Diverging signal tandem doppler probe

A dual transducer probe for measuring blood flow by Doppler effect measurement in which quantitative flow data can be compiled and flow constriction points as small as 10% and less can be pinpointed with relative ease. The probe is formed with two transducers, oriented at about 45.degree. from a bottom contacting surface of the probe so that transmission signals diverge at an angle of about 90.degree.. A first transducer directs pulsed sound waves into the direction of blood flow and a second transducer directs pulsed sound waves with the direction of blood flow. The transducers act as receivers for the respective pulses. Fluid velocity can be calculated due to the Doppler effect. By analyzing the signal frequency shifts from the two transistors the blood flow at a very minute region between the transducers, can be determined. Constrictions, corresponding to changes in flow can be pinpointed by moving the probe along the blood vessel.

BACKGROUND OF THE INVENTION 
The invention relates generally to a probe for measuring fluid flow and 
more particularly to a dual transducer probe for measuring fluid flow by 
employing diverging ultrasound signals according to the Doppler effect 
principle, especially through blood vessels. 
In the field of medical practice, it is often desirable to obtain data 
regarding blood flow within an individual's blood vessels such as the 
arteries, veins and capillaries to assist in medical diagnosis, prognosis 
and treatment. Early microsurgeons appreciated the need for reliable and 
accurate direct evidence of anastomotic patency such as flap coloration, 
capillary refill, or peripheral bleeding. Often, surgeons had to rely on a 
subjective assessment of various types of pulsations postanastomotically. 
A dual forceps or "milking" procedure was proposed by J. W. Hayhurst, et 
al. in "An experimental study of microvascular technique, patency rates 
and related factors.", Br. J. Plast Surg 28:128-32 (1975). However, this 
method is disadvantageous due to its traumatic nature and occurrences of 
proven thrombotic reduction of the lumen by 75-95% in which the 
anastomoses were classified as fully patent. 
An apparatus for measuring the speed of blood flowing through channels by 
the use of ultrasound according to the Doppler effect principle is 
described in U.S. Pat. No. 3,766,517 to Fahrbach, the contents of which 
are incorporated herein by reference. Fahrbach describes a probe having 
two transmitting/receiving transducers that project a converging signal in 
which the sending-receiving directions of the transducers form an angle of 
90.degree.. 
Continuous Wave Doppler Ultrasound was described as being useful to assess 
microvascular anastomotic patency by Van Beek, et al. in "Ultrasound 
evaluation of microanastomosis", Arch Surg 100:945-949 (1975) Van Beek, et 
al. describe several velocity profile waveform parameters that they felt 
would be predictive or ultimate anastomotic patency, but made no 
substantial attempt to relate these criteria to quantitation of luminal 
narrowing. 
Luminal narrowing was addressed by Freed, et al. in "High frequency pulsed 
Doppler ultrasound: a new tool for microvascular surgery", J.Microsurg 
1:148-153 (1979), the contents of which are incorporated herein by 
reference. Freed, et al. applied High Frequency Pulse Doppler Ultrasound 
(HFPDU) to the field of microsurgery. Freed, et al. attempted to 
quantitate arterial stenoses ranging from 25-99% in vessels having a 
diameter less than 1 mm using a simple velocity ratio equation: 
EQU % Area Reductions=100[1-Vp/Vs] 
wherein Vp and Vs represent pre-stenotic and stenotic velocities, 
respectively. 
The Freed, et al. technique involves meticulous scanning of a 
microanastomosis at 0.5 mm intervals with a pencil-type probe. A 
needle-mounted pencil-type probe 20 shown in FIG. 2, including the 
piezoelectric-crystal transducer 14' shown in FIG. 1 is employed. 
Transducer 14' is formed of a 1 mm.sup.2 piezoelectric crystal 11' 
electrically coupled to a pair of electric lead wires 12a and 12b. A 
styrofoam acoustic baffle 13' is disposed on an inner surface of crystal 
11'. 
As shown in FIG. 3, transducer 14 is mounted flush to the end of a twenty 
gauge needle 21 and is positioned at an angle of 45.degree. to a blood 
vessel 31 by hand or with a micromanipulator. Transducer 14 is 
sequentially scanned across the microanastomosis. This technique has been 
found to be impractical in a clinical setting. 
