A method of indicating a stenosis in a vessel, the method comprising the steps of: injecting pulsed contrast boluses into the vessel at a known frequency by injecting a first bolus of contrast media into the vessel over a first time, terminating injection of contrast media over a second time that is after the first time, and injecting a second bolus of contrast media into the vessel over a third time that is after the second time, terminating injection of contrast media over a fourth time that is after the third time and injecting a third bolus of contrast media into the vessel over a fifth time that is after the fourth time; analyzing image data representing a first region of the vessel to determine a distance between the first bolus of contrast media and the second bolus of contrast media in the vessel; analyzing image data representing a second region of the vessel to determine a distance between the second bolus of contrast media and the third bolus of contrast media in the vessel; calculating the blood flow velocity for the first region of the vessel by multiplying the determined distance between the first bolus of contrast media and the second bolus of contrast media by the known frequency; calculating the blood flow velocity for the second region of the vessel by multiplying the determined distance between the second bolus of contrast media and the third bolus of contrast media by the known frequency; and characterised by comparing the calculated blood flow velocity for the first region of the vessel to the calculated blood flow velocity for the second region of the vessel to determine whether a difference between the calculated blood flow velocity for the first region of the vessel and the calculated blood flow velocity for the second region of the vessel exceeds a predetermined flow velocity differential threshold. a stenosis in a vessel, the method comprising the steps of: injecting pulsed contrast boluses into the vessel at a known frequency by injecting a first bolus of contrast media into the vessel over a first time, terminating injection of contrast media over a second time that is after the first time, and injecting a second bolus of contrast media into the vessel over a third time that is after the second time, terminating injection of contrast media over a fourth time that is after the third time and injecting a third bolus of contrast media into the vessel over a fifth time that is after the fourth time; analyzing image data representing a first region of the vessel to determine a distance between the first bolus of contrast media and the second bolus of contrast media in the vessel; analyzing image data representing a second region of the vessel to determine a distance between the second bolus of contrast media and the third bolus of contrast media in the vessel; calculating the blood flow velocity for the first region of the vessel by multiplying the determined distance between the first bolus of contrast media and the second bolus of contrast media by the known frequency; calculating the blood flow velocity for the second region of the vessel by multiplying the determined distance between the second bolus of contrast media and the third bolus of contrast media by the known frequency; and injecting pulsed contrast boluses into the vessel at a known frequency by injecting a first bolus of contrast media into the vessel over a first time, terminating injection of contrast media over a second time that is after the first time, and injecting a second bolus of contrast media into the vessel over a third time that is after the second time, terminating injection of contrast media over a fourth time that is after the third time and injecting a third bolus of contrast media into the vessel over a fifth time that is after the fourth time; analyzing image data representing a first region of the vessel to determine a distance between the first bolus of contrast media and the second bolus of contrast media in the vessel; analyzing image data representing a second region of the vessel to determine a distance between the second bolus of contrast media and the third bolus of contrast media in the vessel; calculating the blood flow velocity for the first region of the vessel by multiplying the determined distance between the first bolus of contrast media and the second bolus of contrast media by the known frequency; calculating the blood flow velocity for the second region of the vessel by multiplying the determined distance between the second bolus of contrast media and the third bolus of contrast media by the known frequency; and characterised by comparing the calculated blood flow velocity for the first region of the vessel to the calculated blood flow velocity for the second region of the vessel to determine whether a difference between the calculated blood flow velocity for the first region of the vessel and the calculated blood flow velocity for the second region of the vessel exceeds a predetermined flow velocity differential threshold.