2.1.2 Scattering

Introduction

  • Determines the volume distribution of light intensity in the tissue.
  • The primary step of tissue interaction, which is followed by absorption and heat generation.
  • Scattering structure of the tissue can be macroscopic like muscle fibres, skin layers or dentin tubules (microscopic structure)
  • Scattering in tissue an come in the form or Rayleigh or Mie scattering.

Application

  • Possibility to measure the light distribution inside the tissue by introducing a miniaturized probe of 100-um diameter. Via a hollow needle of the syringe.
  • Increased fluency rate in the outcome since scattering photons will overlap with the incident photons.
  • Penetrating depth will also be doubled with increased diameter from 1mm to 5mm. Useful in dermatological application.
  • The depth of penetration of laser light into tissue is greatest in the wavelength range of 700 - 900nm. Blood, water and melanin are the main absorbing components in the tissue.

2.1 Optical Properties of Tissue

When photons strike the surface of the tissue, because of the refractive index change, a portion (4–10%) of the photons are reflected according to the angle of incidence.

Photons penetrating the surface initially are refracted, obeying the law of Snellius,'s law
image

which states that photons entering a medium with a higher refractive index are refracted towards the vertical axis to the surface

2.1.1 Absorption

Absorption of a photon takes place only when its energy, E=hv, corresponds to the energy difference between such quantized states.

Absorption of a photon by a chromophore causes:

  • quantized change in the distance between charges (electron transition, ultraviolet or visible spectrum
  • quantized change of vibrational modes of the molecule (vibration transition, near infrared [NIR]

In the NIR and mid-infrared (MIR) region, tissue absorption is dominated by water absorption, with the maximum at 3 µm.

Vibration transitions of a water molecule with its absorption bands image

Scheme of electronic excitation after photon absorption image

Done by: Allen Paul David & Siti Syafirah