FRANKLIN, VATNER, HIGGINS, PATRICK, KEMPER AND VAN CITTERS 
1121 
Exciter 
Typically, an electrical signal of approxi- 
mately 3 V peak to peak at the resonant fre- 
quency of the crystal is used to generate the 10 
MHz sound beam. The crystal material has an 
extremely high dielectric constant such that 
a practical transducer may have a shunt ca- 
pacitance as high as 10-^ farads. At resonance 
the resistive component of the impedance is on 
the order of 100 ohms. Thus, the crystal exciter 
must be capable of producing a 10 MHz signal 
with an output power on the order of 20 mw 
into an impedance on the order of 100 ohms. 
Variable acoustical standing waves will be gen- 
erated by the crystal-transducer when im- 
planted, such that the impedance of the pro- 
jector crystal will vary. This varying impedance 
can be reflected back to the electrical generator 
and modify the frequency of the oscillator. This 
varying frequency signal will be indistinguish- 
able from a Doppler shift so that provision for 
buffering the crystal from the oscillator should 
be provided to minimize spurious measure- 
ments. With these considerations in mind, a low 
power, 2 stage exciter is usually used. This 
consists of an oscillator, variable in frequency 
from 7-10 MHz, followed by a buffer amplifier 
stage which is matched to the impedance of the 
crystal. 
Receiver 
The electrical signal at the receiver crystal 
consists of two components. One component is 
due to the direct coupling of sound from the 
projector crystal across the blood vessel to the 
receiver crystal. This signal is unmodulated, 
i.e., it is at the same frequency as the projected 
signal frequency, and is relatively high in am- 
plitude, i.e., 1-10 mv. The second signal is the 
signal backscattered from the moving erythro- 
cytes. This signal is modulated in frequency in 
proportion to the velocity of the moving ery- 
throcytes. Its amplitude is relatively small, i.e., 
1-30 fjLV. The receiver crystal is identical to the 
projector crystal and appears electrically as a 
generator with approximately a 50 ohm source 
impedance. The first stage of the receiver is 
then a low-noise radiofrequency amplifier with 
a gain of 30 and with an input impedance of 
approximately 50 ohms. The amplified com- 
posite radiofrequency signal is then passed 
through a mixer to generate an audiofrequency 
signal with a frequency equal to the difference 
in frequency between the large directly coupled 
signal and the signal backscattered from the 
erythrocytes. A power detector is used for this 
mixing and audio amplification. The resultant 
signal is filtered to eliminate the 10 MHz carrier 
signal and the sum frequency signal. The output 
of the receiver is typically a 30 mv peak to peak 
signal varying in frequency from 0-10 kHz as 
the blood velocity varies from 0-100 cm/sec. 
In other words, a frequency modulated, audio 
frequency signal is produced, the frequency of 
which is proportional to the flow of blood 
through the implanted transducer. 
Pressure Measurement 
Pressure is measured by the chronic implan- 
tation of a miniature pressure gauge within the 
apical lumen of the left ventricle or within the 
lumen of the thoracic or abdominal aorta. The 
pressure gauge is a disc-like structure 6 mm in 
diameter x 2 mm in thickness with the wires 
exiting from the back of the disc. The front of 
the disc is formed by a thin titanium diaphragm 
on the back of which are deposited four sili- 
con strain gauges connected electrically in the 
form of a wheatstone bridge. As the diaphragm 
is deformed by pressure, the strain gauges are 
stressed and their electrical resistance changes 
in proportion to the pressure applied to the 
metal diaphragm. The voltage output from the 
gauge, when excited with 5.6 v DC, varies from 
0 to 45 mv as the applied pressure varies from 
0 to 300 mm Hg. The gauge is resistively tem- 
perature compensated, such that the zero base- 
line and sensitivity are independent of tempera- 
ture over a wide range (less than 0.5 mm 
Hg/°C). 
In practice, the power for excitation of the 
bridge is provided by an 8 v battery. The output 
of the battery is voltage regulated to provide a 
constant 5.6 volts to the bridge. The voltage 
from the pressure gauge is amplified to provide 
0.3 volts full scale output. 
The gauge is calibrated prior to implantation, 
but this calibration is used only as a guide and 
