Estimates of minimum pulse width and maximum modulation frequency for diffusion optical tomography

The validity of the time-dependent diffusion equation (DE) requires a condition to be imposed on the time derivative of the radiant current density vector. In the published literature this condition has been studied in connection with forward problems. Using a self-consistency criterion we derive a new condition on the time derivative of the radiant current density for inverse problems. From this condition, and using simple models of the time dependence of the radiant current density vector, we obtain estimates of a minimum pulse width and a maximum modulation frequency for time- and frequency-domain optical tomography of diffuse media. For imaging of biological tissues in the near infrared, with optical parameters (mu)a(r)(almost equal)0.01 mm^-1.(mu)s(r)(almost equal)1 MM^-1, and n(almost equal)1.4, the estimated minimum pulse width and maximum modulation frequency are 930 ps and 340 MHz, respectively. The relevance of these results to the practical application of diffusion theory to optical tomography is discussed.