Laser thermal therapy: utility of interstitial fluence monitoring for locating optical sensors

Multipoint optical fluence measurements can potentially be used to detect c oagulation-induced changes in optical propagation during interstitial laser thermal therapy. Estimating the dimensions of coagulation using on-line op tical monitoring, which is applicable to treatments where the tip of the so urce fibre is not precharred, may be limited by the accuracy of the placeme nt of optical sensors with respect to source fibres. A strategy has been de veloped to determine accurately the position of a four-sensor linear array, prior to treatment, using optical fluence data obtained from the sensors f or low-power (less than or equal to0.5 W) irradiation. A minimum of four se nsors in an array was required in order to develop a mathematical formulati on for position determination that did not require tissue optical propertie s or laser power as input. Optical propagation was based on diffusion theor y for homogeneous tissues in spherical geometry. Low input laser power is n eeded to ensure that there are no thermally induced changes in tissue optic al properties not accounted for in the mathematical description. Experiment al evaluation was performed in a tissue-equivalent liquid phantom using 0.5 W of 805 nm optical energy and a translatable isotropic optical sensor. Fo r sensor locations with 2 mm spacing, placement accuracy of 0.67 mm was ach ieved. The accuracy improved to 0.13 mm as the sensor spacing increased to 5 mm.