A brief review on recently developed non-invasive techniques for gas-liquid and gas-liquid-solid systems is presented first. The ultrasonic computed tomography (UCT) developed for measuring the time-averaged cross-sectional distributions of gas and solid holdups in a slurry bubble column is then described. The ultrasonic tomography is a coupling of the earlier developed transmission-mode ultrasonic technique with two-parameter sensing (the energy attenuation and the velocity change) and a tomographic reconstruction technique using a filtered back-projection algorithm. The ultrasonic tomographic technique provides a method capable of measuring the cross-sectional distributions of the gas and the solid concentrations simultaneously in a gas-liquid-solid system. The technique has been applied to a slurry bubble column to get the general structures of the distributions of gas bubbles and solid particles in the column. The macroscopic flow scheme of the slurry bubble column is illustrated based on the structure of the gas holdup distribution. And the solid particle concentrating mechanism in the slurry bubble column along with the effects of gas velocity, solid loading and particle diameter is discussed.