SCIENTIFIC PRINCIPLES IN RELATION TO ENDOSCOPIC, LASER AND RADIOLOGICAL EQUIPMENT

Transcription

1 opchp11.qxd 10/09/99 14:08 Page SCIENTIFIC PRINCIPLES IN RELATION TO ENDOSCOPIC, LASER AND RADIOLOGICAL EQUIPMENT INTRODUCTION THE ELECTROMAGNETIC SPECTRUM. 123 THE ATOM THE TRANSMISSION OF ELECTROMAGNETIC RADIATION ENDOSCOPES THE LASER RADIOLOGICAL EQUIPMENT RECOMMENDED READING G. Grice

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3 opchp11.qxd 10/09/99 14:08 Page 123 SCIENTIFIC PRINCIPLES IN RELATION TO ENDOSCOPIC EQUIPMENT 123 INTRODUCTION Endoscopes, lasers and radiological equipment all utilize the harnessing and manipulation of the physical properties of the electromagnetic spectrum, especially light. Although they have become commonplace in operating theatres, the rapid development of technology in the latter part of this century has greatly increased the scope of surgical practice. Most of this technology has been developed in other fields and then adapted for medical use later. In practice, the application of such technology greatly improves surgical technique and allows much more information to be gathered and utilized by the operation of ever more sophisticated hardware. The rapid increase in its use has not only had the benefit of safer and more successful surgery but has also expanded the range of procedures available. A more recent consideration is the trend towards less invasive and non-invasive surgery where such technology, although initially very expensive, can greatly benefit patients by involving them in less trauma and thus decreasing the time spent in hospital. This also reduces the cost of treatment. THE ELECTROMAGNETIC SPECTRUM Having briefly placed these particular tools into context as far as their use in surgery is concerned, we can now examine the underlying scientific and technological principles involved in their use. To fully appreciate the fundamentals of the electromagnetic spectrum we must understand a little about particle physics, quantum theory and the theory of special relativity. Although the in-depth examination of these subjects is essential to the pure physicist it can be a little daunting to those of us who merely use the technology. However, if we are to understand how these tools work it is first important to examine the behaviour and characteristics of light. Light is the medium by which visual information about the universe is transmitted via the eye to the brain. This is what we all know and experience as visual light but it occupies only a small area within the spectrum of electromagnetic energy (Fig. 11.1). The primary source of electromagnetic energy is radiation that is emitted from the sun and the stars as a product of nuclear fusion. This process is essentially the conversion of matter into energy, due to the reaction and fusion of the nuclei of predominantly hydrogen atoms. The energy released in such reactions is enormous and fundamentally supports the biological life on our planet. Developments this century have enabled man to duplicate the release of nuclear energy for both constructive and destructive ends. Other sources of electromagnetic radiation mainly involve the electronic forces surrounding the nucleus of an atom. An example of the production of radiation is the chemical reaction of burning material, which produces a flame, or the passing of an electric current through a narrow metal filament (electric light bulb). The electromagnetic radiation produced by such sources is the result of the absorption and transmission of energy at the atomic and molecular level of matter, without altering or releasing the huge amounts of energy within the nucleus of the atom. Energy produced in such processes emanates in a random manner from different parts of the electromagnetic spectrum, travelling as waves and identified by its wavelength and frequency (Fig. 11.2). Most of the energy that our bodies sense is broadly heat and light but other parts of the spectrum may be detected, identified and utilized by instruments of technology. THE ATOM The smallest packet of electromagnetic energy is called a photon. Atoms have a positively charged nucleus orbited by negatively charged electrons, the number of which determines the characteristics of the element. An element is the smallest number of atoms that can exist in a stable form by themselves. Thus two atoms of hydrogen form the element hydrogen (H 2 ) and there is one atom of helium in the element helium (He). In a normal state the charge on the nucleus of an atom is balanced by the electron charge and this is called the ground state. When the electrons are allowed to absorb electromagnetic energy (photons) they jump to a higher orbit, said to be the excited state. The atom then returns to its ground state and during the process spontaneously and randomly emits photons. In this way, it can be said that photons are transmitted as messenger packets of energy (Fig. 11.3). THE TRANSMISSION OF ELECTROMAGNETIC RADIATION As previously stated, electromagnetic energy is transmitted in waves and its wavelength and frequency identify the type of radiation. Wavelength is measured in nanometres (0.000,000,001 m or one thousand millionth of a metre) and the frequency is measured in hertz (Hz) or cycles per second. The speed or velocity at which a wave is transmitted is a product of the wavelength and the frequency, so that if wavelength increases, its frequency decreases and vice versa.

