DOI: 10.1148/rg.282075705
RadioGraphics 2008;28:407-416
© RSNA, 2008
Velocity-encoded Cine MR Imaging in Aortic Coarctation: Functional Assessment of Hemodynamic Events1
Jeffrey J. Hom, MD,
Karen Ordovas, MD, and
Gautham P. Reddy, MD, MPH
1 From the Department of Radiology, Thoracic Imaging Section, University of California, San Francisco, Box 0628, L-325, 505 Parnassus Ave, San Francisco, CA 94143-0628. Received February 5, 2007; revision requested May 15 and received June 10; accepted June 17. All authors have no financial relationships to disclose.
Address correspondence to G.P.R. (e-mail: Gautham.Reddy{at}radiology.ucsf.edu).
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Abstract
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Velocity-encoded cine magnetic resonance (MR) imaging is becoming the modality of choice for the clinical evaluation of aortic coarctation, a congenital narrowing of the thoracic aorta, in which a functional assessment of hemodynamic obstruction is as important as anatomic delineation. A flow-sensitive phase-contrast technique, velocity-encoded cine MR imaging is based on the principle that moving protons change phase in proportion to their velocity. Because it enables precise hemodynamic characterization, the technique is especially useful for evaluating the severity of aortic coarctation. By enabling a qualitative assessment of the presence and direction of collateral circulation, velocity-encoded cine MR imaging provides information about the presence and severity of obstruction. It also allows accurate quantitation of key hemodynamic parameters such as flow velocity, flow volume, and pressure gradients across the coarctation—functional information that is clinically useful for both preoperative planning and postinterventional monitoring. The results of recent experience indicate that velocity-encoded cine MR imaging also may be applicable for the detection of recurrent stenosis after stent placement or angioplasty.
© RSNA, 2008
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Introduction
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Aortic coarctation is a discrete narrowing of the aortic arch, just distal to the left subclavian artery. It is the most commonly encountered congenital anomaly of the thoracic aorta, with an estimated frequency of occurrence of five per 100,000 in the general population. In about 20% of those affected, the disorder is diagnosed during adolescence or adulthood (1). Major late complications of aortic coarctation may include systemic hypertension, accelerated atherosclerosis, and intracranial aneurysm rupture.
In the past, the radiologic diagnosis of aortic coarctation was based on the results of conventional arteriography. However, the drawbacks of this invasive imaging technique, including exposure to ionizing radiation and potentially nephrotoxic contrast media, have limited its use. In recent years, spin-echo (SE) magnetic resonance (MR) imaging has emerged as the standard of reference for the anatomic imaging–based diagnosis of aortic coarctation. SE MR imaging accurately depicts the location and degree of obstruction (Fig 1). However, it does not provide flow-sensitive hemodynamic measurements. This is an important distinction because in aortic coarctation, as in other forms of congenital heart disease, the evaluation of function is at least as important as morphologic delineation (2).
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Figure 1a. Anatomic delineation of aortic coarctation at SE MR imaging. (a) Axial image shows the oblique sagittal plane (white line) selected to depict both the ascending and the descending aorta. (b) Oblique sagittal image shows a focal stenosis of the aorta below the origin of the left subclavian artery.
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Figure 1b. Anatomic delineation of aortic coarctation at SE MR imaging. (a) Axial image shows the oblique sagittal plane (white line) selected to depict both the ascending and the descending aorta. (b) Oblique sagittal image shows a focal stenosis of the aorta below the origin of the left subclavian artery.
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For these reasons, velocity-encoded cine MR imaging is emerging as the modality of choice for the functional assessment of hemodynamic compromise in aortic coarctation and other forms of congenital heart disease. In this phase-contrast technique, which is based on the principle that the phase or spin angle of protons in motion changes in proportion to their velocity, a magnetic gradient is used to phase encode the velocity of flow. (3). The acquisition technique yields two MR signal data sets: one dependent on stationary spins and another derived from moving spins. By combining the MR signal data acquired in these two acquisitions, a magnitude image can be reconstructed. Alternatively, a phase image may be created from the phase difference (phase contrast) between stationary spins and moving spins (4,5). The gated MR signal data acquired throughout the cardiac cycle also can be segmented into multiple time-resolved (cine) images depicting the circulating blood flow within the thoracic aorta. Velocity-encoded cine MR imaging allows the measurement of blood flow in the heart and great vessels as well as the quantitation of collateral flow, pressure gradients, stenosis, and flow dynamics in congenital heart disease. 
