We regard this method as elementary in concept but several of the optical devices needed are not off-the-shelf. We designed these ourselves and are seeking industrial partners to make them, in some cases, anew. For large arteries (> 1.0 mm) we postulate that the optical fiber conducting the laser should be centered in the artery with a UV-transparent balloon catheter. Below these features are seen. The ring beam emitted from a conical-tip optical fiber (see below) is tailored to irradiate the inner wall with a uniform intensity not to exceed 20 watts/cm2. These components were available 15 years ago but now need to be reconstituted.
Fig. 7. Centering a conical-tip silica optical fiber within a semi-compliant, UV-transparent balloon (Conic Vascular) inserted into an intracranial artery. Gadolinium is transparent to 355 nm laser light and this allowed real-time observation of the dilation effect, which occurred in seconds. A clear optical path is ensured by balloon expansion just up to the inner wall and must be maintained by Gd infusion so ensure the seal and prevent blood from absorbing the UV light very intensely. The wall is thus dilated non-mechanically.
We have several designs for conical-tipped optical fibers. Previous work utilized a beam emitted at 36° to the arterial axis, but the very apex of the tip was subject to cracking. We realized this problem could likely be alleviated by designing a blunter tip, to emit a ring beam at 96° full angle.
Fig. 8. Old design of silica conical-tip fiber. The apex of the conical tip is missing with only minimal effect on the ring beam, but no mechanical residue can be tolerated. An improved design is needed, and is presented in Fig. 9.
We note that an increased angle of emission can be obtained if a conical segment made of diamond is optically coupled to the tip of the silica fiber. The tip will more blunt and certainly much stronger. For external diamond conical tip, the maximum angle α is 32.5º, and the maximum angle of emission β(α) = 71.5º.
Two more unusual designs occurred to us. The first one is an inverted conical tip (Fig. 10 below). This was invented to limit physical exposure of the tip to external media.
The second idea is a diffractive tip, in which a pattern is etched onto a surface to yield a ring beam, similar to what an axicon lens would produce. Fig. 11 below shows an etching pattern used on an optical lens. This idea is the basis of our provisional patent, in which a diffractive optical fiber is included inside a guidewire. This modification will reduce the thrombectomy time by halving the number of procedural steps. Certainly either conical tip design will also work, but we thought there might be some advantage to using a fiber tip which is essentially flush with the guidewire tip. The current technical problem is that nobody has ever made a diffractive optic this small (100 um). We have been quoted 200 um as the smallest possible from one company.
Fig. 11. Pattern etched on a flat silica lens which will produce a ring beam. The scale we require is much smaller, however. From Holo/or, Israel.