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G. McCallum, C. Roberts, E. Herderick; Quantify Motion of Intraocular Structures Using Hall–Effect Sensors . Invest. Ophthalmol. Vis. Sci. 2005;46(13):712.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To characterize the response and investigate the repeatability of using a Hall–effect sensor to quantify intraocular motion. Methods: A cylindrical, permanent magnet (4mm diameter x 1mm thick) was fixed to the end of a plastic rod (4mm diameter) using cyanoacrylate. The rod was secured horizontally to a three–axis positioning system. A single–axis, axial Hall–probe (Lakeshore, Model 410) was fixed in place and aligned with the center of the magnet. Porcine eyes (n=3) were secured in a holder and positioned so the tip of the Hall–probe touched the sclera. An incision was made in the opposite side and the rod advanced into the eye until a value of 80mT was observed. The magnet was moved in increments of 250µm until a displacement of 2.0mm was achieved and then moved back to the starting position. At each increment the magnetic field strength was recorded. This process was repeated five times for each eye and also for a free–air control case. A regression analysis was performed to fit a quadratic model using indicator variables to compare magnetic field strength for a given eye to the free–air measurements. Results:The regression analysis indicated the magnetic field strength is quadratic (Adj. R2=0.97) as a function of distance from the magnet axis for both eyes and free–air. The standard deviation across all magnet displacements was less than 1.0 mT. Conclusions: Ocular structures do not alter magnetic field strength when compared to free–air measurements with excellent repeatability. The strong quadratic behavior of the magnetic field strength reduces the sensitivity and increases variability at the magnet axis and edge, respectively. Optimally, the Hall–probe should be aligned to a point that is one–half the radius from the magnet’s axis. Applications for this sensing method include measuring displacement in accommodating IOLs and also evaluating and investigating ciliary muscle contraction.
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