Advantages of the technique include the ability to visualize the moving anatomy with high spatial resolution and accuracy without the use of radiographic contrast agents.
In contrast to echocardiography, the results of which are operator dependent, the data obtained with velocity-encoded cine MR imaging are generally reliable and reproducible. However, velocity-encoded cine MR imaging does have potential disadvantages. The velocity and volume of blood flow may be underestimated if the vessel of interest is not imaged in a plane exactly perpendicular to the direction of flow (6). Aliasing, or erroneous reconstruction of the MR signal, may occur if the expected maximum velocity is lower than the actual peak velocity at any time during the cardiac cycle. In addition, the evaluation of very small vessels with velocity-encoded cine MR imaging is suboptimal because such vessels occupy relatively few pixels. Furthermore, pressure gradient measurements are subject to error because they are calculated on the basis of the peak velocity, which may be underestimated when temporal resolution is lower than real time (7).
The purpose of this article is to describe the capabilities of phase-contrast velocity-encoded cine MR imaging to provide a functional assessment of hemodynamic compromise in patients with aortic coarctation. Those capabilities include determining the presence of collateral circulation, the direction of flow within a vessel (an indicator of collateral circulation), the volume of collateral circulation, the pressure gradients across the coarctation, and other flow parameters both before and after surgical repair, stent placement, or other intervention.
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Identifying Collateral Circulation
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Collateral circulation arises in patients with aortic coarctation because of perfusion demands below the level of coarctation. Normally, the intercostal and internal mammary arteries are a low-pressure, low-flow system, and blood flow is greater in the proximal than in the distal part of the aorta. However, in patients with moderate to severe aortic coarctation, the normal physiologic flow patterns are reversed, and blood flow is greater in the distal aorta. Additional blood is forced through the internal mammary arteries; flows retrograde via the intercostal arteries, sometimes producing the appearance of rib notching on chest radiographs; and, finally, reaches the distal descending aorta and the lower body.
The direct depiction of enlarged collateral vessels has proved a reliable indicator of the hemodynamic significance of aortic coarctation (8). The amount of collateral flow and its effect on the volume of blood flow in the descending aorta reflect the hemodynamic severity of the obstruction. Determining the amount of blood flow increase from the proximal to the distal descending aorta provides a direct measurement of the volume of collateral perfusion to the lower body (5).
In an experimental study in which a juxtaductal aortic stenosis (model coarctation) was surgically constructed in juvenile pigs, collateral circulatory pathways were either recruited or developed rapidly soon after the creation of the obstruction (9). Velocity-encoded cine MR imaging was used in that investigation to determine whether collateral flow was present. Likewise, in human patients, collateral vessels can be detected both with gadolinium-enhanced MR angiography and with phase-contrast cine MR imaging (Fig 2). Given the high signal intensity achieved with the use of gadolinium, contrast-enhanced MR angiography is especially useful for detecting small collateral vessels that may not be visible at SE MR imaging (10).
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Figure 2a. Depiction of collateral circulation at MR angiography and phase-contrast velocity-encoded cine MR imaging. (a, b) MR angiograms show the intercostal (arrowheads) and internal mammary (arrow) arteries in two patients. In a, an image obtained in a patient with a hemodynamically significant aortic coarctation, the arteries appear enlarged in comparison with those in b, an image obtained in a patient without aortic coarctation. (c) Axial magnitude image obtained with velocity-encoded cine MR imaging (same patient as in a) also shows the enlarged intercostal (arrowheads) and internal mammary (arrows) arteries.
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Figure 2b. Depiction of collateral circulation at MR angiography and phase-contrast velocity-encoded cine MR imaging. (a, b) MR angiograms show the intercostal (arrowheads) and internal mammary (arrow) arteries in two patients. In a, an image obtained in a patient with a hemodynamically significant aortic coarctation, the arteries appear enlarged in comparison with those in b, an image obtained in a patient without aortic coarctation. (c) Axial magnitude image obtained with velocity-encoded cine MR imaging (same patient as in a) also shows the enlarged intercostal (arrowheads) and internal mammary (arrows) arteries.
