Head Position Monitoring System - BIOSIGNAL 2012: Conference

BIOSIGNAL 2010

Head Position Monitoring System

P. Cech1, J. Dlouhy1, M. Cizek1, J. Rozman1, I. Vicha2 1 Brno University of technology/Department of Biomedical Engineering

2 University Hospital Brno/Department of Ophthalmology [email protected]

Abstract. Presented paper is focused on description of a system for head position monitoring. The head must be kept in proper position after special microsurgical operation called Vitrectomy. In the final phase of this operation a certain amount of vitreous is removed and replaced by an expansive gas (SF6) or oil bubble. The position of the treated eye and therefore the whole head is important for successful treatment. The presented system is capable to sensing the head position using 3D MEMS accelerometer. In case of improper head position the patient is immediately warned by sound alarm. This bio-feedback helps the patient in maintaining proper head position. The device also provides measured data logging for further analysis. Based on monitoring results the postoperative complications caused by improper head positioning can be distinguished from those caused by medical factors.

1 Introduction Human eye can be affected by many diseases or by injuries. Especially damaged retina can

lead to partial or full blindness. Some injuries can not be treated by medicament and therefore must be treated surgically. Pars plana vitrectomy is a special kind of retinal microsurgery operation. In the final phase of this operation a certain amount of vitreous is removed and replaced by an expansive gas (SF6 or C3F8) or oil bubble (Fig. 1). This bubble causes local pressure increasing which presses the retina parts together allows faster retina recovery. During several weeks the bubble is gradually absorbed and replaced by liquid. For optimal treatment results following the vitreoretinal surgery a patient must often maintain a head posture which allows the injected oil or gas bubble to push on the affected place on retina [1][4].

Fig 1. Position of the gas bubble inside the postoperative eye is dependent on resulting

acceleration vector direction.

In is almost impossible to monitor the bubble inside the eye. Because the eye is swollen after operation so its relative position to the rest of the head is constant. Therefore sensing the

ISSN 1211-412X 197 Analysis of Biomedical Signals and Images; 20: 197-202

BIOSIGNAL 2010 head position is a way to monitor the bubble placement inside the postoperative eye.

A head mounted electronic device described in the presented paper is capable of sensing the head position and warning the patient in case of an incorrect head position. The measured head position data are collected into a flash memory and can be analyzed and visualized by the affiliated software application.

2 Principle of measurement The position of the bubble inside the postoperative eye is mainly influenced by the

resulting direction of the applied acceleration (Fig. 1). Triple axis MEMS accelerometer is capable of sensing accelerations in 3 basic Cartesian components of the resultant applied acceleration in a frequency range from DC to several hundreds of Hz. The ability to sense acceleration in all three axis in low-frequency range makes them ideal component for measuring the resulting acceleration vector direction (Fig. 2).

Fig 2. The basic 3 acceleration vectors in Cartesian coordinates and resulting applied

acceleration vector.

For the purposes of head posture monitoring it is necessary to calculate the angle between the actual acceleration vector and an optimal acceleration vector. The optimal acceleration vector was measured in the beginning of the treatment with patient’s head in the correct position which was determined by the clinical specialist. Angular deviation from the desired head position is calculated by the microcontroller firmware using vector algebra:

1

11

0

00

01 )arccos(

a

aa

a

aa

aa

nn

nn

==

⋅=α (1)

Where α is the computed angular deviation, a0 is the acceleration vector initially measured

with the patient’s head in the correct position, a1 is the actual measured acceleration vector, a0n and a1n are the normalized vectors a0 and a1.

Triple axis MEMS accelerometer often introduces an offset error in one or more axes. The error is eliminated by calculating the offsets at the time of initial setup of the device and including them into further computations as correction constants. The method used for offset calculation is based on solution of the general equation of a sphere from 4 points on its surface [5].


BIOSIGNAL 2010

3 Measurement system design The basic idea is to design system with minimum weight and size, maximum operating

time and wireless communication allowing telemetric monitoring. A complete block diagram of the head position measurement system can be seen in Fig.3. The system consists of two parts: telemetric monitoring unit (provides measurement and data logging) and communication unit (provides communication between monitoring unit and PC).

Fig 3. The complete bocl diagram of the designed monitoring system.

The designed monitoring unit is based on a low power 8-bit microcontroller ATMEGA 164PV-10AU from Atmel. The special control software application is developed in c++ language so it can be implemented in another type of microcontroller.

The triple axis MEMS accelerometer LIS3LV02DL is used for measuring acceleration vector components. This accelerometer communicates trough I2C bus and is able to compensate nonlinearity, scale and offset errors in all three axes from constants either programmed by its manufacturer or supplied later by the user. Adaptive measurement range is set to ±2g. Averaging of the signal is implemented in the microcontroller for noise reduction. This low-pass filtering also reduces number of possible false alarms caused by rapid movements or short-term deviations from the desired head posture.

The device periodically wakes up from power-saving mode and samples accelerometer data. After calculation the angle of tilt, the deviation from optimal position is compared with allowed deviation (threshold angle) which was set by the ophthalmologist in the beginning of the treatment. An acoustic alarm is activated whenever the patient holds the head outside the desired range of positions. This sort of bio-feedback actively helps the patient in maintaining the optimal head position.

External Flash memory 24LC512 is used for storing data obtained during periodic head position measurement. The period of measurement sampling is user configurable and its default value is 1 second. The time and actual angle of tilt are stored to memory immediately


BIOSIGNAL 2010 in the case of improper head position. Otherwise the storing period is 5 minutes and it is also configurable. For acquiring real time information a PCF8583 circuit is used.

A 3,6V Lithium cell LS14250 is used as a power supply. Thanks to power-saving mode of the MCU and wireless module the power consumption is approximately 0,6 mA. Therefore only a single battery is sufficient for powering the device during whole monitoring period.

