## 4 Discussion
### Measurements on the VFs  
The described measurement on the VFs was a new method and to our knowledge the first of its kind. That makes a direct comparison difficult. We compared our approach with alternative methods: with other medical imaging techniques, with different placements of the US transducer, and with mechanical measurements on porcine VFs. First, we will compare our results to other medical imaging techniques, like OCT and magnetic resonance imaging (MRI) [55]. The optical methods (e.g., videostroboscopy, videobyography, and videoendoscopy) measure the movements and strains on the VF's surface [56]; whereas in our study, quasi-static loads on the VFs were evaluated. Due to the different nature of these methods, a reasonable comparison of the two approaches is not possible. Consequently, we will only discuss medical imaging techniques, which are able to image tissue in depth, notably US, OCT, and MRI. The OCT examination of the VFs is a promising approach, due to the high-resolution of the images [57; 58; 59; 60; 61; 62]. Imaging the different layers of the VFs, the OCT surpassed the US imaging [59; 62]. However, the penetration depth in the mentioned studies was a maximal of 2.6 mm, whereas the penetration depth of the US was much higher, in our case 20 mm [59]. As a consequence, deeper layers can be displayed. Therefore, the M. vocalis was clearly visible on our images (Figure 6c). The resolution loss through US can be compensated by using higher frequencies. Up to 50 MHz, with a penetration depth of 5 mm, have been used to examine the VF tissue [63]. The MRI images of the phonation process measured the anatomical structures of the VFs. Measurements of the elastic properties with MRI have been shown in other organ systems [64]. Furthermore, the vocal tract [65; 66] and artificial VFs [67] have been examined with MRI. The tissue needs to be excited by an internal or external source [64]. This would be one of the challenges of using magnetic resonance elastography to examine the VFs. Ultimately, it was not clear that there was a superior method. All methods had their advantages and disadvantages. The US had a deeper penetration depth, whereas the OCT had a good resolution, and MRI was able to show the VFs during phonation. Further research in the elastic imaging of the VFs is necessary, in order to choose the best method [9]. Our USE approach had the advantages of being minimally invasive, not exposing the patient to radiation, using a standard US machine, and being well studied. In addition, we will compare different transducer placements. In our study, we placed the transducer directly on the VFs, the so-called direct approach. However, other researchers placed the transducer on the skin of the patient, which is called the transcutaneous approach [10; 11; 13; 14; 15; 54; 68; 69; 70]. In the direct approach, it was easy to identify the VFs, whereas in transcutaneous approaches, the visualization rate was only 36.7% for the true VFs [71]. Those difficulties were based on two main factors: the VFs's anatomical position posterior of the thyroid cartilage and the physical limits of the US method itself. The penetration depth of the US decreased with the frequency, while the resolution increased [13]. Consequently, it was difficult to gain high-resolution images of the VFs in a transcutaneous approach. In the direct approach, we overcame both limitations: this allowed us to exactly place the transducer on the VFs, and we could use higher frequencies for good spatiotemporal resolution. Adversely, the spatial conditions in the human larynx are quite small, so a specially-designed transducer for in vivo measurements would be necessary. Additionally, in clinical use, the patient would have to be anesthetized, which makes the examination more invasive than the transcutaneous approaches. Furthermore, we compared our results to mechanical measurements of the VF's elastic properties.