USE OF DOPPLER EQUIPMENT TO MEASURE blood flow
Experiment 1: Investigation of the relation between maximum flow velocity and pumping speed
With the restriction screws set to produce maximum clarity of oscilloscope trace, find the minimum pump speed that gives observable signals on the scope. Measure the height of the
forward flow trace on the oscilloscope. This trace height may be assumed to be proportional to the voltage output from the Dopplex, which is in turn proportional to the flow velocity. Use the oscilloscope trace to determine the pumping speed in cycles per second. Repeat these measurements for a number of pumping speeds between the minimum and maximum to gather enough data to be able to plot a graph of trace height against pumping speed. How are these quantities related? Is this what you would expect?
N.B. If you have to change the sensitivity controls on either the Dopplex or the oscilloscope during these measurements be sure to take this into account.
Experiment 2: Investigation of the relation between measured flow velocity and angle between probe and flow
With the equipment set up as in experiment 1, set the pumping speed to the maximum available. By changing the angle of the probe, determine the range of angles that give observable signals on the scope. Plot the variation of trace height with angle for values in this range. Plot a graph of trace height against the cosine of the angle. Is the result what you would expect from theory?
Experiment 3: Measurement of flow rate
Set the pumping speed as in Experiment 2 and adjust the Dopplex sensitivity control to x8 (3 bars on the LCD). Set the angle to about 45 degrees. You will use a digital oscilloscope to make your measurements. This has the advantage that you can save waveforms to a USB drive for later analysis. Ensure that you are able to save the waveforms in text format and that you are able to read the saved data back into Excel spreadsheet files. You may need one of the USB sticks compatible with the oscilloscope. You must return it at the end of the session! Select Save/recall button on oscilloscope. Accept the file name convention offered by the oscilloscope (it will have a .csv extension) and, if saving multiple data sets, keep a note of the content of each file. Each new file you save will be given a new number. You will find that the file contains typically 10,000 data points, with two columns of data, one representing time and the other the measured voltage, and, to the left, some information on the oscilloscope settings. Ensure that your forward trace is clear and that it shows the complete pulse as the pump acts, without any clipping. Repeat if necessary, adjusting the sensitivity until you get a satisfactory trace. You will need this trace to complete the remainder of this experiment. Without altering the pump speed measure the flow rate (m 3 s –1 ) by collecting the blood mimicking solution in a volume–calibrated beaker for a measured time. Repeat this several times to get good statistics of the measurement. Now examine your saved trace and by using the voltage and time data scales from the oscilloscope estimate the average signal for one pump pulse. By using the Doppler shift relation in Equation 1, taking the velocity of the ultrasound in the solution as 1500 ms –1, and given that the internal diameter of the tube is 6 mm, calculate the sensitivity of the transducer (i.e. how to convert its output voltage to Doppler shift frequency) using your measured flow rate. Once you have obtained the sensitivity, calculate, from the oscilloscope trace, the peak velocity of the fluid.
Experiment 4: Examination of arterial blood flow
SAFETY NOTE. It is essential in this part of the experiment to keep the probe head away from your eyes. Remove the probe from the flow rig and squeeze a good blob of coupling gel over its end. Locate your own or your partner’s carotid artery. This can be done by depressing your neck gently with a thumb or finger on either side of the trachea. Place the probe head over the artery with the probe making an angle of about 45 degrees to the skin. Move the probe around until the Dopplex LCD arrows indicate strong forward and reverse flow signals. Record a sample of the signal using the oscilloscope as before. Note any particular features of this trace and explain them in terms of flow in the artery concerned. Refer
to the Dopplex manual for examples.
N.B. Avoid getting a signal from a vein instead of from the artery – you can tell the difference from the blood flow direction.
The probe you have been using operates at a frequency of 5MHz, but the manufacturers can also supply probes that work at 2, 3, 4, 8, and 10 MHz, the 2 and 3 MHz ones being for obstetric examinations and the higher frequency ones for vascular work. Suggest why this range of frequencies is required, and account for the differences in responses they might produce.