The telemetric monitoring unit contains communication interface. The communication in the license free 433 MHz band by NFM transceiver RXQ1 has been chosen. The data transmission rate is 20 kb/s. The transmitted data are interference secured with CRC16.

The communication unit contains identical wireless module RXQ1 and it is connected to PC via USB. The serial interface on RXQ1 module is transformed to USB interface via the FT232 circuit.

Fig 4. Hardware prototype: communication unit with USB (left) and telemetrich monitoring

unit (right).

4 Computer software A special software application (Head Position Monitor) was developed for system control

(Fig. 5). This application provides two main tasks. Firstly it provides communication with the device and secondly it provides measurement data analysis and visualization.

The communication between telemetric monitoring unit and communication unit is provided by wireless interface as mentioned above. For this purpose a special communication protocol has been developed. The communication mode is based on master-slave model, where the communication unit is the master and the monitoring unit is a slave device. It is possible to communicate with more than one monitoring unit. This is important when online monitoring is required. Typical situation of online monitoring is during hospitalization.

Once the communication between monitoring device and computer (via communication unit) is established, the user is informed about monitoring device status and can access the device.

Setting up the device must be done before new treatment period. The ophthalmologist determines the maximum allowed deviation from the optimal position and this value is set to the device as the angular threshold. The function of the monitoring device can by adjusted by following parameters:

• Sampling period – represents time period between each measurement (sampling) and it is set to 1 second.

• Storing period – represent time interval of records in memory. Each record consists of time and actual angle of tilt.

• Improper head position time before alarm activation – minimum time of improper head position before the alarm initiation.


BIOSIGNAL 2010 • Proper head position time for alarm termination – minimum time of proper head

position for alarm termination. The communication software is also capable of downloading measured data from the

device. Following a successful retrieval the data are saved to an external file for further analysis. The data file consists of basic patient information and whole treatment monitoring. Each patient has a single file so the analysis can be done any time it is necessary.

The Head Position Monitor also serves for data visualization and analysis. The measurement data can be loaded from device or from external file so the application can operate in online or offline mode. The application window (Fig. 4.) consists of an information area (left) and a graph area. The behaviour of measurement in time is displayed in the graph area. Time and date of measurement are shown on the horizontal axis. The tilt angle deviation is shown on the vertical axis. This area can be individually zoomed. The threshold angle defined by ophthalmologist is shown in the graph as horizontal line with specific value. All points of measurement exceeding the threshold angle are marked by a small square sign.

Fig 5. The main window of the Head Position Monitor with list of measured data.

The information panel on left side of application window contains device and monitoring data. User is informed whether the communication unit and the monitoring unit are connected. The monitoring device battery status is also displayed. Monitoring data panel contains information about patient and monitoring. All important characteristics of measurement are calculated based on the measured data. Following parameters are important for treatment efficiency assessment:

• Threshold angle – defines maximum angular deviation from required head position and was set by ophthalmologist in the beginning of monitoring.


BIOSIGNAL 2010 • Start of monitoring – date and time of beginning of monitoring. • End of monitoring – date and time of end of monitoring. • Overall monitoring time – is calculated as difference between the End of monitoring and

the Start of monitoring. • Out of threshold time – is calculated as a sum of all intervals when the angular deviation

is exceeding the threshold angle. This value is also expressed as a percentage of overall monitoring time.

5 Conclusion Presented system is focused on head position monitoring after special microsurgery

operation called Vitrectomy. For good treatment results, the treated eye and therefore the patients head must be fixed in corresponding position. This position can be described by sensing angle of tilt. Based on real testing the MEMS accelerometers appear to be an efficient method for head tilt sensing. Achieved accuracy of tilt angle measurement is approximately 1 degree, which is sufficient for clinical use demanding resolution of 10 degrees.

The device is designed to warn the patient by a sound alarm in case of improper head position. Such bio-feedback system may help the patient maintain proper head position so it can improve postoperative treatment. It also gives the patient possibility of free movement instead of special positioning apparatuses which are commonly used.

Based on monitoring data analysis, the postoperative complications caused by improper head position can be separated from those caused by relevant clinical factors. The monitoring system can provide a great feedback for the ophthalmologists, so they can assess the effectiveness of the treatment more precisely.

The clinical test are in progress since 12/2009. Te system is currently optimized based on feedback from continual clinical test. The measurement data analysis will be extended with statistical analysis. Also proper head fixation will be optimized.

Acknowledgement This work has been supported by Czech Science Foundation project No. 102/08/1373 and

by the research plan MSM 0021630513.

References [1] Vicha I., Rozman J., Vlkova E., Girgle R., Cizek M., Dlouhy J., Vaclavik V.: A New

Electronic system for Postoperative Monitoring of Patient’s Head Position. Abstracts of 8th European Vitreo-Retinal Society Congress, Prague 2008.

[2] Cullen R. Macular hole surgery: helpful tips for preoperative planning and postoperative face-down positioning. J. Ophthal. Nursing. Technol., 1998, 17, s. 179-181.

[3] Dahl A., Stöppler M. Retinal Detachment Causes, Symptoms, Signs, Treatment and Risks. MedicineNet.com. Online: <www.medicinenet.com/retinal_detachment/article.htm>. 2007.

[4] Jacobs P. M. Vitreous loss during cataract surgery: prevention and optimal management. Eye. Online: <http://www.nature.com/eye/journal>. Feb. 2008.

[5] Bourke P. Equation Of A Sphere from 4 Points On The Surface, Online: <http://local.wasp.uwa.edu.au/~pbourke/geometry/spherefrom4>, June 2002.


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Head Position Monitoring System - BIOSIGNAL 2012: Conference