Biomedical subjects

Christoffer Abrahamsson

Publications and source records attributed to Christoffer Abrahamsson.

3 recordsLinked to original sources

PubMed2005

Least-squares support vector machines modelization for time-resolved spectroscopy.

By use of time-resolved spectroscopy it is possible to separate light scattering effects from chemical absorption effects in samples. In the study of propagation of short light pulses in turbid samples the reduced scattering coefficient and the absorption coefficient are usually obtained by fitting diffusion or Monte Carlo models to the measured data by use of numerical optimization techniques. In this study we propose a prediction model obtained with a semiparametric modeling technique: the least-squares support vector machines. The main advantage of this technique is that it uses theoretical time dispersion curves during the calibration step. Predictions can then be performed by use of data measured on different kinds of sample, such as apples.

Algorithms↗

PubMed2005

Time-resolved NIR spectroscopy for quantitative analysis of intact pharmaceutical tablets.

Near-infrared (NIR) spectroscopy is a useful technique for quantitative measurements of intact tablets, but it suffers from limitations due to the fact that changes in the physical properties of a sample strongly affect the recorded spectrum. In this work, time-resolved transmission NIR spectroscopy was utilized to conduct quantitative measurements of intact tablets. The technique enables separation of the absorption properties of the sample from the scattering properties and can therefore handle changes of the physical parameters of the samples in a better way than conventional NIR transmission spectroscopy. The experiments were conducted using a pulsed Ti:sapphire laser coupled into a nonlinear photonic crystal fiber as light source. The light transmitted through the sample was measured by a time-resolving streak camera. A comparison of the results from the time-resolved technique with the results from conventional transmission NIR spectroscopy was made using tablets containing different concentrations of iron oxide and manufactured with different thicknesses. A PLS model made with data from the time-resolved technique predicted samples 5 times better than a PLS model made data from the conventional NIR transmission technique. Furthermore, an improvement to predict samples with physical properties outside those included in the calibration set was demonstrated.

Calibration↗

PubMed2005

Scatter correction of transmission near-infrared spectra by photon migration data: quantitative analysis of solids.

The scope of this work is a new methodology to correct conventional near-infrared (NIR) data for scattering effects. The technique aims at measuring the absorption coefficient of the samples rather than the total attenuation measured in conventional NIR spectroscopy. The main advantage of this is that the absorption coefficient is independent of the path length of the light inside the sample and therefore independent of the scattering effects. The method is based on time-resolved spectroscopy and modeling of light transport by diffusion theory. This provides an independent measure of the scattering properties of the samples and therefore of the path length of light. This yields a clear advantage over other preprocessing techniques, where scattering effects are estimated and corrected for by using the shape of the measured spectrum only. Partial least squares (PLS) calibration models show that, by using the proposed evaluation scheme, the predictive ability is improved by 50% as compared to a model based on conventional NIR data alone. The method also makes it possible to predict the concentration of active substance in samples with other physical properties than the samples included in the calibration model.

Algorithms↗