Methods of Measurement and Evaluation of Eye, Head and Shoulders Position in Neurological Practice Part 3

Measurement of eye and head position

Despite the fact that an accurate method for measuring the head position and the eye position could contribute to diagnosis of vestibular system, this issue has not been systematically studied (Hozman et al, 2008).

Horizontal and vertical eye movements can be measured by an image of the eye (Moore et al, 2006) by detecting the edges of the pupil (iris) and fitting them into an ellipse (Li, 2006). The main aim of the analysis of eye movements is to obtain the centre of the pupil or the iris. The torsion measurement needs high quality iris description. The video system PAL (NTSC) record video with frequency 50 Hz (60 Hz) non-interlace. These video systems are too slow to capture images of the eye movements, e.g. torsion iris description. In medical practice documented eye movements were with frequency approximately 200-250 Hz. These movements present angular change approximately 400 – 450 °/s.

We used the detection method which searches interface points between the pupil and the iris or between the iris and the sclera. The points are base of the mathematical function (e.g. circle or ellipse). Goal of our solution is to use eye movements’ detection in comparison with the stimulation scene. The scene can be showed on the LCD screen or through the special HMD display unit in 2D or 3D space.

We used a new system based on finding the outline pupil of the eye. We applied modified Starburst (Duchowski et al, 2009) algorithm (Ruian et al, 2006), which we used in the IR spectrum or in the visible spectrum. The system and algorithm was first published by Iowa University in 2006 (Li, 2006). Thanks to the special 3D HMD projection displays we used Starburst algorithm for measuring in the IR spectrum and appropriate LED diode to illuminate the eye. The method is called active, because the eye is scanned in the infra red spectrum. Goal of eye movements’ measurement was location of the centre of the pupil area. Method of finding the margins of the pupil (IR spectrum) or the iris (visible spectrum) is limited by quantity of the rays. The starting point shoots the rays to generate candidate pupil points. The candidate pupil points shoot rays back towards the start point to detect more candidate pupil points. This two-stage detection method takes advantage of the elliptical profile of the pupil contour to preferentially detect features on the pupil contour.

Location of the pupil center with the rays.

Fig. 6. Location of the pupil center with the rays.

The circle (see Figure 6) shows the centre of the pupil after the second iteration (and after the next iteration) and includes determined points. The iteration to find the centre of the pupil was stopped when the detected centre of the new points turned less than d=10 pixels. Thanks to exponential calculation the error of the pupil centre is about ±10 pixels of the whole circle bearings and it is important from the point of view of the found points’ fit resulting ellipse. At the end, we can find centre of the pupil from the resulting ellipse.

There are more possible methods how we can put together the resulting ellipse. We chose method (Ruian et al, 2006) Random Sample Consensus (RANSAC) to solve the problem with large error points (Lee & Park, 2009). The method RANSAC is an efficient technique for completion of the model in the presence of large, but unknown percentage outlines in sample measurement. In our case all internal found points are probable points that correspond with outline of the pupil. RANSAC algorithm was used to estimate parameters of the mathematical model from a set of observed data which contains outliers. On the basis of the MATLAB documentation we used optimal mathematical model to create the ellipse -Nelder-Mead’s algorithm.

For the eye stimulation we used commercial HMD system eMagin Z800 3DVisor personal display with integrated head tracker which can measure head position in the 3D space. The Z800 3DVisor is the personal display system to combine two OLED (organic light-emitting diode) micro displays with stereovision 3D capabilities. Stereo vision refers to the human ability to see in three dimensions and most often refers to depth perception (the ability to determine the approximate distance of objects). Stereovision 3D provides this experience by delivering two distinct images simultaneously on two separate screens, one for each eye. The Z800 3DVisor personal display is used to stimulate the eye in 2D or 3D space. The position of eye and the position of head can be recorded simultaneously by the video camera and integrated head tracker to the laptop.

