# Review on Retrospective Procedures to Correct Retinal Motion Artefacts in OCT Imaging
## 5 Discussion  
Retinal imaging has shown a growing potential in the early diagnosis of pathologies that are related to the physiology and inner structure of the retina and its adjacent tissues [99; 100; 101]. The rapid development of better imaging devices, as well as the research in image processing to enhance and correct the acquired data, will certainly lead to the development of new retinal biomarkers. In the upcoming years, these biomarkers will aid the diagnosis and prognosis of not only ophthalmic, but also cardiovascular and neurodegenerative diseases [102; 103]. As shown in this review, one of the most important research lines on retinal image processing is the motion correction, as it is relevant not only for the correct display and visualization of the volumetric data, but also for the reliability of retinal imaging biomarkers. Motion correction approaches can be roughly divided in prospective and retrospective. On one hand, prospective techniques use specific hardware to collect additional eye motion information during the acquisition of the OCT data. This is probably the most common motion correction approach in the clinic, since many commercial systems use SLO or fundus images to correct the OCT data. A number of studies have also presented specific prospective techniques to correct retinal motion. For instance, [PERSON] et al. [14] used an SLO simultaneously with OCT imaging in combination with axial motion correction. Axial motion was corrected measuring the position of the corneal apex using an additional spectral domain partial coherence interferometer integrated into a transverse scanning OCT system. With this information, the reference arm length of the imaging interferometer was adapted to correct for axial motion and allowed for axial tracking with an accuracy below the axial resolution of the system [104]. [PERSON] et al. [41] performed transverse motion tracking by correcting the actuations of the horizontal/vertical-scanner in a polarization sensitive OCT system. This was done according to the detected retinal position shifts measured by a simultaneously operated line-scanning laser ophthalmoscope. Using the cross-correlation values of the parallel acquisition, the best match was found, and the horizontal/vertical shifts were obtained. A correction signal proportional to the computed shift information was then sent to the \(XY\)-scanner of the OCT. Tracking rates up to 60 Hz were achieved with this system. However, the introduction of AO-OCT requires a more demanding retinal stabilization and, consequently, further research on prospective approaches. [PERSON] et al. [105] presented a customized retinal tracking module integrated into the sample arm of an OCT system, able to correct eye movements up to 100 Hz, and hence reduce residual motion to 10 \(\upmu\)m root mean square. Despite the in situ correction, prospective approaches may cause additional strain to subjects with fixation limitations, as they have to go through a longer exposure. For example, an instrument with online correction stops recording if a misalignment is detected, leading to a longer exposition on subjects with fixation problems. However, in the absence of fixation problems, such as in healthy individuals, one could argue that this solution is appropriate for correcting retinal motion, as it is done online instead of as part of the post-processing. Nevertheless, eye trackers do not remove the need of using motion-correction algorithms [106]. Prospective techniques also require specific additional hardware, which must be customized to a number of applications, increasing the complexity of the imaging device. On the other hand, retrospective techniques, which are the focus of this review, perform a correction based on assumptions about the retinal motion patterns. It is an offline procedure that may include additional references and, unlike prospective approaches, it can be applied to OCTdata acquired previously, allowing its inclusion in longitudinal studies.