During the more than 25 years that have passed since the laser source of coherent light made its appearance at physical and chemical laboratories a deeper insight has been gained into the interac- tion of light with atoms and molecules in the gas phase and condensed media. Typical of the last decade is the extension of these studies to cover more complex structures, such as biological mole- cules and tissue. This has been dictated by the need to substantiate more comprehensively the nu- merous applications of laser light in biology and medicine.16 The extensive use of lasers at labo- ratories has made very urgent the problem of laser radiation dosimetry from the standpoint of laser action on the human organism.
Prof. Milner has nicely introduced the interrelationship of laser radiation parameters, environmen- tal parameters and tissue parameters which influence the biological tissue result. His analogy of a black box is quite appropriate.
Using lasers in medicine, we have on one side the laser as a source of electromagnetic radiation in the visible, UV and IR region and on the other side the human absorbing tissue as a target hit from the laser radiation.
Unlike penetrating ionizing radiation, optical radiation is generally absorbed very superficially. Ex- cept for a narrow band of visible and near-infrared (IR-A) radiation from approximately 400-1400 nm, skin and other biological tissues are nearly opaque to optical radiation. For this reason, volumetric or mass based concepts of absorbed dose are of little value. Additionally, the abosorbed radiant energy is conducted out of the absorbing site and for this reason thermal effects depend largely upon the size and location of the absorbing site as well as exposure and exposure rate. Con- cepts of exposure dose are therefore most useful and practical.
After reviewing the possible mechanisms leading to the biological responses provoked by non-ioni- zing radiation, a new dose unit for application in phototherapy, the nior, is introduced. Then the results of simulation studies to clear up some misunderstandings concerning the interaction of non- ionizing radiation with living materials is discussed. This is followed by a review of those parame- ters of both the non-ionizing radiation and the tissue which have to be considered if a versatile dosimeter for non-ionizing optical radiation is to be developed.
During the SPIE Institue meeting a lot of consideration was given to the "Black Box Model", the concept that one has during medical application of laser radiation, a box which contains the biological tissue with all its parameters and properties, very often including even environmental influences. Laser radiation is fed into this box as an input and the output is the medical result. The discussions showed that too much is put into the black box, so that the proper description of the effects taking place in the biological tissue, the basis for their understanding, is not possible. It is therefore important to specify as far and as exactly as possible the parameters which may influence the result of a medical treatment and to use internationally recognized terms and units which are unambiguous, to describe them.
Optical radiation and the quantities used for this and for actinic radiation are defined. The actinic spectra and methods for their measurement are treated. The methods of measurement of actinic radiation are mentioned and possibilities for the characterization and evaluation of radiometers for the measurement of actinic radiation are reported.
The Monte Carlo method is rapidly becoming the model of choice for simulating light transport in tissue. This paper provides all the details necessary for implementation of a Monte Carlo program. Variance reduction schemes that improve the effiency of the Monte Carlo method are discussed. Analytic expressions facilitating convolution calculations for flat and Gaussian beams are included. Useful validation benchmarks are presented.
The optical properties of biological media are in general very complex and they are characterized by local inhomogeneties in the light velocity and absorption. The local variations in the optical ve- locity are due to differences in the optical polarizability between the cells and their surroundings as well as between the various parts of each individual cell.
During the last few years, several promising modalities for the treatment of neoplastic tissue with non-ionizing radiation have been developed. Hyperthermia as a clinical modality is based upon a selective tumor response during a very mode- rate heating of tissue. Selective tumor response has been reported by heating the malignant tissue to a moderate temperature in the region 41-47 Ã‚Â°C for some tens of minutes, typically 20-30 Min.1'7,9 The temperature rise is therefore usually below the threshold for coagulation and denaturation of proteins, and the optical power density is certainly at a level well below the threshold value for non-linear mechanisms such as plasma formation and shock wave generation.
The energy fluence rate in a slab of tissue has been calculated for a monodirectional and unpola- rized incident light beam of infinite diameter perpendicular to the tissue surface. Published values for the optical constants of human dermis are used and the angular dependence of scattering is de- scribed by the Henyey-Greenstein function. Results are given for index matched and index mis- matched situations. The P3-approximation to the transport equation appears to be a considerable improvement over the P1 or diffusion approximation. Another improvement is obtained when a delta function is added to the scattering function. In the diffusion model this is called the delta- Eddington approximation. In the P3-approximation this addition yields a near perfect agreement with results of Monte Carlo calculations. The improvements are particularly apparent at the boundary where the incident beam enters the tissue. The relevance of these results for dosimetry in Photodynamic Therapy is discussed.
