SWAY MONITOR

Abstract: A sway monitor for clinical use having two or more ultrasound transmitters (1A, 1B) and associated receivers. Rapid pulses of ultrasound are directed towards a subject (2) whose swaying movements towards or away from each transmitter are assessed by measuring the time taken for the reflected beam to be detected. The data may be continuously analysed and stored by means of a computer system (17) and/or recording equipment.

Description: SWAY MONITOR This invention relates to a sway detecting and monitoring apparatus, particularly to such an apparatus for use in assessing drug effects on human volunteers or patients. The upright stance in the human is an unstable position and therefore always accompanied by involuntary motion. This postural sway is a reflection of the noise and regulatory activity of the several control loops involved in the maintenance of balance. The principal sensory systems involved in this are proprioception, the vestibular system and vision, and their afferent neuronal pathways within the central nervous system. The afferent and efferent neuronal pathways involve spinal cord, brainstem, cerebellum, midbrain and sensorimotor cortex. Interference in any of these systems can therefore affect balance, and body sway is often used as an index of psychotropic drug activity in normal subjects. It has also been proposed as an objective index of the extent of progression of, and drug effect on, diffuse brain diseases such as dementia [Visser, H. (1983) Age and Ageing, 12, 296-301]. Numerous methods have been used to obtain a measure of body sway, and have included: the attachment of a thread with weight and pointer to the subjects clothing, the use of pulleys, the employment of balance platforms that resolve sway components from vertically applied forces, the use of cine photography or more recently video recordings, doppler radar ranging, and collimated light sources with linear photodiode arrays. Most of these methods are expensive, cumbersome and require complex preparatory procedures before useful results can be obtained. Ultrasound has been used previously to measure axial eye movement [Haines, J. (1977) Ultrasound in Med.Biol., 3 ^ , 39-45], and facial movement [Haines, J. (1983) Med.Biol.Eng.Comput. , 21_, 105-106], The present invention provides an ultrasound technique for measuring and monitoring body sway. According to the present invention there is provided a sway detecting and monitoring device comprising a) two or more ultrasound transmitters arranged to direct an ultrasound beam across a subject area b) an ultrasound receiver associated with each transmitter to detect ultrasound reflected from a subject in the subject area c) ranging means to compare transmitted and received ultrasound from each transmitter/receiver pair and to output position data related to the position of a subject in the subject area, and d) display means to display position data output from the ranging means and/or e) recording means to record position data output from the ranging means. The term "ultrasound beam' as used herein is intended to describe a series of rapid pulses of ultrasound frequency, rather than a continuous emission of ultrasound, and detection of the swaying movement is based on the principle that the time taken for the ultrasonic pulses to travel from the transmitter and return to the receiver after reflection from the subject will vary as the subject sways. Accordingly, more specifically, the present invention provides a sway detecting and monitoring device comprising a) two or more ultrasound transmitters arranged to direct an ultrasound pulse across a subject area and generate a first timing signal when the pulse is transmitted b) an ultrasound receiver associated with each transmitter to detect ultrasound reflected from a subject in the subject area and generate a second timing signal when a reflected pulse is detected c) ranging means to assess the time delay between the first and second timing signals from each transmitter/receiver pair and, based on the time delay, to output position data related to the position of a subject in the subject area, and d) display means to display position data output from the ranging means and/or e) recording means to record position data output from the ranging means. Normally each transmitter/receiver pair is in the form of a single combined unit commonly known as an ultrasonic 'sensor'. In such sensors, the transmitter and receiver are associated with a transducer such that during transmission, the transducer acts as an emitter of ultrasonic pulses. After transmission, the transducer is switched to operate with the receiver (the transducer being used as a microphone); after receiving reflected ultrasound, the transducer is switched to operate with the transmitter again, and so on. In normal practice, two sensors are used, each set at right angles to the other. Thus, for human subjects, the sensors may be placed at right-angles, suitably some 700mm from the body and at a height of approximately 1300mm. This arrangement permits recording of both lateral (sideways) and saggital (front/rear) components of sway. Suitably the frequencies emitted by the transmitters are in the range 50 - 70 KHz, either as a 'chirp' (i.e. a burst of pulses comprising several frequencies), or as a pulse of a single frequency. Normally, each transmitter is activated (i.e. caused to emit an ultrasonic pulse or burst of pulses, typically of ca. 1 millisecond duration) approximately 15 times per second. It is desirable that each transmitter be