Participants were seated in a chair placed within the C-arm of the fluoroscope. A calibration disc of known size was taped to the participant’s lateral face for calibrated post hoc measurement. Ultrasound and videofluoroscopic imaging was performed concurrently. Data collection included measures of hyoid excursion and thyrohyoid approximation (Fig. 1). Order of data acquisition was randomised between the two measures. Data acquisition involved swallows of saliva, 5 mL water, and 5 mL apple sauce (Wattie’s™). To minimise radiation exposure, participants performed each measure once only per consistency (six swallows per participant in total). The order in which bolus types were presented was kept consistent across participants and sessions as the sequence may influence the findings. Saliva was presented first followed by water and puree to reflect the order routinely followed in clinical practice. Quantities were measured with a syringe; liquid boluses were offered in a 20 mL plastic cup and puree boluses with a spoon. Participants were instructed to hold the bolus in the mouth. Once the scanner was placed, they were asked to swallow as naturally as possible, without accommodating head position to the scanner. The principal researcher collected data from all participants. Ultrasound data collection involved scanning, image selection from the recorded video and measurement. Image selection and measurement were performed online on the iPad immediately after each swallow. A randomly selected 20% of swallows per measure and instrument were measured offline on a second occasion by the principal researcher for assessment of intra-rater reliability. A period of at least 11 days between data collection and offline measurement was implemented to avoid recall. A second rater measured another randomly selected 20% of the data offline for assessment of inter-rater reliability. For both instruments, reliability assessment included image selection from the recorded cine and measurement. Raters were blinded to the measurements performed by the other rater. Both researchers were trained in ultrasound and videofluoroscopic data extraction for purpose of this study. Training was consensus based rather than standardised. Prior to assessment of inter-rater reliability, raters agreed on measurement techniques with a written reference document with measurement guidelines established.

Radiographic image depicting the structures of interest

Aquasonic transmission gel was used for acoustic coupling before the ultrasound scanner was placed manually on the skin surface of the participant’s neck. Consistent scanner position was maintained and minimal pressure against the skin was applied throughout data acquisition [12] to minimise the impact of the scanner on swallowing function. Specific pre-set exam types with customised features, such as scanning depth or gain were selected per measure. For optimal image quality, manual accommodation of depth, gain, and display brightness was performed per measure and participant if required. Structures were viewed upside down according to previous studies assessing validity and/or reliability of hyoid excursion [13] and thyrohyoid approximation [8]. Swallowing events were recorded as individual video-clips of 20 s. For imaging hyoid excursion, settings included a depth between 7 and 10 cm, a frequency of 4 MHz, and single focus. Sagittal sonography was performed with the scanner positioned at right angles to the floor of mouth muscles. The scanner was placed midline, capturing the acoustic shadow of the mandible on one side and on the other side the acoustic shadow of the hyoid [13]. Inferiorly, the surface of the tongue was visible. For recording of thyrohyoid approximation, settings included a depth between 1–7 cm, a frequency of 5 MHz, and dual focus. The scanner was positioned midline or slightly off midline in sagittal plane approximately overlying the thyrohyoid muscle. This allowed visualisation of key features, including the acoustic shadow of the hyoid on one side, and the acoustic shadow of the thyroid cartilage on the other side [9]. Figure 2 depicts the placement of the scanner during data collection.

Scanner placement for assessment of hyoid excursion (at the left of the image) and thyrohyoid approximation (at the right of the image)

For videofluoroscopic examination, a low dose continuous cine mode was selected. Recordings comprised mandible (anteriorly), nasal cavity (superiorly), cervical spine (posteriorly), proximal oesophagus, and trachea (inferiorly).

For online measurement of ultrasound data, specific images were selected after each swallow by navigating through the recorded video on the iPad. For data extraction of hyoid excursion, two images were selected, one representing the peak of hyoid displacement and one with the hyoid at rest position post swallow. The peak position image was defined as the one showing the smallest distance between shadow cast by the hyoid and shadow cast by the mental spine of the mandible (Fig. 3). The extent which the hyoid travels was expressed as a percentage of the distance at maximal displacement from rest ((rest distance between mental spine of the mandible and hyoid – maximal distance between the two structures)/rest distance between the two structures) × 100 [13]. Data extraction of thyrohyoid approximation involved selection of one image showing hyoid and thyroid cartilage maximally approximated intra-swallow and the other with the two structures at rest, post-swallow (Fig. 4). Thyrohyoid approximation was expressed as a percentage of the distance between thyroid cartilage and hyoid at maximal approximation from rest [8, 9].

Sonogram of the hyoid at rest position (a), and at maximal displacement (b) for evaluation of hyoid excursion. For measurement, the line of best fit (Line A) was drawn along the anterior border of the shadow cast by the hyoid (the shadow at the right of the images). For Line B, one calliper was placed at the posterior border of the onset of the shadow created by the mental spine of the mandible (shadow on the left of the images). The second calliper was placed at the intersection point with Line A at the onset of the shadow cast by the hyoid

Sonogram of the distance (Line D) between hyoid and upper border of thyroid cartilage at rest (a) and at maximal approximation (b) [8, 9] for evaluation of thyrohyoid approximation. One calliper was placed at the beginning of the anterior border of the shadow of the hyoid (shadow on the left of the windows) or at the opacity representing the hyoid. The other calliper was placed at either the onset of the shadow cast by the thyroid cartilage (shadow on the right of the images) or at the bright opacity at the superior border of the thyroid cartilage. Of each of the two points, the one that was visible in both images was selected

Videofluoroscopic recordings were analysed using ImageJ software (U.S. National Institutes of Health, Bethesda, Maryland, USA). Images were calibrated for measurement. For data extraction of hyoid excursion, the percentage change from the position at rest post swallow and at maximal anterior displacement was calculated (Fig. 5). For data extraction of thyrohyoid approximation, one image was selected depicting the hyoid and thyroid cartilages at maximal approximation intra-swallow and one showing these structures at rest, post-swallow (Fig. 6). Percentage approximation was calculated from rest.

Hyoid excursion assessed using videofluoroscopy. Dashed measurement lines for calculation of the distance from hyoid to mandible at rest (a) and at maximal hyoid displacement (b). The white drawings were used to define the measurement point at the mandibular prominence. The inferior-anterior part of the hyoid and the mandibular prominence were used as measurement points to approximate the measurement methods of the radiographic images to those for ultrasound and based on reported methodology in the literature [23]

Thyrohyoid approximation assessed using videofluoroscopy. Dashed measurement lines depicting the distance between anterior-inferior aspect of the hyoid and the anterior edge of the inferior end of the thyroid cartilage at rest (a) and at maximal approximation (b). As opposed to ultrasound, the inferior rather than the superior border of the thyroid cartilage was chosen as the upper border was often not sufficiently distinct for measurement. Additionally, even if it was visible, the upper border of the thyroid cartilage superimposed the hyoid at maximal excursion in some cases; hence, calculation of percentage approximation would yield more than 100%

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