Recently, in diagnosis of pulmonary embolism and pulmonary tumor, CT images are used mainly. In the diagnosis of pulmonary embolism, the information of pulmonary arteries is necessary, and in the discrimination of the benign and malignant pulmonary tumors, the information of pulmonary veins is necessary. So, it is important to classify pulmonary blood vessels into pulmonary arteries and pulmonary veins. But, it is impossible to distinguish easily between pulmonary arteries and pulmonary veins, because there are not differences of pulmonary arteries and pulmonary veins on CT values. Moreover, in the improvement for performance of CT devices, the number of slices of CT images increases. So, it has become the burdens of the radiologists. We have developed an CAD system for classifying pulmonary arteries and pulmonary veins automatically for decreasing the burdens of the radiologists. As stated above, pulmonary blood vessels can't be classified easily by a threshold processing. In this study, we made use of the relation that pulmonary arteries are in parallel to the bronchial tubes. We used the distance transform for the pulmonary blood vessels and the bronchial tubes using this relationship, and tried to classify pulmonary blood vessels based on the information of distance between the pulmonary blood arteries and the bronchial tubes. In our method, we performed classification for pulmonary blood vessels and bronchial tubes except for the pulmonary hilum, because the relation between pulmonary blood vessels and the bronchial tubes can't be used in the pulmonary hilum.
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