The HMD displays eMagin Z800 3DVisor’s integrated head tracker uses MEMS (micro-electromechanical system) accelerometers and gyroscopes to detect motion. The head tracker features three gyroscopes, one each for the x-, y-, and z-axis. In addition, the head tracker contains corresponding compasses and accelerometers to ensure performance over varying forms of motion. Such equipment has not been used before in medical practice. From the point of view of contemporary technology there is a possibility to use more accurate miniature 3D inertial measurement unit/motion sensors (IMU) with accelerometer, magnetometer and gyroscope (for example Xsens motion technologies) and custom made Head Mounted Display (HMD). We used the head tracker for the measurement of head position.

For acquisition of the head motion we programmed special software based on Z800 3DVisor SDK 2.2. The software retrieves position of the head from the build-in head tracker through the USB connection and saves the measured results in to the CSV (comma-separated values) file. The result of measurement can be presented graphically as a graph of the head position. By this set we are able to measure eye and head movements continuously and simultaneously.

During the measurement problematic parts had to be solved before the next biomedical tests. The problems were with weight, sharp edges on the semi-permeable mirrors and minimal place between personal display Z800 3DVisor and the eyes.

On the previous base type we made the new projection displays, which allowed tracing projection on the LCD monitor or the projection screen in visible spectrum. Detection algorithm can measure the eye position and the scene position. The video files of positions are merged into the date file. The second version of projection displays has lower weight, does not contain any sharp edges and includes the cameras, which are connected with the help of USB (Universal Serial Bus) interface and does not use any special recorder. Power supply is solved over the USB port. We used record software TVideoGrabber. The software TVideoGrabber can set capture parameters (30 FPS – Frames Per Second, 640 x 480 pixels, RGB24, data format AVI). We used an external flash from photographic apparatus for synchronisation between two cameras (In the future we will use the TVideoGrabber component with more threads. The threads will start recording from several video sources at the same time).

The next type of our projection displays is designated for measuring in the IR spectrum. The third projection displays use the IR USB cameras which record eye movements. The type of these projection displays must use cameras which support eye movements scans in the IR spectrum because lighting is already poor. This type of projection displays combines a unique system for measuring eye movements and head position with 2D or 3D stimulation.

The specialized glasses - projection displays for neurological examination can be used without the LCD monitor thanks to build-in HMD 3D projections displays eMagin Z800 3DVisor. These projections displays can be used as a mobile system as well.. We use experimental systems for monitoring eye movements with different luminous conditions (in the visible spectrum or in the IR spectrum) or using different stimulation sources (e.g. record eye movements at specific activities – „eye Holter", long time eye movements record, 2D and 3D stimulation et al.).

Interpretation and evaluation of eye, head and shoulders position

For analysis of the positions we use simple methods based on combination of the basic methods of evaluation of individual body parts. Below we describe methods for accurate interpretation of the measured data. Given that the systems provide the processed data, the assessment is simple for physicians. Physicians only need to observe the conditions of measurement, such as the precise adjustment of the system before the measurement. They also need to respect the maximum certified accuracy of systems.

Evaluation of head and shoulders position

It is mathematically a simple problem to determine the inclination, rotation and flexion/extension of the head from photographs and by gyro-accelerometer sensor. The angle values are measured and transformed automatically to physical coordinate system. The measurement process is usually carried out according to a predefined procedure to be followed, see Figure 7. The process is based on two main steps/parts of measurement and computational algorithm. First part of algorithm is designed for the precise adjustment of the system. The second part is intended to measure patient’s body segments and to calculate of the angles in physical coordinate system.

Flowchart of clinical measurements using designed camera system.

Fig. 7. Flowchart of clinical measurements using designed camera system.

The described systems provide direct information for physicians on the current position of the patient’s head and patient’s shoulders represented by the angles. There is no further information processing and the physician may use the data to evaluate patient’s health. The designed systems measure head position with precision of 0,5° (Hozman et al, 2008) in three planes (rotation -yaw, flexion-pitch and inclination-roll). Our experimental measurement of the head position was completed with measurement of subjective perception of vertical (SPV). The subject tried to align a needle to vertical position when peering into white sphere. Final angle of the needle was measured. The measured data shows that healthy subject holds his head aligned with physical coordinate system in the range of ±5 degrees for inclination. The set of data was measured on recruited volunteers. The results also predict that there is a correlation between values of inclination and SPV.