Laser development in opthalmology is ever more using extended sources or optics producing large retinal images. Such applications require a retinal dosimetry for minimizing the possiblity of damage. Some biological data show that present limit values for large image sizes are not safe. A formulation is proposed allowing one to specify an appropriate retinal dosimetry suitable for the use for very large images.
Available information on the clinical response of portwine stains to laser irradiation raises the question whether simple physics is adequate to suggest an optimal treat- ment protocol. This paper reviews previous assumptions that led to the millisecond, 577 nm dye laser as the laser of choice to treat portwine stains. Reconsideration of some of these assumptions suggests that a wavelength of about 590 rim may be bet- ter, in agreement with recent clinical findings which indicate that 585 nm is the opti- mum wavelength for treatment.
The delivery device attached to any medical laser system is the very important, if somewhat neglec- ted, "Cinderella" of the system. It is the part that the surgeon or operator interarts with and yet de- spite occupying this key position there are few, if any, objective standards to which the delivery de- vices have to conform. There is a requirement, for example, that if the power output from a CO2 laser varies outside the limits set at + 20 % a system should at least indicate a fault. Despite providing some indication of the output power calibrated in such a way as to represent the output power of the end of the articulating arm there is no requirement on continuous monitoring at the end of the arm and therefore, should a mirror fail through being contaminated with smoke that variation would not be recorded nor any action taken unless the drop in power was noted by the surgeon. I am not necessarily argueing in favour of distal power monitoring but simply make the point that failures of the delivery system have not yet been incorporated into formal regulations.
Electro-optic filters with a transmittance variable in time are used for eye protection in electric welding applications. These filters change their luminous transmittance automatically from an initial maximum value to a minimum value when the welding arc is ignited. In the light state, the work-piece and thus the location where the electrode is to be set can be observed. After the ignition of the arc, within a certain switching time, the filter changes to its dark state in order to make the welding process visible.
A miniature spectroradiometer for dosimetry is proposed. The design goal is a small and easily portable device which is intended to be used with and operated by a personal computer or a single-board computer. It has several applications. It may be used as a general purpose spectroradiometer in the ultraviolet, visible, and near infrared spectral regions capable of measuring different optical radiation hazards or other wavelength-dependent effects or phenomena. It may therefore operate as a radiometer, dosimeter or hazard monitor, with user-defined spectral response. Or, it may be used as a spectral monitor for feedback infor- mation on tissue response to (laser-) irradiation. The spectroradiometer is primarily intended to analyze broad-band radiation with compactness and speed of response as priority items.
Until recently dosimetry in photobiology and phototherapy was restricted to measure the physical parameters of the non-ionizing optical radiation source used, and the biological parameters of the tissue were not taken really into account. After discussing the role of tissue equivalent phantoms in dosimetry some unconventional ideas are presented which, when realized, may lead to dosimetry that renders values expressing not only the physical parameters of the non-ionizing optical radia- tion but also the tissue dependent responses.
A basic problem when using lasers in medicine is that of dosimetry. The definition of the terms dose, effective value etc. will be dealt with in Chapter 2. This chapter is intended to give an insight into the problems of basic dosimetry and its technical realization within the field of photocoagulation, an established method used to treat the retina, or some skin diseases. Until now the coagulation process was assessed to be completed when the irradiated area became blanched. However in terms of dosimetry, it must be possible to predict or at least to monitor the biological effect using well-defined parameters for the laser or in achieving an objective measure for a feedback loop. In the case of coagulation, a prediction in this form is not possible. There are two ways of pro- ceeding further see Fig. 1. One can either determine the physical effect, i.e. temperature, by some kind of sensors, or even better, use some biological effect as a direct measure of the effective dose applied.
With short intensive laser pulses a thin layer can be removed from high absorbing non-transparent tissue in a manner which may be compared with a small explosion. In the field of laser medicine this effect is called photoablation.
We here review a series of basic concepts of laser-ocular media interaction, useful to establish do- simetry and safety criteria in the use of Nd:YAG nanosecond and picosecond photodisruptors. The role of the laser parameters and of the irradiation geometry is discussed in relation to the effective- ness of the disruption procedure and to the associated potential risks. An overview of temporal and spatial dynamics of plasmas in liquids is presented. Emphasis is also given to the risk of intraocular lens rupture during capsulotomy and vitreal surgery. The significance of the data acquired on mo- dels in assessing the dosimetry of laser photodisruptors is discussed. Finally, general safety con- cepts in the use of these instruments are presented.