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Figure 2c. Depiction of collateral circulation at MR angiography and phase-contrast velocity-encoded cine MR imaging. (a, b) MR angiograms show the intercostal (arrowheads) and internal mammary (arrow) arteries in two patients. In a, an image obtained in a patient with a hemodynamically significant aortic coarctation, the arteries appear enlarged in comparison with those in b, an image obtained in a patient without aortic coarctation. (c) Axial magnitude image obtained with velocity-encoded cine MR imaging (same patient as in a) also shows the enlarged intercostal (arrowheads) and internal mammary (arrows) arteries.
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Determining the Direction of Flow
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The presence or absence of collateral circulation is determined on the basis of the direction of blood flow, information that conventional SE MR imaging cannot relay but that phase-contrast MR imaging is well suited to provide. At phase-contrast MR imaging, the detection of retrograde blood flow through the intercostal arteries toward the descending aorta is indicative of collateral circulation (Fig 3).
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Figure 3a. Determining the direction of blood flow with phase-contrast cine MR imaging. Magnitude (a) and phase (b) images acquired in the axial plane at the level of aortic coarctation show prominent intercostal arteries (arrows). In b, the round low-signal-intensity (dark gray) areas indicate caudal flow, which contrasts markedly with the large round high-signal-intensity area indicative of blood flowing cephalad through the ascending aorta.
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Figure 3b. Determining the direction of blood flow with phase-contrast cine MR imaging. Magnitude (a) and phase (b) images acquired in the axial plane at the level of aortic coarctation show prominent intercostal arteries (arrows). In b, the round low-signal-intensity (dark gray) areas indicate caudal flow, which contrasts markedly with the large round high-signal-intensity area indicative of blood flowing cephalad through the ascending aorta.
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In one study, two-dimensional phase-contrast MR imaging was performed in a cohort of five patients with aortic coarctation (5). Acquisitions were performed in a plane perpendicular to the direction of blood flow in the descending aorta, immediately below the coarctation site, by using an encoding velocity of 1.0 m/sec. The resultant images showed retrograde blood flow in relevant mediastinal or intercostal arteries adjacent to the aorta, a finding indicative of collateral flow. The investigators concluded that the presence of collateral flow in aortic coarctation is a reliable indicator of hemodynamically significant obstruction.
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Measuring Collateral Circulation
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Although the anatomic finding of aortic narrowing at SE MR imaging helps establish the diagnosis of aortic coarctation, an assessment of the clinical significance of coarctation in a given patient depends on the functional evaluation of the degree of hemodynamic obstruction. Historically, arm-leg blood pressure gradients were used to estimate the functional hemodynamic severity of aortic coarctation (11). This method was based on the assumption that normal volunteers or patients with a nonsignificant stenosis after a coarctation repair have no difference in systolic blood pressure measured in the arm or in the leg (12). However, the presence of collateral circulation may reduce the measured arm-leg pressure gradient, and this may create the misleading impression that coarctation is less severe than it actually is. Conversely, the arm-leg blood pressure gradient may be elevated by residual changes in sympathetic upregulation of blood pressure, even after a successful repair of aortic coarctation. Furthermore, there is only a poor correlation between the degree of anatomic obstruction and the arm-leg pressure gradient (8,13,14). The arm-leg pressure gradient thus is an unreliable indicator of the severity of hemodynamic obstruction in aortic coarctation. For similar reasons, estimation of the pressure gradient across the coarctation site by using Doppler ultrasonography (US) or invasive catheter-based measurements of pressure above and below the stenosis may yield unsatisfactory information about the hemodynamic severity of coarctation: These techniques may underestimate the degree of obstruction when a large amount of collateral flow is present (5,13). Echocardiography also is hampered by technical limitations, including a limited acoustic window and difficulties in interpreting complex flow patterns.
