Summary form only given. 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. We reported that when using the system shown, there is an error of less than 1 cm in the position of the permanent magnet when the distance between the MI sensor plane and the permanent magnet is less than or equal to 11 cm. However, the estimation error exceeds 1 cm when the distance exceeds 11 cm. This is believed to be due to errors in the calculation of the lead-field matrix. In other words, there is an array mismatch because of the existence of ferromagnetic materials surrounding the measurement system, e.g., beds and steel frames inside the building. In a hospital setting, because of the variety of human body sizes, the error in the estimated magnet position must be less than or equal to 1 cm even when the distance approaches 15 cm. To meet this requirement, we use pre data that are obtained by placing a permanent magnet at each position and in each direction to improve the accuracy with which the position of the permanent magnet is estimated. In this study, we apply the pre data to the minimum variance beamformer (MVBF), which is used for the MI sensor system.
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