energy; emission of photons [4]. In steady-state conditions within reflection, the back-scattering of energy away from the an atom, electrons orbit the nucleus; when the atom is tissue surface; scattering, the internal reflection of energy stimulated by an external energy source, electrons can by various tissue components, resulting in diCuse absorpchange their orbit to an excited state and the atom tion; transmission, when energy is not fully absorbed, becomes metastable. The reversion (decay) of electrons but rather penetrates through tissues, resulting in less to the steady state is accompanied by the emission of transfer of energy to the tissue. energy in the form of photons. The three characteristic The contact of laser energy with various tissues is properties of the laser light which distinguish it from accompanied by one or more of the following eCects. There may be a thermal eCect, whereby when energy is absorbed it is transformed to heat and consequently the Accepted for publication 31 March 1999