Surgery – Instruments – Light application
Reexamination Certificate
1998-10-14
2004-03-09
Shay, David M. (Department: 3739)
Surgery
Instruments
Light application
C606S005000, C606S013000, C351S210000
Reexamination Certificate
active
06702809
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to tracking systems for aiming a laser beam and/or positioning projected light patterns at a known relation onto moving targets. In particular, the invention is concerned with the detection of, measuring of and compensating transverse movements of optical targets such as an eye during ophthalmic laser surgery as well as non-surgical diagnostic procedures. The present invention is particularly powerful when taken in conjunction with a method for capturing, measuring and compensating for movements along the axial direction such as disclosed by Wm. D. Fountain in U.S. Pat. No. 5,162,642 which is assigned to the same party as the present invention.
Methods of integration of a lateral tracker with depth ranging techniques were disclosed in copending patent application Ser. No. 843,374, entitled “Automated Laser Workstation for High. Precision Surgical and Industrial Interventions”, which was filed on Feb. 27, 1992, and which is incorporated herein by reference. In that disclosure, fully automated means to acquire and track randomly moving targets in three dimensions were described, along with methods for interfacing the acquisition and tracking means with a beam aiming and targeting sub-system and a target viewing subsystem, all of which are elements of a complete laser workstation. It is noteworthy that the system and method of said patent application also included, in an alternate embodiment, the capability to distinguish between translational and rotational movements of the eye as an integral part of the full three dimensional tracker.
By comparison with the two earlier disclosures cited above, the present invention emphasizes those aspects and specific embodiments of a transverse 2D tracker that are most critical in allowing a laser beam or projected light patterns to be correlated with and/or directed to a specific location on the target regardless of its lateral movement. Since the application to eye surgery places the most stringent requirements on the tracker, the present invention is described mostly in reference to this application. However, it is to be understood that the invention is broadly applicable to any situation involving precision diagnostic measurements and/or laser operations on moving targets, including industrial applications, such as in semiconductor processing where laser annealing and other techniques call for precise alignment of a mask onto a substrate in the presence of vibrations.
In ophthalmic surgery, the ability to optically track or follow the movement of the patient's tissue—not only the voluntary movements which can be damped with specialized treatment, but also the involuntary movements which are more difficult to control on a living specimen—is recognized as a highly desirable element in laser delivery systems designed to effect precision surgery in delicate ocular tissue. According to Adler's Physiology of the Eye, even when the patient is holding “steady” fixation on a visual target, eye movement still occurs. Such involuntary motion compromises the efficacy of certain ocular surgical procedures requiring great precision. This is true even with total immobilization of the eye, which is not fully effective in suppressing involuntary eye motion while being rather uncomfortable for the patient. Implementation of automatic tracking by remote means would therefore alleviate the need for such immobilization, while offering a method for more effectively accommodating all types of eye motion. Thus, augumenting surgery with on-line eye tracking option can improve significantly upon the accuracy and speed with which old surgical procedures could now be performed as well as enabling new procedures to be carried out for the first time.
In ophthalmology, it is also often desirable to image the tissue simultaneousely with positioning the treatment beam. Effective utilization of an imaging system capable of freezing on a display images or data relating to the configuration of the target during laser treatment requires that the target area be stabilized with respect to both imaging and the laser focal region, thus enhancing the accuracy of energy deposition in tandem with viewing sharpness. The ability to stabilize a video image of a moving target during the surgery procedure itself is especially desirable in those high precision laser interventions employing an instantaneous full image rather than a series of scanned images, such as described in co-pending U.S. patent application Ser. No. 843,374, which is incorporated herein by reference, and in U.S. Pat. No. 5,098,426.
In still other applications relating to diagnostics of targets, tracking can serve an important function in allowing cross-registration of successive readings taken across a moving target. By correlating the true positions of given target segments at the time the readings are taken, the effects of target motion can be compensated for via programming in the computer (i.e., software). In application to corneal surface mapping, utilization of transverse tracking, especially in concert with a depth tracking method that can keep the distance to the eye constant, e.g., as was disclosed in co-pending U.S. patent application Ser. No. 945,207, opens up the prospect of performing true point-by-point thickness and curvature measurements with standard scanning techniques. For example, by aligning separate readings relative to each other, accurate reconstruction of both anterior and posterior surfaces of ocular tissue such as the cornea or the lens can be feasible by scanning the eye with just a slit illuminator coupled to a CCD camera to detect each surface's reflections. Using such simple instrumentation to perform simultaneous surface and thickness measurements was not possible prior to this invention.
Prior attempts to derive simultaneous pachymetry and topography information such as by D. J. Gormley et. al. in
Cornea
, vol. 7, pp. 30-35, 1988 using a scanning laser slit lamp and a photokeratoscope were clearly hindered, among other factors, by the lack of cross-referencing that only tracking can provide. Thus, to compensate for eye movement, each slit lamp image reading consisted of an average of a series of measurements, a procedure which could take up to several minutes. To map the entire cornea in this manner would then clearly require an inordinately long time, especially since, to maintain a common reference point between successive readings, the patient would have to stay fixated throughout the entire scan. By adding means for on-line tracking, the number of required readings can be significantly reduced to where a combined method of depth and surface profiling becomes practical, even allowing the possibility of utilizing a slit illuminator alone for both measurments.
In general, stabilization of a moving target requires defining the target, characterizing the motion of the target, and readjusting the aim of the optical system repeatedly in a closed-loop fashion. For ophthalmic surgery, requirements for a tracking system are set by the type of eye motion, which fall into three categories: microsaccades, drift and high frequency tremor. The high frequency tremors, of about 90 Hz set an upper limit to the frequency, but are of a very small amplitude (up to 40 seconds of arc). Microsaccades are highly accelerated motions with constantly changing directions but lower frequencies (a few Hz). These small but rapid eye movements, combined with slow drift (about 1 minute arc/sec), prevent the retinal image from fading. Analysis of measurements of peak velocity-magnitude-duration parameters by e.g., A. Terry Bahill et al. in
Invest. Ohthalmol. Vis. Sci
., Vol. 21, pp. 116-125, 1981, indicate that requirements set for lateral eye tracking should include, as a goal, the ability to respond to movement with accelerations of up to 40,000 deg/sec
2
. This translates to amplitudes of about 1 degree at maximum frequencies of 100 Hz and increasing to nearly 15 degree at 20 Hz. Meeting these response goals while maintaining accuracies on the
Hoffman Hanna J.
Knopp Carl F.
Orkiszewski Jerzy
Wysopal Jan
Shay David M.
Thompson Lynn M.
Townsend and Townsend / and Crew LLP
Visx, Inc.
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