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A combined transcutaneous pulse-echo and Doppler ultrasonic technique for the measurement of brachial artery peak velocity
A COMBINED TRANSCUTANEOUS PULSE-ECHO AND DOPPLER ULTRASONIC TECHNIQUE FOR THE MEASUREMENT OF BRACHIAL ARTERY PEAK VELOCITY Susan Moss
David Bennett
ABSTRACT The
Parks
Model
806
Doppler
to have a linear voltage frequencies velocities
of up to SkHz.
of 70 ems-t
blood velocity
meter was shown
output with respect to input It was lineor
at angles
of 50°
and accurate and 60*
to
to the flow, on
INTRODUCTION
Despite the increasing use of instrumentation in the monitoring of acutely ill patients, there remains a need for a reliable, non invasive, inexpensive technique for the routine assessment of cardiovascular function. The ultrasonic Doppler technique applies the Doppler principle which states, that if there is relative motion between a transmitter and receiver there will be a change in frequency of a wave detected by the receiver which is proportional to the velocity of the medium. The linear changes of frequency can be additive or subtractive depending on whether the reflecting surface is moving away or towards the transmitter (Wells 1969). The frequency shift of the back-scattered ultrasound is proportional to the blood velocity according to the formula: v=
AF
c
2
F cos B
flow rig, and shown
to have a linear frequency
response
to 9Hz.
pulse echo and Doppler
transducer arterial
system
A combined
has been developed
allowing
quontitation
of
peak veiocity.
echo and Doppler transducer measurement of this angle.
enabling
precise
It has been the aim of the present study to modify existing techniques in order to quantitate brachial artery peak velocity and present an on-line print-out of the data. MATEXAS
AND METHODS
The Parks Model 806 directional Doppler ultrasonic velocity meter was characterized with respect to known frequencies. Sine waves ranging from lkHz3kHz were fed into the zero-crosser input of the Doppler velocity meter. The analogue voltage output from the zero-crossing ratemeter was recorded on a Medelec fibre-optic ultraviolet recorder, the linearity of which had been previously checked. Evaluation of the Parks Model 806 Doppler Velocity meter on a stwdy flow rig
where v = velocity of the blood (cm+‘),, AF = frequency shift (Hz), c =velocity of sound in tissue (15.4 x lo4 ems-l), F = transmitted frequency (Hz), and e = angle of incidence between the ultrasonic beam and the arterial axis.
The development of the directional transcutaneous Doppler ultrasonic blood velocity meter (McLeod 1967) has permitted qualitative assessment of blood velocity signals. However, in order to quantitate blood velocity from the Doppler equation, knowledge of the angle of incidence between the ultrasonic beam and the arterial axis is required. To date, several techniques have been developed to measure this angle (Duck and Hodson 1974; Hansen et al. 1976; McCarty and Woodcock, 1974). Recently, a simple, reliable noninvasive system has been devised by Olson (1974), and uses a combined pulse Department of Medicine, St. George’s Hospital Medical School, Tooting, London. Susan Moss: Present Address: Department of Cardiovascular Medicine, The Radcliffe Infirmary, Oxford.
0141~5425/79/010005-07
a steady
$02.00
The Parks Doppler velocity meter was studied under steady flow conditions to establish the linearity of the response and to compare Doppler velocities to velocities calculated from known flow rates. The apparatus was set up as shown in the diagram (Figure I). A water pump connected to a reservoir and a Winchester bottle via Portex tubing of diameter 0.6 cm. maintained a constant 2 metre head
Stainless
lPlosticine mounting
Figure
I
Reservar
Diagram of the steady flow rig.
J. Biomed.
Eng. 1979, Vol. 1, January
5
Ultrasonic measurement of brachial artery peak velocity: S. Moss and D. Bennett
of pressure. The system was primed with a particulate solution of 25 litres of 0.9% sodium chloride and one pint of outdated human blood. The outlet tap of the Winchester was connected to a length of Portex tubing of diameter 0.6 cm. In order to maximise the reflection of ultrasound from the solution, it was necessary to ensure that the thickness of the interface was less than one wave length of the transmitted ultrasound. Consequently, 30 cm. from the end of the Portex tubing a stainless steel tube of internal diameter 0.64 cm. was inserted. One wall of this tube had been machined flat to a thickness of approximately 0.1 mm (wave length of transmitted frequency = 0.16 mm). For added stability, and to minimize the effect of vibration, the tube was mounted in plasticine. A hollow/was made in the plasticine, filled with ‘Aquasonic’ gel, and the probe inserted towards the direction of flow. The angles of 50”, 60”, and 70” from the axis of flow were studied. Flow rates were varied by a gate clip on the proximal tube. Release of the clamps on the distal tube enabled flow rates to be measured using a measuring cylinder and a centisecond timer (Griffin Triton Ltd.), At a given angle, flow rates were varied in stages and the Doppler analogue voltage output shift recorded on the Medelec fibre-optic ultra-violet paper recorder. The Doppler velocity meter was calibrated with a 1OkHz signal fed into the forward channel. The actual velocity was calculated by dividing the flow rate by the internal cross sectional area of the stainless steel tube and results compared with those calculated from the Doppler output.
