Article Text

Non-invasive imaging
Role of tomographic imaging in preoperative planning and postoperative assessment in cardiovascular surgery
  1. Andrew C Y To1,2,
  2. Paul Schoenhagen1,
  3. Milind Y Desai1
  1. 1Department of Cardiovascular Medicine, Heart and Vascular Institute, Cleveland Clinic, Cleveland, Ohio, USA
  2. 2Department of Cardiovascular Medicine, North Shore Hospital, Auckland, New Zealand
  1. Correspondence to Dr Milind Y Desai, Tomsich Department of Cardiovascular Medicine, Heart and Vascular Institute, Cleveland Clinic, J1-5, 9500 Euclid Avenue, Cleveland, OH 44195, USA; desaim2{at}ccf.org

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Cardiovascular surgeries have become progressively more sophisticated and complicated, due to factors such as the increasingly advanced age of patients, significant comorbidities, and increasing surgical complexity—minimally invasive procedures, reoperations and simultaneous procedures such as combined valve surgery and revascularisation, multiple valve replacements, combined valve and vascular surgery, as well as complicated aneurysm surgeries. Traditionally imaging studies with echocardiography and conventional angiography are increasingly complemented by tomographic techniques such as multidetector CT and MRI. In this context, echocardiography and angiography have significant shortcomings due to limited field of view focused on the examined cardiovascular structures, without significant insight into their spatial relationship vis-à-vis surrounding structures. The strength of CT and MRI is in the assessment of the spatial relationship of cardiovascular and extracardiac structures. In addition, the broadening of the scope of practice of both imaging and interventional cardiologists has made it more imperative to understand the various aspects of conventional and minimally invasive cardiovascular surgery. This article discusses the role of tomographic imaging in preprocedural planning and postoperative assessment of cardiac and vascular surgical patients.

Basics of tomographic imaging

MRI techniques

MRI provides high resolution anatomic images without the use of ionising radiation and iodinated contrast agents. The commonly used pulse sequences utilised in cardiac MRI include: spin echo or ‘black blood’ images which provide good tissue contrast and anatomic detail, useful for visualising morphology; and gradient echo or ‘white blood’ images, including steady state free precession sequences which are commonly used for cine sequences. Gradient echo images are useful for the quantification of ventricular volume and function, as well as the visualisation of turbulent flow due to valvular disease or intracardiac shunts. Phase contrast velocity encoded imaging quantifies both the velocity and flow of blood through an area of interest. Accurate velocity and flow quantification allows the estimation of gradients across valves, regurgitant volumes, as well as shunt fractions such as Qp/Qs ratios. Magnetic resonance angiography using gadolinium based contrast (most commonly gadolinium DTPA) allows better visualisation of cardiac and extracardiac vasculature, which is particularly useful before vascular surgery. Contrast enhanced MRI enables the assessment of myocardial perfusion at rest and after adenosine stress, as well as myocardial viability using delayed enhancement imaging. However, the use of gadolinium based contrast is contraindicated in severe renal impairment due to the risk of nephrogenic systemic fibrosis.1

CT techniques

The multidetector CT scanner—the most common type of CT scanner used today—consists of a rotating assembly of an x-ray tube mounted opposite a series of detectors on a gantry around the patient. Image acquisition has traditionally been performed using spiral or helical acquisition, where the patient is moved at a fixed speed, or pitch, through a constantly rotating gantry. When used in cardiac imaging, ECG gating is performed retrospectively, which means that data are collected throughout the entire cardiac cycle, but image is reconstructed according to the ECG signal. Cine sequences could be reconstructed at 5–10% intervals throughout the cardiac cycle, although for most applications, mid-late diastolic images have the least motion artefact and are most commonly used for image analysis. Dose modulation algorithms reduce the x-ray tube current during systole, as a dose saving strategy.

Newer scanners have the option of data acquisition with prospective ECG triggering, where the table moves sequentially through the long axis of the body. Radiation is triggered by an ECG signal and data are acquired within just over half of a complete gantry rotation during the desired phase of the cardiac cycle, which minimises radiation dose. In the new 320-slice scanner with its long z-axis coverage, axial sequential acquisition mode covers the entire heart in one heartbeat without potential step artefact. Alternatively, the dual source scanner utilises two simultaneous x-ray sources so that data are acquired within just over one-quarter of a gantry rotation, doubling the temporal resolution and minimising motion artefact.