Conventional Doppler effect fluid flow measuring probes have disadvantages. 
While the probes can be effective for determining whether fluid is flowing 
through a vessel in some clinical settings, they can be inappropriate for 
determining quantitative fluid flow values or for pinpointing constricted 
areas. Thus, conventional probes are not fully satisfactory and have 
inadequacies due to their shortcomings. 
Accordingly, it is desirable to provide an improved fluid flow probe which 
avoids the shortcomings of the prior art. 
SUMMARY OF THE INVENTION 
Generally speaking, in accordance with the invention, a dual transducer 
probe for measuring blood flow by Doppler effect measurement is provided 
in which quantitative flow data can be compiled and fluid flow 
constriction points can be pinpointed with relative ease. The probe is 
formed with two transducers, oriented at preferably about 45.degree. from 
a bottom contacting surface of the probe so that transmission signals 
diverge at an angle of preferably about 90.degree.. A first transducer 
directs pulsed sound waves into the direction of blood flow and a second 
transducer directs pulsed sound waves with the direction of blood flow. 
The transducers act as receivers for their respective pulses. Fluid 
velocity can be calculated because of the Doppler effect. By analyzing the 
signal frequency shifts from the two transducers, the blood flow at a very 
minute cross section of the blood vessel between the transducers can be 
determined. Constrictions as small as 10% and below, corresponding to 
changes in flow, can be pinpointed by moving the probe along the blood 
vessel. 
Accordingly, it is an object of the invention to provide an improved probe 
for measuring fluid flow. 
Another object of the invention is to provide an improved probe and method 
for pinpointing minute perturbations in flow constrictions or faulty 
repairs in blood vessels. 
A further object of the invention is to provide an improved probe and 
method for determining flow patterns through blood vessels having small 
diameters. 
Still another object of the invention is to provide an improved probe for 
measuring flow through blood vessels which is easy and convenient to use. 
Still a further object of the invention is to provide a probe to study end 
to end repairs as well as end to side repairs of any angle. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specification and drawings. 
The invention accordingly comprises the several steps and the relation of 
one or more of such steps with respect to each of the others, and the 
apparatus embodying features of construction, combinations of elements and 
arrangement of parts which are adapted to effect such steps, all as 
exemplified in the following detailed disclosure, and the scope of the 
invention will be indicated in the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A tandem doppler probe (TDP) for measuring fluid flow in accordance with 
the invention is constructed with two sound transmitting/receiving sources 
positioned to transmit the sound signals at a diverging angle into the 
flowing fluid. A particularly well suited tandem doppler probe 40 
constructed in accordance with the invention is shown generally in FIGS. 4 
and 5. Probe 40 includes a pair of transducers 41a and 41b, each oriented 
at an angle F to a bottom surface 42 of probe 40. Angle F is preferably 
about 45.degree. but other angles are also suitable. Bottom surface 42 is 
adapted to be placed in contact with a blood vessel 43 that can have a 
diameter as small as about 1 mm or less. Probe 40 can be formed with a 
pair of guide ridges 46a and 46b to assist in aligning probe 40 with blood 
vessel 43 so that transducers 41a and 41b can be conveniently aligned with 
the direction of blood flow. 
Bottom surface 42 may also be configured differently so the bottom edges of 
transducers 41a and 41b which are perpendicular to guide ridges 46a and 
46b are not parallel, to study flow through branched vessels or repairs 
done in "end to side" fashion. In this configuration, guide ridges 46a and 
46b would be bent at an angle and transducers 41a and 41b would be aligned 
with the bent ridges to match the configuration of a forked or curved 
blood vessel. 
Referring to FIG. 4, blood flows in the direction of an arrow B. 
Transducers 41a and 41b are oriented to transmit signals diverging at an 
angle G. Angle G is preferably 90.degree., but other angles are also 
suitable. The absolute angle of each transducer is not as critical as the 
relative angle between transducers 41a and 41b. 