_.--Common solid mounting for verrtcol rods
Figure 2 Diagram of the flulsatile rig. glass water bath. The Doppler analogue voltage output was recorded simultaneously. The Doppler velocity was calculated from the Doppler equation assuming the angle of incidence of the ultrasonic beam to be O”, and the velocity of ultrasound in distilled water to be 14.8 x lo4 ems -l (Wells, 1969) at room temperature. The actual peak velocity of the probe was calculated from the motor speed and the sweep radius according to the formula: v = 2nfr where V = peak velocity (ems-l), f = frequency (Hz), and r = sweep radius (ems). The sweep radius was set at 1 cm. The frequency response of the Parks velocity meter was established by comparing the Doppler velocity with the actual velocity over a range of frequencies. Development
Evaluation of the Parks Doppler model 806 on a pulsatile rig
The Parks Doppler velocity meter was studied under pulsatile conditions in order to assess the frequency response and to compare Doppler velocities with predetermined velocities. The apparatus was set up as show in Figure 2. The shaft of a variable speed motor (Parvalux Electric Motors Ltd.). was connected to a rotating arm. An adjustable pin on the rotating arm allowed the sweep radius to be varied. The pin was connected to a linear bearing which moved horizontally on a bar. The bar was constrained to move in the vertical plan@ by additional bearings moving on two vertical steel rods mounted in a solid base. The Doppler probe was held vertically in a fixed position on the horizontal bar. Thus, rotation of the motor shaft caused vertical sinusoidal movement of the Doppler probe. The magnitude and frequency of the probe movement of the Doppler probe. The magnitude and frequency of the probe movement was varied by selection of the motor speed and sweep radius. The frequency of the motor was monitored by a micro switch, triggered each revolution, by a screw on the motor flywheel. The electrical pulse was recorded on an Elcomatic ultraviolet paper recorder. The Doppler probe was immersed in distilled water at room temperature. Sound waves were reflected from the base of the
6
J. Biomed. Eng. 1979, Vol. 1, January
of the angle measuring
technique
As has been mentioned earlier, in order to quantitate blood velocity using the Doppler equation, it is necessary to evaluate the angle of incidence between the probe and the axis of the underlying artery. The present system is a technique modified after Olson (1974). The simple hand held unit, shown, diagrammatically in Figure 3 was made in the physics laboratory at St. George’s Hospital. Pulse eCh0
probe
Figure 3 Alignment of the probes showing the area of maximum sensitivity (shaded) of the Dopfiler probe. The 8 MHz A-scan transducer (Kretztechnik Series 4100 MG) was mounted perpendicular to the polythene holder. The Doppler transducer (9.5 MHz)
Ultrasonic
measurement
was set at an angle of 45” to the A-scan transducer. Both transducers could be moved along the holder and locked in any position. The distance between the ultrasonic Doppler crystals and the pulse echo crystal (AB) was adjusted so that the interaction of both beams was approximately at the depth of the artery. For brachial artery peak velocity measurements the distance between the crystals was maintained constant for all individuals at 8mm.
Having palpated the brachial artery and placed ‘Aquasonic’ gel over the area, the position of the Ascan transducer was adjusted so that maximal pulse echo signals were received from the posterior and anterior walls of the artery. The Doppler probe was then manipulated on the skin surface around the pulse echo probe to maximise the audio Doppler shift. Only in this unique position was it possible to quantitate the angle of interaction of the ultrasonic beam to the axis flow, in this case 45”, and consequently evaluate peak velocity. Assessment
of the angle
measuring
technique
In order to assess the degree of accuracy with which the developed technique measured the angle of incidence of the ultrasonic beam to the artery, it was necessary to confirm that maximal signals were obtained from the pulse echo transducer only when perpendicular to the artery. The apparatus was set up as shown in the diagram (Figure 9). The tube of internal diameter 5 mm, held firm in a shallow dish, formed a closed system which was filled with milk. A peristaltic pump (WatsonMarlow H.R. flow inducer) simulated blood flow in an artery. Attached to the supporting dish was a protractor and a horizontal pointer set at 0“. The tube was set level with the pointer. The pulse echo ---Pulse Apuoson~c’
gel
1
I
echo probe
of brachial
artery peak velocity: S. Moss and D. Bennett
Assessment of the peak height analyser
To assist analysis of the Doppler velocity trace, a peak height analyser was constructed in the physics laboratory of St. George’s Hospital to the author’s specification. This was designed to detect the peak of the velocity signal and using the constant derived from the Doppler equation for an angle of 45’, present a two and a half digit display of peak velocity (ems J). Data was evaluated on a beat to beat basis, or as an average of ten consecutive beats. Appropriate settings of the threshold allowed only the peak velocity to be recorded, extraneous signals being excluded. For the purposes of brachial artery peak velocity measurements, the peak height analyser was set up to detect maximum zero-crossing frequency shifts of 5 kHz which at 4.5” is equal to a digital readout of 57 or 58 ems -t (57.4 ems -I calculated). The peak height analyser was connected to a fast thermal paper printer. In order to test the linearity of the peak height analyser, sine waves of known frequencies were fed into the Doppler velocity meter and velocity data recorded. To assess the accuracy of the digital display, the meter velocities were compared with peak velocities calculated from the Doppler equation and those evaluated from a paper trace of a brachial artery signal. The integroted
systems
Figure 5 shows diagrammatically the complete system used in the transcutaneous measurement of bmchial artery peak velocity. The Doppler and pulse echo transducers were placed on the skin overlying
Doppler zero crosser
-)
DC
amplifier
_
Oscilloscope
I
dis’mr
1
Digital printer
-
1
I
1
Il&?$-+~~
I
A-scan Supporting
-L
Peak height anolyser
I
I I
Tope recorder
-
izon+ol pointer
Figure 4 Diagram of the apparatus used in the assessment of the angle measuring technique. transducer was set by parallax at exactly 90” to the horizontal. When correctly aligned over the tube, ‘Aquasonic’ gel was placed on the tube and the pulse echo transducer set approximately 4 mm from its surface. The A-scan signals received on the oscilloscope at that position were photographed using a Polaroid camera. The tube was then moved through +2O relative to the vertical probe and the Ascan recorded. The tube was then repositioned at 90” and the A-scan again recorded. This procedure was repeated for +%Oand -2O and -go in the opposite direction. All results were recorded photographically.
display
Figure 5 Diagram of the complete system the brachial artery, which had been previously located by palpation. ‘Aquasonic’ gel was used as the coupling media. The probes were manoeuvred until maximal signals were received from the pulse echo transducer. The shifted frequency heard via stereo headphones was also fed in to the frequency to D.C. voltage zero-crosser system. The analogue voltage output was amplified via a D.C. differential amplifier displayed on an oscilloscope (Digitimer Ltd.) and fed on to a four channel Sony cassette data tape recorder (Model C - 4). Instantaneous beat by beat values of peak velocity were displayed on the peak height analyser and printed via a Date1 fast thermal printer. At a later date the stored information was played through the analyser using the averaging mode.
