Photon Propagation in Scattering Media

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary reason for the intensity of a beam of light to follow exponential decay when passing through a scattering medium?

The primary reason is that some of the light is scattered out of the beam into different directions.

How do light emitting diodes (LEDs) differ from lasers in terms of light emission?

LEDs emit light through spontaneous recombination of electrons and holes, while lasers emit coherent light due to stimulated emission.

What role do He and N₂ play in the operation of a carbon dioxide laser?

Helium and nitrogen are included to improve the laser efficiency by facilitating better energy transfer between molecules.

Why is the infrared light produced by carbon dioxide lasers particularly effective for vaporizing tissue?

The infrared light is absorbed very strongly by water, which is a major component of tissue. Signup and view all the answers

In transport theory, what does it mean to describe the passage of particles through a scattering medium statistically?

It means providing statistical insights about the average behavior of large populations of particles rather than tracking individual particles. Signup and view all the answers

What is the typical power range of clinical laser systems?

10-50W Signup and view all the answers

What is the primary active medium used in an Nd:YAG laser?

Yttrium-aluminum-garnet (YAG) doped with neodymium (Nd3+ ions) Signup and view all the answers

For what clinical applications is the Nd:YAG laser primarily used?

Photodisruption in posterior capsulotomy, photoacoustic imaging, and coagulation. Signup and view all the answers

How do excimer lasers like the ArF operate at a molecular level?

They emit laser light from an excited gas of diatomic molecules that dissociate upon returning to a ground state. Signup and view all the answers

What is the pulse duration and energy range of pulsed excimer lasers?

~10ns pulses with 0.2-1 J energy. Signup and view all the answers

What distinguishes diode lasers in terms of design and functionality?

They are semiconductor lasers with a resonant cavity, known for being compact, inexpensive, and having long lifetimes. Signup and view all the answers

What major surgical application utilizes ArF excimer lasers?

Corrective laser eye surgery (LASIK). Signup and view all the answers

What is the primary application of the CO₂ laser with a wavelength of 10,600 nm?

It is used for deep laser resurfacing. Signup and view all the answers

Which wavelength range of LED light is recognized for anti-aging properties and stimulating collagen?

600-650 nm. Signup and view all the answers

Describe the unique property of blue to green light (495-570 nm) in dermatological treatments.

It fights off and kills bacteria, particularly useful in treating acne. Signup and view all the answers

What significant advantage do optical fibers provide when using laser light in treatment?

They allow precise and easy delivery of light to targeted areas. Signup and view all the answers

What skin conditions can violet to deep blue light (380-495 nm) effectively treat?

It can treat psoriasis, dermatitis, and vitiligo. Signup and view all the answers

What is the primary application of the Argon laser despite its absorption characteristics?

It is rarely applied in aesthetic or dermatologic treatments. Signup and view all the answers

How does the absorption property of the Ruby laser influence its applications?

It is used for laser hair removal and treatment of pigmented lesions due to its strong absorption by melanin. Signup and view all the answers

What wavelength range does the Diode laser operate within, and what is its primary use?

The Diode laser operates between 800-900 nm and is primarily used for hair removal. Signup and view all the answers

Which laser is specifically noted for its application in treating vascular lesions?

The Pulsed Dye laser is specifically utilized for treating vascular lesions. Signup and view all the answers

What is the therapeutic role of the He-Ne laser in clinical situations?

It is employed in vitiligo management and repair of nerve injuries and wound healing. Signup and view all the answers

Identify the laser that allows deeper skin penetration while being absorbed by melanin.

The Alexandrite laser allows deeper skin penetration. Signup and view all the answers

What wavelength is associated with Nd:YAG laser, and what medical procedure is it used for?

The Nd:YAG laser operates at 1064 nm and is used for photocoagulation. Signup and view all the answers

What unique feature of the Er:YAG laser contributes to its use in cosmetic procedures?

It emits a mid-infrared beam suitable for ablating tissue. Signup and view all the answers

Explain how the wavelength range of 577-585 nm laser light affects its absorption targets.

The wavelength range of the Pulsed Dye laser is absorbed particularly by hemoglobin. Signup and view all the answers

Which laser type is less commonly applied in dermatology due to its absorption properties?

