A method of delivering light energy to target matter in a mammalian body is described. The method may include inserting at least a portion of a catheter into a patient's vasculature, wherein the catheter comprises an open distal tip, a lumen extending proximally from the open distal tip, and at least one optical fiber within the lumen, wherein the at least one optical fiber has a distal end. The method may include flowing a liquid light guide medium through the open-ended catheter tip, wherein the liquid light guide medium flows beyond the distal end of the at least one optical fiber, wherein the liquid light guide medium comprises a magnesium chloride solution having an ion concentration that is isotonic with blood and tissue. The method may include forming a fluid optical channel with the liquid light guide medium between the catheter and the target matter. Other methods are described.