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EE 5340/7340, SMU Electrical Engineering Department, ©2004
1
Carlos E. Davila, Electrical Engineering Dept.Southern Methodist University
slides can be viewed at: http:// www.seas.smu.edu/~cd/ee5340.html
EE 5340/7340 Introduction to Biomedical Engineering
Ultrasound Flowmetry II
EE 5340/7340, SMU Electrical Engineering Department, ©2004
2
A More Realistic Geometry
u
red bloodcell (RBC)
transmitted ultrasound
reflected ultrasound
t
r
ucos r
ucost
affects firstfrequency shift
affects secondfrequency shift
EE 5340/7340, SMU Electrical Engineering Department, ©2004
3
A More Realistic Geometry (cont.)
fc u
c ufr
tt
cos
cos
r
fuc
fd t r t cos cos
(see HW problems)
(1)
(2)
Doppler frequency shift: fd = fr - ft
EE 5340/7340, SMU Electrical Engineering Department, ©2004
4
Vector Notation
transmitted ultrasound
reflected ultrasound
t
r
kt
ukr
k kt t
t 2
k kr r
r
2
EE 5340/7340, SMU Electrical Engineering Department, ©2004
5
Vector Notation (cont.)
fc u
c ufr
tt
cos
cos
r
(1)
can be written as: f
ck
u k
ck
u k
frt
t
rr
t
1
1
since: x y x y cos
x
y
angle between thesevectors > 90o
(3)
EE 5340/7340, SMU Electrical Engineering Department, ©2004
6
Vector Notation (cont.)
fck
u kck
u k frr
rt
t t11
11
(3) becomes:
since: ck 2fand
we get: r r t tu k u k
or: d r t r tu k k
EE 5340/7340, SMU Electrical Engineering Department, ©2004
7
Transmitter/Receiver Configurations
the geometric configuration of the transmitter and receivercan greatly affect the sensitivity of the flowmeter:
d r tu k k
kt
kr
u
k kr t
d negative, sensitivity is maximized
assume k kr t , then Doppler shift depends primarily on angle between and
u
k kr t
u
EE 5340/7340, SMU Electrical Engineering Department, ©2004
8
Transmitter/Receiver Configurations (cont.)
kt
kr
u
k kr t 0
no sensitivity
kt
kr
u
k k ur t
no sensitivity
d r tu k k
EE 5340/7340, SMU Electrical Engineering Department, ©2004
9
Transmitter/Receiver Configurations (cont.)
kt
kr
u
tr
d r tu k k
u
k kr t
d negative, moderatesensitivity, also sensitive totransversal vessel wall motion
EE 5340/7340, SMU Electrical Engineering Department, ©2004
10
Angle of Transmitted Ultrasound
hand-held flow probes are calibrated assuming t = 45o.
if t is too small, ultrasound undergoes too much attenuation since it has a longer path from transmitter to reflector and back to sensor.
if t is too large (close to 90o), sensor becomes too sensitive to transverse motion.
error due to mispositioning the sensor away from 45o is not too severe (see HW).
EE 5340/7340, SMU Electrical Engineering Department, ©2004
11
Frequency Spectrum of Received Ultrasound
f
reflection from stationary objectstransverse vesselwall motion
forward blood flowreverse blood flow
sign
al p
ower
ft fd > 0 fd < 0
EE 5340/7340, SMU Electrical Engineering Department, ©2004
12
Doppler Frequency Shift Varies due to:
Acoustic beam illuminates a relatively large volume of blood within the vessel nonuniform velocity profile across section of vessel (e.g.
laminar flow has lower velocity near vessel walls) acoustic beam is nonuniform, effective angles t and r
vary over different locations If acoustic beam is focused so as to illuminate a very small
volume, then the reflector is seen for only a short period of time, this produces spreading in the frequency domain (e.g. in the limit the Fourier transform of an impulse function is a constant for all frequencies).
EE 5340/7340, SMU Electrical Engineering Department, ©2004
13
Continuous Wave (CW) Doppler Flowmeters-transmitter is excited continuously
skin
vessel
oscillator
transmitterRF amp
mixer audio
amp
speaker
zero-crossingdetector
LPF
recorder
LPFvr
vt vd va
2cos t t
2
EE 5340/7340, SMU Electrical Engineering Department, ©2004
14
CW Doppler Flowmeters (cont.)
v tt tcos
v tr t d cos
v t t
t td t t d
d t d
2
2
cos cos
cos cos
used trig identity:
cos cos cos cosu v u v u v 12
EE 5340/7340, SMU Electrical Engineering Department, ©2004
15
CW Doppler Flowmeters (cont.)
after LPF: v ta dcos note that flow direction is lost since
cos cos d dt t reverse blood flow cannot be distinguished from forward flow for ft in the 2 - 10 MHz range, fd varies from 10 Hz to 10 kHz,
corresponding to audio frequencies audible output devices are small, hand-held devices which are easy to
use. useful for diagnosis of vascular occlusion
EE 5340/7340, SMU Electrical Engineering Department, ©2004
16
Zero-Crossing Detectors
v ta dcos
zero-crossing detector output:
LPF output, crude flow signal:
t
t
t
hysteresis band
va must pass through entire hysteresis band to produce a pulse
EE 5340/7340, SMU Electrical Engineering Department, ©2004
17
Choice of Transmission Frequency
higher transmission frequencies tend to undergo better reflection (backscattering) when the reflectors are small relative the the wavelength of the ultrasound, as is the case with RBC’s, which produce Rayleigh scattering.
