OPTIMAL IMAGING

The role of anterior segment imaging in femto-cataract surgery is vital but there is still a long way to go before the perfect imaging system is developed, Ioannis Pallikaris FRCS, FRCOphth, told a session of the XXXV United Kingdom & Ireland Society of Cataract & Refractive Surgeons (UKISCRS) Congress. Discussing the swift evolution of cataract surgery in recent years, he noted that lower phacoemulsification energy is now used, as are smaller incisions. Adjunctive astigmatic techniques have been developed and IOL designs have improved, while patient state-of-mind and expectations have changed.

However, imaging technology remains key for visualisation and customisation when carrying out femto-cataract surgery, he said. “The image guidance system is a critical part of femto-cataract surgery. It determines the location and dimension of ocular structures and guides the surgeon in customising the placement of laser incisions and lens fragmentation,†said Prof Pallikaris, professor in ophthalmology, Rector University of Crete, Greece.

The ideal image guidance system must be able to generate references for the size and centration of the capsulotomy, determine corneal thickness, and detect the iris boundaries and the posterior surface of the lens.

He reminded delegates that laser cataract surgery poses unique imaging challenges for surgeons. The imaging technology must have superior performance at a wide range of depths from superficial to deep structures, it must be able to image the boundary of the thin and clear posterior capsule, and be able to “see through†opaque lenses. Surgeons also must have 100 per cent confidence in image registration and spatial integrity, he noted.

There are two main ways to image the anterior segment using the technology available today – optical coherence tomography (OCT) and Scheimpflug imaging, Prof Pallikaris explained.

There are a number of different OCT technologies, which is based on the principle of low coherence interferometry where a Michelson-type interferometer is used so back scattered and back-reflected light recombines to produce a coherent interference pattern. Axial position and signal strength of back-scattered tissue is determined by the known reference path length that the light has travelled in the reference arm.

The original time domain OCT has been around for some time, and in this technology the movement of mirror produces an axial scan (A-scan), while lateral scanning produces 2-D. The reference arm moves mechanically and the echo time delays are measured one at a time. The highest resolution is about 18 μm axially and 60 μm laterally in this technology.

In the more recently developed Fourier frequency domain (FD) OCT, the broadband signal is broken into a spectrum using a grating or linear detector array, and depth is determined from the Fourier transform of the spectrum without motion along the reference arm. This system has a better resolution of 5 μm axially and laterally 15 μm compared to time-domain OCT.

Depending on the system used, Prof Pallikaris said one may observe the anterior/posterior corneal surface, the anterior lens surface, ciliary body, iridocorneal angle, Descemet’s membrane, trabecular meshwork, and Schwalbe’s line. These systems allow assessment of anterior chamber biometry, and corneal and lens thickness.

However, there are limitations to this technology he acknowledged. That aside, Prof Pallikaris said FD OCT is the more accurate and precise form of OCT.

Scheimpflug imaging is an optical system in which the object plane, lens plane, and observation plane are not parallel to each other. The Scheimpflug system images the anterior chamber with a camera at an angle to a slit-beam creating an optic section of the cornea and lens, and the multiple pictures taken in segments can be combined to provide a 3-D image. The advantages of this system are the increased depth of field and image analysis as well as the ability to identify the cataract grade, in order to guided the laser energy to the appropriate level for the emulsification.

Prof Pallikaris discussed the commercial imaging systems that are on the market for refractive surgery. LenSx and OptiMedica use FD-OCT for three-dimensional, high-resolution viewing of ocular structures, while Customlens uses real-time OCT that enables planning (placement of cuts) and monitoring (position of cuts) of the cataract procedure. LensAR, meanwhile, uses a 3-D confocal structured illumination scanning transmitter very similar to Scheimpflug technology.

Prof Pallikaris said there is a long way to go with this type of imaging technology and the industry is likely to produce much more advanced and automatic systems in the future.