# Side-Illuminating Optical Fiber for High-Power-Density-Mediated Intraluminal Photoacoustic Imaging  
[PERSON]  
1 Department of Medical Biophysics, University of Toronto, Toronto, ON M5S 1A1, Canada;  
[EMAIL_ADDRESS]  
[PERSON]  
1 Department of Medical Biophysics, University of Toronto, Toronto, ON M5S 1A1, Canada;  
[EMAIL_ADDRESS]  
[PERSON]  
2 Sunnybrook Research Institute, Toronto, ON MN 3M5, Canada;  
[EMAIL_ADDRESS]  
[PERSON]  
1 Department of Medical Biophysics, University of Toronto, Toronto, ON M5S 1A1, Canada;  
[EMAIL_ADDRESS]  
[PERSON]  
2 Sunnybrook Research Institute, Toronto, ON MN 3M5, Canada;  
[EMAIL_ADDRESS]  
and \(\Phi(r)\) is the local fluence (energy per unit area). As the light travels through the tissue, it is scattered and absorbed, resulting in a gradual reduction in the fluence with distance from the light source; hence, the PA signal is limited by the fluence reaching it. This can degrade the PA imaging performance when large-volume imaging is required or if the imaging target is situated deep in the anatomy [8; 9; 10]. PA imaging systems for non-invasive imaging often use optical fiber bundles flanking the ultrasound transducer to illuminate tissue over the ultrasound imaging plane with high peak power laser pulses, typically in MWs. However, the fiber bundle adds to the bulk of the ultrasound probe [1; 11]. The PA field of view can match the ultrasound imaging field if the fiber bundles deliver sufficient fluence at depth while being distributed along the length of the US probe. However, for applications that have limited access, such as image-guided intervention, intraluminal, or endovascular imaging, both the ultrasound probe and the light delivery device must be compact. Single optical fibers, with forward illumination from the end or side-fire with a prism, mirror, or level at the tip have been developed to investigate these applications [9; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. Small-diameter fibers (e.g., \(<\)5 mm) can also enable through-transmission illumination aligned with the ultrasound image plane, for example, intraluminal light delivery separated from the US probe [28; 29; 30]. While compact side fire devices have yielded good PA imaging depth [28; 29; 31], the imaging field of view is restricted by the small spot size and weakly diverging beam, requiring the optical fiber to be mechanically swept to illuminate the full region of interest. This may significantly increase the imaging time and make it difficult to use in a clinical setting. Further, the laser pulse energy may have to be restricted so that the fluence at the small aperture is below safety limits to avoid localized tissue heating or damage. Therefore, a compact light delivery device with an extended illumination aperture is needed. Radial fiber diffusers deliver light circumferentially along a section of the fiber [17; 27; 32]. Such designs can have an extended aperture and provide large illumination volumes. However, the cylindrical beam is not an efficient use of the laser energy for PA imaging using a finite ultrasound imaging plane. Furthermore, diffusers based on light scattering centers in the fiber core, such as those used for photodynamic therapy applications [32; 33] are not able to withstand high peak powers. Therefore, we propose a compact side-fire fiber with an elongated illumination aperture that can carry high peak power (MW) ns pulses. Elongated side-fire illuminators have been developed by creating multiple notches along the fiber to reflect the light [34], by depositing materials with different refractive indices on the core [35], by laser-ablating micro-holes the generate multiple side-fire beams [36; 37], or by adding a conical mirror to a cylindrical diffuser [38]. However, high power laser pulse transmission has not been demonstrated with the first three, while the diameter of the latter [38] is 8 mm. The challenge of generating uniform side-fire illumination from a compact (e.g., \(<\)3 mm diameter) device capable of carrying high pulse power remains.