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Photon transport in biological tissue can be equivalently modeled numerically with Monte Carlo simulations or analytically by the radiative transfer equation (RTE). However, the RTE is difficult to solve without introducing approximations. A common approximation summarized here is the diffusion approximation. Overall, solutions to the diffusion equation for photon transport are more computationally efficient, but less accurate than Monte Carlo simulations. ==Definitions== The RTE can mathematically model the transfer of energy as photons move inside a tissue. The flow of radiation energy through a small area element in the radiation field can be characterized by radiance . Radiance is defined as energy flow per unit normal area per unit solid angle per unit time. Here, denotes position, denotes unit direction vector and denotes time (Figure 1). Several other important physical quantities are based on the definition of radiance:〔 *Fluence rate or intensity *Fluence *Current density (energy flux) . This is the vector counterpart of fluence rate pointing in the prevalent direction of energy flow. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Radiative transfer equation and diffusion theory for photon transport in biological tissue」の詳細全文を読む スポンサード リンク
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