**3. Imaging technique**

While utilization of modified unmounted tabletop OCT scanners has been reported in literature, this is challenging due to the limited portability of the device [11, 13]. Commercially available portable handheld and arm-mounted OCT systems made it feasible to obtain imaging in premature infants, but at this time they are still not widely used in clinical practice [14, 15].

The eye of a premature infant and the adult eye have many structural and optical dissimilarities. The axial length of the premature eye undergoes precipitant growth during neonatal period and then slows progressively afterwards. The cornea has steeper curvature in neonates compared to adults [15, 16]. Refractive error pattern switches from slight myopia in neonatal period towards slight hyperopia in infancy. In addition, newborn eyes have greater astigmatism [15]. If these features are not taken into consideration, difficulties, such as poor image clarity and clipping artifacts can be encountered. Thereby, imaging protocols must be configured to account for these age-specific properties [17]. OCT systems with shorter acquisition times such as spectral domain (SD) and swept source (SS) are faster making them more suitable for infants [9]. OCT angiography (OCT-A) is a rather new quick and non-invasive imaging technique to perform visualization and quantitative analysis of retinal vasculature as well as the evaluation of retinal blood flow without the need for an intravenous or intravitreal injection of a contrast agent [18–20]. This provides an alternative to fluorescein angiography. OCT-A can be used to generate various foveal vascular characteristics including vessel perfusion density, vessel length density and vascular diameter index [21]. A limitation of OCT/OCT-A is that they do not easily capture the peripheral retina where stages of ROP occur.

The imaging can be done with minimal to no discomfort to the patient. A speculum is generally needed [20, 22]. Oral sucrose may be given to comfort the patient. Arm mounted imaging systems may greatly facilitate the task as there would be no need to support the weight of the scanner. A second person would operate the software and capture the images [17, 22]. Ocular lubrication should be applied before imaging to create a stable tear film for clearer images. Scleral depression may be used to manipulate the eye position and improve peripheral views [23]. Ultimately, it has been reported that OCT imaging may even be less stressful than indirect ophthalmoscopy examinations that are routinely done to evaluate for ROP [24]. Implementation of age-specific techniques was evaluated by Maldonado et al. and it was found that the average time per imaging session decreased and there was no significant change of vital signs from baseline [15].
