An imaging catheter for measuring an internal structure in a body of a patient, the imaging catheter comprising: a hollow, flexible elongate member having a proximal portion and a distal portion with a central lumen extending therebetween, wherein the central lumen is sized to accommodate the passage of a guidewire; an imaging device coupled to the distal portion of the elongate member; a radiopaque element; and a plurality of inked markers, characterized in that: the radiopaque element is a flexible radiopaque element wound about the distal portion of the elongate member, the radiopaque element including a single length of material that is coiled into a coil having an alternating series of tightly wound sections and loosely wound sections, wherein the tightly wound sections comprise areas of greater radiopacity than the loosely wound sections such that the tightly wound sections effectively form radiopaque markers separated from each other by the loosely wound sections thereby allowing the imaging catheter to serve as an internal measuring device, wherein the elongate member comprises an inner tubular member defining the central lumen and an outer tubular member surrounding the inner tubular member, comprised of an optically clear material through which the inner tubular member may be seen, and defining an annular lumen between the inner tubular member and the outer tubular member, wherein the radiopaque element is disposed within the annular lumen, and wherein the inner tubular member includes said plurality of inked markers (505, 563, 564) at the proximal portion of the elongate member. a hollow, flexible elongate member having a proximal portion and a distal portion with a central lumen extending therebetween, wherein the central lumen is sized to accommodate the passage of a guidewire; an imaging device coupled to the distal portion of the elongate member; a radiopaque element; and a plurality of inked markers, characterized in that: the radiopaque element is a flexible radiopaque element wound about the distal portion of the elongate member, the radiopaque element including a single length of material that is coiled into a coil having an alternating series of tightly wound sections and loosely wound sections, wherein the tightly wound sections comprise areas of greater radiopacity than the loosely wound sections such that the tightly wound sections effectively form radiopaque markers separated from each other by the loosely wound sections thereby allowing the imaging catheter to serve as an internal measuring device, wherein the elongate member comprises an inner tubular member defining the central lumen and an outer tubular member surrounding the inner tubular member, comprised of an optically clear material through which the inner tubular member may be seen, and defining an annular lumen between the inner tubular member and the outer tubular member, wherein the radiopaque element is disposed within the annular lumen, and wherein the inner tubular member includes said plurality of inked markers (505, 563, 564) at the proximal portion of the elongate member. the radiopaque element is a flexible radiopaque element wound about the distal portion of the elongate member, the radiopaque element including a single length of material that is coiled into a coil having an alternating series of tightly wound sections and loosely wound sections, wherein the tightly wound sections comprise areas of greater radiopacity than the loosely wound sections such that the tightly wound sections effectively form radiopaque markers separated from each other by the loosely wound sections thereby allowing the imaging catheter to serve as an internal measuring device, wherein the elongate member comprises an inner tubular member defining the central lumen and an outer tubular member surrounding the inner tubular member, comprised of an optically clear material through which the inner tubular member may be seen, and defining an annular lumen between the inner tubular member and the outer tubular member, wherein the radiopaque element is disposed within the annular lumen, and wherein the inner tubular member includes said plurality of inked markers (505, 563, 564) at the proximal portion of the elongate member.