In velocity-encoded cine MR imaging, a flow-sensitive phase-contrast sequence is applied in a plane orthogonal to the direction of blood flow. Flow-sensitive imaging techniques permit the measurement of flow as either velocity or volume per unit of time (7). Mean spatial velocity and cross-sectional vessel area measurements are obtained at two levels of the descending thoracic aorta: just distal to the site of coarctation and just proximal to the diaphragm (Fig 4). The flow volume in each vessel cross section is calculated by multiplying the spatial mean velocity by the cross-sectional area of the vessel at that level. The flow volume of the collateral circulation is then obtained by subtracting the flow volume in the proximal descending aorta from that in the distal aorta. The percentage increase in flow from the proximal to the distal aorta bears a direct linear relationship to the percentage of aortic stenosis measured on MR images. The percentage increase in flow from the proximal to the distal descending thoracic aorta is therefore a reliable indicator of the degree of hemodynamic compromise (6,11).
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Figure 4a. Velocity-encoded cine MR imaging for quantification of collateral circulation. (a) Oblique sagittal SE MR image shows the planes (white lines) selected for velocity-encoded cine MR acquisitions. Both planes are perpendicular to the direction of blood flow in the descending aorta; one is just below the site of coarctation, and the other, at the level of the diaphragm. (b–e) Magnitude (b, d) and phase (c, e) images obtained from velocity-encoded acquisitions in proximal (b, c) and distal (d, e) locations of the descending aorta (indicated by white lines in a) show the placement of the region of interest (white circle) around the aorta. (f) Flow volume–time curves show the values measured in the proximal (squares) and distal (diamonds) locations during an average cardiac cycle. The volume of collateral flow was estimated by subtracting the proximal flow volume from the distal flow volume. In this patient the distal flow volume was greater than the proximal flow volume, a finding indicative of hemodynamically significant coarctation.
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Figure 4b. Velocity-encoded cine MR imaging for quantification of collateral circulation. (a) Oblique sagittal SE MR image shows the planes (white lines) selected for velocity-encoded cine MR acquisitions. Both planes are perpendicular to the direction of blood flow in the descending aorta; one is just below the site of coarctation, and the other, at the level of the diaphragm. (b–e) Magnitude (b, d) and phase (c, e) images obtained from velocity-encoded acquisitions in proximal (b, c) and distal (d, e) locations of the descending aorta (indicated by white lines in a) show the placement of the region of interest (white circle) around the aorta. (f) Flow volume–time curves show the values measured in the proximal (squares) and distal (diamonds) locations during an average cardiac cycle. The volume of collateral flow was estimated by subtracting the proximal flow volume from the distal flow volume. In this patient the distal flow volume was greater than the proximal flow volume, a finding indicative of hemodynamically significant coarctation.
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Figure 4c. Velocity-encoded cine MR imaging for quantification of collateral circulation. (a) Oblique sagittal SE MR image shows the planes (white lines) selected for velocity-encoded cine MR acquisitions. Both planes are perpendicular to the direction of blood flow in the descending aorta; one is just below the site of coarctation, and the other, at the level of the diaphragm. (b–e) Magnitude (b, d) and phase (c, e) images obtained from velocity-encoded acquisitions in proximal (b, c) and distal (d, e) locations of the descending aorta (indicated by white lines in a) show the placement of the region of interest (white circle) around the aorta. (f) Flow volume–time curves show the values measured in the proximal (squares) and distal (diamonds) locations during an average cardiac cycle. The volume of collateral flow was estimated by subtracting the proximal flow volume from the distal flow volume. In this patient the distal flow volume was greater than the proximal flow volume, a finding indicative of hemodynamically significant coarctation.
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Figure 4d. Velocity-encoded cine MR imaging for quantification of collateral circulation. (a) Oblique sagittal SE MR image shows the planes (white lines) selected for velocity-encoded cine MR acquisitions. Both planes are perpendicular to the direction of blood flow in the descending aorta; one is just below the site of coarctation, and the other, at the level of the diaphragm. (b–e) Magnitude (b, d) and phase (c, e) images obtained from velocity-encoded acquisitions in proximal (b, c) and distal (d, e) locations of the descending aorta (indicated by white lines in a) show the placement of the region of interest (white circle) around the aorta. (f) Flow volume–time curves show the values measured in the proximal (squares) and distal (diamonds) locations during an average cardiac cycle. The volume of collateral flow was estimated by subtracting the proximal flow volume from the distal flow volume. In this patient the distal flow volume was greater than the proximal flow volume, a finding indicative of hemodynamically significant coarctation.
