Method of calculating the image resolution of a near-infrared time-of-flight tissue-imaging system.

A new method for calculating the image resolution for a near-infrared time-of-flight tissue-imaging system is presented. The image resolution is calculated from the full width at half-maximum of the photon-path function along the midplane of the medium, integrated over all times of flight, and weighted by the time-resolved detector response. Detailed treatment of the optical gating mechanism shows that for some types of gating mechanism, there exists an optimal gating time beyond which the image resolution is not improved by arbitrarily decreasing the gating time. This theory predicts a limiting image resolution of~20% of the medium thickness, which is consistent with the research of others.

Optical transmission properties of homogenised milk used as a phantom material in visible wavelength imaging.

Using the collimated output beam from a Helium-Neon laser, operating at 632.8 nm, transmission intensities through various thicknesses and concentrations of homogenised milk have been measured for the purpose of determining the suitability of milk as a phantom for visible wavelength imaging. Also, theoretical predictions to the laser beam attenuation in milk based on diffusion theory have been calculated, giving substantial agreement with experiment for milk concentrations greater than 10%. The transmission properties of milk have been compared with a commonly used optical phantom, namely Intralipid, and the scattering coefficient mu(s) measured for each phantom. The scattering coefficients for undiluted milk and 20% solids Intralipid are 52 +/- 1, and 139 +/- 6 mm-1, respectively. With appropriate normalisation of the physical parameters, the effective physical properties of the phantom materials vis a vis light transmission become the same within limits imposed by experimental error. Comparison of measured transmission properties of homogenised milk with tissue confirm its suitability as a phantom vis a vis scattering properties determined by mu(s) and its mean cosine of scatter g, but that its absorption coefficient mu(a) is approximately two orders of magnitude less than for tissue. Appropriate attenuators, such as India Ink, have been suggested as additives to increase the value of mu(a).