Three dimensional image analysis

Both CT and MRI provide high resolution three dimensional (3D) data for analysis (figure 1). Multiplanar reformation (MPR) describes the creation of thin planes through the 3D data volume. An unlimited number of planes can be reconstructed in any orientation from the 3D dataset. This provides the most accurate measurement of vessel calibre. Curved MPR images allow reconstruction of curved planes following the entire course of tortuous vascular structures, defined by placement of seed points (figure 1A). The course and shape of the plane is determined by the centre line of vascular structures, which is defined manually or semi-automatically. Maximum-intensity projection images are orthogonal or oblique planes or slabs with increased thickness, displaying the information within a stack of MPR images. These images provide more comprehensive assessment of vascular structures, similar to conventional angiograms. Volume rendered imaging techniques assign specific colour and opacity values to every voxel inside a volume of interest. Users can interactively highlight groups of voxels for display, for example, distinguishing the aorta versus the chest wall and vice versa. Finally, images acquired with cine MR or using a retrospectively ECG gated helical mode on CT can be reconstructed at multiple phases throughout the cardiac cycle to view as a cine loop and provide functional assessment of ventricular, valvular and vascular structure. Table 1 summarises and contrasts the techniques and indications of MRI and CT in the assessment of cardiac and aortic surgeries.

Table 1

Utility of MRI and CT in the assessment of cardiac and aortic surgeries

Figure 1

Different reconstruction and display techniques for assessment of aortopathies using CT and MRI. (A) Curved multiplanar reformatted contrast enhanced CT image with a centre line in a patient with descending aortic aneurysm (B) measuring the aneurysm dimensions using multiplanar reformatting. (C) Volume rendered contrast enhanced CT image in a patient with Marfan syndrome. (D) Maximum intensity projection contrast enhanced MR angiogram in a patient with coarctation of aorta.

Clinical applications of tomographic imaging in cardiac surgery

Preprocedural planning for primary and redo cardiac surgery

It is increasingly common for patients to present at a later age for their initial cardiac surgery due to improvements in treatments. Older patients frequently have advanced thoracic aortic atherosclerosis, associated with postoperative adverse events, morbidity and mortality. At the Cleveland Clinic, CT of the thoracic aorta is routinely performed to evaluate for the extent of atherosclerosis and exclude extensive ascending aortic calcification (‘porcelain aorta’), as this would preclude its cannulation for cardiopulmonary bypass. At the same time, the subclavian arteries are assessed as possible alternative sites for cannulation and surgical side grafts. A recent study demonstrated the independent relationship between aortic atherosclerotic plaque burden as assessed by CT and long term mortality.2 While non-contrast CT will only demonstrate aortic calcifications without the details on aortic atherosclerosis, it is often used as an alternative in those patients with significant renal dysfunction. MRI is less useful for this clinical indication, due to calcium related susceptibility artefacts.

Over the last decade, the rate of complicated reoperative cardiac surgery has also steadily increased, approaching 30% of all cardiac surgeries performed at our institution, where comprehensive tomographic evaluation, most commonly with CT, is routinely performed preoperatively. This has also been recommended by the latest appropriateness criteria.3 Important high risk findings include the close proximity (<1 cm) and/or adherence of the right ventricle or aorta to the chest wall (figure 2), bypass grafts crossing the midline within 1 cm in anteroposterior direction and/or adherence to the sternum, as well as significant aortic calcification. This knowledge triggers the use of preventive surgical strategies, most commonly the extrathoracic vascular exposure before sternal incision; initiation of peripheral cardiopulmonary bypass, with or without deep hypothermic circulatory arrest; use of non-midline incision; as well as (rarely) the cancellation of surgery.4 ,5 Preoperative tomography is also associated with shorter perfusion and crossclamp time, total time in intensive care unit, and lower volume of postoperative blood transfusion. In patients with renal impairment, non-contrast CT or non-contrast MRI may be performed to provide similar information.

