British Journal of Radiology 74 (2001),1150-1158 © 2001 The British Institute of Radiology
Imaging the post-operative thoracic aorta: normal anatomy and pitfalls
P Riley, MRCP, FRCR1,
S Rooney, MRCP, FRCS2,
R Bonser, FRCS2 and
P Guest, MRCP, FRCR1
Departments of 1Radiology and 2Cardiothoracic Surgery, Queen Elizabeth Hospital, Edgbaston, Birmingham B15 2TH, UK
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Abstract
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Following surgical repair or replacement of the thoracic aorta, interpretation of CT and MRI scans of the thorax can be confusing. It is important to be aware of the variety of appearances that can be encountered. There is usually a surgical explanation and close collaboration with surgical colleagues is required. An appreciation of the normal post-operative appearances allows recognition of the abnormal. Potential pitfalls in interpretation are discussed.
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Introduction
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Following surgical repair or replacement of the thoracic aorta, patients are imaged using either CT or MRI. The aim of imaging is to exclude potential complications and to document baseline appearances for future comparison. A baseline study is especially important in patients in whom pathological segments of aorta remain, as these may require further surgery at a later date. Furthermore, patients who have undergone repair of thoracic aneurysms are at risk of a second non-contiguous aneurysm, necessitating imaging of the whole aorta [1]. Interpretation of these images can be confusing. In some patients, variations in anatomy seen after surgery may suggest complications, while pathology specific to the post-operative aorta is identified in others. We aim to clarify some of the anatomical variations seen after surgery as well as demonstrating examples of pathology that we have encountered in the remaining native aorta.
CT (GE ProSpeed; GE Medical Systems, Milwaukee, WI) and MRI (Siemens Magnetom 1.5 T; Siemens, Erlangen, Germany) studies were performed on patients following surgical repair of the thoracic aorta. Indications for surgery included: aneurysm of ascending aorta or arch; aortic dissection (predominantly Type A); thoracoabdominal aneurysm (Crawford extent I–IV); leaking descending aortic aneurysm; coarctation; and sinus of Valsalva aneurysm rupture. The cases illustrate both normal variation and pathology encountered in the post-operative thoracic aorta.
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Surgical technique
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Either prosthetic grafts or homografts are used for repair or replacement of the various components of the thoracic aorta. Two widely used techniques involve placement of an interposition graft or an inclusion graft. In the former technique the aortic root, ascending aorta and aortic arch are replaced, with excision of the diseased segment, either singly or in combination. The inclusion graft technique involves closure of the remaining diseased aorta around the graft, creating a potential space between the graft and the aortic wall. This perigraft space may contain thrombus, flowing blood or both [2].
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The aortic root and ascending aorta
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Reconstruction of the aortic root involves aortic valve replacement and re-implantation of the coronary arteries (Figure 1a
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Figure 1. (a) Aortic root replacement with re-implantation of coronary arteries. (b) Repair of ascending aorta. IA, innominate artery; RCA, right coronary artery; LCA, left coronary artery; AV, aortic value.
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MRI is demonstrated following ascending aortic repair for Type A dissection (Figure 2). This is a static image from an MRI cine study that shows flowing blood as high signal intensity. The segment of ascending aortic graft is clearly seen, with minimal constriction of the vessel wall at the sites of proximal and distal anastomosis. There is abnormal dilatation of the aortic root above the aortic valve. A residual Type B dissection is also demonstrated, with slow flow in the false lumen appearing as intermediate signal intensity. To ensure perfusion of any branches arising from a patent false lumen, the proximal end of the dissection flap is tacked down to the aortic wall and a fenestration is created in the flap distal to this site.
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Figure 2. ECG-gated oblique sagittal MRI following repair of a Type A dissection. The dilated residual aortic root is seen (arrow) as well as the false lumen of a residual Type B dissection (arrowheads).
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Another Type A dissection repair is illustrated in Figure 3. Here the root and ascending aorta have been replaced with a Starr–Edwards valve and Vascutek graft. ECG-gated T1 weighted oblique sagittal MRI (Figure 3a) demonstrates a considerable amount of soft tissue intensity material around the proximal end of the aortic graft. This is due to considerable bleeding from the anastomotic site during surgery. The adventitia of the native aorta was closed over the graft. This was further supported by suturing the right atrium over the bleeding area to achieve haemostasis. A chronic Type B dissection is also seen. The oblique sagittal gradient echo image also demonstrates the perigraft wrap (Figure 3b). Note the signal void due to the metal prosthetic valve and the sternal wires.
