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Photothermal Therapy

Photothermal Therapy. Nicholas Ellens MBP1028 28 September 2010. Outline. Context Photothermal mechanisms and delivery Absorption and power deposition in tissue. Context • Delivery • Absorption and Power Deposition. Hyperthermia has been used for thousands of years

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Photothermal Therapy

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  1. Photothermal Therapy Nicholas Ellens MBP1028 28 September 2010

  2. Outline • Context • Photothermal mechanisms and delivery • Absorption and power deposition in tissue

  3. Context• Delivery • Absorption and Power Deposition • Hyperthermia has been used for thousands of years • Photothermal therapy emerged shortly after the invention of the laser • Laser treatments range in orders of magnitude in both treatment time and intensity

  4. Context• Delivery • Absorption and Power Deposition Boulnois, J-L, “Photophysical Processes in Recent Medical Laser Developments: a Review” in Lasers in Medical Science, 1986.

  5. Context• Delivery • Absorption and Power Deposition • Driving limitation in phototherapy is the absorption, is quantified by a, the absorption coefficient (units of cm-1) • Depends on medium and frequency • Penetration depth, a-1, ranges from fractions of mm (UV) to a few mm (near IR) • As such, therapy is suitable for topical applications or deeper via fibres delivered an endoscope or percutaneously

  6. Context• Delivery • Absorption and Power Deposition • Further control provided by pulsing the light • If the pulse length is less than the thermal relaxation time, the treatment can be made more localised • Feedback usually involves direct observation • Smoke, blanching • Where this is not achieved, other methods are required

  7. Context• Delivery • Absorption and Power Deposition Boulnois, J-L, “Photophysical Processes in Recent Medical Laser Developments: a Review” in Lasers in Medical Science, 1986.

  8. Context• Delivery • Absorption and Power Deposition • Thermal treatment is a three stage process: • Conversion of heat to light • Thermal transfer • Tissue Response

  9. Context• Delivery • Absorption and Power Deposition Brunetaud, J et al., “Non-PDT Uses of Lasers in Oncology” in Laesrs in Medical Science, 1995. Covered elsewhere in course

  10. Context• Delivery • Absorption and Power Deposition • Reflection: • An air-tissue (n≈1.41) interface causes substantial internal reflection, further depositing energy • Scattering: • Scattering length is typically 100-1000 times less than absorption length • Upon entering tissue, photons are scattered many times before being absorbed • Effectively, this increasesthe light intensity close tothe tissue surface Burgholzer, P. “Photoacoustic tomography: Sounding out fluorescent proteins” in Nature Photonics 3, 2009.

  11. Context• Delivery • Absorption and Power Deposition • Conversion of light to heat • Absorption: • A + hu A* • Deactivation: • A* + B(E)  A + B’(E+DE) • Efficacy depends on high probability of collision and high number of accessible vibrational states • Though sensitive to wavelength, less so than chemical processes

  12. Context• Delivery • Absorption and Power Deposition • Heat diffusion • As per the bioheat transfer equation • Affected by thermal properties of tissue, timescale, blood perfusion • Thermal effects • Hyperthermia • Coagulation • Vaporisation

  13. fini

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