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Figure 4e. Velocity-encoded cine MR imaging for quantification of collateral circulation. (a) Oblique sagittal SE MR image shows the planes (white lines) selected for velocity-encoded cine MR acquisitions. Both planes are perpendicular to the direction of blood flow in the descending aorta; one is just below the site of coarctation, and the other, at the level of the diaphragm. (b–e) Magnitude (b, d) and phase (c, e) images obtained from velocity-encoded acquisitions in proximal (b, c) and distal (d, e) locations of the descending aorta (indicated by white lines in a) show the placement of the region of interest (white circle) around the aorta. (f) Flow volume–time curves show the values measured in the proximal (squares) and distal (diamonds) locations during an average cardiac cycle. The volume of collateral flow was estimated by subtracting the proximal flow volume from the distal flow volume. In this patient the distal flow volume was greater than the proximal flow volume, a finding indicative of hemodynamically significant coarctation.
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Figure 4f. Velocity-encoded cine MR imaging for quantification of collateral circulation. (a) Oblique sagittal SE MR image shows the planes (white lines) selected for velocity-encoded cine MR acquisitions. Both planes are perpendicular to the direction of blood flow in the descending aorta; one is just below the site of coarctation, and the other, at the level of the diaphragm. (b–e) Magnitude (b, d) and phase (c, e) images obtained from velocity-encoded acquisitions in proximal (b, c) and distal (d, e) locations of the descending aorta (indicated by white lines in a) show the placement of the region of interest (white circle) around the aorta. (f) Flow volume–time curves show the values measured in the proximal (squares) and distal (diamonds) locations during an average cardiac cycle. The volume of collateral flow was estimated by subtracting the proximal flow volume from the distal flow volume. In this patient the distal flow volume was greater than the proximal flow volume, a finding indicative of hemodynamically significant coarctation.
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For an accurate assessment of collateral flow, velocity-encoded cine MR images must be reviewed carefully for evidence of aliasing. When images are degraded by aliasing artifacts, the misregistration of areas of high-velocity flow may lead to the underestimation of flow volume. Aliasing is easily corrected by increasing the encoding velocity for subsequent image acquisitions (Fig 5).
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Figure 5a. Aliasing artifact at phase-contrast velocity-encoded cine MR imaging in a patient with aortic coarctation. Both images were acquired in the same plane, perpendicular to the direction of blood flow through the descending aorta, at a level distal to the site of coarctation. (a) Image acquired with an encoding velocity of 3.5 m/sec shows a high-signal-intensity aliasing artifact (arrow) within an area of lower signal intensity. (b) Image obtained with a corrected encoding velocity of 4.5 m/sec shows the absence of the aliasing artifact (arrow).
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Figure 5b. Aliasing artifact at phase-contrast velocity-encoded cine MR imaging in a patient with aortic coarctation. Both images were acquired in the same plane, perpendicular to the direction of blood flow through the descending aorta, at a level distal to the site of coarctation. (a) Image acquired with an encoding velocity of 3.5 m/sec shows a high-signal-intensity aliasing artifact (arrow) within an area of lower signal intensity. (b) Image obtained with a corrected encoding velocity of 4.5 m/sec shows the absence of the aliasing artifact (arrow).
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Estimating Pressure Gradients
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Traditional measurements of the arm-leg blood pressure gradient are likely to be unreliable indicators of the presence and severity of aortic coarctation. However, estimates of pressure gradients across a coarctation, when they are obtained with velocity-encoded cine MR imaging, may be clinically useful. Data obtained from phase-contrast velocity-encoded cine MR imaging in planes perpendicular to the direction of blood flow through the aorta (through-plane acquisitions) can be used to measure peak flow velocity (v) across the area of maximal narrowing (Fig 6). The pressure gradient (
P) then can be estimated by using the modified Bernoulli equation P = 4(v2). The estimated pressure gradient may be useful for deciding whether surgical management of the coarctation is indicated. However, the threshold value for hemodynamic significance of aortic coarctation is somewhat arbitrary and may be influenced by the patient’s overall clinical status. One group of investigators suggested a pressure gradient of 15 mm Hg as an appropriate threshold for intervention (6). In another study, in which pressure gradients were measured with Doppler echocardiography, a threshold of more than 20 mm Hg was used to define moderate to severe coarctation (11).
