1. A computer-implemented method for generating a positron emission tomography (PET) attenuation correction map from magnetic resonance (MR) images, said method performed by the computer comprising the steps of: segmenting a 3-dimensional (3D) magnetic resonance (MR) whole-body image of a patient into low-signal regions, fat regions, and soft tissue regions; classifying the low-signal regions as either lungs, bones, or air by identifying lungs, identifying an abdominal station, and identifying a lower body station; and generating an attenuation map from the segmentation result by replacing the segmentation labels with corresponding representative attenuation coefficients, wherein identifying an abdominal station comprises identifying an only large low-intensity connected component crossed by a line connecting a trachea with a crotch as a bladder; sorting all low intensity connected components along the posterior-anterior direction according to the position of their center-of-mass in the anterior-posterior direction; selecting a point along the anterior/posterior axis; and labeling a low-intensity component beyond the selected point as bone, and labeling a component before the selected point as air.