494 Principles of Optical Fiber Grating Sensors the FBG is its small form factor, as the diameter of the hypodermic needle is only slightly larger than the optical fiber ($0.2 mm), minimizing tissue damage and discomfort to the patient. This technique benefits from the fact that the small form factor of the sensors do not interfere with other diagnostic tools such as ultrasonic imaging, although they are still visible to x-rays for manipulation and monitoring. 10.4.2 Respiration Monitoring Chest strain can be used for monitoring patients, as breathing is extremely difficult to monitor. In this application, an FBG is attached to an elastic belt, which is held in position just above the breast. As the patient breathes, the tho- rax cage distends and deflates rhythmically, providing a method for monitoring respiration. In addition, the frequency of the signal can be used to trigger corrective action should the patient be under stress [103]. Figure 10.47 shows a schematic of the respiratory sensor. Optical Fiber FBG Elastic Belt Figure 10.47 FBG respiratory belt to monitor breathing in patients. (Adapted from reference [103].) 10.4.3 Oil, Gas, and Mining More recent applications are in the oil, gas, and mining industries. For exam- ple, it is necessary to know when an oil or gas pipeline might fail. Thus, a quasi- continuous monitoring procedure is necessary to detect impending problems. A distributed FBG system is attached to the pipe and continuously monitored for vibration and or strain. Weakening of the pipe structure is detected through a sudden change in strain or vibration signature. Preventive action is thus possible before major oil or gas leaks occur. Temperature monitoring is also important for oil wells. Here, the tempera- ture requirements for FBG sensors are quite demanding as the temperature