1. A method, comprising: obtaining angiographic image data of a vessel segment from an imaging device positioned external to the vessel segment; creating a three-dimensional image of the vessel segment based upon the obtained angiographic image data; obtaining intravascular ultrasound (IVUS) image data of the vessel segment from an imaging device positioned within the vessel segment, wherein the IVUS image data comprises a series of intravascular images acquired as the imaging device positioned within the vessel segment is moved through and along the vessel segment; correlating IVUS image data to the three-dimensional image of the vessel such that each of the images of the series of intravascular images of the IVUS image data is correlated to a portion of the three-dimensional image of the vessel created based upon the obtained angiographic image data, wherein correlating the IVUS image data to the three-dimensional image of the vessel includes aligning the images of the series of intravascular images of the IVUS image data both axially and circumferentially to the corresponding portions of the three-dimensional image of the vessel, wherein the images of the series of intravascular images of the IVUS image data are axially aligned with the three-dimensional image of the vessel by determining a best axial fit between a lumen measurement of the vessel segment as depicted in the images of the series of intravascular images of the IVUS image data and a lumen measurement of the vessel segment as depicted in the three-dimensional image of the vessel segment based on the obtained angiographic image data, and wherein the images of the series of intravascular images of the IVUS image data are circumferentially aligned with the three-dimensional image of the vessel after the images of the series of intravascular images of the IVUS image data are axially aligned with the three-dimensional image of the vessel and wherein the images of the series of intravascular images of the IVUS image data are circumferentially aligned with the three-dimensional image of the vessel by determining a best angular fit between a lumen measurement of the vessel segment as depicted in the images of the series of intravascular images of the IVUS image data and a lumen measurement of the vessel segment as depicted in the three-dimensional image of the vessel segment based on the obtained angiographic image data; rendering simultaneously on a display an image of the series of intravascular images of the IVUS image data and at least the correlated portion of the three-dimensional image of the vessel as determined by the correlating step; wherein the lumen measurement is a cross-sectional area; and wherein determining the best axial fit between the cross-sectional area of the vessel segment as depicted in the images of the series of intravascular images of the IVUS image data and the cross-sectional area of the vessel segment as depicted in the three-dimensional image of the vessel segment based on the obtained angiographic image data comprises determining a minimum of the sum of the squared differences between the cross-sectional area of the vessel segment as depicted in the images of the series of intravascular images of the IVUS image data and the cross-sectional area of the vessel segment as depicted in the three-dimensional image of the vessel segment based on the obtained angiographic image data.