4 opchp11.qxd 10/09/99 14:08 Page FUNDAMENTALS OF OPERATING DEPARTMENT PRACTICE Figure 11.1 The electromagnetic spectrum. The characteristics of a wave are dependent on the source and the medium through which it is being transmitted. Electromagnetic energy can travel through a vacuum (space) at a constant velocity of 186,282 miles per second, i.e. the speed of light. When it encounters matter (the atmosphere or water), which can absorb some or all of the energy, it is transmitted in the form of waves. The characteristics of these waves will then be dependent on the type of matter through which they are travelling, i.e. gas, liquid or solid. Electromagnetic wavelengths will only transmit through certain mediums, i.e. X-rays will transmit through flesh and bone but not lead, while visual light will transmit through glass but not a brick wall. The behaviour and interaction of light is thus a complex affair but there are some physical laws and properties that can be applied. Wave transmission can be reflected, refracted or diffracted and can exhibit interference. Figure 11.2 waveform. Characteristics of the electromagnetic REFLECTION Light originates from primary sources such as the sun, a burning flame or a light bulb, which we visualize directly. It also reflects off surfaces, enabling us to see the object in question. Reflection of light (Fig. 11.4) depends on the type and surface of an object. Smooth, flat, polished surfaces reflect parallel rays of light which

5 opchp11.qxd 10/09/99 14:08 Page 125 SCIENTIFIC PRINCIPLES IN RELATION TO ENDOSCOPIC EQUIPMENT 125 Figure 11.4 Types of relfection Figure 11.3 Formation of the excited state in the atom. remain parallel (specular reflection) while rough, uneven surfaces reflect parallel rays in different directions (diffuse reflection). A flat, polished surface, i.e. a mirror, reflects light in a uniform way and demonstrates the laws of reflected light (Fig. 11.5), which are 1. The incident ray, the reflected ray and the normal ray are all in the same plane. 2. The angle of incidence is equal to the angle of reflection (i = r). A ray of light will reflect in the same way from a curved mirror, except that for each ray the norms will be different. Parallel light from a distant object will produce a clear image at the focal length of a concave curved mirror (Fig. 11.5). In this way images can be manipulated by reflecting rays of light from different shaped mirrors. Figure 11.5 REFRACTION Light reflection from a mirror Light waves passing from one medium to another, e.g. from air to water or glass, change direction (bend) and this is called refraction (Fig. 11.6). Refraction is caused by a change in speed of the transmission of light while the angle of refraction is dependent on the medium. The latter is constant for a particular substance and is known as the refractive index. This property of light is the basis for manipulating images through lenses and is used in the production of optical devices.

6 opchp11.qxd 10/09/99 14:08 Page FUNDAMENTALS OF OPERATING DEPARTMENT PRACTICE Incident ray N O R M A L R Angle of incidence i A Y Glass of or water r Angle of refraction Refracted ray Figure 11.6 Refraction DIFFRACTION Light diffracts when passing through a small aperture. It bends and becomes diffuse, producing an image that has light and dark rings surrounding it (the so-called Newton s rings). The reader should consult a standard physics textbook for a more detailed description. INTERFERENCE Electromagnetic waves are capable of interfering with each other. If two waves are in phase their troughs and peaks coincide with each other, the wave energy summates and the amplitude of the combined wave is increased. If two waves are out of phase they tend to cancel each other out and there is a decrease in overall amplitude. If two identical waves are exactly out of phase the resulting amplitude will be zero. This is illustrated in Fig Having examined the fundamental properties of electromagnetic waves we should be able to understand how these properties are utilized in our technological instruments. ENDOSCOPES The basic principles underlying the use of endoscopes involve the science of optics, which started with the invention of the telescope. Light is used to illuminate the object to be viewed, thus allowing it to be seen and interpreted by the human brain. Optical lenses are used to manipulate the returning light in order to produce an enhanced and clearly focused image of that object. Endoscopes may be classified as rigid or flexible. Figure 11.7 Interference (Courtesy of Dr Paul Atherton) RIGID ENDOSCOPES Rigid endoscopes vary from simple metal tubes with a light source at the distal end to operating telescopes that can enlarge, reduce or bend the image and are used to perform complex surgical procedures in enclosed areas of the body. The anatomy and position of the internal organ or bodily orifice that is to be examined will determine the design of the simple endoscope. Examples include the oesophagoscope, sigmoidoscope and laryngoscope. These instruments are used for examination under direct vision and the field of vision is limited to the area immediately in front of the end of the endoscope. They have changed very little over the years except for the way that the light, originating from an electric lamp, is delivered to the subject. Until relatively recently, a light carrier consisted of a metal

