A medical imaging system (100) includes a stationary gantry (102) and a rotating gantry (104) that rotates around an examination region (108) about a z-axis. An air bearing (106) rotatably couples the rotating gantry (104) to the stationary gantry (102). A radiation source (110) is affixed to the rotating gantry (104) and rotates with the rotating gantry (104) and emits radiation that traverses the examination region. A detector array (112) is affixed to the rotating gantry (104) on an opposite side of the examination region with respect to the radiation source (110) and detects radiation traversing the examination region (108). A dynamic z-axis imbalance determination system (116) determines an imbalance of the rotating gantry (104) in the z-axis direction directly from the air bearing (106), and the determined imbalance is used to position a balance mass (114) affixed to the rotating gantry (104) in the z-axis direction, thereby balancing the rotating gantry (104) along the z-axis.