Failure-Avoidance Analysis: A Brief Introduction for Neuroradiologists

Neuroradiologic special procedures represent extraordinary feats of craftsmanship. To show respect for our patients, and for our teachers, we seek to bring forth the finest expression of our craft in each procedure. We all have our own little halls of fame, composed of revered individuals, who for us personify the essence of the craft. We have developed our own techniques over years, composites of what we have been taught and what we have learned by experience, often intu­ itively. Neuroradiologic special procedures represent extraordinary technical accomplishments. Using a superb battery of devices, we venture almost imperceptibly into once inviolable reaches of the body, sometimes to examine, sometimes to mod­ ify. The development and deployment of those devices are exercises in engineering. What are we? Are we craftspeople or are we engineers? Consciously or unconsciously, we have elements of both. Medicine has been, and to this day is, dominated by the tradition of craftsmanship. We are, after all, descended in part from barbers. We are familiar with the craftsperson 's paradigm: if I understand the prop­ erties and capabilities of my materials, and if I have developed a rational plan for the case, and if I prepare the patient properly, and if I have everything I might possibly need, then I hope to succeed. The engineering paradigm is somewhat less familiar to us. Engineers understand that all ma­ terials and all devices fail; they seek to define failure conditions. Physicians tend to personalize failure, and interpret it as a sign of personal shortcomings. Engineers tend to accept failure as a familiar phenomenon. They unemotionally dis­ cuss functions like the MTBF, the mean time between failures , an acceptance of the inevitability of failure. Engineers learn about the failure modes of materials and devices by testing, often by destructive testing. Destructive testing is an­ other way of saying breaking things. By breaking things many times, in many ways, under con­ trolled conditions, performance characteristics can be determined. The paper by Drs. Schueler and Ruefenacht, published in this edition of the American Journal of Neuroradiology (1), is an engineering analysis of some devices used in interventional neurora­ diology. They have measured the performance characteristics of endovascular balloons under standardized conditions in the laboratory. Since the laboratory conditions are similar to conditions found in the human body, the performance char­ acteristics defined in the laboratory can be ex­ trapolated to performance characteristics in living systems. In the laboratory we can learn the burst­ ing pressures of balloons, the aspect ratios (the ratio of length to diameter) that presage rupture, and other failure conditions. Armed with an en­ gineering analysis of the balloons, we can work within their performance envelopes, avoiding bal­ loon failure. By making observations during pro­ cedures on human patients, it would take much longer and be far more dangerous to develop the same data, and we might never understand them with the same certitude as we can with controlled laboratory experimentation. Engineering analysis of complex systems has been driven , in large part, by the aerospace in­ dustry. Enthusiasm for aviation has historically been tempered by concern over spectacular and appalling failures. In response to the failures, reliability theory and fault tree analysis were de­ veloped, beginning in the 1950s. Reliability the­ ory, which originated in the engineering sciences and has been developed by mathematicians and