7 opchp11.qxd 10/09/99 14:08 Page 127 SCIENTIFIC PRINCIPLES IN RELATION TO ENDOSCOPIC EQUIPMENT 127 tube with an internal wire connected to a bulb at the distal end of the carrier. The carrier would be inserted down the endoscope, illuminating the area immediately around the light source, which became hot with use. The power source was either a battery or transformed mains current. Today, almost universally, light carriers consist of optical fibres through which light is transported and which deliver a cold, more powerful, reliable and even illumination. Fibreoptic transmission of light is dependent on the phenomenon of total internal reflection. Light reaching a glass air or similar interface will either be refracted or internally reflected. If the light strikes the glass air junction at an angle greater than 42 it will be totally reflected (Fig. 11.8). Optical fibres are long strands of glass bundled together to produce a fibreoptic cable. All the strands transmit light because as the light travels along the glass fibre the rays strike the surface at an angle greater than the critical angle of 42. The rays are therefore reflected totally, cannot escape through refraction and remain within the glass fibre. Thus the light travelling along these cables not only illuminates the object but can be used to transmit all kinds of images as information. More sophisticated rigid endoscopes such as arthroscopes and laparoscopes are inserted surgically into joints and cavities, producing an enhanced image using lenses and prisms. The image may be viewed directly through the endoscope or displayed via a small camera onto a television screen. This particular development has facilitated so-called keyhole surgery, which is far less invasive and the ability to televise such procedures greatly enhances the teaching of this surgical technique. FLEXIBLE ENDOSCOPES Figure 11.8 Flexible endoscopes, as the name suggests, can bend and flex within the orifices, lumina and tracts of the body. The main difference is that the flexible endoscope has the ability to transmit an image as well as light along optical fibres, thus enabling the tube to bend considerably without compromising the performance of the instrument. (This also permits a much more thorough examination with less chance of missing pathology.) Flexible endoscopes may be linked to television and recording equipment, enabling review and of course opportunities for teaching. They can be much longer than rigid endoscopes and are fitted with channels for suction, blowing, irrigation and biopsy depending on the type of endoscope. The advent of flexible endoscopes has enabled the use of some types of laser, transmitted down the optical fibres, especially in the field of gastrointestinal treatment when bleeding can be accurately and safely coagulated within the alimentary tract. THE LASER Fibreoptic transmission of light. Laser is an acronym for Light Amplification by Stimulated Emission of Radiation and to understand how it works, we must look at each component.

