Accommodation is the act of the eye to adjust the refractive power to bring near objects into sharp focus. Accommodation occurs through possible controlled changes in the crystalline lens shape and thickness, and in distances between the major refractive surfaces. The mechanism of accommodation and its relevance to presbyopia has been debated for more than 150 years since the debut of Helmholtz’s theory of accommodation [1
]. Researches into understanding accommodation and restoring accommodative function of the eye for millions of people with presbyopia and cataract surgery have attracted great interest among the ophthalmic research community [2
A quantitative high resolution imaging technique that can image the whole anterior segment (the cornea, iris, and crystalline lens) in real time is highly desired and will greatly benefit the research in accommodation. However, currently there is no in vivo
imaging technology that can meet all the needs for measuring the modifications of the anterior segment during accommodation, which we believe is one of the major hurdles for the research on accommodation and reasons why the arguments about the mechanisms are still surprisingly vehement [2
]. The conventional technologies for anterior segment imaging are Scheimpflug photography, A / B scan ultrasound, UBM (Ultrasound Biomicroscopy), slit scanning topography, and Purkinje imaging [5
]. With these technologies, it is necessary to stimulate the fellow eye in order to observe the variations of the analyzed eye. The techniques using ultrasound can only be used with contact systems or with water baths that will modify the anatomical dimensions or the pressure of the anterior segment. Geometrical reconstructions are necessary with the Scheimpflug photographic technique while it cannot be used in certain axes. The eye also needs to be dilated for Scheimpflug photography, which is not a natural condition for accommodation. MRI (magnetic resonance imaging) was also used to image the anterior segment of the eye by some authors [12
]. But it may not be routinely used.
Optical coherence tomography (OCT) is a low-coherence interferometer-based noninvasive medical imaging modality that can provide high-resolution cross sectional images of biological tissues [14
]. Anterior segment OCT (AS-OCT) provides the advantage of producing non-contact images of the anterior segment in static and dynamic conditions [16
]. AS-OCT was first reported by Izatt et al.
using a time-domain system with a superluminescent diode light source at 800 nm [16
]. Baikoff et al.
] and Richdale et al.
] explored the modifications of the anterior chamber and lens thickness with aging and accommodation using time-domain AS-OCT. However, simultaneous imaging of the whole anterior segment (cornea, iris, anterior chamber and crystalline lens) cannot be realized with commercial time-domain AS-OCT system because of the limitation of imaging depth as well as imaging speed. Because all the surfaces of the anterior segment cannot be imaged simultaneously, the contribution of the posterior surface curvature of the lens to accommodation, which may also play a significant role, was not discovered in their studies. Furthermore, when imaging the posterior surface of the lens with conventional AS-OCT usually only one specular reflection spot appears in the image, which makes it impossible to calculate the curvature of the surface. The accuracy of the measurement of the lens thickness is also not reliable when only one spot on the posterior surface can be detected.
The application of spectral domain OCT (SD-OCT) in imaging the anterior segment of the eye has been reported by several groups [17
]. SD-OCT provided unprecedented imaging speed, and thus made possible 3D imaging of the anterior segment of the eye. However, the limited imaging depth in SD-OCT also made imaging the whole anterior segment formidable. To improve the imaging depth in SD-OCT several groups have developed techniques to eliminate the mirror image [19
]. The most recent development by Grulkowski et al.
] represents a big step toward real time imaging of the whole anterior segment of the eye using a high-speed CMOS camera. However, if only single OCT channel is used, these techniques reduce the imaging speed the same system can achieve since multiple images need to be acquired to remove the complex ambiguity in SD-OCT. Another limitation in imaging the whole anterior segment is the limited depth of focus of the objective lens. Unless dynamic focusing is employed, which is not suitable for high speed imaging, the depth of focus cannot cover the depth of the whole anterior segment. Multiple-channel [23
] and multiple-focus OCT [35
] offer possible solutions to the problem.
In the present work, we developed a dual channel dual focus SD-OCT system to image all the surfaces of the anterior segment of the eye simultaneously for studying accommodation of the eye. This technique provides a powerful imaging tool for research on the mechanism of accommodation and presbyopia.