J. Biomed.
Eng. 1979, Vol. 1, January
7
Ultrasonic
measurement
of brachial
artery peak velocity: S. Moss and D. Bennett 2001
RESULTS
180 Assessment velocity
of the Parks Model
806
Doppler 160 t
meter
/
Graphs were plotted of input frequency against Doppler velocity meter voltage output at two gain settings of the Doppler (Figure 6). Results showed that the Doppler velocity meter voltage output was linear with respect to increasing input frequencies.
4020-
I/.
OO
I
1
20
40
I
60
a
I
60 Actual
I
I
I
I
I
100 velocity
120
140
160
180
20(
I00
120 ,
160 1
160 t
20(,
,
(cm -I)
200r 180 Doppler
Figure 6
voltage
i
output (V)
Assessment of the Parks Model 806 Doppler velocity meter - the relationship between increasing input frequency and voltage output at two gain settings.
Results from the steady flow rig experiment showed there to be a linear relationship between the actual velocity and the calculated Doppler velocity for each of the three angles (Table I). Using the chi-squared probability test for the goodness of fit, there was found to be no significant difference between observed and expected velocities for 50° (0.9 > P >O.g), and 60° (0.5 > P > 0.2). However, there was a significant difference between the two lines when considering an angle of 70” (0.01 > P > 0.001)
20
1
OO I/
201
40,
801 Actual
b
(Figure 7a, b, c).
60,
velocity
(ems-‘)
I401
.
Table 1: Data from the Steady Flow Rig Experiment.
Run No.
Vol mls
Time Sets
Flow mls-’
Velocity V = F/A cm+
39 42 72 170 210
10 5 5 5 5
3.9 8.4 14.4 34.0 42.0
12.2 26.3 45.0 106.3 131.3
6 :;:
34 49 68 140 240 300
10 5 5 5 5 5
3.4 9.8 13.6 28.0 48.0 60.0
10.6 30.6 42.5 87.5 150.0 187.5
6 :;’
35 :;
10 5 5 5 5
3.5 8.8 16.4 33.0 44.0
10.9 27.5 51.3 103.1 137.5
Deflection mms
F HZ
Velocity Doppler ems-L
L = 50” 1 2 3 4 5
L=60° 1 2 3 4 5 6
L = 700 1 2 3 4 5
8
165 220
J. Biomed. Eng. 1979, Vol. 1, January
870 2029 4058 8261 9855
11.0 25.6 51.2 104.2 124.3
42 70 76
870 2029 3043 6087 10145 11014
14.1 32.9 49.3 98.7 164.5 178.5
4 11 18 35 49
580 1594 2609 5072 7101
13.7 37.8 61.8 120.2 168.3
:
Ultrasonic
meosurement
of brochiol
artery peok velocity: S. Moss ond
D. Bennett
200 f 180-
/ 0
/ / / /
7 140 160# i
/
/ /
" 120 5 '; 100 %
/' / / / /
& 80 % z
/ / / //
40 60
/
/ O/ /
20- k /
I/
I
OO 20 C
Figure
I
I
I
I
40
60
80 Actual
100
I
I
120
velocity
140
I
160
I
I
160 200
Lems-‘)
7 Doppler velocity against actual velocity from the study flow rig for a probe angle of (a) 50° (b) 60° (c) 70”.- - - Line of no difference. Regression line (a) Vdop = 0.95 Vact + 2.54; r = 0.99; (6) Vdop = 0.98 Vact + 6.98; r = 0.99; (c) Vdop = 1.19 Vact t 1.78; r = 1.0.
OL 0
Frequency
Figure 8
Actual and Doppler velocities were calculated from the pulsatile rig experimental results, and are presented in Table 2.
Run No.
12
of probe
OSCillOtiOn
1
I
I5
I8
(HZ)
Frequency response of the Parks Model 806 Doppler velocity meter under pulsatile conditions.
Assessment of the angle measuring technique
The pulse echo signals received at the various angles studied were recorded on Polaroid photographs (Figure 10). Results show that with the probe at right angles to the tube, signals of approximately
Rig Experiment. Doppler
Actual velocity Electric Pulse Distance cm
I
I
9
error of the Parks Model 806 in measuring velocity is t2.8ems -1, However, it is possible that a percentage of this error is due to the mechanical limitations of the pulsatile rig in providing an accurate figure for the actual velocity. It is, therefore, considered that the maximum error in the measurement of blood velocity in vivo using the integrated system described in the text is 2.8 ems -r + 8%.
A graph of probe oscillation frequency against Doppler velocity was plotted (Figure 8). The frequency response of the Parks Doppler was shown to be linear to approximately 9 Hz. The results in the linear region of the graph were correlated with the corresponding actual velocities (r = 0.99, y = 1.08 x t2.84). A graph of this regression line was drawn and compared with the line of no difference (Figure 9). The Doppler velocity meter was found to over-estimate by 2.8 ems -* at an actual velocity of zero and by 6.8 ems-1 at an actual velocity of 50 ems-*. If it is assumed ,that the mechanical model is correct (i.e. there are no errors in calculating the ‘actual’ velocity from the mechanical model), the
Table 2: Data from the P&utile
/ 6
’ 3
Frequency HZ
Velocity ems-’
velocity Doppler Shift Hz
Deflection mms
1 2 3 4
91.0 47.5 33.5
1.1 2.1 3.0
0 6.9 13.2 18.8
15.0 21.5 32.5
: 7 8 9 10 11
23.5 16.5 13.5 11.0 8.5 8.0 7.5
4.3 6.1 7.4 9.1 11.8 12.5 13.3
27.0 38.3 46.5 57.2 74.1 78.5 83.6
42.0 54.5 71.0 80.0 92.5 93.5 96.5
.
