Optical Parameters N(r,ŝ): At a given point r, is defined as the number of photons per unit volume moving in the direction of unit vector ŝ. Radiance: Used to describe the propagation of photon power. Is related to power flowing through infinitesimal area , located at r, in the direction of unit vector ŝ. Unit [W/m2] Fluence rate: At a given point in space, the radiant power incident on a small sphere, divided by the cross sectional area of that sphere. Radiant Energy: Delivery of energy described in terms of radiant power, power emitted or transferred or received as radiation. Radiant Emittance: Energy radiant flux leaving an element of the surface divided by the area of that element. Irradiance: At a point of a surface , the radiant energy flux incident on a element of the surface divided by the area of the surface. Source term: The amount of photon energy per second absorbed locally in tissue. Unit [W/m3] Laser tissue Interaction Different types of lasers react differently with tissue. Interaction depends on: •The wavelength of the laser •Power density and exposure time •Optical and thermal property of the tissue being irradiated •Laser beam size on the tissue •If exposure is CW or pulsed wave radiation Some common interactions…. Electromechanical Causes dielectric breakdown in tissue caused by shock wave plasma expansion resulting in localized mechanical rupture. Photoablative Causes photo dissociation or breaking of the molecular bonds in tissue. Photothermal Converts light energy into heat energy. This causes the tissue to heat up and vaporize. Photochemical Very low-power irradiation inactivates cell function by means of induced toxic chemical processes Causes target cells to start light-induced chemical reactions. Laser-tissue interaction – absorption: Absorption spectra of main absorbers: melanin in skin and hemoglobin (Hb/HbO2) in blood Absorption……. •The broad absorption band of water in the IR can also be used to transfer optical energy into kinetic energy (heat) in tissue •In the visible range, light absorption by water is low, which can be used for in vivo interrogation of tissue •In UV, light scattering in tissue increases significantly - less penetration depth in tissue Laser-tissue interaction – thermal Thermal effects of laser radiation (raised temperature in tissue) Note that protein and collagen denature at around 60°C Heating tissue at 60°C can lead to cell necrosis - can be used to kill cancer cells Thermal effects….. Can also be used to join or bond soft tissues such as blood vessels, skin, nerve fibers, various tubes/tracts etc. In Helium laser Lithotripsy, the principal interaction is thermal Laser-tissue interaction – Laser ablation •With enough photon energy, laser can be used to ionize molecules in biological tissue. •Even better, tissue can be removed very precisely without any appearance of thermal damage using short pulses of laser •The absorption of laser energy in tissue follows Beer-Lambert’s law •There exists a threshold at which the absorbed energy is high enough to cause decomposition of tissue Applications: •Ultra short laser pulses can be used to cut biological tissue into thin slices without any previous preparation of the tissue. •Focusing ultra short laser pulses on biological tissue, optical breakdown Occurs within the focus of the laser pulse •Laser eye surgery - Ablation of corneal tissue is achieved by using a pulsed ArF laser (λ=193nm) •Wrinkle removal (skin resurfacing) and pigment removal from skin Lasers in Ophthalmology LASIK (laser-assisted in situ keratomileusis LASIK is a painless,patchless 15 minutes procedure which aims at permanently altering the focusing power of the eye by reshaping the corneal surface with the laser light. 1. Anesthetic drops are used and a lid speculum is used to hold your eyelids wide open. 2. No injections are used and you will not feel any pain, only some discomfort may be felt. 3. A ring is placed on your eye and a slight pressure will be applied to create suction. NORMAL EYE CORNEAL RING 4. Your doctor will then make a corneal flap. 5. The suction ring is removed. FLAP MAKING 6. The doctor will then lift the flap and hold it back on its hinge and dry the exposed tissue. CORNEAL FLAP 7. The laser beam will then be focused over your eye and you will be asked to stare at a green blinking light. This light is to help you to fixate your eyes on one spot, once the laser is started. 8. The laser light will reshape the cornea. 