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Isotropic detectors with spherical scattering tips are commonly used for in-vivo dosimetry of light fluence rate during photodynamic therapy (PDT). These detectors are typically calibrated in-air. It has been well established that the response of an isotropic detector is a function of the refractive index (n) of the surrounding medium when it is surrounded by an infinite medium of uniform n. However, there are few, if any, studies of the isotropic detector response when the detector is placed in a secondary medium, such as air, before it is placed inside the infinite uniform medium. This condition often arises when one places the isotropic detector inside an air-filled catheter which is then inserted into a turbid medium, such as tissue. We have performed theoretical and experimental studies to determine the correction factors in water (n = 1.33), which has a refractive index similar to that of tissue (n = 1.4). We found that the resulting correction factor is almost the same (within 20%) as the correction factor for the outermost medium (the water) rather than the immediate medium surrounding the isotropic detector (air). The detector correction factor is also a function of the index of refraction of the probe material. For a 1-mm diameter probe from CardioFocus, the detector correction factor varied from 1 (in air) to 1.09 (at air-water interface) to 1.49 (completely submerged in water). At the air-water interface the spherical bulb of the isotropic detector is placed half in air and half in water. For a 0.5-mm diameter probe from the same company, it varied from 1 (in air) to 1.32 (at air-water interface) to 1.87 (in water). For a 0.3-mm diameter probe from the same company, it varied from 1 (in air) to 1.32 (at air-water interface) to 1.71 (in water). We have also found that the detector response changes by less than 10% when the detector position is varied from touching the catheter wall closest to the light source, to not touching, to touching the catheter wall farthest from the light source. The calibration factors between individual isotropic detectors of the same type varied within 5% for all detector types. Thus mean correction factor can be used for each individual isotropic detector of the same type.
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