high frequencies are attenuated more than low frequencies. for shallow vessels, typically use ft = 4 - 10 MHz
for deeper penetration, use ft = 2 MHz
2 conflicting criteria:
EE 5340/7340, SMU Electrical Engineering Department, ©2004
18
Bi-directional Flowmeters: Quadrature Phase Detection
skin
vessel
oscillator
transmitterRF amp
mixer
vr
vt
LPFvd va
2cos t t2
90o phaseshift
2sin t t
2
LPFvdq vaq
-1
tan-1
d t
EE 5340/7340, SMU Electrical Engineering Department, ©2004
19
Quadrature Phase Detection (cont.)
v ta dcos
as seen before:
v tr t d cos
v t t
t tdq t t d
d t d
2
2
sin cos
sin sin
after LPF: v t taq d d sin sin
used trig identity: sin cos cos sinu v u v u v 12
EE 5340/7340, SMU Electrical Engineering Department, ©2004
20
Quadrature Phase Detection (cont.)
tan-1d t
sin
cos
d
d
t
t
t
d
sign of d is retained
EE 5340/7340, SMU Electrical Engineering Department, ©2004
21
Hardware for d Computation
+_
va
0
+_
vaq
0MMV
+_ LPFcomparator
comparator
differenceamp
arctan function not implemented in practice
d
“one shot”
a
b c
a’
d
e
EE 5340/7340, SMU Electrical Engineering Department, ©2004
22
Timing Diagram: Flow Towards Transducer
t
t
ta
a’
b
c
d
e
va
vaq
t
t
t
t
t
EE 5340/7340, SMU Electrical Engineering Department, ©2004
23
Timing Diagram: Flow Away From Transducer
t
t
ta
a’
b
c
d
e
va
vaq
t
t
t
t
t
EE 5340/7340, SMU Electrical Engineering Department, ©2004
24
Pulsed Doppler Flowmeters
Rather than CW excitation, the transducer is excited for a short time interval, a pulse of ultrasound is transmitted.
The resulting echoes reflect the Doppler shifts occurring along different locations within the vessel.
Can use the pulse echo information to image the flow profile along a section of the vessel.
Typically, transmit a 1 s pulse of 8 MHz ultrasound. Can use a single transducer for both transmission and
reception.
EE 5340/7340, SMU Electrical Engineering Department, ©2004
25
Pulsed Doppler Flowmeters (cont.)
transmitted pulse:
echo:
near-wallreflection
far-wallreflection
reflectionfrom vessel
center
vt
vr
eight cycles of 8-MHzultrasound
1 s
t
t
va va changes much slower than vrt
EE 5340/7340, SMU Electrical Engineering Department, ©2004
26
Pulsed Doppler Flowmeters (cont.)
skin
vessel
oscillator
transmitterRF amp
mixer audio
amp
LPFvr
vt vd va
2cos t t
21-2n DEMUX
n-bitcounter
quadrature phase detection can also be used
“gating”
EE 5340/7340, SMU Electrical Engineering Department, ©2004
27
Pulsed Doppler Flowmeters (cont.)
audio amp
va
1-8 DEMUX
3-bit counter
va scanned once for every transmitted pulse
scan
-1sc
an-2
scan
-3
cos d tn
t
vessel center
near wall
far wall
EE 5340/7340, SMU Electrical Engineering Department, ©2004
28
Pulse Repetition Rate
to avoid aliasing, t must satisfy the Nyquist criterion:
but 1/t is the pulse repetition rate, this imposes constraints on the maximum range, Rmax:
ft
fp d 1
2
Rcf
c t
pmax
2 2
EE 5340/7340, SMU Electrical Engineering Department, ©2004
29
Transit-Time Flowmeters
vesseluD
downstream transit time: tD
c udown cos
upstream transit time: tD
c uup cos
invasive
EE 5340/7340, SMU Electrical Engineering Department, ©2004
30
Transit-Time Flowmeters (cont.)
t t tDu
c u
Du
cdown up
2 2
2 2 2 2cos
cos
cos
since c u t is proportional to u
fudge factors: u u133.
u u107.
laminar flow
turbulent flow
u: average velocity along cross section of vessel
EE 5340/7340, SMU Electrical Engineering Department, ©2004
31
Stethoscope Pocket Doppler Flowprobe
Built-in probe Approximate 8 MHz frequency Operates on a standard 9 volt battery Stethoscope fitted to a Telex earphone outlet
courtesy of Park Medical
EE 5340/7340, SMU Electrical Engineering Department, ©2004
32
Pocket Doppler Flowprobe
Pocket Doppler FlowprobeBuilt-in signal processing for displaying recordings Factory set amplitude control Plug-in probes Dimensions: height: 3.8 cam width: 7.9 cam depth: 14.6 cam Built-in speaker Auto-shutoff Approximate 8 MHz frequency Operates on a standard 9 volt battery Auxiliary earphone output
courtesy of Park Medical
EE 5340/7340, SMU Electrical Engineering Department, ©2004
33
Features:
Auto-shutoff Built-in speaker Auxiliary earphone output Rechargeable battery Low battery indicator Recorder output 8 MHz to 9.7MHz frequencies available Various welded-metal cases available Nondirectional
courtesy of Park Medical
EE 5340/7340, SMU Electrical Engineering Department, ©2004
34
Bi-directional Flowmeter
Chart recorder Two chart speeds 5 mm / sec 25 mm / sec 40 mm chart paper Large speaker Auxiliary earphone output Rechargeable battery AC coupled pneumoplethysmograph DC coupled pneumoplethysmograph AC coupled photoplethysmograph DC coupled photoplethysmograph Dual frequency Approximate 8 MHz frequency
Approximate 4 MHz frequency
courtesy of Park Medical
EE 5340/7340, SMU Electrical Engineering Department, ©2004
35
Some systems combine B-mode scanning and Doppler flowmetry
© ATL