Figure 2

Sagittal contrast enhanced CT image in a patient being considered for redo cardiac surgery. Arrow demonstrates tethering of the right ventricle to the chest wall.

Preprocedural planning in valvular surgery

While echocardiography remains the mainstay of valvular assessment, various MR techniques can be useful in valvular disease, including the quantification of chamber volumetrics and the assessment of stenosis and regurgitation mechanism and severity. Similarly, CT is finding an increasing role in valvular assessment, especially with the advent of minimally invasive, robotic assisted surgery, as well as percutaneous aortic valve procedures6–8 (figure 3). Such procedures are characterised by lack of direct procedural exposure, reduced direct visualisation, and novel approaches of vascular access. For example, robotic surgery establishes cardiopulmonary bypass by retrograde perfusion via the femoral vessels, so that the absence of atherosclerotic disease is important to reduce the risk of cerebral embolism or retrograde aortic dissection. Preoperative aortic assessment with CT angiography is frequently used.8 In percutaneous aortic valve procedures, tomographic imaging is extremely important to understand the complex 3D anatomic relationships of the left ventricular outflow tract, aortic annulus, aortic root, and coronary arteries.7

Figure 3

(A) Multiplanar reformatted short axis contrast enhanced CT image of a heavily calcified aortic valve in a patient with severe aortic stenosis. (B) Same orientation following transfemoral aortic valve replacement. (C) Volume rendered images of a well placed transfemoral aortic valve.

In endocarditis, while single centre studies have suggested CT may be as accurate as transoesophageal echocardiography (TOE) in diagnosing vegetations,9 the main role of CT is in the diagnosis of perivalvular abscesses, which often complicate endocarditis and are more commonly seen in infection of the aortic valve and the adjacent annulus. Anteriorly located abscesses can also be difficult to visualise on echocardiography. CT is also useful in determining the extent of destruction by the infection and therefore surgical planning. Assessment of prosthetic valve endocarditis by TOE is sometimes inadequate, especially in cases where acoustic shadowing affects assessment. MRI is less often used for this indication because of its inferior spatial resolution and valve related artefacts.

Another emerging indication for CT is the assessment of prosthetic valves (see online supplementary movie S1), as echocardiography and MR are often challenging due to excessive metal shielding artefacts.10 ,11 Helical acquisition with retrospective gating by CT reconstructs cine images that can accurately determine mechanical leaflet motion and opening angle. In addition, it can depict prosthetic complications including ‘frozen’ leaflets from thrombus or pannus, valve dehiscence, pseudoaneurysm, infective endocarditis, or paravalvular abscess.

Assessment of cardiac masses

Generally, echocardiography provides the initial diagnosis of cardiac masses, but tomographic imaging has incremental value in the detection of concurrent lesions, tissue characterisation, diagnosis or exclusion of local invasion and/or distant metastasis, as well as evaluation of neighbouring structures for additional pathology.12 While CT and MRI can aid in differentiating benign from malignant lesions, MRI with the use of different scan sequences is often the preferred tomographic modality. CT, on the other hand, is useful for the evaluation of calcification and fat content within a mass.10 ,13 Furthermore, CT is also useful in the staging of malignant tumours (figure 4).

Figure 4

Multiplanar reformatted contrast enhanced CT image of the heart in a patient with a cardiac sarcoma invading through the left atrium into the lung.

Assessment of pericardial disorders

MRI, while not the first line modality for diagnosing pericardial disorders, is an excellent modality for periprocedural planning.14 It accurately demonstrates the location and extent of pericardial thickening, effusion, pericardial cysts, pericardial oedema, and inflammation. One area where MRI has significant limitations is in the assessment of pericardial calcification, due to the lack of signal from calcium on MRI sequences. CT, whether gated or non-gated, with or without iodinated contrast, visualises the pericardium and assesses the location and extent of pericardial thickening, calcification and effusion.

