Non-Linear Image Distortions in Flexible Fiberoptic Endoscopes and their Effects on Calibrated Horizontal Measurements Using High-Speed Videoendoscopy

Abstract

Laryngeal images obtained via high-speed videoendoscopy are an invaluable source of information for the advancement of voice science because they can capture the true cycle-to-cycle vibratory characteristics of the vocal folds in addition to the transient behaviors of the phonatory mechanism, such as onset, offset, and breaks. This information is obtained through relating the spatial and temporal features from acquired images using objective measurements or subjective assessments. While these images are calibrated temporally, a great challenge is the lack of spatial calibration. Recently, a laser-projection system allowing for spatial calibration was developed. However, various sources of optical distortions deviate the images from reflecting the reality. The main purpose of this study was to evaluate the effect of the fiberoptic flexible endoscope distortions on the calibration of images acquired by the laser-projection system. Specifically, it is shown that two sources of nonlinear distortions could deviate captured images from reality. The first distortion stems from the wide-angle lens used in flexible endoscopes. It is shown that endoscopic images have a significantly higher spatial resolution in the center of the field of view than in its periphery. The difference between the two could lead to as high as 26.4% error in calibrated horizontal measurements. The second distortion stems from variation in the imaging angle. It is shown that the disparity between spatial resolution in the center and periphery of endoscopic images increases as the imaging angle deviates from the perpendicular position. Furthermore, it is shown that when the imaging angle varies, the symmetry of the distortion is also affected significantly. The combined distortions could lead to calibrated horizontal measurement errors as high as 65.7%. The implications of the findings on objective measurements and subjective visual assessments are discussed. These findings can contribute to the refinement of the methods for clinical assessment of voice disorders. Considering that the studied phenomena are due to optical principles, the findings of this study, especially those related to the effects of the imaging angle, can provide further insights regarding other endoscopic instruments (eg, distal-chip and rigid endoscopes) and procedures (eg, gastroendoscopy and colonoscopy).

Keywords: Flexible fiberoptic endoscopy; Horizontal calibrated measurements; Image distortion; Imaging angle; Laryngeal imaging; Laser calibrated endoscope; Voice assessment.

FIGURE 1.

Optical principles of image formation. (A) Parameters of the Snell’s law (B) Image formation in the Gaussian optics model.

FIGURE 2.

Effects of tilting the target surface on the geometry of the acquired images.

FIGURE 3.

The employed setup for benchtop recordings.

FIGURE 4.

A schematic for measuring the tilting angle.

FIGURE 5.

Automatic detection of the grid lines. (A) Recording from 1 mm grids at the working distance of 10 mm. (B) The binary image showing locations of the minima. (C) Fitted second-order polynomials on the locations of the minima.

FIGURE 6.

Groupings for experiments 1 and 2. (A) The solid red blocks and the patterned blue blocks denote the center and the periphery groups. (B) The selected sides of an example image. Center of the image-FOV is denoted by a green cross mark.

FIGURE 7.

Variation in pixel size for different working distances and groups.

FIGURE 8.

Boxplots of the pixel size for different groups and working distances.

FIGURE 9.

(A) Selected line segments are shown in green dashed line, and the center of the image-FOV is denoted with a red cross mark. (B) Dependence of a pixel size on its distance from the center of the image-FOV and the working distance. The negative distance means blocks that were below the center of the image-FOV.

FIGURE 10.

Groupings for experiment 3. Solid red lines denote the back group, dotted green lines denote the middle group, and dashed blue lines denote the front group. (A) Groupings at the working distance of 5 mm (B) Groupings at the working distance of 15 mm.

FIGURE 11.

Values of mean and standard deviation of pixel size. (A) Working distance of 5 mm, (B) Working distance of 10 mm, (C) Working distance of 15 mm, (D) Working distance of 20 mm.

FIGURE 12.

(A) The selected line segments are shown in green dashed line, and the center of the image-FOV is denoted with a red cross mark. (B) Dependence of a pixel size on its distance from the center of the image-FOV and the tilting angle at the working distance of 15 mm.

FIGURE 13.

Dependence of location with the highest spatial resolution on the tilting angle.