By a News Reporter-Staff News Editor at Journal of Engineering -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventor Raksi, Ferenc (Mission Viejo, CA), filed on December 23, 2014, was published online on September 26, 2017.
The patent's assignee for patent number 9770362 is NOVARTIS AG (Basel, CH).
News editors obtained the following quote from the background information supplied by the inventors: "Surgery with femtosecond ophthalmic lasers is based on generating a pulsed laser beam and delivering the laser pulses by a scanning delivery system through a focusing optics to a sequence of focus spots along a scan-pattern in a target region of an ophthalmic tissue. Each laser pulse creates a plasma or cavitation bubble in the target tissue at the focus spot of the laser beam when the beam intensity or energy density exceeds a plasma or photodisruption threshold. During surgery, the focus spot of the laser beam is scanned along a three dimensional scan-pattern, creating a sequence of these bubbles to form macroscopic surgical cuts or photodisrupted regions.
"During the surgery, however, the laser beam can also cause unintended collateral damage away from the focus spot such as excessive heating and shockwaves in the target tissue and light poisoning in the retina. Therefore, surgical systems are designed to deliver the laser beam with an energy density that exceeds the photodisruption threshold, but only marginally to achieve the surgical functionality while minimizing the collateral damage.
"The energy density or beam intensity is determined by the energy, duration and repetition rate of the individual laser pulses and the size of the focus spot. Modern surgical laser systems provide high precision and control by using precisely controlled laser sources, refined optical designs, high quality optical parts and an objective with a large numerical aperture to focus the laser beam down to a diffraction limited focus spot with a diameter of a few microns, and do so at all points of the scan-pattern within a surgical volume, or at all scanner positions of the surgical laser system. This high precision makes the modern laser surgical systems capable of maintaining the beam intensity marginally above the plasma threshold along the entire scan-pattern within the surgical volume in ideal or model targets.
"Unfortunately, in spite of all the design and manufacturing effort spent on optimizing the laser sources and optics, the focus spot in the ophthalmic target region is often still larger than its diffraction limited value because the target tissue itself often gets distorted, making it different from the ideal or model targets used during the design of the laser optics. Distortions can be also caused by imperfections of the scanning delivery system and the focusing optics. The enlarging of the focus spot caused by any of these distortions can lead to failing surgical performance since it lowers the pulse energy density or beam intensity below the plasma threshold and thus prevents the scanning laser beam from forming the planned surgical cuts, leaving uncut lines or regions in the target region.
"This problem of failing surgical performance can become particularly acute during surgical cuts where the targeted tissue is very thin such as a capsulotomy of the thin lens capsular bag during a cataract surgery. Since the targeted tissue is thin, the laser beam scans it only once or only a few times along a loop, as this scan-pattern should be already capable of cutting through the entire thickness of the capsular bag. However, if any one of the above distortions reduces the beam intensity below the plasma threshold along a section of the loop then that section can remain uncut. This uncut section of the capsular bag needs to be cut and separated manually, possibly leading to a tearing of the capsular bag and thus to a substantial lowering of the precision of the cataract surgery.
"Therefore, there is a need for surgical laser systems that can deliver the laser beam with a pulse energy density that is marginally higher than the plasma threshold in the entire surgical volume even if distortions are present along the beam path either in the target region or in the optical system itself, as such laser systems are capable of cutting the target region according to the scan-pattern in the entire surgical volume without leaving uncut regions or lines."
As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventor's summary information for this patent: "FIG. 1A illustrates that an objective of cataract surgery is to direct or deliver a surgical laser beam 10 into an eye 1 through its cornea 2 and anterior chamber 3 to photodisrupt a cataractous target region in a lens 5. FIGS. 1A-E illustrate some of the problems caused by beam distortions in cataract surgery. FIG. 1A illustrates that many surgical laser systems have a patient interface (PI) 20 attached to a distal end of an objective of the laser scanning delivery system. The PI 20 can include a contact lens 22 that makes contact with the cornea 2 to allow a well-controlled entry of the surgical laser beam 10 into the cornea 2. The PI 20 is often outfitted with a suction ring 24 and a vacuum hose 26 for creating a reliable mechanical coupling with the eye 1.
"FIG. 1B illustrates that the PI 20 and its contact lens 22 can be coupled to the cornea 2 reliably by applying suction to the vacuum hose 26 that presses the contact lens 22 to the cornea 2. Sometimes, the PI 20 and its contact lens 22 can be additionally pressed against the cornea 2 by its own weight or by a mechanical system such as spring loading.
"FIG. 1B also shows that, unfortunately, the pressure caused by the vacuum suction and the mechanical pressure can create wrinkles 7 in the cornea 2 which can cause the above mentioned beam distortions.
"FIG. 1C illustrates a mathematical formulation of the distortions or aberrations of the laser beam 10. It is customary to define an aberration as the deviation of a wavefront of the laser beam 10 from a conceptual Gaussian reference sphere segment S of radius R. The Gaussian sphere segment S can be centered on the geometrical focal point P.sub.0 of the laser beam 10 and formed by the intersection of the laser beam 10 and an entire Gaussian reference sphere. In many cases, the reference sphere segment S is the pupil of the laser system. The two main classes of distortions or aberrations are phase and amplitude distortions/aberrations. The formulation is presented here for the more typical phase aberrations. Amplitude aberrations can be described in an analogous manner.
