Clinical Radiology (1990), 41, 75 76
Editorial Lasers in Radiology There is an increasing number of applications for the use of laser energy in medicine. Lasers are established as the treatment method of choice in eradication of early cervical and oropharyngeal tumours: they have been used for many years in ophthalmology and dermatology and have a role in the management of gastro-intestinal bleeding and the palliative treatment of bronchogenic carcinoma. The treatment of non-resectable tumours by interstitial hyperthermia using laser energy is still at an experimental stage and, should it be found to be valuable, radiologists will probably be required to position the laser fibres and monitor the effects of thermal necrosis using ultrasound or CT control (Stege~ et al., 1989). The involvement of radiologists with lasers stems from 1985 when they were first used in the cardiovascular system for percutaneous transluminal angioplasty (PTA) (Ginsburg et al., 1985). Using wavelengths ranging from mid infra-red to ultraviolet, laser energy can vaporise atheroma and the aim was to recanalise occlusions and reduce the recurrence of atheromatous disease, particularly in smaller arteries with a relatively high recurrence rate, such as the femoral and coronary arteries. However, for laser energy to be successfully used within arteries it has to be delivered to the target site down an optical fibre. Until recently this has limited the source of energy to Argon and N d : Y A G using continuous waveforms. Pulsed waves of laser energy are now emerging as sources suitable for PTA (Cross et al., 1987). In theory, pulsing the energy allows heat to dissipate before the next pulse arrives, so minimising thermal damage to normal tissue when the pulses are short relative to the thermal relaxation time of the tissue. Until recently it has proved difficult to deliver pulsed energy down an optical fibre. The wavelengths used in practice have little selective absorption by atheroma: although there is preferential absorption of blue light by carotenoids in atheroma, this is insufficient for selective clinical action (Prince et al., 1986). Increasing this preferential absorption by prior drug treatment (photodynamic therapy) is at an experimental stage (Prevosti et al., 1988). Initially a bare optical fibre was used down a catheter. However, it produced a high vessel perforation rate and provided only a very narrow channel (Gir~sburg et al., 1985; Cumberland et al., 1986). This led to modification of the fibre tip, either by putting a metal tip on to the end of the fibre and so converting all the energy to heat (the laser probe), or a tip with a sapphire lens to allow a combination of direct laser and thermal interaction with tissue. Both devices have been evaluated clinically and found to be safe (Cumberland et al., 1986; Lammer et al., 1988; Belli et al., 1989) and have been successfully used in recanalising some occlusions where the conventional guide-wire and catheter technique had failed. However, neither of these probes is useful in negotiating heavily calcified obstructions. At our institution, a 2 mm diameter thermal laser probe Was used percutaneously in 141 peripheral arteries (Cumberland and Belli, 1989). The probe successfully traversed 75% of 110 femoral/popliteal occlusions of mean length 9 .crn (range 1 to 35 cm). Thirty-six of these occlusions were Judged 'impossible' for conventional angioplasty, either
because a previous attempt had failed or they had adverse anatomical features (e.g. popliteal occlusion extending into proximal tibial artery). Fifty per cent of these 'impossible' occlusions were successfully traversed, with subsequent balloon dilatation. Although the vessel lumen is of better size than that produced with the bare optical fibre, we do not consider its diameter adequate to maintain patency without adjunctive balloon dilatation. The laser probe has working temperatures of between 300 and 600°C within the artery; these are achieved by using 10 Watts witha 2 mm probe. In air this will heat the probe to 1000°C within seconds. The damage produced by direct conventional laser energy is blamed on its uncontrolled thermal effect, and this has led to the quest for energy which cuts without extensive thermal damage, e.g. pulsed Nd: YAG, dye or excimer lasers (Cross et al., 1987; Wollenek et al., 1988; Lawrence, 1989). However, the damage produced by the thermal laser probe is confined to a narrow underlying zone of injury and there is evidence that the arteries heal and