Eye and head movement analysis

The Goal of our new designed methods is to use eye movements’ detection together with the stimulation scene. Thanks to the special 3D HMD projection displays, we used Starburst algorithm for measuring in the IR spectrum and appropriate LED diode for illumination of the eye (Charfreitag et al, 2008). The Goal of the eye movements’ measurement was to locate the centre of the pupil area (Stampe, 1993). We used a new system based on finding the contour line of the pupil of the eye. Finally, at the end, we can find the centre of the pupil from the resulting ellipse at the camera coordinate system i.e. shots, see Figure 8.

The second part was to use the headtracker to measure the head position. The first measured values were used as initial, i.e. zero and were used as correction for all subsequent values. The new systems provide direct information for physicians on the current position of the pupil centre represented by pixels or millimetres and patient’s head represented by three angles, see Figure 9. There is no further information processing and the physician may use the data to evaluate patient’s health. The head position was measured by modified 3D HMD (Z800 3DVisor ) with precision of 1.0° in three planes (Charfreitag et al, 2009). Thus, we can study the three dimensional motion of head defined by three angles – inclination, rotation and flexion/extension. By this method we can also study, analyze and measure eye and head movements continuously and simultaneously.

Example of graph of pupil center movements

Fig. 8. Example of graph of pupil center movements

Example of graph of head movements

Fig. 9. Example of graph of head movements


In this topic we have described related works and designed special equipment and measurement methods for very accurate evaluation of eye, head and shoulder position in neurological practice. Possible applications and perspectives for clinical practice are also described in the topic.

We have described systems and sets of procedures for evaluation of the inclination, flexion and rotation of the head and the inclination and rotation of the shoulders with resolution and accuracy to 2°. This accuracy is the minimum accuracy required in clinical practice. The described ways of measuring and evaluating eye, head and shoulder positions could also be applied in other areas of medicine and science.

Our designed systems are based on cameras or/ possibly on gyro-accelerometer (inertial) sensors. The new two or three camera equipment designed to measure the head and shoulders positions is cheaper and more accurate than sophisticated systems which use accelerometers and magnetometers. The second advantage of our camera system over conventional and commercial systems such as Zebris motion analysis system (zebris Medical GmbH), LUKOtronic AS100/AS200 (Lukotronic Lutz-Kovacs-Electronics Oeg.) or sonoSens Monitor (sensomotion, Inc.) is that it can measure a patient without the influence of mechanical elements on patient’s body segments or that the system allows direct detection of anatomical axes of patient’s head and shoulders, which cannot be done when using current systems (Hozman et al, 2005). The systems based on two cameras have cameras placed on both sides (lateral profiles) or in front and above the patient. This is a very important advantage for medical doctors, because they can make various examinations which require open space in front of the face. Our systems based on combination of infrared cameras and inertial systems are also sufficient and more accurate and cheaper than commercial systems for broader use than just to analyze the position of the head and shoulders.

The measurement results of mean values of the head position being (100 healthy controls): retro flexion 21.7°; inclination to the right 0.2°; head rotation to the left 1.7°. The rotation measurement has a greater error in comparison with the inclination and flexion/extension measurement (Kutilek & Hozman, 2009).

We have also described related systems and designed system for monitoring eye movements. Our equipment designed for measurement of the eye and head movements is based on display units – specialized glasses with eMagin 3DVisor. We modified the specialized projection displays for neurological examination which can perform measurements using a variable set of visual stimuli and active head movements. The solution combines system for measurement of the eye movements and the head posture in the 3D space with 2D or 3D eye stimulation. We came to the conclusion that it is possible to join together the two important and closely related methods for the measurement of the human vestibular system.

A result of this study is the recommendation to use the video cameras with higher frequency (approximately 200 Hz) for the measurement of eye movements and the head tracker with lower dynamic error (less than 0.3°/s) for the measurement of head position. The overall accuracy of our designed system could increase significantly because the accuracy of the method alone is in eights of degree per the ten measurements. This is the dynamic error due to the low-cost head tracker which needs long time to stabilise after the previous measurement.

Above described and designed ways of measuring eye, head and shoulder position and motion could also be applied in other areas of engineering, medicine and science. Our systems can be used anywhere to study the posture of a person.

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