Mieke Bus

40 Chapter 3 excitation S 0 S 1 emission wavelength (nm) spectrum excitation emission Light source Detector Wavelength Filter scope accumulated dye Figure 5: Photodynamic Diagnosis is based on fluorescence of externally applied dyes that accumulate in tumours. When a dye absorbs light, it is excited from the ground state (S0) to a higher energy level of the first excited state (S1), before relaxing to the lowest vibrational energy level of S1. Subsequently, the dye returns to the ground state while emitting a photon of lower energy (higher wavelength) than used for excitation. Appropriate filtering separates the emitted light from the excitation. Insert shows a PDD image of urothelial carcinoma in the ureter. Since a tangential light beam is almost inevitable in the ureter, result interpretation is hampered and causes more false positives. largely compensates for varying blood absorption and accumulated 5-ALA in malignant tissue appears pink/red on a blue background. In the upper urinary tract several impediments exist for application of this technique. Concern is optimal delivery of fluorochromes and visualization in the setting of anatomic obstacles. Delivery of fluorochromes seems technically possible by nephrostomy or urinary stent, but equal administration of fluorochromes and optimal duration of urothelial expo- sure to fluorochromes are more difficult to establish because of the small cavity sizes. (16, 18) Furthermore, visualizing fluorescent thin, normal urothelium of the ureter requires a per- pendicular viewing angle. The ureteroscope’s longitudinal axis creates a tangential viewing angle in which the urothelial layer appears as a thicker layer with a corresponding increase in fluorescence, potentially resulting in false positives.