remain functionally intact (Verdaasdonk et al., 1988). In our experience there has been no increase in adverse effects using the laser probe followed by balloon angioplasty. Acute re-occlusions have occurred, but with no increase in incidence when compared with conventional balloon angioplasty. A one Year cumulative patency rate in a study combining Sheffield and Boston patients appears to be better than figures quoted in the literature for conventional PTA (Sanborn et al., 1988). It is possible that a somehow modified response of the arterial wall to balloon dilatation after laser 'debulking' of atheroma is beneficial. A prospective randomised trial is currently under way to confirm this potentially beneficial effect. We are now using a 2.5 diameter 'hybrid' probe designed by Abela and first used intra-operatively (Abela et al., 1986), which has a window at the tip allowing 18% of laser energy to emerge and interact directly with the atheroma. Most of the energy is still converted to heat in the metal probe. The emerging energy appears to produce a 'pilot channel' for the probe to follow. It produces a significantly larger channel than the purely thermal probe. Other systems modifying the delivery of laser energy are currently being evaluated. The direct Argon laser system consists of a bare fibre with a divergent light beam which is kept centrally within the lumen by a high profile balloon. This device successfully recanalised 23 out of 26 femoral occlusions and 10 out of 10 high grade femoral stenoses (Nordstrom et al., 1988). The complications encountered were two cases of distal emboli and one arterial wall perforation. A dual laser system has also been clinically evaluated. The dual system has been developed in an attempt to further reduce the perforation rate by discriminating between atheroma and healthy arterial wall. Atheroma is detected by laser induced fluorescence using ultraviolet light. The fluorescent light is transmitted to a computer via the same optical fibre and spectral analysis determines whether disease is present and, if so, a treatment laser (pulsed dye laser) is triggered which ablates the atheroma. Better tissue selectivity means that the levels of energy
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CLINICAL RADIOLOGY
required should be low e n o u g h n o t to h a r m n o r m a l tissue. This device has been used to successfully recanalise 19 o u t of 19 femoral occlusions, some calcified, with n o laser induced perforations, b u t one guidewire p e r f o r a t i o n did occur (Geschwind et al., 1989). So far, the l u m e n p r o d u c e d by these various modifications is still n o t sufficient to obviate the need for b a l l o o n dilatation. However, in smaller vessels such as the c o r o n a r y a n d tibial arteries, laser probes have been used to recanalise occlusions w i t h o u t the need for s u b s e q u e n t b a l l o o n d i l a t a t i o n (Linnemeier a n d C u m b e r l a n d , 1989; S a n b o r n et al., 1989). Early experience in laser c o r o n a r y artery angioplasty suggested a n increased acute re-occlusion rate using a r g o n - p o w e r e d laser p r o b e therapy c o m p a r e d with conventional angioplasty due to either thermal or m e c h a n i c a l t r a u m a ( C u m b e r l a n d et al., 1986). N d : Y A G laser sources, which can deliver a higher wattage t h a n a r g o n sources a n d led to fewer acute re-occlusions, represented a significant a d v a n c e in laser c o r o n a r y angioplasty (Linnemeier a n d C u m b e r l a n d , 1989). F u r t h e r modifications include the use of a 0.018 inch 'laser wire' with which early experience in the recanalisation of chronic c o r o n a r y occlusions has been f a v o u r a b l e (Bowes et al., 1989). Lasers are still new in the field of P T A a n d further i m p r o v e m e n t s can be expected. D e v e l o p m e n t s in the design o f delivery systems a n d laser sources c o n t i n u e at a rapid rate. However, the high e q u i p m e n t cost will prevent the widespread use o f lasers except in centres with a large P T A practice, a n d where the laser generator can be shared between departments. L o n g term patency rates have yet to be d e t e r m i n e d a n d r a n d o m i s e d trials are n o w being set up to c o m p a r e the results of laser assisted angioplasty with c o n v e n t i o n a l P T A to assess whether it does have a n y a d v a n t a g e over c o n v e n t i o n a l means.
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