Before updating this entry, a reader asked a question about a statement I'd made about the flexibility of the Boston Scientific holmium fibers AccuMax™ and Flexiva™ that I answered in the updated Part 2 entry. I am leaving my answer to that question in this entry, for congruity. Pardon the redundancy.
Q: ‘How do you know Boston’s AccuMax™ and Flexiva™ are stiffer than ProFlex 200 if, as you say, you’ve never seen one of Boston’s fibers?’
A: Updated answer -- we measured the glass OD of the Boston fibers and, as expected, it is 290 microns (242 microns x 1.2 CCDR). This is just 10 microns smaller than a 273 micron fiber (273 microns x 1.1 CCDR) and a full 50 microns larger than a true 200 micron fiber (200 x 1.2 CCDR). More discussion of CCDR and the fiber size conundrum may be found in a later part, not yet written. And we will be adding actual ASTM flexibility testing to our Fiber IQ Test as well, so stay tuned...
This question affords segue into the attributes of an ideal fiber for kidney stones, one of which is flexibility1. Modern flexible ureteroscopes allow a maximum angular deflection of about 275 degrees2. Addition of any optical fiber larger than a true 200 micron core reduces the maximum deflection, e.g. a 273 micron core fiber reduces maximum deflection to as little as 221 degrees3 or roughly 20% where smaller core fibers have little effect, e.g. a 200 core fiber reduces deflection by about 5%3. (Arguably, this is the sole rationale for producing a 150 micron core fiber -- zero reduced deflection in the rare cases where that is critical.)
Fiber addressing lower pole stone
Per the above illustration, 220 degrees may appear to be just barely enough deflection to access the lower pole stone, but the available deflection within the calyx (open space in the kidney) is also reduced by the total curvature of the ureteroscope pathway. With all other systems and personnel in place and ready to shock the rock, I imagine that it can be very frustrating to discover that the fiber you’ve been provided is incapable of reaching your target.
Why are these smaller fibers so hard to come by? I mean real 200 micron core fibers. Boston Scientific doesn’t make one (although they now have AMS’ SureFlex™ 200, which is a good design even though it is decades old, but AMS' version does not perform as well as when IQ made them, with maximum labeled power at 12W). Convergent doesn’t make one. Lumenis doesn’t make one. Dornier doesn’t make one. Trimedyne doesn't make one (but I make one for their lasers). Lisa has one in the expensive reusable model, but it's restricted to the Sphinx Litho platform. The answer is that making a safe and effective, true 200 micron core fiber (or smaller) is just not that simple to do. The basic problem IS quite simple; the laser focal spot is bigger than 200 microns under laboratory conditions and can bloom to 400 microns or larger in protracted surgical cases.
Coupling a 250 micron laser focal spot into a 273 micron core fiber is not as simple as it sounds. First, the definition of focal spot size ignores ~15% or more of the laser energy that is outside of the defined diameter and the spot diameter shrinks and swells while dancing the mambo across the fiber connector in prolonged use. And have you ever considered the energy density that we're dealing with? 0.25 microns is 0.025 cm so the area of the spot is just 0.0005 square centimeters. A one joule pulse that's 0.00035 seconds wide (350 microseconds) is 2860 watts so the energy density is almost 6 MW per square centimeter or 60 million times the energy density of the sun on your summer tan. Suffice it to say that this will vaporize steel very easily. At 1 J. In one pulse.
The technology for coupling to 200 micron and smaller core fiber is (a) beyond the capabilities of most companies and (b) the best ways of doing it are patented. SureFlex™ is covered by four US Patents. I wrote them and AMS and now Boston owns them. Last year I wrote two more patents, independent of SureFlex and owned by IQ; U.S. Pats. 9,122,009 and 9,233,089 cover IQ’s new ProFlex™ fibers.
Tapered input fibers – good or bad?
Boston Scientific and others deride tapered input fibers as a solution for coupling small core fibers to large laser foci:
“Tapered fibers are more prone to failure than similar sized, non-tapered fibers (citing reference 3, listed below). The connector tends to convert off-axis rays into higher order rays that might exceed the fiber NA and cause failure (citing reference 4, below).”
-- Boston Scientific brochure “6 Characteristics of an Ideal Laser Fiber”6
What is excluded from Boston’s recitation of the reference is the following:
“Tapered fibers have some potential advantages over non-tapered fibers, in that tapered fibers may better couple with lasers with 1) a larger focus, e.g. the Dornier MediLas, or 2) those that are poorly maintained and have misaligned optics or pitted blast shields. In essence the taper acts as a funnel, channeling energy into the fiber core. However, a well-made taper is difficult to achieve and it risks the conversion of photons to higher order rays (citing reference 5). Often the process is performed by hand by a single individual. If there are any defects in the assembly, the taper can become the source of fiber failure.”