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Figure 6a. Velocity-encoded cine MR imaging for measurement of pressure gradients across an aortic coarctation. (a, b) Magnitude (a) and phase (b) images acquired at the site of coarctation, in a plane parallel to the direction of blood flow (in-plane acquisition), show the placement of the region of interest (black contour line in b) around the flow jet to allow measurement of the peak velocity of the blood flow across the stenosis. The peak velocity was 2.9 cm/sec, which corresponds to a pressure gradient of 33 mm Hg estimated with the modified Bernoulli equation. (c, d) Magnitude (c) and phase (d) images acquired for comparison with a and b, at the level of coarctation and in a plane perpendicular to the direction of blood flow (through-plane acquisition), show a peak velocity of 2.3 cm/sec (estimated pressure gradient, 21 mm Hg) within the prescribed region of interest (white circle). The higher peak velocity value, which in this patient was that obtained from the in-plane acquisition, should be the one used for clinical decision making.
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Figure 6b. Velocity-encoded cine MR imaging for measurement of pressure gradients across an aortic coarctation. (a, b) Magnitude (a) and phase (b) images acquired at the site of coarctation, in a plane parallel to the direction of blood flow (in-plane acquisition), show the placement of the region of interest (black contour line in b) around the flow jet to allow measurement of the peak velocity of the blood flow across the stenosis. The peak velocity was 2.9 cm/sec, which corresponds to a pressure gradient of 33 mm Hg estimated with the modified Bernoulli equation. (c, d) Magnitude (c) and phase (d) images acquired for comparison with a and b, at the level of coarctation and in a plane perpendicular to the direction of blood flow (through-plane acquisition), show a peak velocity of 2.3 cm/sec (estimated pressure gradient, 21 mm Hg) within the prescribed region of interest (white circle). The higher peak velocity value, which in this patient was that obtained from the in-plane acquisition, should be the one used for clinical decision making.
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Figure 6c. Velocity-encoded cine MR imaging for measurement of pressure gradients across an aortic coarctation. (a, b) Magnitude (a) and phase (b) images acquired at the site of coarctation, in a plane parallel to the direction of blood flow (in-plane acquisition), show the placement of the region of interest (black contour line in b) around the flow jet to allow measurement of the peak velocity of the blood flow across the stenosis. The peak velocity was 2.9 cm/sec, which corresponds to a pressure gradient of 33 mm Hg estimated with the modified Bernoulli equation. (c, d) Magnitude (c) and phase (d) images acquired for comparison with a and b, at the level of coarctation and in a plane perpendicular to the direction of blood flow (through-plane acquisition), show a peak velocity of 2.3 cm/sec (estimated pressure gradient, 21 mm Hg) within the prescribed region of interest (white circle). The higher peak velocity value, which in this patient was that obtained from the in-plane acquisition, should be the one used for clinical decision making.
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Figure 6d. Velocity-encoded cine MR imaging for measurement of pressure gradients across an aortic coarctation. (a, b) Magnitude (a) and phase (b) images acquired at the site of coarctation, in a plane parallel to the direction of blood flow (in-plane acquisition), show the placement of the region of interest (black contour line in b) around the flow jet to allow measurement of the peak velocity of the blood flow across the stenosis. The peak velocity was 2.9 cm/sec, which corresponds to a pressure gradient of 33 mm Hg estimated with the modified Bernoulli equation. (c, d) Magnitude (c) and phase (d) images acquired for comparison with a and b, at the level of coarctation and in a plane perpendicular to the direction of blood flow (through-plane acquisition), show a peak velocity of 2.3 cm/sec (estimated pressure gradient, 21 mm Hg) within the prescribed region of interest (white circle). The higher peak velocity value, which in this patient was that obtained from the in-plane acquisition, should be the one used for clinical decision making.
