Objective     To investigate on the possibility of automatic segmentation of lung tissue in the thoracic MRI images.     Methods     According to the features of thoracic MRI images

an algorithm for the segmentation of lung tissue was proposed. It was characterized by the initial rough segmentation and then followed by fine segmentation. Initially

the region of lung tissue was extracted roughly in the threshold method. Then the background was unified by means of the connected region labeling. Finally

the morphological method was adopted to smooth the edge of the segmented lung tissue

fill the internal small defects and remove the attached trachea as well as main bronchus.      Results     The edges of the automatically segmented lung tissues in all images were distinct and almost matched the original ones. The results were also consistent with the manually segmented ones. The average dice similarity index was beyond 89%

and could be identified as effective segmentation.     Conclusion     The algorithm is accurate and valid for automatic segmentation of lung tissue in the thoracic MRI images collected at different instants in a respiratory cycle.