A low-cost telescope for enhanced stimulus visual field coverage in functional MRI

Highlights

• A low costs MR safe device can be constructed to expand the amount of visual field being examined in the scanner.

• This can be attached to existing head coils.

• It works well when refractive error is fully corrected and participant can maintain binocular fusion resulting in an increase in cortical activation that is measurable.

Abstract

Background

A common limitation of typical projection systems used for visual fMRI is the limited field of view that can be presented to the observer within the scanner. A wide field of view over which stimuli can be presented is critical when investigating peripheral visual function, in particular visual disorders or diseases that lead to the loss of peripheral vision.

New method

We present a relatively low-cost Galilean telescopic device that can be used in most MRI scanners to double the effective visual field being presented. The system described is non-ferromagnetic, and compatible with most standard methods of visual presentation in MRI environments. The increase in area of visual cortex activation was quantified by comparing the extent of visual activity evoked by observing flickering checkerboards with and without the telescope in place.

Results

In all three observers that reported image fusion from the telescope, the extent of cortical activation was greater with the telescope, while in the fourth observer there was no difference between the two methods due to a lack of fusion.

Conclusion

The telescope is a low cost, easy to implement solution in situations where changes to the existing equipment or setup are not feasible.

Introduction

The early visual system is mapped in retinotopic coordinates, such that points that are adjacent to each other in the visual scene are represented by neighbouring neurons. This is the case from the photoreceptor cells in the retina through to the cerebral cortex, and provides the theoretical basis for retinotopic mapping; the identification of visual areas based on having one whole representation of visual space (Bridge, 2011; DeYoe et al., 1996; Sereno et al., 1995; Tootell et al., 1998).

Functional magnetic resonance imaging (fMRI) studies exploit retinotopy to identify different visual areas noninvasively. However, while the visual field of view is around 150° in natural vision, the limited space within the MRI environment significantly reduces the region visible from inside the scanner bore. Although the central visual field has considerably greater neural representation, known as cortical magnification (Wandell et al., 2007), there are many situations when peripheral stimulation is important (DeYoe et al., 2015). Firstly, in retinotopic mapping, the greater the stimulated region, the larger area of visual representation mapped in the cortex (Tootell et al., 1998). Secondly, some extrastriate visual areas, such as V6 and V6A primarily contain representations of the peripheral visual field, and cannot be accurately localised without far peripheral stimulation (Pitzalis et al., 2013). Thirdly, a lack of peripheral stimulation is a particular challenge when mapping the visual field in participants with eye disease that affects the periphery such as glaucoma, and rod-cone dystrophies (Brown et al., 2016; Silson et al., 2018). In the early stages of these conditions, vision loss can begin outside the area measurable by standard presentation methods. Therefore the cortical representation of this vision loss is not measured until the late stages of disease progression (Brown et al., 2016; Dumoulin and Knapen, 2018).

A number of different approaches are used to provide visual stimulation within the scanner environment, with one of the most common being a digital display placed at the end of the scanner bore. The screen is typically viewed using a standard mirror placed above the head of the scan participant, who lies supine on the scanner bed. This setup is limited by physical constraints, such as the scanner bore size, distance between the mirror and screen, and the physical apertures for the eyes provided in the head coil. A head-mounted goggle system can provide larger visual field coverage by using screens close to the eye (8,9). The main drawback of this type of system is the cost, which is significantly greater than other presentation systems, and may not be suitable for participants with larger head sizes due to the space in the head coil. Another approach used to increase the stimulated field of view is a projector system with a target screen set close to the participant inside the scanner bore (Roby et al., 2000), but constraints on space make this option impossible for some scanning environments. Furthermore, errors can be induced by the close proximity of the screen to the eyes inducing convergence or accommodative errors. Solutions that require permanent installation of equipment are not feasible in scanner sites shared between multiple teams. The options available for presenting stimuli dichoptically have been previously comprehensively reviewed (Choubey et al., 2009), and include a system relying on Keplerian binoculars to separate the images to the two eyes (Neri et al., 2004).

In visual stimulation experiments using a screen outside the scanner bore, the stimulus typically extends up to around 13 degrees radius, though this varies on the specific setup in any particular center (Dumoulin and Wandell, 2008), with some set-ups reaching a maximum field of view with a radius up to 15 degrees (Baseler et al., 2011). Using a wide projection screen viewed directly without a mirror can increase the visual field up to 100 degrees (Pitzalis et al., 2013). However, this type of wide projection screen is difficult to set up due to the space requirements.

Telescopic systems allow the magnification of the stimulus, and therefore a greater area of the visual field to be covered by the stimulus. They can be attached directly to a standard MRI mirror, thus keeping the cost of the system low, and the time to implement short. Telescopes consist of two lenses set at specific distances from the eye and from each other in order to achieve optical magnification of the image. There are two telescopic designs that allow the creation of an image that is both magnified, and correctly oriented. Keplerian (or astronomical) telescopes as implemented in binoculars used previously (Backus et al., 2001; Neri et al., 2004) allow for greater levels of magnification but are generally large, and create an inverted image that must be rectified (albeit rectified by commercial binoculars). Nonetheless, the binoculars used previously relied on use of a surface coil on the occipital lobe, rather than the modern 64-channel head coils. Galilean telescopes are shorter and create a non-inverted image, therefore allowing for a more compact construction that is able to fit into the space available in the MRI environment between the eye and the head coil (Dickinson, 2002). The latter design was therefore chosen to fit in the limited space available (Fig. 1). The Galilean design is used in low vision devices such as the Eschenbach Max range which forms the inspiration for the design proposed.