Optimized processing has revealed spatial patterns for age related preservation and losses in brain SPECT that indicate their origin is primarily structural. Correction for structural effects in optimized SPECT is needed to confirm whether any regional ageing effects derive from changes in rCBF.

Use of a normal database in quantitative regional analysis of brain single-photon emission tomography (SPET) facilitates the detection of functional defects in individual or group studies by accounting for inter-subject variability. Different reconstruction methods and suboptimal attenuation and scatter correction methods can introduce additional variance that will adversely affect such analysis. Similarly, processing differences across different instruments and/or institutions may invalidate the use of external normal databases. The object of this study was to minimise additional variance by comparing reconstructions of a physical phantom with its numerical template so as to optimise processing parameters. Age- and gender-matched normal scans acquired on two different systems were compared using SPM99 after processing with both standard and optimised parameters. For three SPET systems we have optimised parameters for attenuation correction, lower window scatter subtraction, reconstructed pixel size and fanbeam focal length for both filtered back-projection (FBP) and iterative (OSEM) reconstruction. Both attenuation and scatter correction improved accuracy for all systems. For single-iteration Chang attenuation correction the optimum attenuation coefficient (mu) was 0.45-0.85 of the narrow beam value (Nmu) before, and 0.75-0.85 Nmu after, scatter subtraction. For accurately modelled OSEM attenuation correction, optimum mu was 0.6-0.9 Nmu before and 0.9-1.1 Nmu after scatter subtraction. FBP appeared to change in-plane voxel dimensions by about 2% and this was confirmed by line phantom measurements. Improvement in accuracy with scatter subtraction was most marked for the highest spatial resolution system. Optimised processing reduced but did not remove highly significant regional differences between normal databases acquired on two different SPET systems.

The spatial resolution achievable in time-resolved optical transillumination imaging through a turbid (scattering and absorbing) medium has been reassessed theoretically. The temporal point spread function was constructed assuming a delta function input pulse, a approximately 50 mm thick medium and a small detector with zero risetime. Temporal profiles were derived from an indeterministic Monte Carlo simulation for different time scales. From the temporal point spread function (TPSF), an analytic edge response function from which the spatial resolution was determined was derived. Previous analytical methods for determining the spatial resolution are approximations for very short flight times (sub-100 ps time region). The results show that a spatial resolution of about two millimetres is possible under ideal signal-to-noise ratio conditions and with detector gate times of the order of ten picoseconds. If this predicted spatial resolution can be achieved in an imaging system, it may be possible to improve the diagnosis of breast tumours.

A time-resolved indeterministic Monte Carlo (IMC) simulation technique is proposed for the efficient construction of the early part of the temporal point spread function (TPSF) of visible or near infrared photons transmitted through an optically thick scattering medium. By assuming a detected photon is a superposition of photon components, the photon is repropagated from a point in the original path where a significant delay in forward propagation occurred. A weight is then associated with each subsequently detected photon to compensate for shorter components. The technique is shown to reduce the computation time by a factor of at least 4 when simulating the sub-200 picosecond region of the TPSF and hence provides a useful tool for analysis of single photon detection in transillumination imaging.