Biomedical Engineering Reference
In-Depth Information
for a bed, which is incorporated with the detector of vibration signal of mea-
sured of measured object into the system,
23
is a supersensitive condenser
microphone for detecting the external noise of the measuring environment,
which is included with
22
in the detector of the vibration signal of the envi-
ronmental noise, and which is fixed in the same way as the laser source head
and is incorporated into the system with the detector of the vibration signal
of the measured object.
Furthermore, in Fig. 1.50,
24
is a consultation bed,
25
is the subject,
26
is
an electrocardiogram,
27
is a filter that has a high-pass filter and a low-pass-
filter,
28
is an amplifier, and
29
is a DAT (digital audiotape) recorder. The
united detector of the vibration signal of measured object
21
is placed apart
from the subject
25
, and may be attached to a fixed support
30
or be made
to slide along the fixed support
30
. The murmurs and internal and external
noises are detected by the detector of the vibration signal of the measured
object and the detector of the vibration signal of the environmental noise,
and the vibration signal containing the murmur and the internal and external
noises is sent to the filter
27
. The filtered vibration signal is amplified by the
amplifier
28
and is recorded on the DAT recorder
30
.
1.8.10
Data Acquisition of the Vibratory Signal
of the Anterior Chest Wall
It is possible to measure in a lying position (supine) or in a sitting position
and others because the measurement is done using a noncontacting technique.
It is measured by fixing a projecting laser beam to the left sternal border
of the left fourth intercostal space, which corresponds to the front of the
heart, or by sliding a number of laser source heads or one laser source head
over the left back in a position without lung respiration. The reason for
measuring without respiration is to avoid the effect of vesicular sound from
the anatomical respiration. In our earlier research, it has been found that the
frequency band of the stenotic vibration deriving from the coronary artery
stenosis ranges from almost 200 Hz to 1000 Hz. The signal is filtered by
the high-pass filter at 200 Hz and by the low-pass filter at 1000 Hz, and
it is possible to set the system from 100 Hz to 1000 Hz. According to the
superscription, it is possible to acquire the above frequency band with an
excellent signal-to-noise ratio. The gain of the amplifier
28
is approximately
set from 200 times to 1000 times. The measurement is done for ten heartbeats
of a subject; that is, for about 10 s. This measured signal is recorded with
the electrocardiogram signal in the DAT 29.
1.8.11
Signal Processing
The acquired signal is converted from analog to digital at 2 kHz as a sampling
frequency, and is incorporated into the computer. The maximum entropy
method (MEM) is used for analyzing in the frequency domain. The objective