9. A computer controls the amount of laser energy delivered to your eye. LASER LIGHT RESHAPED CORNEA 10. The flap is put back into position. 11. A black goggle will be placed over your eye at the end of the procedure to protect your eyes 12. There are no stitches. FLAP BACK IN PLACE CORNEA HEALED Therapeutic and Diagnostic applications of Lasers in Ophthalmology Transmission and Absorptive properties of Ocular tissues - Light entering the eye can be reflected, transmitted, scattered or absorbed. -In the visible and the near infrared spectrum the absorption characteristics of ocular tissues is determined by group of chromophores within the tissue. -Ocular chromophores include melanin located in the retinal pigment epithelium, iris pigment epithelium, hemoglobin located in red blood cells within blood vessels. - Due to its transmission characteristics cornea provides effective window for vision, photocoagulation, photodisruption, imaging and examination of intraocular structures. Photothermal Laser applications A] Mechanisms of photothermal laser tissue interaction: 1. Occur when laser energy is absorbed by target tissue and converted into heat. 2. Low temperature elevations cause cell damage without causing structural alterations to the tissue (photoheating). 3. Greater temperature elevations cause thermal coagulation of tissue with cellular death and structural damage to the tissue (photocoagulation). 4. To minimise unwanted thermal damage, a wavelength should be selected that is preferentially absorbed by the target tissue and the laser exposure duration should be shorter than the thermal relaxation time of the tissue which is given by Clinical and research applications for photothermal lasers in ophthalmology Photodisruptive laser tissue interaction Photochemical Laser applications: Photoablation and photodynamic therapy Photodynamic therapy -Utilizes a selective laser wavelength to activate a photosensitizing agent that in turn causes damage or destruction to malignant or abnormal tissue. -A sensitizer is a molecule or compound that produces singlet oxygen or free radicals when irradiated by a particular wavelength. -Phototoxicity results from the biochemical interaction of tissues with the singlet oxygen and radicals and is a function of photosensitizers quantum efficiency and absorption spectra, its concentration in tissue and wavelength and fluence of the irradiation. Photosensitizer -Hematoporphyrin derivative -A complex mixture of monomeric and aggregated porphyrins . -It is a tetrapyrrole aromatic macrocycle related to heme -Protoporphyrin IX absorbs light, exciting it to an excited singlet state. -Resulting in excited triplet protoporphyrin IX -Energy is transferred from triplet protoporphyrin IX to triplet oxygen, resulting in excited singlet oxygen. -Singlet oxygen reacts with biomolecules, fatally damaging some cells in the treatment area. DIAGNOSTIC LASER APPLICATIONS 1. Visual Acuity Measurement: -Spatial coherence of lasers is used. -The retinal functional health is evaluated when it is obscured by cataract. -Helpful in deciding whether or not cataract surgery will be benefit to the patient. 2. Laser Doppler Velocimeter: -Light scattered by moving blood cells is shifted in frequency and is used to measure the rate of blood flow in the veins and arteries of the retina. -Temporal coherence of lasers is used. 3.ScanningLaserOphthalmoscope: - Useful in viewing retina and its supporting structures including blood vessels, nerve bundles and underlying layers. 4. Spectroscopic diagnosis of Ocular diseases: -Uses fluorescence spectroscopy and Raman spectroscopy to detect various abnormal states of ocular tissues and ocular diseases. - Understanding of biochemical changes associated with cataract formation. Lasers in Dermatology The choice of laser depends on the colour, depth and chemical nature of the tattoo ink. Two to ten treatments are often necessary. Black: QS ruby, alexandrite or Nd:YAG Blue and green*: QS ruby, alexandrite Yellow*, orange*, red: QS Nd:YAG or PDL Tatoo Removal By QS Ruby laser QS Alexandrite laser QS Nd -YAG laser PORT WINE STAIN -A port-wine stain is a vascular birthmark consisting of superficial and deep dilated capillaries in the skin which produce a reddish to purplish discolouration of the skin. -The flash lamp pumped dye laser, a yellow light laser, has been the most successful at destroying stains in infants and young children. The neodymium YAG laser is used to treat the nodules that may develop in some adult port-wine stains.