Imaging of the Thoracic Aorta with Time-Resolved Three-Dimensional Phase-Contrast MRI: A Review Thomas A. Hope, MD,* and Robert J. Herfkens, MD † Time-resolved three-dimensional (3D) phase-contrast (PC) magnetic resonance imaging (MRI), or four-dimensional (4D) flow, is able to provide robust 3D images with three- directional velocities. This review discusses the technique and application of 4D flow in the imaging of thoracic aortic pathologies. It has been instrumental in describing normal flow patterns throughout the cardiac cycle in the ascending and descending aorta and has shown the variety of flow patterns that exist in ascending aortic aneurysms. Semin Thorac Cardiovasc Surg 20:358-364 © 2008 Elsevier Inc. All rights reserved. KEYWORDS 4D flow, phase contrast, thoracic aorta I maging has become critical in the management of thoracic aortic pathology. Computed tomography is the preferred modality for anatomic evaluation. Simple measurements such as aortic diameter can provide prognostic information about the risk of rupture and have become central to manag- ing ascending aortic aneurysms. Configuration of the aneu- rysm is also important, for example, in distinguishing my- cotic and atherosclerotic aneurysms. To date, information about flow patterns has not been clinically used in evaluation of thoracic aortic pathology. This is partly due to the fact that echocardiography is limited to measurements of unidirec- tional velocities perpendicular to the transducer and is lim- ited by the lack of acoustic window for the thoracic aorta in many cases. Although magnetic resonance imaging (MRI) is second to CT in anatomic evaluation, it is able to provide functional information that may have utility in thoracic aortic disease management. Two-dimensional (2D) phase contrast (PC) provides information about flow through prospectively prescribed planes but does not describe three-dimensional (3D) flow patterns. Time-resolved 3D PC MRI, or four di- mensional (4D) flow, is able to provide robust 3D images with three-directional velocities at each pixel in the volume (Fig. 1). 4D Flow Technique Magnetic Resonance Imaging (MRI) and Phase Contrast (PC) MRI is based on imaging of hydrogen nuclei in a strong magnetic field. Radiofrequency pulses are used to localize the distribution of hydrogen in the human body, and signal in- tensity is based on the chemical properties of the tissue, spe- cifically their T1 and T2 relaxivities. Tissue movement results in alteration of the MR signal that is taken advantage of with PC imaging to measure velocities. 1 In PC, a bipolar magnetic gradient is applied to the tissue, which encodes motion en- abling MR to measure velocities. In static tissue, the two equal but opposite poles of the bipolar gradient cancel out but if a tissue moves between the timing of the two poles the phase of the tissue will be shifted proportional to the tissue’s velocity. By comparing images with and without the applied bipolar gradient, one can accurately calculate velocities at each pixel in an image. Phase Contrast to 4D Flow Initially, phase contrast was limited to measuring a single velocity component, usually perpendicular to a plane of in- terest. This technique was used to show skewed flow in the ascending aorta. 2 PC images provide information about di- rection and when integrated over time give accurate flow quantification. 2D PC has become an important tool in eval- uation of congenital heart disease and valvular abnormali- ties. 3-5 With time, all three velocity components were mea- sured along a single plane, which was used to depict the normal flow patterns in the thoracic aorta, including the first description of the right hand helical flow pattern in the as- *Department of Radiology, University of California San Francisco, San Fran- cisco, California. †Department of Radiology, Stanford University, Stanford, California. Address reprint requests to Robert J. Herfkens, MD, Department of Radiol- ogy, Stanford University, Lucas MR Center, 1201 Welch Road, Stanford, CA 94305-5488. E-mail: herfkens@stanford.edu 358 1043-0679/08/$-see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1053/j.semtcvs.2008.11.013