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Figure 3. MRI after Type A dissection repair and aortic valve replacement. (a) Oblique sagittal T1 weighted image shows the perigraft wrap (arrowheads) and a chronic Type B dissection (arrow). (b) Gradient echo sequence demonstrates signal void created by the sternal wires and valve prosthesis (arrowheads).
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The imaging appearances may occassionally be far more complex following aortic root surgery. Figures 4a,b are from an ECG-gated T1 weighted MRI study in a patient with an aortic root homograft. An initial prosthetic valve and aortic root replacement became infected and a false aneurysm originating from the anastomotic site was repaired with omental translocation to fill the infected space and wrap the aorta. The persistent 8 cm false aneurysm posterior to the sternum was treated by further aortic root replacement with homograft material. The axial image (Figure 4a) demonstrates soft tissue intensity material posterior and to the left of the ascending aorta, seen as a wedge of soft tissue on the oblique sagittal scan (Figure 4b). This most likely represents omentum. Another patient (Figure 5) underwent aortic root replacement and subsequent omental translocation to aid haemostasis at the anastomotic site. A considerable amount of low attenuation material in the anterior mediastinum represents omental fat that has been pulled up into the thorax. There is a small residual air-filled space posterior to the sternotomy wound.
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Figure 4. ECG-gated T1 weighted axial (a) and oblique sagittal (b) MRI. The perigraft wrap (arrows) is demonstrated as soft tissue intensity posterior to the ascending aorta.
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Figure 5. Contrast enhanced axial CT following omental translocation (arrows). Residual air is seen in the sternotomy wound (arrowhead).
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The ascending aorta and aortic arch
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Total arch replacement involves re-implantation of the head and neck vessels, usually as a single patch incorporated into the graft (Figure 6), while hemi-arch replacement involves bevelling the graft and anastomosing this to an appropriately bevelled distal native aortic arch that incorporates the head and neck vessels and left subclavian artery (Figure 7). These procedures may or may not involve replacement of the aortic root. In the next case (Figure 8), repair of a Type A dissection involved aortic root, ascending aorta and total aortic arch replacement with persistent Type B dissection. The appearances of the aortic arch on dynamic contrast enhanced axial CT are complex, with small out-pouchings adjacent to the lumen. These represent small diverticula at the anastomotic site where re-apposition of the true and false lumina using biological glue and suturing has been attempted. They develop from the wall of the false lumen distal to the anastomotic site when this lumen remains perfused. The features of a residual intimal flap in the descending aorta are clearly seen on MRI (Figure 9). An oblique sagittal image following aortic root and ascending arch replacement for a Type A dissection demonstrates signal void in both channels. An area of discontinuity in the dissection flap is likely to be a site of re-entry of the dissection into the true lumen.
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Figure 6. Total arch replacement with Stage 1 elephant trunk. The single patch incorporating the head and upper limb vessels is demonstrated.
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Figure 8. Contrast enhanced CT showing anastomotic pseudoaneurysms (arrow) arising from distal aortic arch.
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Figure 9. ECG-gated oblique sagittal MRI demonstrates persistent Type B dissection (arrow). There is signal void in both the true and false lumina.
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The Stage I elephant trunk technique is employed in patients with a diffuse aneurysmal process, which may warrant replacement of all of the aorta at some stage, i.e. in dissection or Marfan's disease. This procedure involves graft replacement of the ascending aorta and aortic arch with or without a separate valve replacement. A free segment of graft material is left projecting into the proximal end of an invariably diseased descending aorta (Figure 10). This allows subsequent repair of the descending aorta at a later date. Coronal MRI through the aortic arch (Figure 11) demonstrates appearances following aortic root and total arch replacement with a Stage 1 elephant trunk technique for aortic root and ascending aortic aneurysm. The walls of the distal graft segment project into the lumen of the distal aortic arch, which remains aneurysmal.
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Figure 10. Stage 1 elephant trunk procedure. The distal free-lying segment of graft is seen dangling in the proximal aneurysmal descending aorta. The head and upper limb vessels have been incorporated into the arch as a single patch.
