Fs-lasers are well established in ophthalmic surgery as high precision tools for corneal flap cutting during laser in situ keratomileusis (LASIK) and increasingly utilized for cutting the crystalline lens, e.g. in assisting cataract surgery. For addressing eye structures beyond the cornea, an intraoperative depth resolved imaging is crucial to the safety and success of the surgical procedure due to interindividual anatomical disparities. Extending the field of application even deeper to the posterior eye segment, individual eye aberrations cannot be neglected anymore and surgery with fs-laser is impaired by focus degradation. Our demonstrated concept for image-guided vitreo-retinal fs-laser surgery combines adaptive optics (AO) for spatial beam shaping and optical coherence tomography (OCT) for focus positioning guidance. The laboratory setup comprises an adaptive optics assisted 800 nm fs-laser system and is extended by a Fourier domain optical coherence tomography system. Phantom structures are targeted, which mimic tractional epiretinal membranes in front of excised porcine retina within an eye model. AO and OCT are set up to share the same scanning and focusing optics. A Hartmann-Shack sensor is employed for aberration measurement and a deformable mirror for aberration correction. By means of adaptive optics the threshold energy for laser induced optical breakdown is lowered and cutting precision is increased. 3D OCT imaging of typical ocular tissue structures is achieved with sufficient resolution and the images can be used for orientation of the fs-laser beam. We present targeted dissection of the phantom structures and its evaluation regarding retinal damage.
Although fs-laser surgery is clinically established in the field of corneal flap cutting for laser in situ keratomileusis, surgery with fs-laser in the posterior part of the eye is impaired by focus degradation due to aberrations. Precise targeting and keeping of safety distance to the retina also relies on an intraoperative depth resolved imaging. We demonstrate a concept for image guided fs-laser surgery in the vitreous body combining adaptive optics (AO) for focus reshaping and optical coherence tomography (OCT) for focus position guidance. The setup of the laboratory system consist of an 800 nm fs-laser which is focused into a simple eye model via a closed loop adaptive optics system with Hartmann-Shack sensor and a deformable mirror to correct for wavefront aberrations. A spectral domain optical coherence tomography system is used to target phantom structures in the eye model. Both systems are set up to share the same scanner and focusing optics. The use of adaptive optics results in a lowered threshold energy for laser induced breakdown and an increased cutting precision. 3D OCT imaging of porcine retinal tissue prior and immediately after fs-laser cutting is also demonstrated. In the near future OCT and AO will be two essential assistive components in possible clinical systems for fs-laser based eye surgery beyond the cornea.
In ophthalmic microsurgery tissue dissection is achieved using femtosecond laser pulses to create an optical breakdown. For vitreo-retinal applications the irradiance distribution in the focal volume is distorted by the anterior components of the eye causing a raised threshold energy for breakdown. In this work, an adaptive optics system enables spatial beam shaping for compensation of aberrations and investigation of wave front influence on optical breakdown. An eye model was designed to allow for aberration correction as well as detection of optical breakdown. The eye model consists of an achromatic lens for modeling the eye’s refractive power, a water chamber for modeling the tissue properties, and a PTFE sample for modeling the retina’s scattering properties. Aberration correction was performed using a deformable mirror in combination with a Hartmann-Shack-sensor. The influence of an adaptive optics aberration correction on the pulse energy required for photodisruption was investigated using transmission measurements for determination of the breakdown threshold and video imaging of the focal region for study of the gas bubble dynamics. The threshold energy is considerably reduced when correcting for the aberrations of the system and the model eye. Also, a raise in irradiance at constant pulse energy was shown for the aberration corrected case. The reduced pulse energy lowers the potential risk of collateral damage which is especially important for retinal safety. This offers new possibilities for vitreo-retinal surgery using femtosecond laser pulses.
