Early and chronic stroke patients tend to have trouble swallowing . To ensure that they are able to receive the required nutrition, a nasogastric tube is usually used to pour nutrients into the stomach. However, when the tube is inserted through the patient's nostril into their stomach, it may be accidentally inserted into the lungs, and this may be fatal if the nutrients are released into the lungs. To prevent this, a method to estimate the position of the tip of the tube has previously been proposed. This method involves attaching a permanent magnet to the nasogastric tube and arranging several magneto impedance (MI) sensors outside the body (Figs. 1 and 2) .