Use of a Precise and Portable Flow Standard to Improve the Accuracy of Medical and Scientific Laboratory Flow Measurements
Sixteenth Southern Biomedical Engineering Conference, Proceedings, 10, (1997)
J.C. CONTI, E.R. Strope, L.D. Spence
Dynatek Dalta Scientific Instruments, PO Box 254, Galena, MO 65656
There is an ever increasing need in both medical imaging, industrial, and research laboratories for the use of any number of new and modern flow probes. Some of these probes are designed to measure flow rate and others, for instance, using laser or ultrasound, utilize Doppler analysis to measure velocity directly and flow rate indirectly. It is an extremely cumbersome operation to fabricate and/or utilize any steady state system with known flow characteristics to calibrate and check the accuracy of the various flow probes and systems that we have discussed. It is even more difficult in a pulsatile system Nonetheless, in spite of the difficulty of fabricating or using a flow standard, it is imperative that a device like this be used in many laboratories to assure accuracy. This is not only because the probes and instruments themselves are very complicated and therefore easily misadjusted, but also because many probes can be affected by normal use or dropping such that somewhat inaccurate measurements will be obtained from an instrument that appears to be functioning just fine. Built-in electronic algorithms do not substitute for a true flow measurement. Therefore the scientist or technologist who is dependent upon accurate flow or velocity measurements is in a quandary as to whether to accept the data that they are receiving or to embark on the known difficult path of using a home-built flow loop. We had been confronted in our laboratory on dozens of occasions with the need to have rapid, convenient, and accurate assessment of various electromagnetic, laser or ultrasonic flow systems. Our approach was to build a portable, pulsatile, and precise flow system that we could easily transport to whatever site was necessary to check probes or flow systems. The design and calibration of this flow standard is the subject of separate paper . We needed to do this to assure the accuracy of various medical device or laboratory systems that we use to assess flow in a number of scientific and research applications.
In order to convey this information in a coherent manner, we will present three different scenarios as separate presentations focusing primarily on the medical ultrasound field and then moving onto electromagnetic systems and then finally to laser flow systems.
I. Medical Ultrasound
Materials and Methods.
Fifteen medical institutions were visited after they agreed to cooperate with a study designed to evaluate the accuracy of their clinical ultrasound flow systems. At these institutions, a total of fifty-seven medical ultrasound systems from various manufacturers utilizing various probes were evaluated. Protocol was to set up the flow standard such that the medical technologist was unaware of the true peak velocity of the system.The technologist was then asked to report the peak velocity produced by that particular combination of instrument and probe. We found that this blinded technique was the only way to assure unbiased adjustment to the instrument. It also would best replicate behaviors occurring in the noninvasive laboratory when a patient was present.
The individual results from the various instruments were recorded and compiled with respect to accuracy of peak determinations regardless of the combination of probe frequency, pulse repetition rate, etc. Table one is a summary of the results and errors that were acquired with these instruments. Note the large range of errors in the determination of peak velocities. The severity of the errors did not appear to be related to the manufacturer or model of the device. Nor was it directly attributable to any particular user, probe type, or any recurring error in the setting of the instruments themselves. There did seem to be relationship between the age of the probe and the accuracy of the overall system. This is probably a result of the gradual orientation of crystals that occurs along with variations that arise with extended aging solid state ultrasonic generation devices.
Results from Clinical Ultrasound Instruments
* Sine wave flow symmetrical around 0 – cm/sec
The technologists were instructed on how the use of a flow standard can be incorporated into a laboratory by simply using a chart that includes a conversion factor generated from the calibration step. In all cases, the technologists seemed to have no trouble incorporating this step in their peak velocity determinations. Once the conversion factors were introduced into the evaluation process, it is not surprising to note that the peak velocity values were close to 100% in agreement with the flow standard. Most of the errors would not be considered catastrophic from a medical diagnosis standpoint (although some were rather large). The problem with moderately inaccurate flow and velocity calculations in this medical situation is that it becomes nearly impossible to track a patient from year to year or month to month to evaluate the progress of a disease. It is well known that timely intervention on the part of the surgeon or cardiologist when dealing with heart valve disease is critical. This timely intervention is even more important for a reoperation. The uncertainties associated with errors in the ten to forty percent range make it unlikely that month to month or year to year quantitation of flow conditions will be traceable.
It has been our experience that the use of a flow standard along with a simple conversion step is all that would be necessary to render all ultrasonic flow analysis nearly 100% accurate.
II. Electromagnetic Flow Probes.
Although calibration steps are not of great interest from the standpoint of data evaluation, we have used this flow standard repeatedly in our laboratory to check the accuracy of electromagnetic flow probes that are used external to the flow tube, as well as electromagnetic and ultrasonic flow probes mounted on catheters that are used in vivo . The utilization of an accurate and dependable flow standard coupled with the incorporation of various pieces of hardware (e.g. Twohy Borst adapter on the end plate) allows us to recheck these probes with little trouble in just a few minutes. Although many probes are sold with accompanying calibration factors, we have found it most beneficial and necessary to check the accuracy of these various probes on a daily basis. We feel the indwelling catheter based ultrasonic and electromagnetic probes need to be checked on a regular basis, particularly if they are used in medical laboratories to evaluate the flow conditions of patients. There is a great deal of controversy at the present time in the use of right heart catheterization to determine the flow conditions. Controversy seems to be focusing on the benefits to the patient of this somewhat invasive procedure. It is our contention that the uncertainty of the numbers generated by these indwelling probes is at least partially at the heart of the controversy.
III. Laser Flow Laboratories.
A recent application of the flow standard was its use in assisting the set-up, debugging and determination of the precision of a new laser flow system. Without the use of this convenient and portable system, it would have been extremely difficult to track down the various problems associated with the initial set-up of this type of system. In fact, once a laser system is set up, if there is no known pulsatile flow system available, it becomes somewhat difficult to assure oneself that all of the alignments, as well as the various support instrumentation, are functioning as expected. In our experience, one cannot be certain of the precision and accuracy of any flow analysis system without the use of a known flow standard. Without the use of a system like this, the operation of a new laser or electromagnetic system is lowered to a matter of faith in the instrumentation. With the use of a convenient and reliable flow standard properly configured for convenience and accuracy, confidence in the data from any flow measuring system is much improved.
We have used this previously described portable and precise flow standard to assist in the evaluation of a number of instruments and technologies that are commonly utilized to determine flow and/or velocity of a liquid in both the medical and non-medical laboratories. We have demonstrated the kind of error that one gets from medical ultrasound equipment when random checking of the equipment is performed. We have determined that it is easy for medical ultrasound technologists to incorporate a factor in their peak velocity or flow calculations to convert their qualitative data into reliable quantitative data.
The portable flow standard has been used extensively in our laboratory to allow quick and accurate checking of various electromagnetic ultrasonic and optical flow evaluation probes. Finally, we have utilized this portable flow standard to assist in the initial set-up and debugging of laser-based hydrodynamic systems and have found it indispensable in these particular applications. More work is ongoing evaluating the long-term reliability of this device.
 Manuscript submitted for publication