A system and method for characterising a narrowing in a fluid filled tube, the system comprising: a probe having a first measurement sensor to take an instantaneous measurement at different locations along the tube； a mechanism to draw the probe through the tube； a position measure to provide location data relating to the location at which a respective instantaneous measurement is taken by the first measurement sensor； a processor to calculate, from the instantaneous measurements, a characteristic of the tube at different locations along the tube.