J. Eiomed.
Velocity IXlS-*
1500 2150 3250
0 11.7 16.7 25.3
4200 5450 7100 8000 9250 9350 9650
32.7 42.5 55.3 62.3 72.1 72.8 75.2
Eng. 1979, Vol. 1, January
9
Ultrasonic
measurement
of brachial
Actual velaclty
Figure 9
artery peak velocity: S. Moss and D. Bennett
(cm-‘)
The correlation between Doppler velocity and actual velocity over the linear range of results obtained underpulsatileconditions.
equal maximal amplitude were received from the anterior and posterior walls. At -+2” from this original position, signals which were reduced in amplitude, were received only from the posterior wall of the tube. At ;-+!4Ofrom the original position, signals were received from both walls but with less than maximal amplitude. It was concluded, using this technique, that angles could be measured to an accuracy of + Howhich at the cosine of 45O + so represents an error of f 1.7% in the calculation of velocity. Assessment of the peak height analyser
Within the error of a two and a half digit display meter ( f 1.8% at 57cms J) of the peak height analyser there was found to be no significant difference (P>O.99) between the observed and calculated velocities. Subsequent analysis of a paper trace recording of a brachial artery blood velocity signal showed the values of peak velocity to concur with those displayed by the peak height analyser. The maximum pulse repetition rate for the digital display update was approximately 20 Hz. DISCUSSION
A commercially available Parks Doppler (Model 806) ultrasonic blood velocity meter has been assessed. The results showed that the output of the velocity meter was linear with respect to input frequencies of up to 5kHz. The velocity meter was linear and accurate to velocities of 70 ems -l at angles of 50” and 60” on a steady flow rig and had a linear frequency response to approximately 9 Hz on a pulsatile rig. Harmonic analysis of a blood velocity curve from the ascending aorta indicates that the majority of the frequency information is contained in the first 7-8 harmonics (McDonald, 1974)
10
J. Biomed.
Eng. 1979, Vol. 1, January
Figure 10
A-scan signals obtained from model. (a) 90’ (b) *XP (c) +2”.
equivalent, at a heart rate of 75 beats per minute to component frequencies of O-10 Hz. In order to quantitate maximum acceleration it is necessary to consider frequencies of up to 20Hz (Bennett et al. 1974). Results, therefore, indicate that the Parks Doppler Model 806 velocity meter is suitable for the measurement of arterial peak velocity, but due to the frequency response is inadequate for the determination of maximum acceleration. Since time
Ultrasonic
measurement
intervals from the velocity signals were not of interest, the technique developed for the assessment of the frequency response did not include the facility for measuring phase shift. Furthermore, it was considered that changes in phase shift of higher harmonics would not lead to significant differences in the measured values of peak velocity. Due to the Doppler ultrasonic technique employed, peak velocity is perhaps a misleading term and could more accurately be replaced by ‘weighted mean peak velocity’, that is, the maximum value of the mean of the velocity profile at the point of measurement. Factors which affect the weighting include the nonuniform distribution of the erythrocytes in the blood vessel, the non-ideal reflecting surface and shape of the red cells, the reduction in beam intensity across the vessel, the varying receiver sensitivity due to the distance from the reflecting particles, and the additional Doppler shift as a result of vessel wall and tissue movements (Wille, 1977). The technique described has now been used in a systematic way to measure peak velocity in a large number of normal subjects, and also in a group of patients admitted with acute myocardial infarction. The results show that most patients with acute myocardial infarction have significantly lower peak velocities than the age matched normal group. This work is being reported elsewhere. It is felt, therefore, that this technique may be of benefit in the assessment of acutely ill patients. CONCLUSION
A combined Doppler and A-scan pulse echo transducer system has been developed, tested and shown to be satisfactory for the measurement of brachial artery peak velocity. The technique has the advantage of being relatively inexpensive, simple,
of brachial
artery peak velocity: S. Moss and D. Bennett
noninvasive, painless and repeatable. It is suggested that the measurement of brachial artery peak velocity may be of use to the physician in the rapid assessment of cardiovascular function, and subsequent management of acutely ill patients. REFERENCES
Bennett, E.D., Else, W., Miller, G.H.H., Sutton, G.C., Miller, H.C., and Noble, M.I.M. (1974). Maximum acceleration of blood from the left ventricle in patients with ischaemic heart disease. Clinical Science Molecular Medicine 46,49-59. Duck, F.A., and Hodson, C.J. (1973). A practical method of eliminating the angular dependence of Doppler flow measurements. In ‘Proceedings of the Second World Congress on Ultrasonics in Medicine,’ Rotterdam. Hansen, L.P., Cross, G. and Light, L.H. (1976). Beatnangle independent Doppler velocity measurement in superficial vessels. Woodcock, J. Clinical blood flow measurements. Sector Publishing Ltd., London. McCarty, I(., Woodcock, J.P. (1974). The ultrasonic Doppler shift flowmeter: a new development. Biomedical Engineering 9, 336-341. McDonald, D.A. (1974). Blood flow in arteries. The Camelot Press Ltd., Southampton. McLeod, F.D. (1967). A directional Doppler flowmeter. Digest of the Seventh Int. Conf. on Medical and Biological Engineering. Stockholm 3587.7. Olson, R.M. (1974). Human carotid artery wall thickness, diameter and blood flow by a non-invasive technique. Journal of Applied Physiology 37, (6), 955-960. Wells, P.N.T. (1969). Physical principles of ultrasonic diagnosis. Academic Press, London and New York. Wille, S. (1977). A computer system for on-line decoding of ultrasonic Doppler Signals from blood flow measurements. Ultrasonics 15, (5), 226-230.
