Influence of spatial camera resolution in high-speed videoendoscopy on laryngeal parameters

Abstract

In laryngeal high-speed videoendoscopy (HSV) the area between the vibrating vocal folds during phonation is of interest, being referred to as glottal area waveform (GAW). Varying camera resolution may influence parameters computed on the GAW and hence hinder the comparability between examinations. This study investigates the influence of spatial camera resolution on quantitative vocal fold vibratory function parameters obtained from the GAW. In total 40 HSV recordings during sustained phonation (20 healthy males and 20 healthy females) were investigated. A clinically used Photron Fastcam MC2 camera with a frame rate of 4000 fps and a spatial resolution of 512×256 pixels was applied. This initial resolution was reduced by pixel averaging to (1) a resolution of 256×128 and (2) to a resolution of 128×64 pixels, yielding three sets of recordings. The GAW was extracted and in total 50 vocal fold vibratory parameters representing different features of the GAW were computed. Statistical analyses using SPSS Statistics, version 21, was performed. 15 Parameters showing strong mathematical dependencies with other parameters were excluded from the main analysis but are given in the Supporting Information. Data analysis revealed clear influence of spatial resolution on GAW parameters. Fundamental period measures and period perturbation measures were the least affected. Amplitude perturbation measures and mechanical measures were most strongly influenced. Most glottal dynamic characteristics and symmetry measures deviated significantly. Most energy perturbation measures changed significantly in males but were mostly unaffected in females. In females 18 of 35 remaining parameters (51%) and in males 22 parameters (63%) changed significantly between spatial resolutions. This work represents the first step in studying the impact of video resolution on quantitative HSV parameters. Clear influences of spatial camera resolution on computed parameters were found. The study results suggest avoiding the use of the most strongly affected parameters. Further, the use of cameras with high resolution is recommended to analyze GAW measures in HSV data.

Fig 1

A) Recording of the vocal fold oscillations via a rigid endoscope being attached to a high-speed camera. B) Superior view of the vocal folds as seen with the endoscope. C) Computed GAW: number of pixels enclosed within the glottis over time.

Fig 2. Reduction of image resolution by pixel averaging and selection of a rectangular subsection.

Fig 3. Scheme of the segmentation process: 1.

Selection of a 1000 frames section, 2. Rough pre-segmentation, 3. Initial grid in the lowest resolution data, 4. First condensing of the grid, 5. Second condensing of the grid, 6. Adjustment of brightness thresholds, 7. Calculation of the partial GAWs, and 8. Extraction of all GAWs.

Fig 4

Relative deviations between all fundamental period measures, of reduced and original resolution data for A) females and B) males. (For some plots the green boxes are not visible because at least one half of the compared values did change only marginally or not at all between resolutions).

Fig 5

Relative deviations between all period perturbation measures, of reduced and original resolution data for A) females and B) males. The y-axis scaling is logarithmic. The * symbol indicates a statistically significant deviation (p≤0.05).

Fig 6

Relative deviations between all amplitude perturbation measures except AVI, of reduced and original resolution data for A) females and B) males. The y-axis scaling is logarithmic. The * symbols indicate statistically significant deviations (p≤0.05).

Fig 7

Relative deviations between all energy perturbation measures, of reduced and original resolution data for A) females and B) males. The y-axis scaling is logarithmic. The * symbols indicate statistically significant deviations (p≤0.05).

Fig 8

Relative deviations between all symmetry measures, of reduced and original resolution data for A) females and B) males. The * symbols indicate statistically significant deviations (p≤0.05).

Fig 9

Relative deviations between all glottal dynamic characteristics, of reduced and original resolution data for A) females and B) males. The * symbols indicate statistically significant deviations (p≤0.05).

Fig 10

Relative deviations between all mechanical measures, of reduced and original resolution data for A) females and B) males. The * symbols indicate statistically significant deviations (p≤0.05).

Fig 11. Change in overall shape of a male GAW with decreasing resolution.