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Figure 11. ECG-gated T1 weighted oblique coronal MRI ("black blood" image) demonstrates appearances following elephant trunk repair. The distal graft segment is in the lumen of the distal native aortic arch.
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There is signal void outside the graft but within the lumen of the remaining native arch. The graft and arch have been anastomosed proximal to this site. This segment of graft represents the elephant's trunk part of the technique and any further aortic graft can be sutured to this free-lying segment. Figure 12 is an axial CT image in a different patient who has undergone aortic arch replacement with the Stage 1 elephant trunk technique. The distal graft projects into the aneurysmal proximal descending aorta, with enhanced blood both within the graft lumen and outside the graft wall but within the lumen of the remaining native aorta; this segment of graft represents the elephant trunk. Intraluminal thrombus has accumulated around the proximal part of the trunk. The proximal aorta is of normal calibre.
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Figure 12. Contrast enhanced axial CT demonstrates the elephant trunk repair and residual aneurysm of the descending aorta. The distal graft lies free in the lumen of the proximal descending aorta. Adjacent thrombus is seen as low attenuation material against the wall of the native vessel (arrow).
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The descending aorta
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Several different techniques apply here, depending on the pathology. Thoracoabdominal aneurysms are classified according to the variable extent of involvement of each component (Crawford Type I–IV). Repair may involve anastomosis of the visceral arteries as a patch incorporating part of the native aortic wall (Figure 13) and re-attachment of lower intercostal and upper lumbar branches to preserve spinal cord supply. After thoracoabdominal aortic replacement for leaking acute Type B dissection, axial MRI through the aorta (Figure 14) at a level caudad to the diaphragmatic hiatus demonstrates an abnormality in the midline, anteromedial to the aortic lumen. Signal void here indicates blood flow with probable thrombus formation peripherally, suggesting either residual dissection or false aneurysm formation at this site. The original dissection involved the intercostal arteries and abdominal visceral arterial ostia. These vessels were incorporated into the graft, but surgery has failed to completely obliterate the false lumen. A small false aneurysm has developed, which should be monitored for expansion.
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Figure 13. Thoracoabdominal aneurysm repair demonstrating anastomosis of the coeliac axis and superior mesenteric arteries as a single patch. The lower intercostal arteries have been incorporated into the graft in a similar fashion.
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Figure 14. T1 weighted axial MRI through the upper abdominal aorta. The false aneurysm can be seen as an area of mixed signal intensity anteromedial to the abdominal aorta (arrow).
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Less complex but apparently abnormal appearances are seen in a different patient (Figure 15) after proximal descending and thoracoabdominal aneurysm replacement. The descending aorta appears enlarged, with circumferential soft tissue attenuation material around the lumen. In this case the aneurysm sac has been closed over the graft and the apparent thickening of the wall of the descending aorta represents both the wall of the native vessel and thrombus formation within it but around the graft (the perigraft space).
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Figure 15. Contrast enhanced axial CT demonstrates an inclusion graft and a thrombosed perigraft space (arrow) at the site of the descending thoracic aorta.
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Differing techniques may be employed in the repair of aortic coarctation. One technique is placement of an interposition graft. An oblique sagittal MRI study (Figure 16) demonstrates some irregularity of the wall of the proximal descending aorta at the site of repair but no significant gradient was shown on flow studies. The ascending aorta is 4.5 cm in diameter. Another technique is the use of a bypass graft. In the next patient a thoracoabdominal coarctation was repaired using an aorto-aortic Dacron graft to bypass the abnormality, extending from the distal aortic arch to the upper abdominal aorta. The graft is easily seen on MRI in the oblique sagittal plane (Figure 17a) but the chest radiograph may cause some confusion (Figure 17b).
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Figure 16. Oblique sagittal gradient echo MRI. Coarctation repair. The site of anastomosis of the interposition graft is indicated (arrows).
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Figure 17. (a) Oblique sagittal gradient echo MRI and (b) chest radiograph of bypass graft for coarctation repair. The graft (arrows) passes posteriorly towards the spine (high signal intensity) from the proximal descending aorta (a).
