Biomedical Engineering Reference
In-Depth Information
in order to monitor the environmental noise deriving from the measuring
environment.
Therefore, the system can monitor the vibration signal of the environ-
mental noise by visualizing it as a referred vibration signal of the environ-
mental noise, and cancel the vibration signal of the environmental noise from
the vibration signal of the body surface as a vibration signal to be measured.
A number of high damping gradient filters (about 1000 dB/octave) are put
on an amplifier with low noise and a high amplification factor. This may form
a system that measured displacement of such a system from the laser source
is extracted as an acceleration signal by combining differentiating circuits,
and is displayed and recorded. As mentioned above, the noise is removed on
the hardware. Each objective amplified signal is converted into the frequency
domain by a signal processing method such as linear prediction processing
and is made to be a power spectrum. Thereafter, the noise signal is canceled
from the objective measured signal and only the frequency of the objective
signal is extracted, displayed, and recorded.
The coronary artery blood flow causing the objective measured signal is
the flow during the heartbeat diastole. The frequency band is designed to be
of 100 Hz width (recently of 10 Hz width), as it is an unsteady quantity with
fluctuation. The domain from (II) heart sound being stop consonant of arte-
rial valve in the heart sound to the time after 100 ms of the (II) heart sounds
in the maximum is recorded for ten heartbeats. Therefore, the measuring
time is about 10 s, and this time is much shorter than the measuring time of
the electrocardiogram, the 24 h electrocardiogram (conventionally called the
Holter ECG), the motion loaded electrocardiogram, or the scintigram in the
conventional noninvasive examination of coronary artery disease.
The software supplements the low signal-to-noise ratio in the hardware in
the measuring system, and the noise element entering from the commercial
power supply is removed by using a filter transformer. There, the measuring
system is the sensor fusion measuring system as a whole.
1.8.9
The Details of the Prototype Device
The measuring principle of displacement of vibration utilizing a pulsed la-
ser beam is shown in Fig. 1.49 (currently, combined with a pulse Doppler
method).
A class 2 laser diode is used as a laser beam, being harmless to humans,
and the near infrared area of 670 nm wavelength is utilized. The displace-
ment is outputed as a displacement magnitude signal (shown as AB and A
B
in Fig. 1.49), which is based on the principle of triangulation. The minute
vibration of the light source is generated in the environment even if the la-
ser beam is used as the signal detecting medium, and this becomes internal
noise deriving from the measuring system. Therefore, measurement with mo-
nitoring is also carried out to cancel this internal noise. Furthermore, as the
minute vibration of the body of the subject also becomes a kind of internal
