A system (100, 200) comprising: a catheter assembly (102, 240, 300, 500) including an intravascular measuring device (108, 309, 300, 500) having a measurement module configured to emit and receive energy and generate measurement data, the catheter assembly configured to introduce a quantity of fluid (410, 530) into a vessel of a patient; a measurement engine in communication with the intravascular measuring device comprising at least one processor, the measurement engine configured to: receive the measurement data from the intravascular measuring device; generate, using the at least one processor, a speckle density value based on the measurement data, the speckle density value corresponding to a concentration of unresolvable structure in the vessel; determine a start time associated with an introduction of the quantity of fluid into a predetermined portion of the vessel; determine, using the at least one processor, an end time based on the measurement data; calculate, using the at least one processor, an elapsed time based on the start time and the end time, the start time and the end time being when the speckle density value crosses a predetermined speckle density threshold (620, 622); and calculate, using the at least one processor, a flow rate through the vessel based on the elapsed time and a travel distance of the quantity of fluid during the elapsed time; wherein the intravascular measuring device (108, 309, 300, 500) comprises an IVUS imaging probe, and; characterized in that: the start time is when the speckle density value crosses a first predetermined speckle density threshold (620, 622) from a first higher speckle density value to a first lower speckle density value, and the end time is when the speckle density value crosses a second predetermined speckle density threshold from a second lower speckle density value to a second higher speckle density value. a catheter assembly (102, 240, 300, 500) including an intravascular measuring device (108, 309, 300, 500) having a measurement module configured to emit and receive energy and generate measurement data, the catheter assembly configured to introduce a quantity of fluid (410, 530) into a vessel of a patient; a measurement engine in communication with the intravascular measuring device comprising at least one processor, the measurement engine configured to: receive the measurement data from the intravascular measuring device; generate, using the at least one processor, a speckle density value based on the measurement data, the speckle density value corresponding to a concentration of unresolvable structure in the vessel; determine a start time associated with an introduction of the quantity of fluid into a predetermined portion of the vessel; determine, using the at least one processor, an end time based on the measurement data; calculate, using the at least one processor, an elapsed time based on the start time and the end time, the start time and the end time being when the speckle density value crosses a predetermined speckle density threshold (620, 622); and calculate, using the at least one processor, a flow rate through the vessel based on the elapsed time and a travel distance of the quantity of fluid during the elapsed time; receive the measurement data from the intravascular measuring device; generate, using the at least one processor, a speckle density value based on the measurement data, the speckle density value corresponding to a concentration of unresolvable structure in the vessel; determine a start time associated with an introduction of the quantity of fluid into a predetermined portion of the vessel; determine, using the at least one processor, an end time based on the measurement data; calculate, using the at least one processor, an elapsed time based on the start time and the end time, the start time and the end time being when the speckle density value crosses a predetermined speckle density threshold (620, 622); and calculate, using the at least one processor, a flow rate through the vessel based on the elapsed time and a travel distance of the quantity of fluid during the elapsed time; wherein the intravascular measuring device (108, 309, 300, 500) comprises an IVUS imaging probe, and; characterized in that: the start time is when the speckle density value crosses a first predetermined speckle density threshold (620, 622) from a first higher speckle density value to a first lower speckle density value, and the end time is when the speckle density value crosses a second predetermined speckle density threshold from a second lower speckle density value to a second higher speckle density value.