J. Biomed. Eng. 1979, Vol. 1, January
11
David Bennett
ABSTRACT The
Parks
Model
806
Doppler
to have a linear voltage frequencies velocities
of up to SkHz.
of 70 ems-t
blood velocity
meter was shown
output with respect to input It was lineor
at angles
of 50°
and accurate and 60*
to
to the flow, on
INTRODUCTION
Despite the increasing use of instrumentation in the monitoring of acutely ill patients, there remains a need for a reliable, non invasive, inexpensive technique for the routine assessment of cardiovascular function. The ultrasonic Doppler technique applies the Doppler principle which states, that if there is relative motion between a transmitter and receiver there will be a change in frequency of a wave detected by the receiver which is proportional to the velocity of the medium. The linear changes of frequency can be additive or subtractive depending on whether the reflecting surface is moving away or towards the transmitter (Wells 1969). The frequency shift of the back-scattered ultrasound is proportional to the blood velocity according to the formula: v=
AF
c
2
F cos B
flow rig, and shown
to have a linear frequency
response
to 9Hz.
pulse echo and Doppler
transducer arterial
system
A combined
has been developed
allowing
quontitation
of
peak veiocity.
echo and Doppler transducer measurement of this angle.
enabling
precise
It has been the aim of the present study to modify existing techniques in order to quantitate brachial artery peak velocity and present an on-line print-out of the data. MATEXAS
AND METHODS
The Parks Model 806 directional Doppler ultrasonic velocity meter was characterized with respect to known frequencies. Sine waves ranging from lkHz3kHz were fed into the zero-crosser input of the Doppler velocity meter. The analogue voltage output from the zero-crossing ratemeter was recorded on a Medelec fibre-optic ultraviolet recorder, the linearity of which had been previously checked. Evaluation of the Parks Model 806 Doppler Velocity meter on a stwdy flow rig
where v = velocity of the blood (cm+‘),, AF = frequency shift (Hz), c =velocity of sound in tissue (15.4 x lo4 ems-l), F = transmitted frequency (Hz), and e = angle of incidence between the ultrasonic beam and the arterial axis.
The development of the directional transcutaneous Doppler ultrasonic blood velocity meter (McLeod 1967) has permitted qualitative assessment of blood velocity signals. However, in order to quantitate blood velocity from the Doppler equation, knowledge of the angle of incidence between the ultrasonic beam and the arterial axis is required. To date, several techniques have been developed to measure this angle (Duck and Hodson 1974; Hansen et al. 1976; McCarty and Woodcock, 1974). Recently, a simple, reliable noninvasive system has been devised by Olson (1974), and uses a combined pulse Department of Medicine, St. George’s Hospital Medical School, Tooting, London. Susan Moss: Present Address: Department of Cardiovascular Medicine, The Radcliffe Infirmary, Oxford.
0141~5425/79/010005-07
a steady
$02.00
The Parks Doppler velocity meter was studied under steady flow conditions to establish the linearity of the response and to compare Doppler velocities to velocities calculated from known flow rates. The apparatus was set up as shown in the diagram (Figure I). A water pump connected to a reservoir and a Winchester bottle via Portex tubing of diameter 0.6 cm. maintained a constant 2 metre head
Stainless
lPlosticine mounting
Figure
I
Reservar
Diagram of the steady flow rig.
J. Biomed.
Eng. 1979, Vol. 1, January
5
Ultrasonic measurement of brachial artery peak velocity: S. Moss and D. Bennett
of pressure. The system was primed with a particulate solution of 25 litres of 0.9% sodium chloride and one pint of outdated human blood. The outlet tap of the Winchester was connected to a length of Portex tubing of diameter 0.6 cm. In order to maximise the reflection of ultrasound from the solution, it was necessary to ensure that the thickness of the interface was less than one wave length of the transmitted ultrasound. Consequently, 30 cm. from the end of the Portex tubing a stainless steel tube of internal diameter 0.64 cm. was inserted. One wall of this tube had been machined flat to a thickness of approximately 0.1 mm (wave length of transmitted frequency = 0.16 mm). For added stability, and to minimize the effect of vibration, the tube was mounted in plasticine. A hollow/was made in the plasticine, filled with ‘Aquasonic’ gel, and the probe inserted towards the direction of flow. The angles of 50”, 60”, and 70” from the axis of flow were studied. Flow rates were varied by a gate clip on the proximal tube. Release of the clamps on the distal tube enabled flow rates to be measured using a measuring cylinder and a centisecond timer (Griffin Triton Ltd.), At a given angle, flow rates were varied in stages and the Doppler analogue voltage output shift recorded on the Medelec fibre-optic ultra-violet paper recorder. The Doppler velocity meter was calibrated with a 1OkHz signal fed into the forward channel. The actual velocity was calculated by dividing the flow rate by the internal cross sectional area of the stainless steel tube and results compared with those calculated from the Doppler output.
