Investigation on the ability of an ultrasound bubble detector to deliver size measurements of gaseous bubbles in fluid lines by using a glass bead model

Detectors based on ultrasonic principles are today's state of the art devic es to detect gaseous bubbles that may be present in extracorporeal circuits (ECC) for various reasons. Referring to theoretical considerations and oth er studies, it also seems possible to use this technology to measure the si ze of detected bubbles, thus offering the chance to evaluate their potentia l hazardous effect if introduced into a patient's circulation. Based on the se considerations, a commercially available ultrasound bubble detector has been developed by Hatteland Instrumentering, Norway, to deliver bubble size measurements by means of supplementary software. This device consists of a n ultrasound sensor that can be clamped onto the ECC tubing, and the necess ary electronic equipment to amplify and rectify the received signals. It is supplemented by software that processes these signals and presents them as specific data. On the basis of our knowledge and experience with bubble de tection by ultrasound technology, we believe it is particularly difficult t o meet all the requirements for size measurements, especially if these are to be achieved by using a mathematical procedure rather than exact devices. Therefore, we tried to evaluate the quality of the offered bubble detector in measuring bubble sizes. After establishing a standardized test stand, i ncluding a roller pump and a temperature sensor, we performed several sets of experiments using the manufacturers software and a program specifically designed at our department for this purpose. The first set revealed that th e manufacturer's recommended calibration material did not meet essential re quirements as established by other authors. Having solved that problem, we could actually demonstrate that the ultrasonic field, as generated by the b ubble detector, has been correctly calculated by the manufacturer. Simply, it is a field having the strongest reflecting region in the center, subsequ ently losing strength toward the ECC tubing's edge. The following set of ex periments revealed that the supplementary software not only does not compen sate for the ultrasonic field's inhomogeneity, but, furthermore, delivers r esults that are inappropriate to the applied calibration material. In the l ast set of experiments, we were able to demonstrate that the signals as rec orded by the bubble detector heavily depend upon the circulating fluid's te mperature, a fact that the manufacturer does not address. Therefore, it see ms impossible to resolve all these sensor related problems by ever-increasi ng mathematical intervention. We believe it is more appropriate to develop a new kind of ultrasound device, free of these shortcomings. This seems to be particularly useful, because the problem of determining the size of gase ous bubbles in ECC is not yet solved.