Characterization of the head-to-trunk orientation with handheld optical 3D apparatus based on the fringe projection technique

Knowing the orientation of the head is important in many different fields such as human-computer interfaces, video compression, face recognition systems, biological experiments and medicine [1–3]. Head orientation measurement techniques are mainly based on a combination of different gyroscopes, accelerometers and electronic compasses. One such system is based on a three-axis accelerometer and a resonator gyroscope mounted on a hard hat [4]. The data acquired by the tilt sensor reached an accuracy of 0.63° and the gyroscope reached an accuracy of 0.52°. Another measuring system was composed of four sensors placed on the person and the stationary electromagnetic transmitter of a Fastrack system [5]. Its static orientation accuracy is 0.15° [6]. Another technique uses two inclinometers attached to each arm of a pair of eyeglasses [7]. Yet another uses two intersecting plastic protractors, a grid pattern and a laser mounted on a helmet [8]. The rotations in the sagittal and transverse plane were calculated employing the location of the laser illumination. The rotation in the coronal plane was calculated from protractors. Nintendo Wii remote controllers fixed to a mechanical frame and an IR beacon, mounted on the head were also used to measure the head posture. The standard deviation of 15 repeated measurements ranged from 0.56° to 2.70° [3]. The head orientation can be also determined by using 2D camera. One technique is based on finding the best translation and rotation of the pre-measured 3D shape of the subject’s face according to the acquired 2D image [2]. The reported standard deviation is 2.3 mm in position and 0.47° in orientation. The second 2D camera based technique uses a template tracking algorithm that automatically extracts neck angles from sagittal videos [9].

Although the presented systems achieve high levels of measuring uncertainty, our concerns are related mostly to the fixation of the different markers on the subject’s body. In our opinion it is very hard to ensure that the junction between the marker and the human body is rigid. The systems are also usually quite large and difficult to transport. Other weakness of the above mentioned methods is impossibility to distinguish between the head rotations caused by the neck and the rotations caused by the torso twisting, because the torso rotations are not monitored.

This paper presents a novel method that enables the measurement of the head posture relative to the trunk. The method has been developed with the aim of characterizing the effects of cervical dystonia, a disease which affects the patient’s head posture and the range of movement as well as their quality of life in many other ways [10, 11]. The main effects on the head posture are the head lateral tilt in the body’s coronal plane (laterocollis), head rotation in transversal plane (torticollis) and flexion (anterocollis) or extension (retrocollis) in the body’s sagittal plane. Most patients suffer from a combination of two or more effects [5]. The presented method utilizes optical 3D measurements of the upper trunk and head to measure these effects. The relative rotation of the head with respect to the trunk is calculated from the alignment of the trunk and head measurements to the reference one.

The uncertainty of the proposed method was assessed by three experiments. In-vitro verification was performed using the mannequin to evaluate the uncertainty of the method by comparing the results with the reference instrument. In-vivo verification was performed in order to evaluate the human influence on the measurement’s uncertainty, and in-vitro vs. in-vivo comparison to prove the relevance of in-vitro results. The gathered data was analysed using standard statistics’ approaches; unless noted otherwise, the precision of the measurements is defined as one standard deviation.

The results of the in-vitro mannequin measurement are shown in Table 1. The reference instrument was the XOT, which recorded the data at each head position for one minute. 3D measurements were repeated twenty times using LS and forty times using HHC. The average Euler angles and standard deviations are shown for each instrument and for each head position. From these results we see that the XOT and LS have approximately the same precision (around 0.12°). This means that the proposed method of the head orientation extraction from the 3D shape works well. Additionally, we noticed that the HHC has somehow a lower precision (around 1.6°), which is attributed to the lower 3D measuring precision. The uncertainty of the calculated Euler angles compared to the XOT is presented by the Bland-Altman plots in Figure 7. The angle differences Δφ, Δθ and Δψ are defined as:

$\mathit{\Delta }\mathit{\phi }={\mathit{\phi }}_{\mathit{\text{XOT}}}-{\mathit{\phi }}_{3D}$

(12)

$\mathit{\Delta }\mathit{\theta }={\mathit{\theta }}_{\mathit{\text{XOT}}}-{\mathit{\theta }}_{3D}$

(13)

$\mathit{\Delta }\mathit{\psi }={\mathit{\psi }}_{\mathit{\text{XOT}}}-{\mathit{\psi }}_{3D}$

(14)

