Medical video recording

When analyzing a single period, it is assessed:

• asymmetry of the work of the vocal folds (right and left fold), including:
  • amplitude,
  • phase difference;
• difference in the opening and closing phases;
• regurgitation, with specification of the length of the vocal folds in which it occurs and the degree of regurgitation;
• closing and opening coefficient (with the note that determining on the basis of one period may be subject to a very large error);
• dynamic parameters (speed of opening and closing);
• the occurrence of a mucosal wave.

When analyzing groups of periods, additionally it is assessed:

• uniformity of the work of the vocal folds, also by determining the jitter and shimmer parameters in the same way as in the case of acoustic analysis;
• frequency of work of the vocal folds;
• averaged closing and opening rates and its spread;
• beginning and end of phonation (the way of starting and ending vibrations).

In the case of the fast film technique, the recording of the work of the vocal folds takes place by recording video frames with a high speed - an average of 2000 frames per second:

In order to achieve the slow-motion effect, a movie recorded at high speed is enough to play at a lower speed, eg the standard 25 frames per second; within 1/10 second of the recording it is possible to record up to 20 cycles of vocal folds operation. Thanks to this, it is possible to register vibrations even during a very short phonation, ensuring maximum reliability of the image of the vocal folds activity, additionally, it enables the examination of momentary phenomena, the beginning and end of phonation, and allows for the correct verification of the irregularity of the vocal folds work.

In the strobe technique, the slow-motion impression is obtained by appropriate exposure of individual frames, recorded at a speed of 25 frames per second. One cycle of vocal folds observed in stroboscopy is composed of frames from many different real cycles that are distant from each other. This is shown in Fig. 10. The synchronization of the flashes of light or the camera shutter with the movement of the vocal folds, necessary to achieve in the case of stroboscopy, is usually obtained on the basis of the analysis of the voice of the examined person recorded in parallel.

Typically 10 seconds of stable phonation are needed to register 10 vocal folds, which is usually not achievable with pathological voices. Correct control of the moments of exposure of frames is possible only with sufficient regularity of vibrations of the vocal folds. Otherwise, it is not possible to precisely select the work cycle so as to obtain a correct record of possible irregularities in the work of the vocal folds, therefore any disturbances in the cyclical work of the vocal folds in stroboscopy will be illustrated only as fraying of the performed kymographic sections, and not as real changes in the length of individual periods.

For this reason, the slow-motion recordings generated using the phenomenon of stroboscopy allow for the analysis of only a single period of vibration to a limited extent. Due to the method of generation, it is practically impossible to evaluate groups of periods. High-speed camera recordings can be assessed both in terms of assessing one period as well as groups of periods, as in most cases they cover much more periods of vocal fold movement (Fig. 11). To highlight these significant differences between high-speed camera kymographs and those obtained from strobe recording, the second are referred to as strobokymograms.

Fig. 11. Horizontal kymogram for high-speed recording

The lack of credibility in generating slow-motion image with the stroboscopic method in the case of pathologies and cyclic disturbances was the main reason why, even despite the imperfections and high costs of high-speed cameras, they were used more willingly in research centers:

Figure 12 shows an example of a pathology that makes it impossible to assess the work of the vocal folds by means of a stroboscopic examination, and where the assessment with virtually any high-speed camera is possible.

Fig. 12. An example of vocal fold pathology (the so-called bamboo folds) illustrating the superiority of even the historical model of the speed camera in the assessment of the function of the vocal folds

World-class specialists agree that the stroboscopic technique is not suitable for the assessment of irregular vibrations, and this is what we deal with in the case of diseases of the voice organ:

Due to the fact that the basis of videostroboscopy is the assumption of a regular cycle of work of the vocal folds, in case of irregular vibrations the results obtained with this technique are distorted. Fig. 13 shows the high speed camera kymographs for the same patient, while the high speed camera kymogram shows the irregular work of the vocal folds - the stroboscopic technique "loses" irregularities between the cycles.

Unfortunately, the stroboscopic technique fails most often in cases where accurate imaging of the work of the vocal folds is particularly desirable - in the case of significant pathologies.

However, even in the case of minor pathologies, the evaluation of the work of the vocal folds must often be carried out on the basis of the visibility of one, sometimes two basic periods and limited to a single-period analysis. It is not possible to evaluate the uniformity of vibrations. It requires 10 or more periods of vocal fold movement to be visible. In the case of stroboscopy, this is at least 10 s of phonation.

Two additional problems then arises:

• rarely any examined person is able to phon for so long during an uncomfortable examination - these results in phonation separated by respiratory phases, which "destroy" the synchronization between the strobe and the vocal folds,
• it is difficult for the examiner during such a long examination to keep the endoscope tip stable - apparent movements of the vocal folds appear on the screen, "spoiling" the appearance of kimograms and making analysis difficult.

Figure 14 shows the strobe video frames that show the problem of the endoscope tip being stably held in one position over the vocal folds, and Figure 15 shows a typical kymogram distortion from a long strobe video due to respiratory phases and endoscope tip movements. Any analysis is practically impossible for this case.

Fig. 15. Strobokymogram for the sample, the cages of which are shown in Fig. 14

If the examination subject cooperates exceptionally well and the examiner has experience and a "steady hand", it is possible to obtain, and in the case of stroboscopy, longer recordings, including even 7-10 periods without the respiratory phase and with moderate movements of the endoscope tip. A rare example is shown in Figure 16a.