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Pre- and Postinterventional Assessment
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Velocity-encoded cine MR imaging may be used to guide the clinical management of aortic coarctation before, during, and after intervention. Current methods of intervention may involve surgical repair with the placement of a synthetic patch, percutaneous balloon dilation, or imaging-guided stent placement. Evidence of hemodynamically significant collateral circulation at velocity-encoded cine MR imaging may be used to decide whether to intervene with surgical repair of a coarctation. If no collateral circulation is found in a mildly hypertensive patient with only mild aortic narrowing, medical antihypertensive therapy may be preferred (5). Given that surgical repair of aortic coarctation often involves aortic cross-clamping, a preoperative finding of absent collateral circulation may necessitate additional intraoperative techniques such as the creation of a left heart bypass, internal shunt, or jump graft to minimize the risk of spinal cord ischemia (11).
Postoperative evaluation of aortic blood flow with velocity-encoded cine MR imaging is useful for monitoring the outcome of surgical correction. The adequacy of synthetic patch repair, angioplasty, or stent placement may be represented by a return to normalcy in aortic flow patterns. Restenosis and aneurysm formation also may occur as serious complications after these procedures. Restenosis is estimated to occur in up to 40% of patients after intervention (8). Velocity-encoded cine MR imaging is useful for monitoring aortic blood flow and flow dynamics after angioplasty and stent placement because it enables the noninvasive detection of restenosis (Fig 7).
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Figure 7a. Hypertension detected at velocity-encoded cine MR imaging after stent placement for correction of aortic coarctation. (a) Oblique sagittal SE image shows the proximal and distal planes (white lines) selected for velocity-encoded cine imaging perpendicular to the direction of blood flow in the descending aorta: one at a level just below the stenosis, and the other at the level of the diaphragm. Note the irregular contour of the aorta, an appearance caused by a stent-related artifact. (b–e) Magnitude (b, d) and phase (c, e) images obtained in the proximal (b, c) and distal (d, e) locations show the prescribed region of interest (white circle) around the aorta. (f) Flow volume–time curve from an average cardiac cycle shows that despite the appearance of a low-grade stenosis in a, the distal flow volume (diamonds) was greater than the proximal flow volume (squares), a finding indicative of hemodynamically significant restenosis. Note the lack of normal phasic flow in comparison with Figure 4f.
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Figure 7b. Hypertension detected at velocity-encoded cine MR imaging after stent placement for correction of aortic coarctation. (a) Oblique sagittal SE image shows the proximal and distal planes (white lines) selected for velocity-encoded cine imaging perpendicular to the direction of blood flow in the descending aorta: one at a level just below the stenosis, and the other at the level of the diaphragm. Note the irregular contour of the aorta, an appearance caused by a stent-related artifact. (b–e) Magnitude (b, d) and phase (c, e) images obtained in the proximal (b, c) and distal (d, e) locations show the prescribed region of interest (white circle) around the aorta. (f) Flow volume–time curve from an average cardiac cycle shows that despite the appearance of a low-grade stenosis in a, the distal flow volume (diamonds) was greater than the proximal flow volume (squares), a finding indicative of hemodynamically significant restenosis. Note the lack of normal phasic flow in comparison with Figure 4f.
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Figure 7c. Hypertension detected at velocity-encoded cine MR imaging after stent placement for correction of aortic coarctation. (a) Oblique sagittal SE image shows the proximal and distal planes (white lines) selected for velocity-encoded cine imaging perpendicular to the direction of blood flow in the descending aorta: one at a level just below the stenosis, and the other at the level of the diaphragm. Note the irregular contour of the aorta, an appearance caused by a stent-related artifact. (b–e) Magnitude (b, d) and phase (c, e) images obtained in the proximal (b, c) and distal (d, e) locations show the prescribed region of interest (white circle) around the aorta. (f) Flow volume–time curve from an average cardiac cycle shows that despite the appearance of a low-grade stenosis in a, the distal flow volume (diamonds) was greater than the proximal flow volume (squares), a finding indicative of hemodynamically significant restenosis. Note the lack of normal phasic flow in comparison with Figure 4f.