8 opchp11.qxd 10/09/99 14:08 Page FUNDAMENTALS OF OPERATING DEPARTMENT PRACTICE LIGHT The light in a laser is that part of the electromagnetic spectrum that extends from the near infrared to the near ultraviolet and includes visible light (see Fig.11.1). STIMULATED EMISSION In the normal course of events atoms spontaneously emit photons when absorbing energy such as light or heat (see Fig. 11.3). With stimulated emission, specific atoms such as carbon dioxide (the lasing medium) are pumped by a high-energy source so that more of the atom s electrons are in a higher orbit (Fig. 11.9) and this is called population inversion. More atoms will then return to a ground state and will emit more photons that collide with other photons, causing a stimulated chain reaction. The outcome is an emission of photons of identical wavelength producing monochromatic light. Because all the wave fronts are in phase (coherent), the beam remains parallel (collimated or in the same direction) and concentrated over long distances. Control and focusing are relatively easy to achieve using mirrors or, in most cases, fibreoptics. Figure 11.9 Stimulated emission AMPLIFICATION Amplification takes place within the tube containing the lasing medium (the resonating tube or optical cavity). Mirrors are placed at each end of the tube so that photons reflect back and forth through the lasing medium, stimulating more atoms to release more photons, thus amplifying the effect (Fig ). One of the mirrors allows a beam of laser light to escape from the tube and this is delivered via optical devices (mirrors, fibreoptics) to the objective. RADIATION Radiation refers to that part of the electromagnetic spectrum (near infrared to near ultraviolet) that produces a laser beam, which can be used to vaporize tissue accurately and precisely. The wavelength and type of beam produced are dependent on the medium used and this medium can vary from gases, such as carbon dioxide and helium/neon, to a liquid dye or a solid such as ruby. Each medium produces a different wavelength of light that has different properties and uses in surgery. The carbon dioxide laser produces a powerful (40 watts output) invisible infrared beam, focusing on a small area ( mm) and is useful for vaporizing tissue cells in surface lesions. Penetration is minimal due to the wavelength of light being readily absorbed by water, which is the main constituent of the cells. This laser is ideal for the bloodless excision of small lesions when minimal damage to surrounding and underlying tissue is required. The only drawback is that currently the beam cannot be transported through fibreoptic cables and can only be used in direct line of sight, transported by a cumbersome articulated arm. Carbon dioxide lasers are mainly used in ENT, maxillofacial surgery inside the mouth and in gynaecological surgery for surface ablation of the cervix. Figure Resonating tube

9 opchp11.qxd 10/09/99 14:08 Page 129 SCIENTIFIC PRINCIPLES IN RELATION TO ENDOSCOPIC EQUIPMENT 129 The argon laser produces a visible blue/green light and is most suitable for coagulation, due to the wavelength being readily absorbed by the haemoglobin in red blood cells. Because the beam will travel through the lens and body of the eye without damaging them, it is ideal for targeting the blood vessels and tissue of the retina. Argon lasers are widely used in the treatment of diabetic retinopathy. An argon laser beam can be transmitted by way of flexible fibreoptics and produces a beam with a spot size of micrometers, but with a smaller power output of 5 watts. Argon lasers are also used to photocoagulate gastrointestinal bleeding by introducing the fibreoptic cable down a flexible gastroscope. A power output of up to 20 watts is available for this purpose. The dye laser produces laser light across the spectrum of visible light by simply changing to a different dye and this allows it to be tuned to the required wavelength. The only drawback is that the power output is relatively low (10 watts). Because the blood vessels within the skin are selectively destroyed without undue surface scarring, it is most useful in the photocoagulation of skin blemishes such as port-wine stains. The Nd-YAG laser is the only common surgical laser using a solid medium. This consists of a crystal rod composed of yttrium, aluminium and garnet infiltrated with neodymium. The energy used to stimulate the laser light is usually provided by a krypton arc lamp. It is the most powerful surgical laser (up to 100 watts) and its wavelength is in the near infrared. The beam can be transmitted by fibreoptic cable and a low-power helium/neon laser aiming beam of visible light is also transmitted along the same fibres. Carbon dioxide lasers use the same guidance system. This wavelength is deeply absorbed in tissue without being colour specific. At a higher power level, the absorption is over a larger area at a lower temperature and it can therefore be used to treat larger areas without causing damage to underlying or surrounding tissue. This laser is less precise than the carbon dioxide laser but can be used in a wider range of surgical procedures such as cataract removal, debulking brain tumours and destroying renal and gall stones. The laser may also be used via an endoscope. HEALTH AND SAFETY Because lasers emit potentially harmful radiation, their use is limited to authorized and approved personnel who have undertaken a special training course. Each hospital must produce local rules for the safe use of lasers and a copy of these rules must not only be kept in the theatre office but also must be attached to the laser in question. As an illustration, the following should be included in any rules written for the use of the SLT CLMD YAG laser. Hazards The red aiming beam does not present a hazard except in the case of prolonged exposure. Staff should not stare into the beam; the normal blink reflex will keep the exposure of any accidental exposure to a safe minimum. The invisible radiation beam produced by the YAG laser is much more powerful and hazards associated with it are as follows: The eye is the critical organ for damage. Radiation at this wavelength (1064 nm) is mainly absorbed at the retina and can cause damage leading to impaired vision. Hazards to the skin (particularly uncovered areas of skin) range from various photochemical effects to severe burns. Fire, emission of toxic gases and explosion may result if unsuitable materials are exposed to the beam. Examples include drapes, clothing, plastic instruments and tubing, flammable anaesthetics and skin preparations. Reflective surfaces may redirect the beam along unexpected paths and reflective curved surfaces may refocus this beam, thus causing hazardous conditions outside the normal beam path. Administrative precautions The employer is ultimately responsible for all protection measures and for the protection of all workers (whether or not they are employed by the trust), patients and members of the public on their premises. In consultation with the Head of Department and the Laser Protection Adviser (by law each geographical area must have a Laser Protection Adviser), the Trust must appoint a Laser Safety Officer from amongst the staff regularly working with the particular laser. Appointments and terminations of employment must be in writing. The Head of Department is responsible for ensuring that the safety measures recommended in the rules are implemented but the Laser Safety Officer has a direct responsibility for: 1. The safe custody of the key to operate the laser. 2. The register of authorized operators.

