A system for characterising one or more narrowings in a fluid filled tube being a blood vessel, the system comprising: a catheter; a probe that emanates from an end of the catheter into the tube having a first pressure sensor to take an instantaneous pressure measurement at different locations along the tube; a further pressure sensor provided on the catheter to take an instantaneous pressure measurement at the end of the catheter, so that two instantaneous pressure measurements are taken, one by the further pressure sensor at a substantially constant location along the tube and another by the first pressure sensor at different locations along the tube; a mechanism to draw the probe along the tube; a position measure to provide location data relation to the location at which a respective instantaneous pressure measurement is taken by the first pressure sensor; and a processor that is configured to calculate, from the instantaneous pressure measurements obtained by the first pressure sensor and the further pressure sensor, ratios of the instantaneous pressure measurements for different locations along the tube to identify narrowings of the fluid filled tube having an occlusive value; wherein the mechanism is a motorized mechanism to draw the probe through the tube, characterized in that the processor is configured to compare a ratio of the instantaneous pressure measurements calculated for a current location along the tube against a moving average mean and standard deviation of the ratios of the instantaneous pressure measurements for proceeding n beats in a cardiac environment and to control the motorized mechanism such that a) the motorized mechanism continues with the draw, if the ratio calculated for the current location falls within a tolerance threshold defined by the moving average mean and standard deviation, and b) the motorized mechanism pauses the draw, if the ratio calculated for the current location falls outside the tolerance threshold. a catheter; a probe that emanates from an end of the catheter into the tube having a first pressure sensor to take an instantaneous pressure measurement at different locations along the tube; a further pressure sensor provided on the catheter to take an instantaneous pressure measurement at the end of the catheter, so that two instantaneous pressure measurements are taken, one by the further pressure sensor at a substantially constant location along the tube and another by the first pressure sensor at different locations along the tube; a mechanism to draw the probe along the tube; a position measure to provide location data relation to the location at which a respective instantaneous pressure measurement is taken by the first pressure sensor; and a processor that is configured to calculate, from the instantaneous pressure measurements obtained by the first pressure sensor and the further pressure sensor, ratios of the instantaneous pressure measurements for different locations along the tube to identify narrowings of the fluid filled tube having an occlusive value; wherein the mechanism is a motorized mechanism to draw the probe through the tube, characterized in that the processor is configured to compare a ratio of the instantaneous pressure measurements calculated for a current location along the tube against a moving average mean and standard deviation of the ratios of the instantaneous pressure measurements for proceeding n beats in a cardiac environment and to control the motorized mechanism such that a) the motorized mechanism continues with the draw, if the ratio calculated for the current location falls within a tolerance threshold defined by the moving average mean and standard deviation, and b) the motorized mechanism pauses the draw, if the ratio calculated for the current location falls outside the tolerance threshold.