## 1 Introduction
Since it helps in minimizing blood presence in the treated areas, the absorption of the excitation light used for PDT is decreased [25]. Since the blood flow interruption is limited in duration, this feature of balloon catheters can be a disadvantage (limited total delivered light dose) or an advantage (limited risk of side effects) [26]. It should be noted that the duration of this interruption might be significantly increased using \"perfusion catheters\" that are designed so that the blood flow is not totally interrupted when the balloon is inflated. Figure 1: (**a**) Generic drawing of a stop-flow double balloon, multi-lumen nylon catheter (courtesy of Acrostak (Schweiz) AG); (**b**) generic drawing of a radial/cylindrical light diffuser for endovascular illumination (lower vessel wall omitted for clarity’s sake, courtesy of Medlight SA, Ecublens, Switzerland). Because they do not interrupt the blood flow, bare radial/cylindrical diffusers can be used for longer durations in small (\(<\)1 mm) vessels, but require that the illumination wavelength be chosen in the longer-wavelength part of the visible spectrum (\(>\)650 nm) to minimize light absorption by the blood. This also results in a comparatively deeper light penetration into tissues, and limits the possible side effects associated with intimal hyperplasia [13] or thrombosis [27]. Instances are reported [28] when the PDT effect described is too weak, a side effect that can be ascribed to the blood absorption of light [28; 29; 30]. In the case of a radial diffuser such as the one described in Figure 1b, it must distribute light with a longitudinal profile as close as possible to a top hat profile [31; 32; 33]. Consequently, the light dose (primary incidence) delivered at a given distance from the diffuser can be calculated relatively easily if the distance (d) between the diffuser surface and the vessel wall is smaller than the irradiation length (L) of the diffuser, d \(<<\) L, see Figure 1b), in particular if its emission is Lambertian. It is noteworthy that, if d \(>>\) L, the parameter \"d\" affects not only the primary irradiance but also the shape of the illumination spot [16; 33; 34]. This second effect is particularly marked if the light emission (brightness) at the surface of the diffuser is not Lambertian (i.e., is backward- or forward-peaked), as described in detail and illustrated in [34]. This may result in a \"translation\" of the illumination spot relative to the light diffuser [11; 34] with potentially dramatic consequences. Importantly, if the illumination is not Lambertian, light propagation in tissues becomes only isotropic at depths larger than 1/\(\upmu_{\text{s}}\)' (where \(\upmu_{\text{s}}\)' is the reduced scattering coefficient). Thus, if the illumination is not Lambertian, the angular distribution of the radiance at the surface of the light distributor impacts the treatment of lesions thinner than 1 mm, since \(\upmu_{\text{s}}\)'-1 in the red part of the spectrum can reach several hundreds of microns [31; 33; 35]. This means that, for a Lambertian source, the decrease in fluence rate in tissues starts close to the vessel surface, whereas for a collimated source, this decrease starts up to 1 mm underneath this surface, i.e., when most photons have been scattered, thus impacting any possible PDT effect [34]. It should be noted that the angular distribution of the radiance at the surface of the light distributor also significantly impacts the fluence rate in deep-seated tissues. Unfortunately, the frequent lack of detailed description of diffusers optical properties makes it somewhat difficult to directly compare different designs and to fully master the light dosimetry associated with a given instrumental configuration [31]. This report describes a system to achieve local, intra-arterial PS delivery (catheter) and illumination (diffuser).