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Jae-hyung Kim, Tae Hwan Moon, Moses Kim, Ju Byung Chae; Measurability of partial coherence interferometry. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2532.
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Although partial coherence interferometry (PCI) provides more accurate and reproducible measurements of ocular axial length (AL) than conventional contact ultrasound methods, PCI is less useful in measuring AL in eyes with severe media opacity. To date, however, no studies have assessed the minimum near infrared ray intensity required for PCI measurements of AL. We therefore measured the minimum near infrared ray intensity required for consistent measurements of AL and analyzed media opacities in patients with media opacities that precluded measurement using PCI.
Two types of PCI (IOLmaster, ALscan) were used, with two polarizing lenses (Kenko PL) attached to the PCI. The near infrared ray intensity of PCI was modified by rotating the axis of one PL at intervals of 5 degrees. The right eye of one volunteer was measured three times, with the AL and signal to noise ratio (SNR) recorded 5 times for each measurement. Reduction of light intensity was theoretically estimated using Malus’ law. We retrospectively reviewed the medical records of patients unmeasurable by PCI (ALscan) from May to September 2013.
AL was measured consistently with both IOLmaster and ALscan, until they reached 55 degrees (83.6% block) and 60 degrees (87.5% block), respectively. In contrast, SNR decreased as light intensity decreased. From May to September 2013, 23 eyes of 237 patients (9.7 %) could not be measured using ALscan due to media opacities, such as severe posterior subcapsular cataract (PSC, 8 eyes), hypermature cataract (2 eyes), and vitreous hemorrhage (13 eyes). The median grades of PSC (LOCS III) and vitreous haze were 4 (range 3-5) and 8 (range 4-8), respectively.
Although 83.6%~87.5% of near infrared rays were blocked or scattered, AL could be measured consistently.
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