_.--Common solid mounting for verrtcol rods
Figure 2 Diagram of the flulsatile rig. glass water bath. The Doppler analogue voltage output was recorded simultaneously. The Doppler velocity was calculated from the Doppler equation assuming the angle of incidence of the ultrasonic beam to be O”, and the velocity of ultrasound in distilled water to be 14.8 x lo4 ems -l (Wells, 1969) at room temperature. The actual peak velocity of the probe was calculated from the motor speed and the sweep radius according to the formula: v = 2nfr where V = peak velocity (ems-l), f = frequency (Hz), and r = sweep radius (ems). The sweep radius was set at 1 cm. The frequency response of the Parks velocity meter was established by comparing the Doppler velocity with the actual velocity over a range of frequencies. Development
Evaluation of the Parks Doppler model 806 on a pulsatile rig
The Parks Doppler velocity meter was studied under pulsatile conditions in order to assess the frequency response and to compare Doppler velocities with predetermined velocities. The apparatus was set up as show in Figure 2. The shaft of a variable speed motor (Parvalux Electric Motors Ltd.). was connected to a rotating arm. An adjustable pin on the rotating arm allowed the sweep radius to be varied. The pin was connected to a linear bearing which moved horizontally on a bar. The bar was constrained to move in the vertical plan@ by additional bearings moving on two vertical steel rods mounted in a solid base. The Doppler probe was held vertically in a fixed position on the horizontal bar. Thus, rotation of the motor shaft caused vertical sinusoidal movement of the Doppler probe. The magnitude and frequency of the probe movement of the Doppler probe. The magnitude and frequency of the probe movement was varied by selection of the motor speed and sweep radius. The frequency of the motor was monitored by a micro switch, triggered each revolution, by a screw on the motor flywheel. The electrical pulse was recorded on an Elcomatic ultraviolet paper recorder. The Doppler probe was immersed in distilled water at room temperature. Sound waves were reflected from the base of the
6
J. Biomed. Eng. 1979, Vol. 1, January
of the angle measuring
technique
As has been mentioned earlier, in order to quantitate blood velocity using the Doppler equation, it is necessary to evaluate the angle of incidence between the probe and the axis of the underlying artery. The present system is a technique modified after Olson (1974). The simple hand held unit, shown, diagrammatically in Figure 3 was made in the physics laboratory at St. George’s Hospital. Pulse eCh0
probe
Figure 3 Alignment of the probes showing the area of maximum sensitivity (shaded) of the Dopfiler probe. The 8 MHz A-scan transducer (Kretztechnik Series 4100 MG) was mounted perpendicular to the polythene holder. The Doppler transducer (9.5 MHz)
Ultrasonic
measurement
was set at an angle of 45” to the A-scan transducer. Both transducers could be moved along the holder and locked in any position. The distance between the ultrasonic Doppler crystals and the pulse echo crystal (AB) was adjusted so that the interaction of both beams was approximately at the depth of the artery. For brachial artery peak velocity measurements the distance between the crystals was maintained constant for all individuals at 8mm.
Having palpated the brachial artery and placed ‘Aquasonic’ gel over the area, the position of the Ascan transducer was adjusted so that maximal pulse echo signals were received from the posterior and anterior walls of the artery. The Doppler probe was then manipulated on the skin surface around the pulse echo probe to maximise the audio Doppler shift. Only in this unique position was it possible to quantitate the angle of interaction of the ultrasonic beam to the axis flow, in this case 45”, and consequently evaluate peak velocity. Assessment
of the angle
measuring
technique
In order to assess the degree of accuracy with which the developed technique measured the angle of incidence of the ultrasonic beam to the artery, it was necessary to confirm that maximal signals were obtained from the pulse echo transducer only when perpendicular to the artery. The apparatus was set up as shown in the diagram (Figure 9). The tube of internal diameter 5 mm, held firm in a shallow dish, formed a closed system which was filled with milk. A peristaltic pump (WatsonMarlow H.R. flow inducer) simulated blood flow in an artery. Attached to the supporting dish was a protractor and a horizontal pointer set at 0“. The tube was set level with the pointer. The pulse echo ---Pulse Apuoson~c’
gel
1
I
echo probe
of brachial
artery peak velocity: S. Moss and D. Bennett
Assessment of the peak height analyser
To assist analysis of the Doppler velocity trace, a peak height analyser was constructed in the physics laboratory of St. George’s Hospital to the author’s specification. This was designed to detect the peak of the velocity signal and using the constant derived from the Doppler equation for an angle of 45’, present a two and a half digit display of peak velocity (ems J). Data was evaluated on a beat to beat basis, or as an average of ten consecutive beats. Appropriate settings of the threshold allowed only the peak velocity to be recorded, extraneous signals being excluded. For the purposes of brachial artery peak velocity measurements, the peak height analyser was set up to detect maximum zero-crossing frequency shifts of 5 kHz which at 4.5” is equal to a digital readout of 57 or 58 ems -t (57.4 ems -I calculated). The peak height analyser was connected to a fast thermal paper printer. In order to test the linearity of the peak height analyser, sine waves of known frequencies were fed into the Doppler velocity meter and velocity data recorded. To assess the accuracy of the digital display, the meter velocities were compared with peak velocities calculated from the Doppler equation and those evaluated from a paper trace of a brachial artery signal. The integroted
systems
Figure 5 shows diagrammatically the complete system used in the transcutaneous measurement of bmchial artery peak velocity. The Doppler and pulse echo transducers were placed on the skin overlying
Doppler zero crosser
-)
DC
amplifier
_
Oscilloscope
I
dis’mr
1
Digital printer
-
1
I
1
Il&?$-+~~
I
A-scan Supporting
-L
Peak height anolyser
I
I I
Tope recorder
-
izon+ol pointer
Figure 4 Diagram of the apparatus used in the assessment of the angle measuring technique. transducer was set by parallax at exactly 90” to the horizontal. When correctly aligned over the tube, ‘Aquasonic’ gel was placed on the tube and the pulse echo transducer set approximately 4 mm from its surface. The A-scan signals received on the oscilloscope at that position were photographed using a Polaroid camera. The tube was then moved through +2O relative to the vertical probe and the Ascan recorded. The tube was then repositioned at 90” and the A-scan again recorded. This procedure was repeated for +%Oand -2O and -go in the opposite direction. All results were recorded photographically.
display
Figure 5 Diagram of the complete system the brachial artery, which had been previously located by palpation. ‘Aquasonic’ gel was used as the coupling media. The probes were manoeuvred until maximal signals were received from the pulse echo transducer. The shifted frequency heard via stereo headphones was also fed in to the frequency to D.C. voltage zero-crosser system. The analogue voltage output was amplified via a D.C. differential amplifier displayed on an oscilloscope (Digitimer Ltd.) and fed on to a four channel Sony cassette data tape recorder (Model C - 4). Instantaneous beat by beat values of peak velocity were displayed on the peak height analyser and printed via a Date1 fast thermal printer. At a later date the stored information was played through the analyser using the averaging mode.