Transducer 41b transmits a pulsed signal in the direction with fluid flow 
and receives the delayed returning signal. Transducer 41a transmits a 
pulsed signal into the direction of fluid flow and receives the signal of 
increased frequency. The delay and increase in signal frequency is caused 
by the Doppler effect. By analyzing the difference in the frequency 
between transmission and reception by conventionally known methods, the 
blood flow across the cross section of the blood vessel at the minute 
region between the transducers can be measured 
Constrictions corresponding to changes in flow can be detected by moving 
the probe along the vessel. The device can be positioned so that one 
transducer reads the flow proximally and the other distal to or at an 
anastomosis to provide two doppler tracings simultaneously for instant 
comparison of blood flow on either side of an anastomosis or stenosis. 
The electronics for a tandem Doppler probe (TDP) such as probe 40 are 
commonly known and are similar to the electronics discussed in the Freed 
article, J. Microsurgery 1: 148-153 (1979) and in several U.S. patents 
such as U.S. Pat. No. 3,987,673 to Hansen, the contents of which are 
incorporated herein by reference. Additional information can be obtained 
from the Cardiovascular Sciences Laboratory of the Baylor College of 
Medicine and other publically accessible sources. 
High Frequency Pulsed Doppler Ultrasound (HFPDU) is particularly well 
suited for measuring blood flow with a tandem doppler probe constructed in 
accordance with the invention. The pulsed technique provides advantages 
for the study of blood flow in vessels within a microsurgical domain. The 
elimination of separate transmitting and receiving crystals in the probe 
head reduces the bulk of the probe head in half and eliminates the 
acoustic window dead-space directly in front of the crystals. This 
construction permits placement of the crystal directly on the probe wall. 
Acoustic scattering is proportional to the fourth power of the frequency. 
Accordingly, the increase in frequency to 20MHz permits the use of 
piezoelectric crystals that are as small as about 0.5 mm squares. By 
addition of the variable delay time between transmission and reception of 
ultrasonic pulses, a discrete sample of volume of less than a cubic 
millimeter can be placed anywhere between about 0.5 and 10 mm from the 
probe face. This high spatial resolution permits generation of complex, 
parabolic velocity profiles by sampling across the lumen of a vessel in 
increments as small as about 0.1 mm. The compact probe face allow a 
meniscus of saline to act as an acoustic coupling agent obviating the need 
for gels or creams. 
HFPDU can be employed to create precise cross-sectional velocity profiles 
in a study of the hemodynamic characteristics of normal anastomoses and 
the effect of topical lidocaine on vasospasm as described in Blair, 
"Hemodynamics after microsurgical anastomoses: the effects of topical 
lidocaine.", J. Microsurg, 2:157-164 (1981), the contents of which are 
incorporated herein by reference. 
Lee, et al. used this modality successfully to compare the waveforms 
traversing microsurgical vein graphs inserted with either continuous or 
interrupted suture technique in "Effect of suture technique on blood 
velocity waveforms in the microvascular anastomoses of autogenous vein 
graph", Microsurgery, 4:151-156 (1983). 
To construct a preferred version of tandem doppler probe 40, transducers 
41a and 41b are first constructed. A 20MHz gold-plated 
piezoelectric-crystal such as those available from the Valpey-Fisher 
Company is formed into 1.0 mm squares such as by using a steel rule and 
Carbide scribe under an operating microscope to yield a crystal 11. Other 
sizes for crystal 11 are also acceptable Crystal 11 is soldered to lengths 
of wire, such as silver wire which can be obtained from braided EKG wire 
to yield four wire leads 44c, 44d, 44e and 44f. Crystal 11 is then coated 
with laminating epoxy such as Fibre-Glast, #88/87 to strengthen and 
insulate the joints. 
It is preferable to degas the epoxy for approximately 30 minutes in a 
vacuum of at least 28 inches of mercury to remove dissolved gases which 
can form minute bubbles when the epoxy cures. The bubbles cause acoustic 
voids in the probe face which can significantly degrade the ultrasound 
signal. A 1 mm square block of styrofoam 13 can be disposed on a backside 
of crystal 11 to serve as an acoustic baffle and minimize stray 
ultrasound. 
Transducers 41a and 41b are carefully melted into the surface of a 
2.times.3 cm piece of Dental Modeling Wax such as by a brief application 
of the tip of a hot soldering pencil to the wax. A small metal shim cut at 
45.degree. can serve as a template for the probe face angle. Transducers 
41a and 41b should be separated by distance of about 1 to 5 mm and 
preferably about 2.5 mm. The absolute angle of each transducer 41a and 41b 
is less significant than their relative position to one another, which is 
important. 