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Imaging may be more difficult to interpret if recovery is complicated. The patient in Figure 18 developed an aortobronchial fistula arising from a false aneurysm at the site of previous coarctation repair. Initial surgery had involved aortoplasty complicated by haemorrhage and subsequent insertion of a Dacron patch. To facilitate repair, an extra-anatomical conduit was inserted from the ascending to the descending aorta prior to exploration and closure of the aortic defect. On the axial CT image (Figure 18a) the conduit is seen passing anterior to the hilum of the left lung. The site of repaired aortic defect was noted to be stenosed at surgery but the conduit will maintain normotensive distal perfusion. The native descending thoracic aorta caudad to the repair site is aneurysmal (4 cm diameter) with eccentric thrombus within the lumen. A three-dimensional maximum intensity projection image (Figure 18b) demonstrates the position of the conduit with respect to other mediastinal vascular structures.
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Figure 18. (a) Contrast enhanced axial CT and (b) three-dimensional maximum intensity projection image. The enhancing extra-anatomical conduit is anterior to the left pulmonary artery (arrows). Aneurysmal native descending thoracic aorta (arrowheads).
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Discussion
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Detailed knowledge of surgical technique and its anatomical consequences are essential for accurately evaluating post-operative imaging. The repaired thoracic aorta may appear unremarkable, particularly in the axial plane, in cases involving the aortic root and ascending aorta, although sites of anastomosis are usually demonstrated in oblique sagittal or coronal planes (Figure 2
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Following repair of a Type A dissection, a persistent intimal flap is seen distal to the graft site in up to 75–100% of cases [3]. Patients with communicating dissections are at risk of secondary aneurysm formation involving the false lumen; in one study, 25% developed aneurysms distal to the graft site [4]. Evidence of flow may be seen in both lumina; if it is not then the false lumen is usually thrombosed. It is therefore important to differentiate thrombus from very slow blood flow (Figure 2), and MRI cine or contrast enhanced studies may be helpful. There is also a risk of anastomotic pseudoaneurysm formation [5]. This results from partial dehiscence of the proximal or distal suture line or the coronary anastomosis if re-implantation was performed [6]. Similarly, this almost invariably explains the presence of flowing blood in a perigraft space, although this situation is sometimes deliberately created surgically [5]. It is essential to monitor patients closely after ascending aortic repair; morbidity and mortality are as high as 75% and 25%, respectively, in patients requiring a repeat operation to repair a graft pseudoaneurysm [7] and higher in the emergency situation. The diverticula demonstrated in Figure 8 are likely to represent early pseudoaneurysms at the distal suture line and will require regular imaging to monitor any increase in size.
Abnormal soft tissue is occasionally demonstrated outside a graft or anastomosis. Although confusing, the appearances can usually be explained once surgical details are known, especially where soft tissue has been wrapped around anastomoses to aid haemostasis (Figure 3) or where the inclusion graft technique has been used (Figure 15).
The length of the graft does not always match the extent of the aneurysm and residual dilatation of the remaining native abdominal aorta is often encountered. The appearances of the descending aorta will be abnormal if an inclusion graft technique is employed and may suggest further aneurysm formation (Figure 14).
Contrast enhanced helical CT and MRI with MR angiography provide excellent diagnostic accuracy for a wide range of thoracic aortic diseases in the non-emergengy setting [8]. Helical CT is more widely available and probably easier to perform. MRI sequences for the thoracic aorta usually include multiplanar ECG-gated spin echo (SE), giving "black blood" images, and an angiographic technique with "bright blood" images. The latter techniques include cine gradient echo (GRE) imaging, 2-dimensional (2D) time-of-flight imaging, 2D gadolinium enhanced rapid GRE imaging and gadolinium enhanced 3-dimensional MR angiography [5]. "Bright blood" techniques enable differentiation of slow flow from thrombus. Phase contrast techniques are used to quantify flow, including measurements of velocity and pressure gradients, for example following coarctation repair. More recently the half-Fourier single shot turbo SE (HASTE) sequence has been deployed for black blood imaging and has several advantages over ECG-gated SE techniques [5].
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Acknowledgments
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Figures 6, 10 and 13 reproduced with kind permission from Crawford E, Coselli S. Thoracoabdominal aneurysm surgery. Sem Thorac Cardiovasc Surg 1991;3:300–22. Figures 1a,b and 7 courtesy of Medical Illustration Department, University Hospital Birmingham NHS Trust.
Received for publication October 18, 1999.
Revision received March 6, 2001.
Accepted for publication April 4, 2001.
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Abstract
Introduction