"It is known from the theory of optical wave propagation that the intensity of light I(P) at a point P in the focal plane that contains the geometrical focus point P.sub.0 is given by the absolute value squared of the electromagnetic disturbance, in essence the electric field, with the fast oscillating e.sup.i.omega.t factor removed: I(P)=|U(P)|.sup.2.
"According to the Huygens-Fresnel principle, the electric field U(P) at the point P is given by an integral of the beam components, or rays, E(Q,P) over the Gaussian reference sphere segment S:
".function..times..lamda..times..intg..intg..times..function..times.d.func- tion..times..lamda..times..intg..intg..times..function..times.eI.times..ti- mes..times..times..PHI..times.d.function..times..lamda..times..intg..intg.- .times..times.eI.times..times..function..times.eI.times..times..times..tim- es..PHI..times.d.function. ##EQU00001##
"Here, E(Q,P) is the propagating electric field, beam component or ray that propagates from a dS(Q) vicinity of point Q on the Gaussian reference sphere segment S to the point P of the focal plane near P.sub.0, the geometrical focus point. This beam component can be decomposed into E.sub.0(Q,P), the propagating electric field in the absence of a phase aberration and into e.sup.ik.PHI., representing the phase aberration by a phase aberration function .PHI.. The undistorted field can be represented as:
".function..times.eI.times..times..function. ##EQU00002## Here, A is the amplitude of the beam component or ray at point Q, reduced during the propagation to point P by 1/s, where s is the length of the QP ray from the point Q to point P. Further, e.sup.ik(s-R) represents the propagating wave phase factor, acquired by the propagating electromagnetic wave E.sub.0(Q,P) in the absence of aberrations. Finally, k=2.pi./.lamda. is the wavenumber and .lamda. is the wavelength of the laser beam 10. Discussing the aberration-free beam, for P=P.sub.0 s=R and thus the phase factors of the beam components that propagate from the different Q points of the reference sphere segment S to the geometrical focus point P.sub.0 add up with maximum constructive interference. Further, as known, the interference remains constructive in a small but finite vicinity of the geometrical focus point P.sub.0, broadening the geometrical focus point P.sub.0 into a diffraction limited focus spot 32.
"FIG. 1D illustrates the beam intensity along a typical scan line in the target region when the laser beam is scanned over an unwrinkled cornea. Since the aberration function is essentially zero in this region, the propagating wave phase factors e.sup.ik(s-R) of the beam components E(Q,P) in the Huygens-Fresnel integral can add smoothly and constructively when reaching P points in the vicinity the geometrical focus point P.sub.0, thus producing a laser beam 10 with a beam intensity that can remain above the plasma threshold along the shown and the other scan lines within the surgical volume. Therefore, as the laser beam 10 is scanned across the scan lines of the scan-pattern, it can create the intended surgical cuts in the entire surgical volume.
"FIG. 1E illustrates that, in contrast to the unwrinkled case of FIG. 1D, if the vacuum suction or the mechanical pressure creates corneal wrinkles 7, then these wrinkles 7 can distort the laser beam 10 by refracting the propagating electric fields, or beam components, to distorted directions, so that their aberration function .PHI. in the Huygens-Fresnel integral become different from zero. The corresponding phase factors e.sup.ik.PHI. can lead to a substantially destructive interference between the beam components, possibly substantially reducing the beam intensity. The magnitude of the corneal phase aberration can be estimated as the product of the amplitude of the wrinkles and the change of the refractive index at the cornea-aqueous humor interface. The refractive index of the cornea is approximately 1.377 while the index of the aqueous humor is 1.337, separated by a difference of 0.04. As an example, for a laser wavelength of 1 micrometer, wrinkles with amplitude of 25 micrometers give approximately 2.pi. phase aberration. Therefore, in general, for .PHI.>.pi./4 the phase aberrations can already substantially reduce the beam intensity, and for .PHI.>.pi./2 the aberrations even reverse the sign of the contributions of the beam components E(Q,P) to the Huygens-Fresnel integral. These destructive interferences can reduce the beam intensity at the focus spot 32 to a value below the plasma threshold and thus preventing the laser beam 10 from photodisrupting the target region and from executing the surgical cuts along the surgical scan-pattern, instead leaving uncut regions behind. In some cases, the single focus spot may even break up into multiple foci.
"FIGS. 2A-B illustrate a related effect of corneal wrinkling. FIG. 2A illustrates that in the absence of corneal wrinkling the focus spot 32 of the laser beam 10 can have a near diffraction limited size of a few microns for a laser beam 10 with wavelength in the 500-1,500 nm range. The scanning delivery system and optics can be designed to deliver the laser beam 10 with an intensity to this focus spot 32 that marginally exceeds the plasma or photodisruption threshold everywhere in the surgical volume and thus is capable of executing the surgical cuts without leaving uncut regions behind.
