An apparatus for generating an image of a blood vessel having a wall, the apparatus comprising: an intravascular imaging system comprising: a catheter for insertion into the blood vessel, a transducer configured to generate a signal indicative of a geometry of the wall, a lumen defined by the wall, and a portion of a side branch joined to the wall; and an image circuit coupled to the transducer, wherein the circuit is configured to construct an intravascular image of the blood vessel and the portion of a side branch from the signal, wherein the intravascular image comprises successive intravascular image frames; an angiography system configured to generate an angiogram showing the blood vessel and the portion of the side branch of the blood vessel; and a computer in data communication with the catheter-based intravascular imaging system and the angiography system, wherein the computer is programmed to: reconstruct an image of the catheter in three-dimensional space from the angiogram; identify blood vessel lumen boundaries including the portion of the side branch of successive intravascular image frames and reconstruct the lumen of the blood vessel by stacking together successive intravascular image frames; correct for any relative rotation between successive intravascular image frames caused by bending or torsion of the catheter to produce a corrected three-dimensional intravascular image cast of the blood vessel corrected for relative orientation; overlay the angiogram and the corrected intravascular image cast; and orient the corrected intravascular image cast such that the portion of the side branch in the corrected intravascular image cast aligns with a corresponding portion of the side branch in the angiogram to provide an image of the blood vessel that is corrected for relative orientation and absolute orientation with respect to a coordinate system of the patient. an intravascular imaging system comprising: a catheter for insertion into the blood vessel, a transducer configured to generate a signal indicative of a geometry of the wall, a lumen defined by the wall, and a portion of a side branch joined to the wall; and an image circuit coupled to the transducer, wherein the circuit is configured to construct an intravascular image of the blood vessel and the portion of a side branch from the signal, wherein the intravascular image comprises successive intravascular image frames; an angiography system configured to generate an angiogram showing the blood vessel and the portion of the side branch of the blood vessel; and a computer in data communication with the catheter-based intravascular imaging system and the angiography system, wherein the computer is programmed to: reconstruct an image of the catheter in three-dimensional space from the angiogram; identify blood vessel lumen boundaries including the portion of the side branch of successive intravascular image frames and reconstruct the lumen of the blood vessel by stacking together successive intravascular image frames; correct for any relative rotation between successive intravascular image frames caused by bending or torsion of the catheter to produce a corrected three-dimensional intravascular image cast of the blood vessel corrected for relative orientation; overlay the angiogram and the corrected intravascular image cast; and orient the corrected intravascular image cast such that the portion of the side branch in the corrected intravascular image cast aligns with a corresponding portion of the side branch in the angiogram to provide an image of the blood vessel that is corrected for relative orientation and absolute orientation with respect to a coordinate system of the patient. reconstruct an image of the catheter in three-dimensional space from the angiogram; identify blood vessel lumen boundaries including the portion of the side branch of successive intravascular image frames and reconstruct the lumen of the blood vessel by stacking together successive intravascular image frames; correct for any relative rotation between successive intravascular image frames caused by bending or torsion of the catheter to produce a corrected three-dimensional intravascular image cast of the blood vessel corrected for relative orientation; overlay the angiogram and the corrected intravascular image cast; and orient the corrected intravascular image cast such that the portion of the side branch in the corrected intravascular image cast aligns with a corresponding portion of the side branch in the angiogram to provide an image of the blood vessel that is corrected for relative orientation and absolute orientation with respect to a coordinate system of the patient.