Date Published: April 22, 2019
Publisher: Public Library of Science
Author(s): Patrick Schlegel, Melda Kunduk, Michael Stingl, Marion Semmler, Michael Döllinger, Christopher Bohr, Anne Schützenberger, Seyedali Mirjalili.
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.
Regardless of the area of research, small factors can often exert a strong influence on the results obtained and the parameters calculated. For example, in mathematical modeling a small change of only one parameter can have a large effect on other parts of the model [1, 2]. In spectroscopy a small difference in a measured spectrum can be a crucial factor for determining the exact kind of material the analyzed light was emitted from  and in medicine slightly different symptoms can indicate completely different diseases . Equally, in laryngeal high-speed video endoscopy (HSV) different factors can significantly influence the outcome of a measurement obscuring the distinction between normal and pathological laryngeal dynamics.
40 HSV recordings of 20 healthy male and 20 healthy female subjects were investigated. The age of the female subjects varied from 17 to 29 years (average 19.9 years) and from 22 to 29 (average age 24.2) for males. The recordings were taken endoscopically from subjects during sustained phonation of the vowel /i/ at a comfortable pitch and loudness level. All recordings were chosen from our existing clinical database and all had a length of at least 250 milliseconds (ms). The study was approved by the ethic committee of the Medical School at Friedrich-Alexander-University Erlangen-Nürnberg (no. 290_13B). Written consent was obtained by the subjects or, in case of underage, by their legal guardians.
Statistical analysis showed clear influences of image resolution on a variety of parameters, although there were distinct differences between the seven different groups of measures. Definitions of parameters vary and in some cases, although the definition is almost identical, the results of the statistical analysis differ. In other cases the definitions look very dissimilar but the results of the statistical analysis are almost the same. However, some of the parameters are related in a simple linear way or their relations are already known. To increase the clarity, we excluded the latter cases from the discussion and only included them in the supporting information. Directly linear or mathematically dependent parameters, which yield identical or almost identical results in the statistical analysis as already discussed parameters, are only briefly mentioned at the end of each subsection. They are also not included in any given numbers of statistically significantly changing parameters. Removed parameters are marked in S1–S3 Tables. Overviews for the relevant parameters of each group are given in Figs 4–10. The structure of these seven figures is as follows:
In total, 18 (51%) out of 35 investigated relevant parameters changed in females and 22 (63%) out of 35 in males. From this, we conclude that reducing the resolution of HSV recordings has a substantial impact on the overall shape of the extracted GAW. The least affected groups of measurements were FPM and PPM; although PPM measures showed strong differences in the effect of decreasing resolution for single subjects as seen in Fig 5. The mostly affected groups were APM and MM. Of the investigated SM and GDC, a major part deviated statistically significantly and the biggest differences between females and males were observed in EPM since they changed statistically significantly in males but were mostly unaffected in females. The only group of measurements that (as the results of this study imply) is truly unaffected by decreasing resolution is FPM.
Several limitations have to be mentioned. The pixel area occupied by the glottis differs between recordings. In our recordings, for men, the GAW maximum fluctuated between 742 and 3214 pixels, for women the range was between 978 and 2057 pixels. This variation can be attributed to different sizes of the glottides but also to a varying distance between endoscope tip and glottis. These effects may be comparable to a changing resolution by pixel averaging. Accordingly, significances may be influenced by “outlier recordings”, which had a particularly low or high endoscope-tip glottis distance or glottis size. Also, image noise may be smoothed for lower resolutions, as this is an unavoidable side effect of the applied pixel averaging method. Moreover, the sample size was rather small.
With this work, another gap in the study of factors influencing GAW parameters was reduced and the understanding of these parameters was improved. It was found that about half of the examined objective GAW parameters were statistically significantly influenced by changing camera spatial resolution. This sensitivity towards the resolution does not only affect the comparability between studies, since the effective resolution also changes with the distance between the glottis and the endoscope tip. That this change probably cannot be neglected is indicated by the number of glottis pixels changing up to fourfold between male recordings. Hence, the comparability of certain parameters between different recordings is not necessarily given when using different spatial camera resolutions.