Pulsed photothermal temperature profiling of agar tissue phantoms

Matija Milanič,Boris Majaron,J Stuart Nelson

doi:10.1007/s10103-007-0455-9

Abstract

We determine experimentally the accuracy of pulsed photothermal radiometric (PPTR) temperature depth profiling in water-based samples. We use custom tissue phantoms composed of agar gel layers separated by very thin absorbing layers. Two configurations of the acquisition system are compared, one using the customary spectral band of the InSb radiation detector (3.0-5.5 microm) and the other with a spectrally narrowed acquisition band (4.5-5.5 microm). The laser-induced temperature depth profiles are reconstructed from measured radiometric signals using a custom minimization algorithm. The results correlate very well with phantom geometry as determined by optical coherence tomography (OCT) and histology in all evaluated samples. Determination of the absorbing layer depth shows good repeatability with spatial resolution decreasing with depth. Spectral filtering improves the accuracy and resolution, especially for shallow absorption layers (~120 microm) and more complex structures (e.g., with two absorbing layers). The average full width at half maximum (FWHM) of the temperature peaks equals 23% of the layer depth.

Highlights

Success of several laser-based therapeutic procedures depends on the physician’s ability to select a suitable combination of treatment parameters on an individual patient basis
Numerical simulations and preliminary experimental data suggest that utilizing the entire spectral band of a typical InSb radiation detector may compromise the accuracy of Pulsed photothermal radiometry (PPTR) measurements in biological tissues due to pronounced spectral variation of the absorption coefficient in water between 3 and 4 μm [12, 14, 15]
We present in this paper a systematic experimental comparison of PPTR temperature profiling utilizing the full and appropriately narrowed acquisition spectral band of the InSb detector

Summary

Introduction

Success of several laser-based therapeutic procedures depends on the physician’s ability to select a suitable combination of treatment parameters on an individual patient basis. Pulsed photothermal radiometry (PPTR) was recognized as a promising technique for non-invasive determination of chromophore distribution in strongly scattering biological tissues and was extensively investigated [3,4,5,6,7,8,9,10,11,12,13]. This technique is based on time-resolved measurement of midinfrared (IR) emission after pulsed laser irradiation of a sample. Numerical simulations and preliminary experimental data suggest that utilizing the entire spectral band of a typical InSb radiation detector may compromise the accuracy of PPTR measurements in biological tissues due to pronounced spectral variation of the absorption coefficient in water between 3 and 4 μm [12, 14, 15]

Methods

Results

Discussion

Conclusion