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Figure 7d. Hypertension detected at velocity-encoded cine MR imaging after stent placement for correction of aortic coarctation. (a) Oblique sagittal SE image shows the proximal and distal planes (white lines) selected for velocity-encoded cine imaging perpendicular to the direction of blood flow in the descending aorta: one at a level just below the stenosis, and the other at the level of the diaphragm. Note the irregular contour of the aorta, an appearance caused by a stent-related artifact. (b–e) Magnitude (b, d) and phase (c, e) images obtained in the proximal (b, c) and distal (d, e) locations show the prescribed region of interest (white circle) around the aorta. (f) Flow volume–time curve from an average cardiac cycle shows that despite the appearance of a low-grade stenosis in a, the distal flow volume (diamonds) was greater than the proximal flow volume (squares), a finding indicative of hemodynamically significant restenosis. Note the lack of normal phasic flow in comparison with Figure 4f.
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Figure 7e. Hypertension detected at velocity-encoded cine MR imaging after stent placement for correction of aortic coarctation. (a) Oblique sagittal SE image shows the proximal and distal planes (white lines) selected for velocity-encoded cine imaging perpendicular to the direction of blood flow in the descending aorta: one at a level just below the stenosis, and the other at the level of the diaphragm. Note the irregular contour of the aorta, an appearance caused by a stent-related artifact. (b–e) Magnitude (b, d) and phase (c, e) images obtained in the proximal (b, c) and distal (d, e) locations show the prescribed region of interest (white circle) around the aorta. (f) Flow volume–time curve from an average cardiac cycle shows that despite the appearance of a low-grade stenosis in a, the distal flow volume (diamonds) was greater than the proximal flow volume (squares), a finding indicative of hemodynamically significant restenosis. Note the lack of normal phasic flow in comparison with Figure 4f.
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Figure 7f. Hypertension detected at velocity-encoded cine MR imaging after stent placement for correction of aortic coarctation. (a) Oblique sagittal SE image shows the proximal and distal planes (white lines) selected for velocity-encoded cine imaging perpendicular to the direction of blood flow in the descending aorta: one at a level just below the stenosis, and the other at the level of the diaphragm. Note the irregular contour of the aorta, an appearance caused by a stent-related artifact. (b–e) Magnitude (b, d) and phase (c, e) images obtained in the proximal (b, c) and distal (d, e) locations show the prescribed region of interest (white circle) around the aorta. (f) Flow volume–time curve from an average cardiac cycle shows that despite the appearance of a low-grade stenosis in a, the distal flow volume (diamonds) was greater than the proximal flow volume (squares), a finding indicative of hemodynamically significant restenosis. Note the lack of normal phasic flow in comparison with Figure 4f.
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In a retrospective evaluation of 36 patients who underwent imaging surveillance for postoperative restenosis or aneurysm formation between 2 and 29 years after surgical repair of aortic coarctation, MR imaging findings were compared with clinical data and with the results of Doppler US (the reference standard), catheterization, and angiography (14). The severity of postoperative restenosis, when noted, was calculated on the basis of MR imaging measurements of the ascending and descending aorta and the aortic isthmus (the site of restenosis). In this investigation, MR imaging was found more accurate than the other imaging modalities for the detection of postsurgical complications, including aneurysms. Aortic restenosis was found to be likely when narrowing of more than 50% at the isthmus was observed at SE MR imaging.
SE MR imaging and velocity-encoded cine MR imaging were recently performed at our institution before and after stainless steel stent placement in 10 patients with aortic coarctation (9). MR imaging, conventional angiography, and stent placement were performed in a combined radiographic and MR imaging suite. A significant reduction (P < .05) was found in both the absolute volume of collateral flow and in the difference between distal and proximal flow in the descending aorta after stent placement. The mean absolute and mean relative collateral flow were 10% ± 7% and 30% ± 28%, respectively, before stent placement, compared with 0% for both after stent placement. The investigators concluded that velocity-encoded cine MR imaging is clinically useful for assessing the success of stent placement. A secondary conclusion is that collateral circulation drops immediately after successful stent placement.
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Summary
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Phase-contrast velocity-encoded cine MR imaging is emerging as the modality of choice for the functional evaluation of hemodynamic compromise in aortic coarctation. This technique allows reliable assessment of the presence and direction of collateral flow and enables accurate quantification of key hemodynamic parameters, including flow velocity, flow volume, and pressure gradients. It is useful both for preoperative planning and for postinterventional monitoring in patients with aortic coarctation. For example, it currently is applied in functional assessments for restenosis after stent placement or angioplasty.
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Footnotes
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Abbreviations: SE = spin echo
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References
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Abstract
Introduction