Position			XOT	LS	HHC
			mean ± std [°]	mean ± std [°]	mean ± std [°]
Reference		φ	0.00 ± 0.12	0.01 ± 0.06	2.75 ± 0.98
		θ	0.00 ± 0.12	0.00 ± 0.05	−1.01 ± 0.56
		ψ	0.00 ± 0.13	0.00 ± 0.14	0.52 ± 0.37
Natural		φ	−8.23 ± 0.11	−8.47 ± 0.05	−5.47 ± 1.09
		θ	−26.68 ± 0.12	−26.63 ± 0.06	−26.14 ± 0.82
		ψ	1.31 ± 0.13	1.62 ± 0.13	1.45 ± 1.24
Rotation	left	φ	1.03 ± 0.11	1.37 ± 0.14	1.60 ± 1.92
		θ	−50.82 ± 0.12	−49.79 ± 0.12	−50.43 ± 1.90
		ψ	7.12 ± 0.12	6.91 ± 0.18	7.91 ± 1.92
	right	φ	−2.02 ± 0.11	−3.31 ± 0.16	−1.69 ± 1.31
		θ	40.00 ± 0.11	41.25 ± 0.07	39.88 ± 0.43
		ψ	−0.53 ± 0.13	0.66 ± 0.21	−0.07 ± 0.69
Lateral tilting	left	φ	3.13 ± 0.11	0.87 ± 0.09	4.06 ± 0.58
		θ	−1.19 ± 0.11	−0.99 ± 0.06	−3.16 ± 0.33
		ψ	35.55 ± 0.14	37.81 ± 0.11	39.81 ± 0.85
	right	φ	−0.04 ± 0.11	−0.02 ± 0.11	2.11 ± 1.05
		θ	1.93 ± 0.11	2.13 ± 0.10	4.60 ± 1.06
		ψ	−33.65 ± 0.12	−35.14 ± 0.14	−34.53 ± 0.88
flexion/extension	flexion	φ	27.87 ± 0.11	27.85 ± 0.06	27.10 ± 0.53
		θ	−0.73 ± 0.10	1.07 ± 0.06	0.04 ± 0.39
		ψ	4.08 ± 0.14	5.59 ± 0.16	6.33 ± 0.36
	extension	φ	−31.58 ± 0.13	−31.36 ± 0.09	−29.56 ± 1.01
		θ	−7.09 ± 0.12	−7.88 ± 0.06	−6.41 ± 0.69
		ψ	−3.7 ± 0.16	−2.02 ± 0.14	−2.69 ± 0.54

where angles with the index XOT were measured with XOT and angles with the index 3D were measured with LS in Figure 7a and HHC in Figure 7b. The average angles of the position were determined as the average of the angles measured with XOT for each position.

Figure 7 indicates that the angle differences measured by LS are approximately half of the ones measured by HHC. This is a consequence of higher resolution of LS relative to HHC as they both use the same 3D surface reconstruction method and algorithm for the head orientation characterization. We expected less accurate results at a higher rotation of the head around the Y axis, due to the smaller area of overlapping between the reference and the current head surface. A healthy person can rotate the head up to 80° left or right [21]. In Table 1 we see that the standard deviation of the measured angles is indeed related to its amplitude. The left rotation (see Table 1, row “left rotation”, column “HHC”) has an amplitude of −50° and a standard deviation of 1.9° in case of using the HHC apparatus. On the other side, the right rotation has an amplitude of 40° and a standard deviation of 0.8° (see Table 1, row “right rotation”, column “HHC”), which confirmed our expectations. This effect can be minimized by adjusting the position of the measuring system in a way, that the impact angles between the optical axis of the measuring system and the surface of the head and trunk are as close as possible; if the head is rotated by 80° in relation to the trunk, the measuring system has to be placed in the direction rotated by 40° in relation to the trunk. The same problem can occur during flexion and extension, but those angles are usually smaller, up to 60° [21]. The least problematic is lateral tilt. Even in the case of large tilting angles, the impact angle between the measured surface and the optical axis of the measuring system does not change much. The problem of reduced overlapping is also insignificant.

The ANOVA analysis of the differences between main measured angles for all measuring instruments showed, that differences were significant at all positions. The critical F(2,57) was 3.1. Calculated F values were 6.7, 10.0, 271.1, 430.6, 35.2, 33.0, 74.6 for natural position, left rotation, right rotation, left lateral tilt, right lateral tilt, flexion and extension, respectively. Bonferroni corrected t-tests also showed that all differences were significant, with the exception of the natural position between XOT and LS (p = 0.12), left rotation between XOT and HHC (p = 0.56), right rotation between XOT and HHC (p = 0.11) and flexion between XOT and LS (p = 0.14). We think that the differences were caused by the non-ideal alignment between CS of the both measuring instruments and by the limited measuring accuracy.

The proposed method enables a satisfactory level of measurement precision (2° for in-vitro and 3° for in-vivo measurement). It is a bit lower compared to the methods, presented in the Background section (from 0.15° to 2.7°) [2–9], but it has other advantages, like compensation of the whole body rotations, higher system portability, and finally no need for any device attached to the patient’s body.

Since the daily clinical practice needs a short, simple and precise tool for rating the improvement or deterioration of the cervical dystonia [22, 23], we believe that the presented method has great potential to replace the currently used measuring systems.

The new non-contact method for measuring head-to-trunk orientation is based on 3D surface acquisition. It enables the compensation of the movement of the measuring instrument or the subject’s body as a whole, is non-invasive, requires little additional equipment and causes little stress for the subject and operator. A subject is measured using the hand-held 3D apparatus which is based on the fringe projection technique. The apparatus is composed of a commercial DSLR camera and custom projection optics, which uses the light from the camera’s built-in flash. It is also highly portable and very user-friendly. The algorithm of the head-to-trunk orientation extraction is based on the separate alignment of the trunk and head on the reference measurement. The final results are the angles of rotation around each coordinate axis. Evaluation of the proposed method shows that the in-vitro measuring uncertainty is no more than 2° and in-vivo accuracy is less than 3°. The compassion of in-vitro vs. in-vivo measurements showed, that only the difference between flexion/extension measured angles was statistically significant. The differences between means were less than 1° for all ranges. The main contribution factors to the measurement’s uncertainty are the accuracy of the 3D apparatus and the subject’s time-varying deformation uncertainty. The measuring uncertainty is acceptable for the clinical use of the system, which can greatly improve the assessment of cervical dystonia in daily clinical practice.