2. The physics of lasers as applied to medicine Sandy Mosse Medical Physics, UCLH National Medical Laser Centre, UCL

3. L ight A mplification by S timulated E mission of R adiation Lasers –basic principles –common types Tissue optics & treatments –scattering & absorption –modes of interaction Safety Contents

4. Spontaneous Emission Energy Levels Photon

5. Stimulated Emission and Absorption

6. Population inversion

7. Lasers mirrors create illusion of a long tube But diode laser “Collimation” Low gain (say 10% per metre) requires long tube

8. Power / Energy Power (Watts) = rate at which energy is delivered Energy (Joules) = amount of energy delivered 1 Watt = 1 Joule per second Energy = power x time,J = Wt

9. Some lasers in medical use CO 2 10.6 micron Er:YAG2.94 miron Ho:YAG2.1 micron Nd:YAG1064 nm Ar488/514 nm Kr647 nm Cu vapour511/578 nm Dyeeg 630/675 nm Diodeeg 600 / 905 nm Excimer> 350 nm

10. Boxes

11. Laser/Tissue interaction Refraction Scatter Absorption

13. Laser penetration depths

14. Photothermal - low power density

15. Mesenteric vessel photothermally coagulated by a 60W Nd:YAG laser

16. Photothermal - high power density. Ablation

17. Tongue before & after CO 2

19. Effect of power density on tissue Power Density W/cm 2 Time (s)

20. Photoablative effect of UV excimer laser light Binding Energy approx. 3.5 eV Photon Energy approx. 6.4 eV Approx. 0.25  m Corneal Tissue Excimer Beam Corneal Tissue Excimer Beam

21. Precision of the Excimer Laser

22. LASIK for Myopic ablation Cornea Desired Corneal surface Tissue to be removed Laser Assisted In Situ Keratomileusis

23. LASIK in Myopia

25. LASIK in Miopia

26. LASIK in Myopia

27. Plasma formed by a focused Q- switch laser

28. Laser fragmentation of a kidney stone

29. Pulse dye laser fragmentation of a kidney stone

30. Photodynamic Therapy - PDT Taken from http://omlc.ogi.edu

31. Dougherty’s pet cat

32. Modern times

33. Photosensitising Drugs Based on porphyrins or similar structures Absorbed photon energy is transferred to form singlet oxygen 1 O 2 highly reactive, reacts within ~ 20 nm Sufficient cell damage causes cell death Should concentrate in the tumour

34. Kills, rather than annihilates, cells. Direct photodamage: 1-2 log kill of tumour cells 1. Treatment needs 6 - 8 log Most cells die as a result of ~ –Damage to microvasculature –Inflamatory reaction –Immune response

35. Advantages The alternatives;~ Surgery Chemotherapy is systemic Radiation serious side effects. Once only. PDT kills cells but does less damage to collagenous tissue structures, and normal cells will repopulate these structures. PDT is potentially a low-cost minimally invasive localised treatment. PDT is repeatable, unlike radiation therapy. Allows wide area shallow treatment

36. Disadvantages Light sensitive patient Poorly defined volume of treatment Access for light Variable potency

37. Light sensitivity

38. Achilles’ heal

43. Laser safety Many types of laser –Local safety rules - written for each laser in each location. Laser light is just light,nothing magic, but may be HIGHLY COLLIMATED –Travel a long way without diverging –Focus to a tiny spot - high power density

44. Laser hazards Exposure >100s photochemical hazard 100  s thermal hazard Nanosecond photoacoustic shock

45. Laser hazards Skin Electrical - Hazard for engineers, not for medics Fire - In particular down endoscope Eye - Retinal or corneal burns –Maybe no sensation of damage at the time –Symptoms include headache watering “floaters ”

46. Eye Laser (visible & NIR)

47. Electromagnetic Spectrum Wavelength (m) Gamma rays 10 -13 10 -10 10 -6 10 3 10 -9 10 0 10 -2 X rays Ultra violet Infra red Radio frequency UHF VHF Radar FM radio Broadcast Power lines Microwaves Angstrom (Å) nano (nm) micro (  m) kilo (Km) centi (cm) 1400 nm 700 nm 400 nm visible near IR

48. Ultraviolet Radiation

49. Eye - visible (400-700 nm) light Focussed to a point - intensified x100,000 Blink protects but takes ~ 1/4 second Safe limit = 1mW, –bright London sunlight  ½ mW/mm 2 Fovea only 3-4% of area of retina

50. Eye - Invisible light NIR transmitted to the retina. No deliberate focussing but, relaxed eye will focus anyway. IR (>1400 nm) absorbed by cornea, lens etc. – glass blower’s or furnace man’s cataract NO BLINK RESPONSE

51. Damage to the Retina

52. Eye Injury Nd:YAG (1064 nm) - permanent damage. Victim saw white flash, heard a click, then immediately a dark spot in visual field

53. Eye Injury Lasers which destroy retinal tissue can cause haemorrhaging into the vitreous - this will eventually clear but the retinal damage is permanent

54. Safety goggles CHECK THEY ARE FOR THE CORRECT WAVELENGTH Usually 4-6 O.D. (1 OD = 1 log reduction = 10% transmission)

55. Fumes & debris Plume of smoke during laser ablation –may contain viruses etc. Always use a fume extractor as close as possible to the treatment area