Fig. 16a. Strobokymogram with eight cycles of vocal folds work. Visible movement of the endoscope tip

In such a situation, it is sometimes possible to additionally stabilize the image in the software. Such functions are provided by the DiagnoScope Specialist software by DiagNova Technologies. Thanks to it, it is possible to "improve" the kymographic cross-section to the form shown in Fig. 16b. Unfortunately, it is practically impossible to remove the problems related to irregularities in the work of the vocal folds, which appear in the strobokymograms as fraying of the edges of the vocal folds (Fig. 16b). It is impossible to determine whether the differences in the shape of the periods in the strobokymogram are due to actual differences or to problems with timing due to non-uniformity of the vocal fold cycles.

Fig. 16b. Strobokymogram in Fig. 16a after image stabilization. Fraying of the edges caused by uneven work of the vocal folds was marked

It should be emphasized that due to the fact that the subsequent lines of the kymogram are created in stroboscopy from image frames distant in time by 1/25 s, which causes significant differences between them. The image stabilization process and any other operations increasing the legibility of the image are then significantly more complicated from the technical point of view and require much more commitment, knowledge and time from the user. Obtaining images such as the kymograms from a high-speed camera shown in Figure 17 is practically impossible in the case of stroboscopy.

Fig. 17. Examples of kymograms from the ALI Cam HS1 high-speed camera for various disorders of the vocal folds (from the top - period asymmetry, different vibration frequencies of both vocal folds caused by, among others, significant regurgitation, slight regurgitation, asymmetry and phase difference of vibrations of both vocal folds, unevenness of work - formation of conglomerates of basic periods

Instead, kymographic sections usually covering one or two cycles are obtained. Work in clinical conditions or in hospital admissions makes it impossible to perform image stabilization (due to its time-consuming nature and the quality of the final effect, depending on the perfect sharpness of the image of the vocal folds). As a result, the images are usually obtained as in Fig. 18a.

Fig. 18a. Examples of strobes without additional time-consuming image stabilization. The number of periods presented corresponds to the maximum possible number of periods from a given recording

Fig. 18b. Examples of kymographic sections for four test persons obtained from a strobe and high-speed camera. The video recording from the strobe was additionally stabilized. The number of periods presented in the strobokymograms corresponds to the maximum possible number of periods from a given recording, the number of periods in kymographic sections from a high-speed camera was selected in order to obtain good visibility. ³

An additional advantage of the examination with a fast camera is the possibility of fully parametrizing the movement of the vocal folds. In the case of stroboscopy, it is limited only to cases in which it was possible to generate kymographic sections, and only to determining parameters from individual periods. In the case of a high-speed video, in most cases it is possible to efficiently parameterize both in terms of a single period of work of the vocal folds (asymmetry, opening factor, amplitude, etc.), and - due to a much larger number of recorded cycles of the vocal folds operation - multi-period. It is then possible to assess the cyclic operation of the vocal folds by determining, as for the acoustic analysis, the fundamental frequency followed by jitter and shimmer, but it can be done for both vocal folds jointly or separately for each.

Figures 19a-19e shows examples of kymographic sections with one second phonations, as well as graphs of the glottis gap and the fundamental frequency of vibration of the vocal folds. The data obtained in this way is easier to analyze than audio data and allows for more precise detection of large irregularities in the length of the basic periods due to the lack of signal disturbances (influence of formants) characteristic of audio data. Thanks to this, it was possible to determine jitter and shimmer, which was added under the drawings.

Fig. 19a. An example of a kymogram, a plot of the fundamental frequency and multi-period parameterization based on phonation lasting about 1 s. No disturbance

Fig. 19b. An example of a kymogram, a plot of the fundamental frequency and multi-period parameterization based on phonation lasting about 1 s. Male. No disturbance

Fig. 19c. An example of a kymogram, a plot of the fundamental frequency and multi-period parameterization. End of phonation, visible periodicity disturbances

Fig. 19d. An example of a kymogram, a plot of the fundamental frequency and multi-period parameterization.
The same subject as on Fig. 19c. End of phonation, visible similar periodicity disturbances at the end of phonation

Fig. 19e.An example of a kymogram, a plot of the fundamental frequency and multi-period parameterization based on phonation lasting about 1 s. Female. No disturbance

Determining the irregularity of the work of the vocal folds seems to be one of the most important future directions in the image analysis of the work of the vocal folds. This is due to the occurrence of such phenomena much more frequently than it has been estimated so far. Due to the fact that their observation is practically impossible in stroboscopy (and evaluation is not possible in any case), these phenomena generally went unnoticed. Meanwhile, they perfectly explain the disturbances recorded in the acoustic analysis. Completely new concepts of "super-cycle" can be introduced, combining several cycles of the vocal folds into repetitive conglomerates, as shown in Figure 20.

Fig. 20. Conglomerates of basic periods creating characteristic "super-cycles". This phenomenon can only be observed when recording with a high speed camera ⁴

Due to the possibility of registering arbitrarily irregular vibrations and many coexisting vibrations, the high-speed camera is also the only tool enabling the evaluation of the work of the pseudo-glottis. In many cases, the regularity of the work of structures generating phonational vibrations is surprising (Fig. 21).

Fig. 21. The image of the pseudo-glottis and the kymographic cross-section of the vibrations showing their surprising regularity ⁵