activated alternately in order to avoid false signals by reflection of ultrasound generated by the other transmitter. Outputs from the ranging means may be digital, i.e. pulses whose duration is proportional to the distance swayed, or made available as analogue voltages. In one embodiment of the invention the deviation of the subject from a fixed 'zero* position may be continuously monitored. Thus the delay between the first and second timing signals is compared with the delay measured when the subject is at a predetermined zero position. In a convenient method of obtaining this comparison, with the subject at the zero position the first timing signal is electronically delayed until it coincides with the second timing signal and the ranging means is adjusted to give a nil output at this coincident position. The swaying of the subject thus causes the second timing signal to be advanced or retarded relative to the first timing signal. The output from the ranging means is related to the advancement or retardation of the second timing signal and is thus directly proportional to the distance of sway from the zero position. In an alternative, preferred, embodiment of the invention, the deviation of the subject from each sensor is continuously monitored. In this embodiment, the delay between the first and second timing signals may suitably be analysed by a microcomputer and a 'sway index' may be generated which quantifies the swaying movements made by the subject with respect to each sensor over a period of time. The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a system diagram of a sway monitoring device in accordance with the invention; Figure 2 is a system diagram of a sway monitoring device in accordance with the invention; and Figure 3 is a system diagram of an integrated micro-processor controlled 'sway index' measurement and display unit. Referring to Figure 1, two ultrasonic sensors 1A (for monitoring saggital sway) and IB (for monitoring lateral sway) are arranged at right angles at an appropriate distance from a human subject 2. Suitable sensors are commercially available, for example from Polaroid U.K. Ltd. The sensors 1A and IB are controlled by an alternate drive clock 3 by means of which they may be activated alternately, each sensor being activated approximately 15 times per second. On activation, the sensors 1A and IB produce a burst of pulses (represented by 'waves' 4A and 4B) at ultrasound frequencies of 50, 53, 57 and 60 KHz. The duration of the pulses produced by each sensor is approximately 1 millisecond. Immediately the pulses 4A or 4B have been transmitted, a 'sound transmitted' signal pulse is sent from the sensor (1A or IB) responsible for the transmission to its respective 'sensor interface' (5A or 5B), and the sensor (1A or IB) is then temporarily switched to become a sensitive receiver using the transducer as a microphone. When an echo from the transmitted pulse (reflected from the subject 2) is detected by sensor 1A or IB, a logic pulse is generated and fed to the corresponding sensor interface 5A or 5B. The leading edge of such an 'echo - received' signal pulse is used, in conjunction with the leading edge of the 'sound - transmitted' signal pulse, to derive from each sensor interface, 5A and 5B, an output signal proportional to the distance between the corresponding sensor, 1A or IB, and the subject 2. When a subject 2 is standing at his or her zero position, variable delay circuits 6A and 6B are adjusted by means of a simple zero control, 7A or 7B, until the 'sound - transmitted* output signal from each sensor interface 5A and 5B is sufficiently retarded for the corresponding 'echo - received' signal to be received simultaneously by respective differential time comparators 8A and 8B. At that point no output pulses are emitted from the differential time comparators 8A and 8B. The differential time comparators 8A and 8B are connected to voltage converters 9A and 9B and thus, in practice, the zero position may be readily defined ' by adjusting the zero controls 7A and 7B until respective panel meters 10A and 10B show no deflection from median positions 11A and 11B. A swaying subject 2 will then cause the 'echo' signal to arrive early (when subject 2 moves towards the sensor 1A or IB) or late (when subject 2 moves away from sensor 1A or IB) with respect to the 'sound - transmitted' signal. The result of such swaying movement is to cause the differential time comparator 8A or 8B to emit pulses, the width of which is proportional to the distance moved by the subject 2, from the zero position, away from or towards the sensor 1A or IB. The digital outputs 12A and 12B from the differential time comparators 8A and 8B may be fed directly to a computer or converted by means of corresponding voltage converters 9A and 9B to output, at points 13A and 13B, analogue voltages that vary linearly with sensor-to-subject distance. These can be recorded independently on a dual channel Yt chart recorder or combined on an X-Y plotter. Such recordings can be immediately visually interpreted for frequency, amplitude and direction of sway and, if desired, recorded on magnetic tape to enable later replay into a computer. Provision of a computer system is advantageous for signal analysis by such methods as the Fourier Transform or 'line-length' evaluation (i.e. determination of the total length of a trace). Since the absolute sensor-to-subject distance is not determined, it is not necessary to know precisely the velocity of the sound waves that comprise the pulse burst. This velocity is dependent