Tomography apparatus comprises: a plurality of electrodes arranged around a perimeter of a cross section of a sample volume for containing a liquid or mixed-phase sample, each electrode being arranged to be in electrical contact with a sample contained in the sample volume; measurement means adapted to perform a set of measurements, each measurement comprising driving a current between a first respective adjacent pair of said electrodes and measuring a voltage developed across a second respective adjacent pair of said electrodes; and processing means adapted to generate a tomogram indicative of sample conductivity over said cross section from said set of measurements. The processing means is arranged to calculate sample conductivity values of a portion of said cross section from said set of measurements and generate said tomogram from said calculated sample conductivity values of said portion using an assumption of symmetry.