Retinal Imaging in Your Hand

Making a cumbersome device more portable often results in broader applicability and greater convenience (think desktop>laptop>tablet). Some miniaturization challenges, however, seem insurmountable: how exactly do you turn an adaptive optics scanning laser ophthalmoscope (AOSLO) – something the size of a billiard table – into a pocket-sized device? After all, AOSLO has to be big to accommodate and integrate the AO components: a wavefront sensor to detect optical aberrations and a deformable mirror to compensate for those aberrations. Without them, you can’t achieve accurate, high-resolution imaging. With them, AOSLO is limted to ‘easy’ patients who can sit upright and fixate for several minutes, which excludes young children and supine or semi-recumbent adults (for example, anaesthetized patients).

Or can they? Now, a team from Duke University (Durham, NC, USA) has managed to reduce AOSLO to the size of a small book (about 10 x 5 x 14 cm). An essential element of this impressive shrinking exercise was the adoption of wavefront sensorless (WS) technology, which  replaces the physical wavefront sensor with an algorithm. This innovation, when combined with a novel opto-mechanical design and a miniaturized deforming mirror, eliminated much of the volume requirement of standard AOSLO. But miniaturization alone wasn’t sufficient; the movement associated with a hand-held device continually changes the path of light through the eye’s optics, so the team had to develop a novel stochastic Zernike gradient descent (SZGD) algorithm to allows dynamic correction.

Sounds great in theory – but how does the hand-held AOSLO (HAOSLO) fare in reality? In healthy volunteers (seven undilated, semi-supine adults and five pharmacologically dilated, supine adults), HAOSLO imaged individual cones close to the fovea. Importantly, HAOSLO also provided images of individual cones in two anesthetized infants – the first known use of AO in young children.

What are the implications? The ability to image cones within or at the edge of the foveal vascular zone, with a portable, hand-held device, could dramatically enhance the study and management of ophthalmological disease. For example, assisting diagnosis of retinal disease, or assessing the efficacy of gene therapy. Furthermore, HAOSLO could be combined with other modalities, such as split detector AOSLO or fluorescence imaging, to provide clinicians with a multifunctional platform technology. Other future developments could also include algorithm modification for use in eyes where light scatter is an issue. Such improvements will be facilitated by the team’s decision to make their optical and mechanical design and software – including the novel SZGD algorithm – open source, effectively putting their breakthrough work into the hands of the community (2).