It is hard to take much of an issue with the above excerpt in that I am the source of that information as well as being the "single individual" referred to therein, but while “by hand” may be accurate for tapers that are drawn and spliced onto fibers, or even IQ’s surface-tension grown tapers of 20-years ago, our current production equipment is state-of-the-art, computer controlled, laser micromachining using the same ultra-precise positioning systems as used by coronary stent manufacturers, down to the brand of the equipment. Each taper is monitored in-process for uniformity by optical incandescence imaging such that the dimensional variability of Pulsar HPC tapers' aperture is now less than that of straight input fibers. IQ spent three years and more than two million dollars redesigning the tapering process from the equipment up, before going to market with ProFlex and the Pulsar™ HPC high power connector.
But unlike prior tapered input fibers, in all but extreme situations the ProFlex Pulsar™ HPC design essentially eliminates the taper from the equation, by minimizing the taper angle and, more importantly, by collimating the bulk of the laser energy into the base fiber core, obviating reliance upon the taper to act “as a funnel”. The taper angle within the ProFlex Pulsar HPC is so low that you'll be hard pressed to 'see' it without instrumentation. The laser light doesn't 'see' it either, not in any traditional sense.
Where tapers promote modes to exceed the fiber base NA, so called 'cladding modes' are launched, but this only happens with the higher angle modes in the laser focus -- those at the periphery of the focal spot. We select these modes out using an annular rejection lens formed about the collimation lens on the face of the Pulsar HPC (see figure, below). When the laser is operating normally, at design specification, these (pink) modes are rare but when the laser blooms they are well populated. Old tapered fiber designs just took them in as offered, promoting them on each bounce off the taper wall to higher and higher orders.
The central, concave lens collimates the desirable laser energy (red) so that it need not rely upon the tapered fiber to couple to the base fiber core while the light is also reduced in mode angle for safer passage through tightly deflected fiber.
Annular lens surrounding collimating lens on Pulsar HPC
This next figure is an optical ray trace taken directly from a geometric optics program. It shows where the rejected rays go inside the Pulsar HPC and illustrates the collimation function of the central collimating lens. If you want to know the main reason the taper is still there it's because the the collimated beam inside the taper is fairly accurately depicted in the image as hitting one side of the taper, at the top near the right edge of the image). Without a taper this would be a big problem.
Pulsar HPC ray trace showing rejection of high angle modes and collimation
The reason the energy collimates inside the taper is the same as why Galileo was able to see the moon of Jupiter. Reversing a Galilean telescope is the same as a beam expander. Reversing it back, with a collimated input -- like the laser rods' output instead of an image of a Jovian satellite -- it is a down collimator. All we are missing was the concave lens so we form that on the fiber input face. This is where fusing the taper into a surrounding quartz ferrule is important -- it produces a larger substrate to form a lens upon with reproducibility. It also allows us to bleed off the stripped-off high order energy into the SMA connector wall in a controlled manner and does a few other things I'll keep to myself, at least for now.
The spiral groove in the light blue quartz ferrule is pitched at an angle that reflects the unwanted rays (pink) away from the fiber, to be absorbed within the connector wall. Other fiber designs purport to use “scattering” elements that redirect overfill energy, but scattering is random, not purposeful, and as such in scatter it is equally probable that the unwanted rays will be redirected toward the fiber at the center; rays crossing the fiber may couple to the fiber at very high angles ('cladding modes'), which is in opposition to the goal of producing a safe fiber.
Relationship of Galilean Telescope/Beam Expander to ProFlex Pulsar HPC
Recapping, in designing the Pulsar HPC, we recognized that not all of the energy within the laser focus was useful in surgery, even were it to be captured by the taper funnel. In fact, indiscriminate capture of all focal angles resulted in predisposition of the SureFlex to failure under power in deflection and we figured out why that was. The Pulsar HPC is designed to separate the useful laser energy from the potentially damaging fraction and only deliver the safe and effective bulk of the laser output to the base fiber for transmission to the surgical site. But even while purposefully separating and rejecting risky rays from coupling to the fiber, ProFlex delivers more laser power to the surgical target than any other 200 micron fiber, including “200 micron” fibers that are actually 273 micron.
That's the design theory of IQ's new ProFlex LLF fibers in a nutshell. If anything is unclear, it might help to refer to the cross-section diagram in the last blog entry. There are additional differences when compared with other fibers out there, but we'll discuss those in context with the other designs when it makes sense to do so.
Next time in “All Holmium Laser Fibers are the Same, Right?” -- Part 4: Other fiber input designs
References:
1) Seto, C., et al. Durability of working channel in flexible ureteroscopes when inserting ureteroscopic devices, Journal of Urology, Volume 20, 2006.
2) Knudsen,B.E,. et al. Performance and Safety of Holmium:YAG Laser Optical Fibers, Journal of Endourology, Volume 19, 2005.
3) Mues, A.C., et al. Evaluation of 24 Holmium:YAG Laser Optical Fibers for Flexible Ureteroscopy, Journal of Urology, Volume 182 2009.
4) Teichman, J., et al. Ho:YAG lithotripsy proximal fiber failures from laser and fiber mismatch, Journal of Urology, Volume 71, 2008.
5) Nazif OA, et al. Review of laser fibers: a practical guide for urologists. Journal of Endourology, Volume 18, 2004.
MediLas is a trademark of Dornier Medtech
ProFlex and Pulsar HPS are trademarks of InnovaQuartz LLC
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