upon the temperature of the air and, to a lesser extent, upon humidity. The formula: velocity = 331.4 x ms" 1 where T is the air temperature in degrees Kelvin, shows that for measurements taken at room temperature between 10 and 30°C the change in velocity is ± 1.7%. This factor is not compensated for in the device described above. In Figure 2, the arrangement of components is the same as shown in Figure 1 as hereinbefore described, but in addition terminals 14A and 14B and 15A and 15B are provided. From these terminals the output signals from the sensor interfaces 5A and 5B may be fed into a microcomputer programmed to record and display data related to the swaying movements of the subject over any given time period. In this embodiment, the variable delay circuits 6A and 6B, zero controls 7A and 7B, and other components 8A-13A and 8B-13B are optional, but are usefully retained so that, if desired, a chart recorder may be used simultaneously with the microcomputer. Referring to Figure 3, the signals from the terminals 14A, 14B and 15A, 15B may be fed into a signal conditioning unit 16 which converts each output into a pulse, the duration of which varies with the distance of the subject from the sensors 1A and IB (Figure 2). The pulses from the signal conditioning unit 16 then pass into a microcomputer 17 which may, for example, be programmed to generate a 'sway index'. The sway index (S) is a numerical value which gives a measure of the swaying motions made by the subject 2 over a given time period and two separate figures - one for lateral sway and one for sagittal sway - are computed. If (as has been found convenient) it is desired to measure the sway index over a period of 1 minute, the microcomputer 17 is programmed to calculate S according to the equation: 6On S = V | x i +l ~xi I (i - l-60n) where n is the number of ultrasonic pulses per second generated by the transmitters 1A and IB, and x^ is the time between the i^ n transmitted and received pulse. The measurement of the sway index may be commenced by manually pressing a start button 18 and at the end of 1 minute, the values of S obtained for both sagittal and lateral sway are displayed on the LCD character display 19. It will be appreciated that sway index data may be generated in this way without any need for the subject to stand at a pre-determined zero position provided he or she stands within the beam path and within the ranging capabilities of the instrument. The computer is programmed to detect any movement out of the beam path and measurement of the sway index cannot be started or will be discontinued if that occurs. The application of ultrasound to the measurement of body sway by means of the present invention provides a compact, portable, inexpensive, objective, non-contact method well suited to use in psychopharmacological studies both in normal healthy volunteers and in patients.

Claims: - 31 - Claims 1. A sway detecting and monitoring device comprising: a) two or more ultrasound transmitters arranged to direct an ultrasound beam across a subject area; b) an ultrasound receiver associated with each transmitter to detect ultrasound reflected from a subject in the subject area; c) ranging means to compare transmitted and received ultrasound from each transmitter/receiver pair and to output position data related to the position of a subject in the subject area; and d) display means to display position data output from the ranging means; and/or e) recording means to record position data output from the ranging means. 2. A device as claimed in Claim 1 in which the ultrasound transmitters generate a first timing signal when the pulse is transmitted; the ultrasound receivers generate a second timing signal when a reflected pulse is detected; and the ranging means assesses the time delay between the first and second timing signals from each transmitter/receiver pair and, outputs position data related to the position of a subject in the subject area based on the time delay. 3. A device as claimed in Claim 1 or Claim 2 in which each ultrasound transmitter and receiver is in the form of a single combined unit or sensor. 4. A device as claimed in Claim 3 in which two sensors are used, each set at right angles to the other. 5. A device as claimed in any one of claims 1 to 4 in which the frequencies emitted by the transmitters are in the range 50-70 KHz. 6. A device as claimed in any one of Claims 1 to 5 in which the deviation of the subject from a fixed zero position is continuously monitored. 7. A device as claimed in any one of Claims 1 to 5 in which the deviation of the subject from each transmitter is continuously monitored. 8. A device as claimed in Claim 7 in which the deviation of the subject from each transmitter is analysed by a computer system. 9. A device as claimed in any one of Claims 7 to 9 in which a 'sway index' S is calculated in accordance with the equation 60n S = L I Xi + 1 - Xi I 1 ' ' wherein i = l-60n, n is the number of ultrasonic pulses per second generated by the transmitters 1A and IB and Xi is the time between the i. -* 1 transmitted and received pulse. 10. A sway detecting and monitoring device substantially as herein described with reference to the accompanying drawings.

Inventors: BATES COLIN DAVID, FRANCIS GRAHAM RUSSELL, MCCLELLAND GRAHAM RALPH, PETTITT KEITH EDWARD ERNEST, BATES, COLIN, DAVID, FRANCIS, GRAHAM, RUSSELL, MCCLELLAND, GRAHAM, RALPH, PETTITT, KEITH, EDWARD, ERNEST
Applicants: BEECHAM GROUP PLC, BEECHAM GROUP P.L.C.
Application year: 1986
Main classification: A61B5
IPC codes: A61B5, A61B8