Grooves 46a and 46b are carved alongside the crystals to provide ridges for 
facilitating probe positioning. A suitable probe-head mold is placed over 
transducers 41a and 41b and melted in place such as by using the tip of a 
soldering pencil. 
A probe handle can be fashioned from a 2 cm long plastic cylinder similar 
to that of a 3 cc syringe barrel or from a disposable electronic 
thermometer sheath (IVAC). It is preferable to attach leads 44c-44f with 
the delicate joints of the crystals already permanently encased in the 
probe head at the appropriate angles. The head and body of the probe can 
be formed of solid epoxy. If the epoxy forming the head is not degassed, 
the resulting minute bubbles act as an effective ultrasound attenuator. 
This permits the elimination of styrofoam piece 13 from the construction. 
The head is easily removed from the mold after it cures. Two pairs of 
insulated copper wire leads are soldered to silver leads 44c-44f and 
heat-shrink tubing can be applied to the closely-packed connections. The 
resulting unit is placed into a longer secondary mold which can be 
fashioned from an entire 3 cc syringe barrel or thermometer probe sheath 
which has had the closed end removed. The remainder of the mold can be 
filled with degassed epoxy and cured. 
After removal from the second mold, the wax is carefully melted from the 
face of probe 40 which can then be ground to its final appearance with a 
high speed grinding wheel. 
Probe 40 is suitable for use with an operating microscope, a dual channel 
oscilloscope and chart recorder and a multichannel HFPDU unit such as the 
Model VF-1 available from Crystal Biotech, Inc., New Englander Industrial 
Park, Kuniholm Dr., Holliston, MA 01746. This unit includes a "Doppler 
Master" which controls synchronization of up to 6 separate flow-channel 
modules for simultaneous, independent operation. The probe is attached to 
two flow channels and the polarities are selected to provide dual positive 
deflections on the oscilloscope or chart recorder. Probe 40 can be either 
hand-held or placed in an articulated arm. 
Use of the tandem doppler probe constructed as described above in 
accordance with the invention in connection with the above described 
set-up will now be explained with reference to the following examples. 
These examples are presented for purposes of illustration only and are not 
intended to be construed in a limiting sense. 
EXAMPLE 1 
20 Sprague-Dawley rats (250-400 grams) were anesthetized with 
intraperitoneal Chloral Hydrate (4%) and their femoral vasculature was 
exposed via a longitudinal incision. Femoral arteries having a diameter of 
0.8 to 1.3 cm were dissected atraumatically from the inguinal ligament to 
the origin of the Superficial Epigastric Vessels. Arteries were bathed in 
a small amount of Ringers Lactate or Normal Saline, which acts as an 
acoustic coupling agent and the probe was carefully placed on the artery. 
The range control of each channel was independently adjusted to obtain 
maximum centerline velocities as assessed by simultaneous oscilloscope 
comparison. The probes provided clear and consistent signals with only 
minimal manipulation. 
Optimal recordings were obtained with the probe just barely touching or 
less than 1 mm away from the vessel wall with a meniscus of saline 
bridging the interface. Wider separation impairs the signal quality and 
adds significant background noise due to artifact from the random motion 
of the saline caused by arterial wall motion. 
EXAMPLE 2 
Several trials were performed in which the TDP measured a variety of graded 
stenoses as well as gradual noose occlusion using 10-0 nylon suture 
material. The stenoses were quantified by measurement of external luminal 
diameters with a microcaliper under high-power magnification (20.times.). 
As shown in FIGS. 6 and 7, the typical peripheral HFPDU waveform obtained 
from the rat shows a large initial systolic component followed by a 
smaller but positive diastolic deflection. An increase in velocity occurs 
during systolic acceleration time (SAT) until the maximum systolic 
velocity (MSV) is reached. Velocity decreases during systolic deceleration 
time (SDT) to the point at which the dichrotic notch (DN) is observed and 
then increases until the maximum diastolic velocity (MDV) is reached. V 
stands for the mean velocity. These animals demonstrate a Dichrotic notch 
in contrast to the negative deflection, representing a short period of 
flow reversal in early diastole seen in the classic Triphasic waveforms of 
larger animals and humans. 
