Photothermal Therapy

Nicholas EllensMBP1028

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

• Photothermal therapy emerged shortly after the invention of the laser

• Laser treatments range in orders of magnitude in both treatment time and intensity

Boulnois, J-L, “Photophysical Processes in Recent Medical Laser Developments: a Review” in Lasers in Medical Science, 1986.

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

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

Context • Delivery • Absorption and Power Deposition

Context • Delivery • Absorption and Power Deposition

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• Thermal treatment is a three stage process:1. Conversion of heat to light2. Thermal transfer3. Tissue Response

Context • Delivery • Absorption and Power Deposition

Context • Delivery • Absorption and Power Deposition

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• 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 increases

the light intensity close tothe tissue surface

Context • Delivery • Absorption and Power Deposition

Burgholzer, P. “Photoacoustic tomography: Sounding out fluorescent proteins” in Nature Photonics 3, 2009.

• 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

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

Context • Delivery • Absorption and Power Deposition

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