10 opchp11.qxd 10/09/99 14:08 Page FUNDAMENTALS OF OPERATING DEPARTMENT PRACTICE 3. Ensuring that the safety measures laid down in the local rules are carried out at all times. The Laser Safety Officer may obtain assistance and advice from the Laser Protection Adviser at any time but must seek this advice if there are major changes to the laser or the working conditions. The names of the authorized operators must be kept in a register and these individuals must satisfy themselves that all those present, including visitors, are fully aware of the hazards and precautions to be followed. They must also sign the register to indicate that they have read and understood the local rules. The laser may only be used in an area approved by the Laser Protection Adviser and the Laser Safety Officer. This laser controlled area must have warning lights at each entrance and no person should be in the room during use unless their presence is required. If there is any suspected overexposure to the main therapeutic beam, the Head of Department must request the Laser Protection Officer to investigate the circumstances. Any recommendations will be included in the subsequent report. If the eyes were affected, an immediate ophthalmic examination must be arranged. Equipment design All medical lasers must incorporate a number of safety features. The local rules should have an appendix listing the safety features and their operation and all operators must be familiar with these features. If there is any possibility of exposing tubes containing oxygen, nitrous oxide or other oxidizing anaesthetic agents to the beam, they must be protected by wrapping them with aluminium tape or by using specially designed tubes with a metallic surround. Extreme care must be taken to avoid the risk of ignition of flammable materials. Personal protection Unless appropriate filters are included in the viewing optics, any person viewing the procedure via an endoscope must wear safety glasses at all times while the laser is in READY mode. If the beam is not completely enclosed within the patient or the external power meter, it is essential that all staff in the laser controlled area wear appropriate safety glasses while the laser is in READY mode. The patient must wear protective glasses if there is any possibility of eye exposure from the laser. Protective glasses must be marked with their optical density at the relevant wavelengths and provide adequate protection from brief exposure but are not designed to protect from prolonged exposure. Protective glasses designed for use with other types of laser are not suitable and MUST NOT be used. Operating procedures 1. Check that all entrance doors are either bolted or fitted with suitable warning signs. Check that the warning lights outside the main doors are illuminated. 2. Carry out the switch on procedure according to the manufacturer s instructions and calibrate the fibre. 3. Verify that the red aiming beam is present. If it is not, treatment MUST NOT PROCEED. 4. Check that all present are wearing protective glasses as necessary. Check that any suction or smoke evacuation equipment is working. 5. Set the desired power level and exposure time and ensure that the fibre is correctly positioned. Select READY, announce that the treatment is about to begin and proceed, using the footswitch. 6. During any pause in treatment, STANDBY must be pressed to disable the footswitch. 7. When the treatment is completed switch OFF with the key before removing the fibre or endoscope. 8. At the end of the session the key must be removed and returned immediately to the custody of the Laser Safety Officer. RADIOLOGICAL EQUIPMENT X-RAYS X-rays are invisible, are situated at the shorter wavelength end of the electromagnetic spectrum (see Fig. 11.1) and are produced by accelerating electrons in an X-ray tube (Fig ). The electrons are induced to accelerate through the high vacuum tube by heating a metal filament (the cathode) with a very high voltage ( Kv). Electrons, forced from the metal atoms by the high voltage, are focused into a beam that is aimed towards a target, the anode, which is a high melting point metal such as tungsten. Ninety-nine per cent of the electron beam s energy produces heat, which is dissipated by pumping oil through a copper rod containing the target metal. The remaining energy is emitted as X-rays through a window in the leadlined tube. X-rays thus produced are penetrating