"FIG. 2B illustrates that when the vacuum suction or pressure of the PI 20 creates wrinkles 7 in the cornea 2, then the wrinkles 7 can redirect and refract some beam components to go through the plane of the focus spot 32, or focal plane, of the unwrinkled case smeared over an enlarged aberration focus spot 32. The increase of the focus spot area decreases the beam intensity, possibly below the plasma threshold. Besides causing destructive interference of the phase factors of the beam components, this focus-spot-smearing is an additional mechanism by which corneal wrinkling can reduce the beam intensity below the plasma threshold.
"Finally, FIG. 2C illustrates a specific case of the problem in FIG. 2B: the formation of a single localized defect or wrinkle 7, created by the vacuum suction or the pressure of the PI 20. As before, the localized defect or wrinkle 7 can redirect or refract the laser beam 10 so that when the laser beam 10 goes through the focal plane, its beam components are smeared out over the enlarged aberration focus spot 32. As before, the increase of the focus spot area decreases the beam intensity, possibly reducing it below the plasma threshold.
"FIGS. 3A-B illustrate the analogous problem for a two dimensional (2D) scan-pattern 50. Such a 2D scan-pattern 50 can be used when an ophthalmic layer is to be cut, or a volume is to be photodisrupted. The laser beam 10 can be scanned along the 2D scan-pattern to create a densely packed layer of photodisrupted bubbles. This photodisrupted layer can effectively cut apart the tissue segments on its two sides. However, if the laser beam 10 is distorted by a wrinkled cornea, at several of the intended spots of the scan-pattern 50 the beam intensity may be reduced below the plasma threshold, and thus the laser beam 10 may fail to create the photodisrupted bubbles, as shown in FIG. 3A.
"FIG. 3B illustrates that the beam intensity may be reduced below the plasma threshold for extended 'no-cut regions' or 'uncut regions' of the size d along a typical scan line 52, where d can be comparable to the size of the corneal wrinkles 7. In typical ophthalmic cases, d can vary from about 10 microns to beyond 1 millimeter. Referring back to FIG. 3A, these uncut regions can have a spatial extent beyond a millimeter in one, two or even all three dimensions. Therefore, when the scanning of the laser beam 10 is finished, the intended surgical cuts will be interrupted by extended no-cut regions.
"The surgeon may attempt to cut these no-cut regions by re-scanning the entire scan pattern or portions of the scan-pattern 50. However, this is not very effective, since the same wrinkles are still present in the cornea, giving rise to the same aberrations. Thus, the same regions will remain uncut during the second scan. Re-scanning is also time-consuming. Every time the surgeon is forced to repeat a surgical step, valuable surgical time is spent, increasing the probability of undesirable outcomes.
"Therefore, the surgeon may be forced to cut the uncut regions manually to complete the surgery, possibly creating jagged edges, leading to the formation of tears in the ophthalmic tissue. These undesirable effects call out for improvements in the surgical laser systems that reduce or eliminate the formation of the uncut regions.
"Briefly and generally, embodiments of the invention offer solutions to the above problems by providing a surgical laser system, comprising: a laser engine, configured to generate a laser beam of laser pulses; a proximal optics and a distal optics, together configured to direct the laser beam to a target region, and to scan the laser beam in the target region through a scanning-point sequence; and an aberration sensor, configured to sense aberration by an aberration layer; a compensation controller, coupled to the aberration sensor, configured to generate compensation-point-dependent phase compensation control signals based on the sensed aberration; and a spatial phase compensator, positioned between the proximal optics and the distal optics, at a conjugate aberration surface, conjugate to the aberration layer, and coupled to the compensation controller, configured to receive the compensation-point-dependent phase compensation control signals, and to alter a phase of the laser beam in a compensation-point-dependent manner to compensate the sensed aberration.
"In some embodiments, a method of reducing aberrations in a surgical laser system comprises: generating a laser beam of laser pulses by a laser engine; directing the laser beam to a target region by a proximal optics and a distal optics; scanning the laser beam in the target region by the proximal optics and the distal optics through a scanning-point sequence; sensing aberration, caused by an aberration layer, with an aberration sensor; generating compensation-point-dependent phase compensation control signals based on the sensed aberration by a compensation controller, coupled to the aberration sensor; and altering a phase of the laser beam in a compensation-point-dependent manner to compensate the sensed aberration by a spatial phase compensator, positioned between the proximal optics and the distal optics, at a conjugate aberration surface, conjugate to the aberration layer, and coupled to the compensation controller to receive the compensation-point-dependent phase compensation control signals."
For additional information on this patent, see: Raksi, Ferenc. Wavefront Correction for Ophthalmic Surgical Lasers. U.S. Patent Number 9770362, filed December 23, 2014, and published online on September 26, 2017. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=9770362.PN.&OS=PN/9770362RS=PN/9770362
Keywords for this news article include: Surgery, NOVARTIS AG, Laser System, Surgical Laser, Medical Devices, Surgical Technology.
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