An investigation using high-precision CCD laser displacement sensor to measure body surface motion induced by heartbeat ()
1. INTRODUCTION
Recently, to avoid the restrictions caused by the presence of lead wires and to remove the burden of attaching electrodes, medical remote sensing techniques that are nonrestrictive, non-invasive and non-contact for monitoring vital signs have been actively researched and developed (here, we refer to such techniques as “unconscious sensing”). Examples include sensors using a strain gauge [1], pressure sensors [2], or film of the piezoelectric polymer polyvinylidene fluoride (PVDF) [3]. In particular, the PVDF-based sensor can non-restrictively and non-invasively measure heartbeat and respiration on a person’s dorsal body surface. In the current study, a method for completely non-contact vital signs monitoring using microwave radar is proposed. The method was originally developed to search for survivors trapped beneath earthquake rubble [4,5]. Microwave radar has the following characteristics: 1) the microwaves can be transmitted through most objects, except metals and water and 2) movement of an object can be detected from distance without actually needing to touch it. By applying this system to the monitoring of human vital signs, motion of the body surface can be observed from a distance without removing any clothing. For example, respiratory rates of examinees in bed were monitored through bedding by using a ceiling-mounted microwave radar [6]. The purpose of the current study is to develop a system not only to reduce the burdens on the elderly which are created by attaching electrodes to the body but also to reduce the burdens on nursing caregivers having to confirm the safety of elderly patients with frequent overnight visits. The microwave radar techniques used in this research have already been applied to monitoring the vital signs of a hibernating bear for more than three consecutive months in a zoological garden [7]. In another trial, a 24- GHz compact microwave radar attached to the rear surface of a chair could monitor not only heart rate but also heart rate variability (HRV) [8].
These unconscious sensing techniques are aimed at detecting motion at extremely minute scales on the body’s surface caused by cardiac motion. An important contribution to this subject area has been made by Lin, Droitcour et al. [9,10] who have developed mathematical theory on non-contact sensing techniques and have constructed mathematical models under the assumption of monitoring body surface motions.
However, motions on the body surface induced by heartbeat are so small that they are barely apparent. As a consequence, the problem in previous research was the extrinsic noise cancellation and the removal of artefacts resulting from respiratory activity and motions caused by arm movement [11]. Nevertheless, none of these studies addressed the question of how many motions of different amplitude appear on the body surface. There are specific body parts that are generally considered to have the characteristics necessary for monitoring by unconscious sensing, owing to there being differences between the conditions on and under the skin for each part of the body.
Therefore, for use as a reference base for unconscious sensing research, the aim of the current study was to measure motions on the body surface induced by heartbeat by using a high-precision charge-coupled device (CCD) laser displacement sensor and to confirm the magnitude of these movements. Through observation of the motion on the surface of the chest wall and dorsal region, the applicability of the unconscious measurement method is discussed.
2. METHODS
2.1. High-Precision CCD Laser Displacement Sensor Specifications
To measure motion on the body surface, an industrial high-precision CCD laser displacement sensor (LK-G35; Keyence Co., Tokyo) was chosen. This red CCD laser has a 650 nm wavelength, a measurement resolution of 0.05 μm and an effective measuring range of ±5 mm; its maximum sampling frequency is 50 kHz and its maximum power output is 4.8 mW. This type of laser displacement sensor is frequently used in measuring the thickness and evenness of a vehicle’s paint finish, and in quality control to find scuffs of extremely minute scales on compact discs and hard disks.
If the laser beam from the laser displacement sensor is irradiated directly onto a person’s skin, the beam, because of its low power, will not penetrate into the body and will not cause skin problems. However, to ensure maximum safety, we placed a piece of white vinyl stretch tape on the body surface and each participant wore laser protection goggles with an optical density of 3.
2.2. Measurement Positions and Participants
The target positions V2, V3 and V5 (see Figure 1) for measurement of body surface motions, were decided from referral to a 12-lead electrocardiogram. Additionally, the left-side carotid artery was monitored (CA in Figure 1), because it is a representative artery that is
(a) (b)
Figure 1. Measurement points. V2, V3 and V5 were decided from referred to 12-lead electrocardiogram. Additionally, the carotid artery in left side was also monitored (CA). Measurement positions on dorsal region V9 were chosen referred to 18-lead electrocardiogram and the elbow height (EH) in lumber area was measured with reference to the industrial standard. (a) Front; (b) Back.
positioned close to the skin’s surface. Conversely, the measurement position on the dorsal region, V9, was chosen as it referred to a 12-lead electrocardiogram [12], and to an earlier study by the authors that monitored the same position [8]. Furthermore, researchers [13,14] have attempted to detect heartbeat from the lumbar area; therefore, the motion on the elbow height (EH on Figure 1) in the lumber area was measured in accordance with industrial standard [15]. This position EH was stirred about 20 mm from midline to left side considered in the backbone.
Body surface motion was measured for eight men (mean age: 21.88 ± 1.13 years; range: 21 - 24 years). Additionally, to observe the relationship between the measurements and the personal physical characteristics of each participant, their subcutaneous fat thicknesses were found in advance (see Table 1). The subcutaneous fat around each measurement position was measured three times with skinfold callipers (MD-550; Yamasa Tolei CO. LTD., Tokyo) in reference to a previous study [16] and the average was calculated.
2.3. Investigation Procedure
To conduct the measurements, a participant, naked from the waist up, lay down on a urethane foam mattress and the laser displacement sensor was set on a steady aluminium frame above the body. The laser sensor was positioned 3 cm from the body surface and the movement measured at each target position (Figure 2). Additionally, to take measurements on the dorsal region, participants lay face down on the mattress. To remove any motion induced by respiration, we instructed each participant to hold a breath for 30 s. For comparison, we also monitored an electrocardiograph (ECG) connected by precordial leads (WEB-5500; Nihon Koden Co., Tokyo) and collected information with a sampling frequency of 100 Hz. The output signal data from the laser distance sensor
Table 1. Individual physical characteristics of eight volunteers.
Figure 2. Schematic diagram of the experimental setup. A high precision CCD-laser displacement sensor was used to measure the motion on the surface of the body, and an ECG was also used as a reference, providing direct measurements with ECG.
were stored in synchronization with the ECG data in a digital recorder (LX-10; TEAC Co., Tokyo).
3. RESULTS
3.1. Motions on the Body Surface
Figure 3 shows 10 s of sample data monitored for subject S1, extracted from the total of 30 s of data. Figure 3(a) shows the output signal from the ECG and Figure 3(b) shows the output signal acquired by the high-precision CCD laser displacement sensor. It can be seen from this figure that the cyclic oscillation in the data acquired by the laser displacement sensor is synchronized with the R-waves from the ECG. Although some of the sensor data included baseline drift resulting from respiratory motion, this synchronous cyclic oscillation with the Rwaves of the ECG was clearly observed in all of the participants. Additionally, this cyclic oscillation has several peaks, similar to the pulse wave. This wave shape was confirmed at all measurement positions for all participants, despite the slight differences in the form of the motion on the body surface. Likewise, some previous