BME 310 Lab 7 Ultrasonic Flowmeter, John G. Webster3/5/00

Introduction: Ultrasonic flowmetry is a commonly used technique for measuring blood velocity in the peripheral arteries. As a result, it can also be used to determine the amount of stenosis, or narrowing, in an artery, since a narrow passage means higher blood velocity. Ultrasonic frequencies are generated and measured using piezoelectric crystals that convert electric signals to acoustic signals, and vice versa. The Doppler effect shifts the frequency of the ultrasonic beam from the oscillator when it intercepts the moving blood by an amount f which can be computed from

where f0 is the fundamental frequency of an ultrasonic wave from the source, traveling at velocity c through the blood. The ultrasonic wave intercepts a stream of moving blood with velocity u crossing the ultrasound beam at an angle  to produce the frequency shift f. The factor of 2 occurs because the Doppler shift arises both on absorption of the sound by the moving blood particles and between the transmitting blood cell and the receiving transducer. Water does not reflect ultrasound so we require particles in the liquid, such as red blood cells in blood or fat globules in milk.

Before the lab: Read material about ultrasonic flowmetry from J. G. Webster (ed.), Bioinstrumentation, Section 8.8.3 at the coursepage

Laboratory Equipment

  1. Huntleigh mini Dopplex VP8 Doppler flowmeter and gel
  2. Fisher Scientific variable speed pump
  3. Earphone plug
  4. Whole milk
  5. Clear tubing
  6. Plastic bowl

At the Lab

  1. Attach tubing to pump and plug earphone jack into the Doppler unit at the plug with the waveform symbol.
  2. Wire the other end of the earphone jack to an oscilloscope probe.
  3. Empty the carton of whole milk into the plastic bowl provided and place both tubes from the pump into the bowl.
  4. Place some gel on the OUT tube from the pump.
  5. Turn the pump on SLOW and FORWARD, setting the dial to 0.
  6. Turn on the Doppler unit and position the probe on the tube at about a 45 angle. Adjust position as necessary to achieve a signal that is free of noise from the pump.
  7. Use the STOP button on the oscilloscope to freeze the screen, and count the number of pulses you see for the 0 setting. Be sure to also record the average voltage amplitude of the peaks, and how high they are above the baseline. If the audio signal is available, record the average number of zero crossings per unit time.
  8. Press RUN and repeat step 7, adjusting the knob to 4, then 8.
  9. Clean out tubes and plastic bowl.
  10. Place some gel on your wrist and obtain a signal for arterial blood flow. Observe the signal.
  11. Perform a Valsalva maneuver, which is a forceful exhalation against a closed glottis. The increased pressure that develops within the thorax decreases venous return and increases heart rate. Record the change in heart rate.
  12. Occlude blood flow by pressing your finger on the artery inside your elbow. Observe the resulting signal.
  13. Release finger and observe the resulting signal.
  14. Place some gel on the inside of your elbow, just below where you have been occluding blood flow. Move the probe to obtain a signal for venous blood flow while avoiding the pulsating arterial signal. Observe the signal.
  15. Occlude blood flow again. Observe the signal.
  16. Release finger and observe the resulting signal.
  17. Inhale deeply, hold your breath briefly, and observe the resulting signal.

Results.

  1. Describe and explain what you observed in steps 7 and 8.
  2. Calculate the velocity of fluid in the tubing, assuming c is 1230 m/s. State any assumptions you make.
  3. Explain why are there two peaks for each flow sound in step 10.
  4. Describe and explain what happens in step 11.
  5. Describe and explain what happens in step 12.
  6. Explain how and why the arterial signal differs from the venous signal.
  7. Describe and explain what you observed in step 15.
  8. Describe and explain what you observed in step 16.
  9. Describe and explain what you observed in step 17.
  10. Describe what you would expect the signal of a partially occluded artery to look like.
  11. Explain why you use gel with ultrasonic probes.
  12. Theoretically, explain what is the best angle to use with a probe to measure frequency shifts.

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