J. Biomed.
Eng. 1979, Vol. 1, January
7
Ultrasonic
measurement
of brachial
artery peak velocity: S. Moss and D. Bennett 2001
RESULTS
180 Assessment velocity
of the Parks Model
806
Doppler 160 t
meter
/
Graphs were plotted of input frequency against Doppler velocity meter voltage output at two gain settings of the Doppler (Figure 6). Results showed that the Doppler velocity meter voltage output was linear with respect to increasing input frequencies.
4020-
I/.
OO
I
1
20
40
I
60
a
I
60 Actual
I
I
I
I
I
100 velocity
120
140
160
180
20(
I00
120 ,
160 1
160 t
20(,
,
(cm -I)
200r 180 Doppler
Figure 6
voltage
i
output (V)
Assessment of the Parks Model 806 Doppler velocity meter - the relationship between increasing input frequency and voltage output at two gain settings.
Results from the steady flow rig experiment showed there to be a linear relationship between the actual velocity and the calculated Doppler velocity for each of the three angles (Table I). Using the chi-squared probability test for the goodness of fit, there was found to be no significant difference between observed and expected velocities for 50° (0.9 > P >O.g), and 60° (0.5 > P > 0.2). However, there was a significant difference between the two lines when considering an angle of 70” (0.01 > P > 0.001)
20
1
OO I/
201
40,
801 Actual
b
(Figure 7a, b, c).
60,
velocity
(ems-‘)
I401
.
Table 1: Data from the Steady Flow Rig Experiment.
Run No.
Vol mls
Time Sets
Flow mls-’
Velocity V = F/A cm+
39 42 72 170 210
10 5 5 5 5
3.9 8.4 14.4 34.0 42.0
12.2 26.3 45.0 106.3 131.3
6 :;:
34 49 68 140 240 300
10 5 5 5 5 5
3.4 9.8 13.6 28.0 48.0 60.0
10.6 30.6 42.5 87.5 150.0 187.5
6 :;’
35 :;
10 5 5 5 5
3.5 8.8 16.4 33.0 44.0
10.9 27.5 51.3 103.1 137.5
Deflection mms
F HZ
Velocity Doppler ems-L
L = 50” 1 2 3 4 5
L=60° 1 2 3 4 5 6
L = 700 1 2 3 4 5
8
165 220
J. Biomed. Eng. 1979, Vol. 1, January
870 2029 4058 8261 9855
11.0 25.6 51.2 104.2 124.3
42 70 76
870 2029 3043 6087 10145 11014
14.1 32.9 49.3 98.7 164.5 178.5
4 11 18 35 49
580 1594 2609 5072 7101
13.7 37.8 61.8 120.2 168.3
:
Ultrasonic
meosurement
of brochiol
artery peok velocity: S. Moss ond
D. Bennett
200 f 180-
/ 0
/ / / /
7 140 160# i
/
/ /
" 120 5 '; 100 %
/' / / / /
& 80 % z
/ / / //
40 60
/
/ O/ /
20- k /
I/
I
OO 20 C
Figure
I
I
I
I
40
60
80 Actual
100
I
I
120
velocity
140
I
160
I
I
160 200
Lems-‘)
7 Doppler velocity against actual velocity from the study flow rig for a probe angle of (a) 50° (b) 60° (c) 70”.- - - Line of no difference. Regression line (a) Vdop = 0.95 Vact + 2.54; r = 0.99; (6) Vdop = 0.98 Vact + 6.98; r = 0.99; (c) Vdop = 1.19 Vact t 1.78; r = 1.0.
OL 0
Frequency
Figure 8
Actual and Doppler velocities were calculated from the pulsatile rig experimental results, and are presented in Table 2.
Run No.
12
of probe
OSCillOtiOn
1
I
I5
I8
(HZ)
Frequency response of the Parks Model 806 Doppler velocity meter under pulsatile conditions.
Assessment of the angle measuring technique
The pulse echo signals received at the various angles studied were recorded on Polaroid photographs (Figure 10). Results show that with the probe at right angles to the tube, signals of approximately
Rig Experiment. Doppler
Actual velocity Electric Pulse Distance cm
I
I
9
error of the Parks Model 806 in measuring velocity is t2.8ems -1, However, it is possible that a percentage of this error is due to the mechanical limitations of the pulsatile rig in providing an accurate figure for the actual velocity. It is, therefore, considered that the maximum error in the measurement of blood velocity in vivo using the integrated system described in the text is 2.8 ems -r + 8%.
A graph of probe oscillation frequency against Doppler velocity was plotted (Figure 8). The frequency response of the Parks Doppler was shown to be linear to approximately 9 Hz. The results in the linear region of the graph were correlated with the corresponding actual velocities (r = 0.99, y = 1.08 x t2.84). A graph of this regression line was drawn and compared with the line of no difference (Figure 9). The Doppler velocity meter was found to over-estimate by 2.8 ems -* at an actual velocity of zero and by 6.8 ems-1 at an actual velocity of 50 ems-*. If it is assumed ,that the mechanical model is correct (i.e. there are no errors in calculating the ‘actual’ velocity from the mechanical model), the
Table 2: Data from the P&utile
/ 6
’ 3
Frequency HZ
Velocity ems-’
velocity Doppler Shift Hz
Deflection mms
1 2 3 4
91.0 47.5 33.5
1.1 2.1 3.0
0 6.9 13.2 18.8
15.0 21.5 32.5
: 7 8 9 10 11
23.5 16.5 13.5 11.0 8.5 8.0 7.5
4.3 6.1 7.4 9.1 11.8 12.5 13.3
27.0 38.3 46.5 57.2 74.1 78.5 83.6
42.0 54.5 71.0 80.0 92.5 93.5 96.5
.
