A system and method for characterising a narrowing or stenosis in a blood vessel is disclosed. Assessment or measurement of the constriction is helpful to review the extent and location of the constriction. The system includes a probe (6) having a first measurement sensor (7) to take an instantaneous measurement at different locations along the tube and a mechanism to draw the probe through the tube. Position (D1-3) is measured to provide data (P1-4) relating to the location at which a respective instantaneous measurement is taken by the first measurement sensor. The system may have a further sensor (9) so that two instantaneous measurements are taken, one by the further sensor at a substantially constant location along the tube and another by the first sensor at different locations along the tube. The line or wire between the two sensors is used to draw the probe through the tube to alter the distance between the first sensor and the second sensor. This information is used to calculate a characteristic, such as the instantaneous pressure ratio, of the tube at different locations along the tube which is useful in identifying a location for a stent.
