Apparatus and method for monitoring a system in which a fluid flows and which is characterized by a change in the system with time in space. A preselected place in the system is monitored to collect data at two or more time points correlated to a system event. The data is indicative of a system parameter that varies with time as a function of at least two variables related to system wash-in and wash-out behavior. A calibration map is made on a calculated basis with each pixel or voxel representative of a color hue indicative of wash-out behavior and a color intensity indicative of wash-in behavior. When a satisfactory map is obtained, the collected data is processed on the basis of the map to obtain an image of the preselected place with each spatial unit thereof correlated with a color hue and a color intensity. Software and a data processing system are provided to develop the calibration map. The calibration map and image of the preselected place are also novel implementations. Two sets of maps and images can be acquired using different sets of time points.