Investigation of plasmas produced by laser ablation using single and double pulses for food analysis demonstrated by probing potato skins.

We report on investigations of plasmas produced by laser ablation of fresh potatoes using infrared nanosecond laser radiation. A twin laser system consisting of two Nd:YAG oscillators was used to generate single or double pulses of adjustable interpulse delay. The potatoes were irradiated under ambient air with moderate pulse energies of about 10 mJ. The expansion dynamics of the ablation plume was characterized using fast imaging with a gated camera. In addition, time-resolved optical emission spectroscopy was applied to study the spectral line emission of the various plasma species. The electron density was deduced from Stark broadening, and the plasma temperature was inferred from the relative emission intensities of spectral lines. The relative concentrations of metals were estimated from the comparison of the measured emission spectra to the spectral radiance computed for a plasma in local thermal equilibrium. It is shown that the plasma produced by double pulses has a larger volume and a lower density. These properties lead to an increase of the signal-to-noise ratio by a factor of 2 and thus to an improved measurement sensitivity.

The effect of scattered radiation in dual-energy analysis.

Dual-energy techniques can be used to provide additional information during x-ray examinations. A recent development has been in the application of dual-energy analysis to fluoroscopic procedures. In this paper the effects of scattered radiation upon the results obtained with a small area, dual-energy probe used during fluoroscopy has been evaluated. The method employed has been the development and application of a Monte Carlo photon transport computer program. Experimental confirmation of the model has been performed and the model extended to study the effects of scattered radiation upon the chemical sensitivity of the dual-energy probe. These effects have been studied using a simplified phantom geometry to represent a homogeneous patient. The results indicate that for tissue analysis where the effective atomic number (Z) is less than 13 the effects of scattered radiation lead to errors in estimates of Z of +/- 0.25 when the acceptance angle of the detected radiation is below +/- 20 degrees.

A real time dual-energy probe for tissue characterization during fluoroscopy.

In the past dual-energy techniques have been applied with a variety of detector systems. However, making clinical decisions during a diagnostic x-ray procedure by using dual-energy information has not been possible. This paper looks at the development of a dual-energy probe that permits real time analysis. The technique is based on the local analysis procedure introduced by Speller and co-workers and uses spectral filtering for energy separation. The split-detector probe is optimized using computer models, and the effects of beam hardening and scattered radiation are considered. It is shown that a 0.25 mm CsI/25 mm NaI combination of detector elements with a 0.3 mm Cu filter offers the best performance. Preliminary results using the probe for in vivo analysis of gall stone composition compare well with the more accepted methods of x-ray diffraction and atomic absorption spectroscopy. The two groups of stones evaluated were found to have mean effective atomic numbers of 5.6 +/- 1.7 and 9.6 +/- 0.5. It is suggested that such a procedure could prove useful in patient management in the use of lithotripsy.