A method for determining an ROI in medical imaging may include receiving first position information related to a body contour of a subject with respect to a support from a flexible device configured with a plurality of position sensors. The flexible device may be configured to conform to the body contour of the subject, and the support may be configured to support the subject. The method may also include generating a 3D model of the subject based on the first position information. The method may further include determining an ROI of the subject based on the 3D model of the subject.