High precision femtosecond laser surgery is achieved by focusing femtosecond (fs) laser pulses in transparent tissues to
create an optical breakdown leading to tissue dissection through photodisruption. For moving applications in
ophthalmology from corneal or lental applications in the anterior eye to vitreal or retinal surgery in the posterior eye the
applied pulse energy needs to be minimized in order to avoid harm to the retina. However, the aberrations of the anterior
eye elements cause a distortion of the wave front and consequently an increase in size of the irradiated area and a
decrease in photon density in the focal volume. Therefore, higher pulse energy is required to still surpass the threshold
irradiance. In this work, aberrations in an eye model consisting of a plano-convex lens for focusing and 2-hydroxyethylmethacrylate
(HEMA) in a water cuvette as eye tissue were corrected with a deformable mirror in combination with a
Hartmann-Shack-sensor. The influence of an adaptive optics aberration correction on the pulse energy required for
photodisruption was investigated. A reduction of the threshold energy was shown in the aberration-corrected case and
the spatial confinement raised the irradiance at constant pulse energy. As less energy is required for photodisruption
when correcting for wave front aberrations the potential risk of peripheral damage is reduced, especially for the retina
during laser surgery in the posterior eye segment. This offers new possibilities for high precision fs-laser surgery in the
treatment of several vitreal and retinal pathologies.
Femtosecond laser induced optical breakdown allows for high-precision cutting of transparent materials with low energy
deposit and little peripheral damage for applications in micromachining and minimally invasive medical surgery. Little
peripheral damage is especially important for laser incisions in the posterior eye due to the vicinity to the retina. When
applying laser pulses through the anterior eye, aberrations are introduced to the wave front, which cause a distortion of
the focal volume and an increase in required pulse energy for tissue manipulation through photodisruption. To decrease
the pulse energy, aberrations need to be corrected to restore a diffraction limited focus. In this work, the influence of an
aberration correction using adaptive optics on the required pulse energy for an optical breakdown was investigated. The
aberrations were introduced in an eye model using HEMA as eye tissue substitute and corrected in an optical setup
including a deformable mirror and a Hartmann-Shack-Sensor. The laser pulses were focused by a plano-convex lens and
the induced impact was compared for the aberrated and the corrected case. The pulse energy required to obtain an effect
was reduced when correcting for aberrations. Therefore, adaptive optics can reduce the risk for potential peripheral
damage during ophthalmic surgery.
Focussed femtosecond laser pulses are applied in ophthalmic tissues to create an optical breakdown and therefore a
tissue dissection through photodisruption. The threshold irradiance for the optical breakdown depends on the photon
density in the focal volume which can be influenced by the pulse energy, the size of the irradiated area (focus), and the
irradiation time. For an application in the posterior eye segment the aberrations of the anterior eye elements cause a
distortion of the wavefront and therefore an increased focal volume which reduces the photon density and thus raises the
required energy for surpassing the threshold irradiance. The influence of adaptive optics on lowering the pulse energy
required for photodisruption by refining a distorted focus was investigated. A reduction of the threshold energy can be
shown when using adaptive optics. The spatial confinement with adaptive optics furthermore raises the irradiance at
constant pulse energy. The lowered threshold energy allows for tissue dissection with reduced peripheral damage. This
offers the possibility for moving femtosecond laser surgery from corneal or lental applications in the anterior eye to
vitreal or retinal applications in the posterior eye.
In ophthalmology, femtosecond laser transections (photodisruption) in the vicinity of the retina need to be
performed with minimized threshold energy to not harm peripheral retinal tissue. However, the aberrations
of the anterior eye elements cause a distortion of the wavefront and therefore a raised threshold energy when
focussing into the posterior segment. We present an optical system that allows for correcting aberrations in eyes
using adaptive optics consisting of a deformable mirror and a Hartmann-Shack-Sensor with a novel light source.
If combined with femtosecond laser pulses this system offers the possibility for minimally invasive laser surgery
in the posterior eye segment.
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