Radiographic Magnification on Videofluoroscopy: An Important Variable to Consider for Scaled Analyses of Swallowing

Abstract

Purpose: Traditionally, kinematic measures on videofluoroscopy require the use of an external scalar (such as a penny) to transform pixels to absolute distances. Videofluoroscopy is subject to image magnification based on the distance of the feature of interest to the X-ray source. However, the impact of the position/location of the external scalar on swallowing measures is unknown. Our goal was to systematically investigate the accuracy of various common external scalar locations in lateral and anterior–posterior (A-P) view. Method: U.S. pennies were taped to a styrofoam head in three positions (on the left and right lateral neck and in midline submentally). Locations were measured to ensure equal left and right, as well as midline, placement. A metal screwdriver (6 mm in diameter) was inserted into the premanufactured hole that is centrally located at the bottom of the styrofoam head. The head was centered on a medical tray and placed in the middle of a Siemens Alpha C-arm Fluoroscope field. ImageJ was used to measure penny length in pixels (three locations) in both lateral and A-P views. Penny length was known (19.05 mm), and, therefore, used to derive screwdriver size (for each location) for comparison to the actual screwdriver size. Results: All scalars overestimated the screwdriver size ranging from 6.55 to 7.87 mm, representing a 9%–31% inflation. Scalars closer to the X-ray source had the largest magnification. Conclusions: Our results confirm that image magnification of external scalars is a significant source of variability that is currently unaccounted for in the swallowing literature. Recommendations for future research design/measurement methods are provided.