11 opchp11.qxd 10/09/99 14:08 Page 131 SCIENTIFIC PRINCIPLES IN RELATION TO ENDOSCOPIC EQUIPMENT 131 Figure X-ray tube. electromagnetic radiation that is absorbed by matter at differing rates depending on the density of that matter. The X-ray tube is lead lined because the X-ray radiation is unable to penetrate this medium. Use of X-rays X-rays are mainly used within the operating theatre as a diagnostic tool but also have a therapeutic use in the treatment of cancer. The penetrating power of X-rays is dependent upon the input voltage, so the shorter wavelengths can be used to destroy cancer cells while longer wavelengths are used for diagnostic purposes. X-rays react with a photographic film in much the same way that light does and, after passing through the body tissue, produce a contrasting picture on a photographic plate. The production of this shadowy picture of the structures inside the body is possible because body tissues absorb electromagnetic energy at different rates. The most common piece of equipment that utilizes X- rays in the operating theatre is the image intensifier (Fig ). Instead of using a photographic plate, the X-rays are passed through a screen coated with zinc sulphide, causing it to produce fluorescence. This screen converts the shorter wavelength X-rays into longer wavelength visible radiation, thus allowing us to actually see a picture, which is then transmitted onto a television screen. The advantages of this system are the convenience of real-time viewing together with the lower level of radiation that is needed. Image intensifiers are used in surgery when information is required before proceeding further but they are also able to take conventional X-ray pictures. Figure The image intensifier. HEALTH AND SAFETY All X-rays are dangerous and must therefore be strictly controlled and monitored when radiological equipment is in use. Administrative precautions The administrative arrangements are exactly the same as described for the laser and include a Radiation Protection Adviser and a Radiation Protection Supervisor. Ultimate responsibility lies with the Chief Executive of the Trust, through the Clinical Director of Radiology. All clinicians who direct medical exposure to ionizing radiation must receive adequate training under the Protection of Persons Undergoing Medical Examination or Treatment 1998 regulations. Everyone who is potentially exposed to ionizing radiation must also wear a radiation-monitoring device under their protective clothing. Radiation protection within the operating theatre The local rules are very extensive and cover working within and outside the X-ray department. This chapter will therefore only give a summary of the rules

12 opchp11.qxd 10/09/99 14:08 Page FUNDAMENTALS OF OPERATING DEPARTMENT PRACTICE involving the use of X-rays within the operating theatre. 1. X-ray equipment must only be operated by a radiographer and directed by an appropriately trained clinician. 2. All doors leading to the X-ray area must have a warning notice stating that X-rays are in use. 3. A controlled area will exist for 2 metres around the irradiated area of the patient and the X-ray tube, together with everywhere that is in direct line of the primary beam or is designated by the radiographer. 4. Only essential staff will remain in this controlled area when X-ray equipment is in use. 5. Any member of staff who is, or suspects that she is, pregnant must wear a lead rubber apron at all times when X-rays are in use and MUST NOT be within the controlled area during X-ray exposure. 6. The patient s reproductive organs should be shielded from radiation. 7. All staff within the controlled area must wear a full-size lead rubber gown and other protection as deemed appropriate, e.g. thyroid protector and lead glasses, and a radiation-monitoring badge, which should be worn on the front of the trunk at waist level beneath the lead gown. 8. If a person needs to hold any part of the patient they must also wear lead lined rubber gloves. 9. All other staff should leave the controlled area and stand behind a lead-lined protective screen during exposure. This screen must be at least 2 metres from the irradiated area and out of direct line of the primary beam. 10. Because the radiation protection badges are used by more than one surgeon, anaesthetist, scrub person, etc., a register must be kept of the users on each occasion. 11. The radiation protection badges are checked monthly by the Radiation Protection Officer. In summary, electromagnetic radiation is a most useful medical tool but, because the radiation is potentially dangerous, it must be used within strictly controlled safety guidelines. RECOMMENDED READING Local rules for the safe use of lasers. Local rules for radiation safety in diagnostic radiology.