J. Eiomed.
Velocity IXlS-*
1500 2150 3250
0 11.7 16.7 25.3
4200 5450 7100 8000 9250 9350 9650
32.7 42.5 55.3 62.3 72.1 72.8 75.2
Eng. 1979, Vol. 1, January
9
Ultrasonic
measurement
of brachial
Actual velaclty
Figure 9
artery peak velocity: S. Moss and D. Bennett
(cm-‘)
The correlation between Doppler velocity and actual velocity over the linear range of results obtained underpulsatileconditions.
equal maximal amplitude were received from the anterior and posterior walls. At -+2” from this original position, signals which were reduced in amplitude, were received only from the posterior wall of the tube. At ;-+!4Ofrom the original position, signals were received from both walls but with less than maximal amplitude. It was concluded, using this technique, that angles could be measured to an accuracy of + Howhich at the cosine of 45O + so represents an error of f 1.7% in the calculation of velocity. Assessment of the peak height analyser
Within the error of a two and a half digit display meter ( f 1.8% at 57cms J) of the peak height analyser there was found to be no significant difference (P>O.99) between the observed and calculated velocities. Subsequent analysis of a paper trace recording of a brachial artery blood velocity signal showed the values of peak velocity to concur with those displayed by the peak height analyser. The maximum pulse repetition rate for the digital display update was approximately 20 Hz. DISCUSSION
A commercially available Parks Doppler (Model 806) ultrasonic blood velocity meter has been assessed. The results showed that the output of the velocity meter was linear with respect to input frequencies of up to 5kHz. The velocity meter was linear and accurate to velocities of 70 ems -l at angles of 50” and 60” on a steady flow rig and had a linear frequency response to approximately 9 Hz on a pulsatile rig. Harmonic analysis of a blood velocity curve from the ascending aorta indicates that the majority of the frequency information is contained in the first 7-8 harmonics (McDonald, 1974)
10
J. Biomed.
Eng. 1979, Vol. 1, January
Figure 10
A-scan signals obtained from model. (a) 90’ (b) *XP (c) +2”.
equivalent, at a heart rate of 75 beats per minute to component frequencies of O-10 Hz. In order to quantitate maximum acceleration it is necessary to consider frequencies of up to 20Hz (Bennett et al. 1974). Results, therefore, indicate that the Parks Doppler Model 806 velocity meter is suitable for the measurement of arterial peak velocity, but due to the frequency response is inadequate for the determination of maximum acceleration. Since time
Ultrasonic
measurement
intervals from the velocity signals were not of interest, the technique developed for the assessment of the frequency response did not include the facility for measuring phase shift. Furthermore, it was considered that changes in phase shift of higher harmonics would not lead to significant differences in the measured values of peak velocity. Due to the Doppler ultrasonic technique employed, peak velocity is perhaps a misleading term and could more accurately be replaced by ‘weighted mean peak velocity’, that is, the maximum value of the mean of the velocity profile at the point of measurement. Factors which affect the weighting include the nonuniform distribution of the erythrocytes in the blood vessel, the non-ideal reflecting surface and shape of the red cells, the reduction in beam intensity across the vessel, the varying receiver sensitivity due to the distance from the reflecting particles, and the additional Doppler shift as a result of vessel wall and tissue movements (Wille, 1977). The technique described has now been used in a systematic way to measure peak velocity in a large number of normal subjects, and also in a group of patients admitted with acute myocardial infarction. The results show that most patients with acute myocardial infarction have significantly lower peak velocities than the age matched normal group. This work is being reported elsewhere. It is felt, therefore, that this technique may be of benefit in the assessment of acutely ill patients. CONCLUSION
A combined Doppler and A-scan pulse echo transducer system has been developed, tested and shown to be satisfactory for the measurement of brachial artery peak velocity. The technique has the advantage of being relatively inexpensive, simple,
of brachial
artery peak velocity: S. Moss and D. Bennett
noninvasive, painless and repeatable. It is suggested that the measurement of brachial artery peak velocity may be of use to the physician in the rapid assessment of cardiovascular function, and subsequent management of acutely ill patients. REFERENCES
Bennett, E.D., Else, W., Miller, G.H.H., Sutton, G.C., Miller, H.C., and Noble, M.I.M. (1974). Maximum acceleration of blood from the left ventricle in patients with ischaemic heart disease. Clinical Science Molecular Medicine 46,49-59. Duck, F.A., and Hodson, C.J. (1973). A practical method of eliminating the angular dependence of Doppler flow measurements. In ‘Proceedings of the Second World Congress on Ultrasonics in Medicine,’ Rotterdam. Hansen, L.P., Cross, G. and Light, L.H. (1976). Beatnangle independent Doppler velocity measurement in superficial vessels. Woodcock, J. Clinical blood flow measurements. Sector Publishing Ltd., London. McCarty, I(., Woodcock, J.P. (1974). The ultrasonic Doppler shift flowmeter: a new development. Biomedical Engineering 9, 336-341. McDonald, D.A. (1974). Blood flow in arteries. The Camelot Press Ltd., Southampton. McLeod, F.D. (1967). A directional Doppler flowmeter. Digest of the Seventh Int. Conf. on Medical and Biological Engineering. Stockholm 3587.7. Olson, R.M. (1974). Human carotid artery wall thickness, diameter and blood flow by a non-invasive technique. Journal of Applied Physiology 37, (6), 955-960. Wells, P.N.T. (1969). Physical principles of ultrasonic diagnosis. Academic Press, London and New York. Wille, S. (1977). A computer system for on-line decoding of ultrasonic Doppler Signals from blood flow measurements. Ultrasonics 15, (5), 226-230.
J. Biomed. Eng. 1979, Vol. 1, January
11