EXAMPLE 3 
FIG. 8 shows waveforms obtained by placing the TDP on a segment of normal 
artery without a stenosis. Two sets of strips were produced and were 
identical except for being out of phase by an amount equal to the 
inter-crystal distance of the probe. The TDP was placed with one crystal 
in a pre-anastomotic position and the other face on or slightly distal to 
the stenoses. When adjusted to display maximum velocities, the probe 
clearly demonstrates graded stenoses as small as a 10% reduction in 
external vessel diameter, as shown in FIG. 9. 
EXAMPLE 4 
FIG. 10 shows waveforms produced by the TDP that result from gradual noose 
occlusion of a 1.0 mm artery. The post-stenotic crystal shows a linear 
increase in the maximum and mean velocities until approximately 50% 
reduction in cross-sectional area. At this point, values from both faces 
progressively decrease until the signal is lost, which occurs at complete 
occlusion. 
EXAMPLE 5 
FIG. 11 shows characteristic changes in individual waveform morphology that 
is seen in progressive arterial stenoses which include: initial increase 
followed by decrease in maximum and mean velocities, slowed systolic 
acceleration time, loss of diastolic component and dichrotic notch and 
progressive rounding and attenuation of the terminal signal component 
until loss. 
TABLE 1 
______________________________________ 
Characteristic post-stenotic waveform morphotology 
obtained with progressive occlusion as shown in FIG. 11. 
Chart letter Morphology 
______________________________________ 
a normal baseline 
b post-stenotic "jet" 
c region of turbulence with flow 
reversal 
d, e, f, g gradual blunting of waveform; 
slowing of 
acceleration/deceleration times; 
loss of diastolic component and 
dichrotic notch 
h residual background signal of 
complete occlusion 
______________________________________ 
The Tandem Doppler probe (TDP) constructed in accordance with the invention 
provides many advantageous features. The two probe faces are accurately 
and permanently related to one another so that only one set-up and reading 
are necessary. This saves time and reduces sampling error. Because data 
represent a ratio of two simultaneous readings, absolute values are 
de-emphasized and variations in systemic conditions such as arterial 
pressure, heart rate and peripheral resistance will not affect the 
results. In addition, because the TDP compares identical pulses at 
different locations, beat to beat variability is eliminated as a source of 
error. Due in part to these features and the fact that velocity and 
waveform changes occur both proximal and distal to the stenoses, the TDP 
can identify pathologies that could be missed or miscalculated by a 
single, post-stenotic monitor. 
A TDP constructed in accordance with the invention permits accurate and 
quantitative determination of luminal narrowing as little as about 10%. It 
can provide the sensitivity necessary to perform detailed waveform 
analysis of various anastomotic catastrophes. It is expected that analysis 
of waveform "signatures" could be used to identify thrombosis, venous out 
flow obstruction, tortion, kinking, back-wall stitches, or extrinsic 
compression by sub-flap hematoma or fluid collection. Computer assisted 
flow analysis and spectral analysis of the microvasculature could add to 
the TDP's effectiveness. 
The TDP provides a simple and accurate technique for examining flow in 
blood vessels of any size including those that can be smaller than 1 mm in 
diameter. The TDP can simultaneously measure both pre and 
post-anastomotically without repositioning or manipulation. The two 
waveforms provided can be compared instantly to quantify anastomotic 
narrowing of as little as 10% and to appreciate subtle waveform changes 
that accompany anastomotic pathology. It is expected that the TDP will be 
able to detect narrowing as small as 5% and below. The TDP permits earlier 
and more precise delineation of anastomotic problems in the operating room 
and can provide an objective assessment of technique for the microsurgical 
trainee and may allow more sensitive post-operative monitoring of free 
tissue transfers in an implantable form. 
A TDP constructed in accordance with the invention can serve as an ideal 
training device. It provides instant, atraumatic, hemodynamic assessment 
of "practice" anastomoses and provides objective feedback. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained and, 
since certain changes may be made in carrying out the above method and in 
the constructions set forth without departing from the spirit and scope of 
the invention, it is intended that all matter contained in the above 
description and shown in the accompanying drawings shall be interpreted as 
illustrative and not in a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention herein described 
and all statements of the scope of the invention which, as a matter of 
language, might be said to fall therebetween.