The Argon laser is less commonly applied in dermatology. Signup and view all the answers

Study Notes

Source and Detectors

Session 4
Light sources: incoherent (LEDs, Superluminescent Diodes) and coherent (Lasers)

Coefficient Scattering

Consider a collimated beam of light traveling in the z-direction through a thin, elastically scattering, non-absorbing medium.
As the beam passes through the medium, some of the light will be scattered out of the beam into different directions, and the intensity of the beam will follow an exponential decay (exp(-μsz)).
The probability that a photon will be scattered while traveling the short distance between z and z + dz is μδz.

Tissue Propagation Light

Transport theory is an area of statistical mechanics that describes the passage of particles (neutrons, photons, molecules) through a scattering medium.
It describes the statistics for a large population of particles.

Tissue Propagation Light continued

It is possible to give some statistics about a large population of particles.

The Processes Included in the Radiative Transfer Equation

Photons are included, and can be generated, absorbed, scattered into or out of a direction (ŝ).

Equations

1 дф / c dt = q − (ŝ · ∇ + μτ)φ + ∫(ŝ, ŝ')(ŝ') dŝ' / S2
μτ = μa + μs

Lasers Medical - Carbon Dioxide Laser

Carbon dioxide laser (λ = 10.6 µm)
Laser levels are vibrational levels of the CO2 molecule, although the active medium also contains He and N2.
Helps to improve laser efficiency.
IR light is strongly absorbed by water, used to vaporize tissue (water content)
Laser scalpels and skin resurfacing.
Typical power is 10–50W.

Lasers Medical - Nd:YAG

Nd:YAG laser (λ = 1064 nm)
A four-level solid state laser with an active medium from an yttrium-aluminum-garnet crystal (YAG, Y3Al5O12).
Doped with neodymium Nd3+ ions.
Pulsed Nd:YAGs can generate pulses of tens to a few hundred mJ.
Clinically used for photodisruption in posterior capsulotomy, photoacoustic imaging, and coagulation.

Lasers Medical - ArF Excimer

ArF excimer laser, wavelength: 193 nm
Gas lasers emitting UV wavelengths.
Excimer is a contraction of excited-dimer.
Active medium is a gas of diatomic molecules.
They are bound only when in an excited state. Atoms separate immediately on return to the ground state.
The molecule is empty in the ground state.
Pulsed excimer lasers typically produce ~10ns pulses with 0.2-1 J energy.
Used in corrective laser eye surgery (LASIK).
Short wavelength allows photoablation of corneal tissue without damaging it.

Lasers Medical - Diode Laser

Laser diodes, in the 650-2000nm range, are semiconductor lasers.
Operation is similar to superluminescent diodes, with a resonant cavity.
Features: compact and inexpensive, long lifetimes, available in varying red-NIR wavelengths.
Output power for NIR diode arrays can be as high as 50W.
Applications: clinical applications, NIR spectroscopy, optical imaging, photodynamic therapy, photocoagulation.

Optics Fibre

One of the significant advantages of using light, and particularly laser light, over other wavelengths of radiation is that it can often be coupled into optical fibres, allowing light to be delivered precisely to the needed location.

Photodiodes

Photodiode sensors for detecting light in the applications of endoscopy and minimally invasive surgery, where small holes are needed to fit the surgical tools.
Optical fibers utilize total internal reflection to transmit light long distances with minimal loss and into otherwise inaccessible areas.
Step-index fibers have a core medium for light travel and a cladding medium for insulation.
The core has a slightly higher refractive index than the cladding, resulting in total internal reflection.
Core diameter in singlemode fibers is 8-10µm, while multimode fibers are significantly wider (~ 50µm).
The numerical aperture (NA), dictates the maximum acceptance angle and the divergence angle (angle at which the light leaves the fiber). NA = √(n_core² - n_cladding²).
Applications such as therapy (destruction of tumors, corneal reshaping) don't necessarily require light detection, whereas imaging and sensors do.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Explore the principles of light sources and their behavior in scattering media in this quiz from Session 4. Understand incoherent and coherent light sources, the exponential decay of intensity, and transport theory related to tissue propagation. Test your knowledge on how particles like photons interact with different environments.

Photon Propagation in Scattering Media - Session 4

Choose a study mode