Quantification of the magnification and distortion effects of a pediatric flexible video-bronchoscope

Abstract

Background: Flexible video bronchoscopes, in particular the Olympus BF Type 3C160, are commonly used in pediatric respiratory medicine. There is no data on the magnification and distortion effects of these bronchoscopes yet important clinical decisions are made from the images. The aim of this study was to systematically describe the magnification and distortion of flexible bronchoscope images taken at various distances from the object.

Methods: Using images of known objects and processing these by digital video and computer programs both magnification and distortion scales were derived.

Results: Magnification changes as a linear function between 100 mm (x1) and 10 mm (x9.55) and then as an exponential function between 10 mm and 3 mm (x40) from the object. Magnification depends on the axis of orientation of the object to the optic axis or geometrical axis of the bronchoscope. Magnification also varies across the field of view with the central magnification being 39% greater than at the periphery of the field of view at 15 mm from the object. However, in the paediatric situation the diameter of the orifices is usually less than 10 mm and thus this limits the exposure to these peripheral limits of magnification reduction. Intraclass correlations for measurements and repeatability studies between instruments are very high, r = 0.96. Distortion occurs as both barrel and geometric types but both types are heterogeneous across the field of view. Distortion of geometric type ranges up to 30% at 3 mm from the object but may be as low as 5% depending on the position of the object in relation to the optic axis.

Conclusion: We conclude that the optimal working distance range is between 40 and 10 mm from the object. However the clinician should be cognisant of both variations in magnification and distortion in clinical judgements.

Figure 1

Photographic Object: A square of graph paper 3 cms × 3 cms with 1 mm × 1 mm units and concentric circles of varying diameter (2 mm, 6 mm, 10 mm and 20 mm) and diameter markings (AE, BF, CG, DH) all generated by Auto CAD.

Figure 2

The end view of the tip of the Olympus BF Type 3160 bronchoscope displaying the "off set" lens and lights and the geometric centre.

Figure 3

Effects of distance from object on the image appearance within the Field of View (FOV) and Light intensity obscuring parts of the image. Note the barrel appearance of the graph paper at the periphery in image "A" while in image "C" these effects are clearly offset or asymmetrical.

Figure 4

Effects of position within the Field of View: Magnification changes from 40 mm to 3 mm from the object displaying the variable exponential changes in magnification close to the object.

Figure 5

Across lens magnification: the near linear changes in magnification across the horizontal plane from the centre to periphery of the lens.

Figure 6

Mean central distortion ± 95% CI for the central or geometrical axis aligned bronchoscope.