Viability and surgical revascularisation

The usefulness of viability assessment in patient selection for revascularisation in ischaemic cardiomyopathy has been the subject of much recent debate since the publication of the STICH (Surgical Treatment for Ischemic Heart Failure) trial.15 While a full debate is beyond the scope of this article, delayed enhancement imaging by MRI plays a central role in viability assessment and appears superior to nuclear techniques in its superior spatial resolution. Recent studies using rest and stress myocardial perfusion imaging by CT have also been advocated as a useful test for myocardial ischaemia and viability.

Specific for surgical revascularisation, MRI and CT are both useful in distinguishing aneurysms versus pseudoaneurysms in complicated ischaemic cardiomyopathy (figure 5), planning for left ventricle reconstruction,16 as well as the use of assist devices. For the latter, CT is a useful adjunct to echocardiography in the diagnosis of post-implantation complications.17 ,18

Figure 5

Cine MR image in a patient with ischaemic cardiomyopathy and a rupture of the left ventricular free wall, who was being evaluated for coronary artery bypass grafting and left ventricular reconstructive surgery.

Clinical applications of tomographic imaging in vascular surgery

In the preprocedural planning of patients with aortopathies, tomographic imaging is routinely performed, more commonly CT, though MRI with or without contrast is also useful (figure 1). Spiral CT scanning without ECG gating usually suffices except if the suspected pathology is in the aortic root or ascending aorta, where motion artefact should be avoided with either retrospective or prospective ECG gating. Assessment of thoracic aortopathies before open surgical repair includes obtaining data on the size, location and extent of the aneurysm; presence of complications such as dissection and intramural haematoma; and the number, patency and location of the aortic side branches. In the increasingly adopted approach of endovascular stent grafting,19 ,20 information on the suitability and length of proximal and distal landing zones is also important, to determine the use of fenestrated versus covered stents, and the need for side stents.

Aortic aneurysms

True aortic aneurysms involve the intima, media, and adventitia and can be found in all segments of the aorta.21 Incidence of thoracic aneurysms is increasing with the aging population, and is estimated to be around 10 per 100 000 patient years.22 Most common disorders leading to aortic aneurysms include atherosclerosis and familial, inflammatory and infectious disorders.

Aortic root aneurysms are usually seen in bicuspid aortic valves and familial thoracic aortic aneurysms. The prevalence of significant ascending aortic aneurysms in patients with bicuspid aortic valve has been estimated to be 20%23 ,24 (figure 6). Aneurysm of the tubular portion of the ascending aorta is common in idiopathic, bicuspid aortic valve, and atherosclerotic and inflammatory arteritis cases. The normal aortic dimensions are shown in table 2. Routine tomographic surveillance of the aortic root and ascending thoracic aorta, based on the size and rate of progression, should be performed according to the latest guidelines.25

Table 2

Normal adult thoracic aortic diameters

Figure 6

Multiplanar reformatted contrast enhanced CT images in a patient with a bicuspid aortic valve and a history of coarctation repair. (A) Bicuspid aortic valve. (B) Aortic root aneurysm with effacement of the sinotubular junction (annuloaortic ectasia). (C) Evidence of surgical coarct repair (arrow). (D) Volume rendered image demonstrating aortic root aneurysm and normal appearance post-coarct repair.

Tomographic assessment of aortic aneurysms should include the nature, location, extent, and dimension of the aneurysm. All measurements should be made in a 3D context using multiplanar reformatting and generation of ‘true’ short axis views. Several imaging features are important to understand:

  • Fusiform aneurysms—the symmetrical dilation of the entire circumference of the aortic wall

  • Saccular aneurysms—the localised outpouching of a portion of the aortic wall

  • Pseudoaneurysms—dilation from a contained rupture of the aortic wall and disruption of the intima and media, such that the aneurysm wall does not contain all three aortic wall layers

  • Annuloaortic ectasia—the effacement of the sinotubular junction, giving the classic tulip bulb appearance, common in Marfan, Loeys–Dietz, Ehlers–Danlos, and other familial aortic syndromes

  • Sinus of Valsalva aneurysm—dilation of the sinus of Valsalva that results from failure of the development of the aortic media elastic component, affecting most commonly the right cusp, followed by the non-coronary cusp.

Acute aortic syndromes

Acute aortic syndromes consist of three interrelated conditions: aortic dissection, intramural haematoma, and penetrating aortic ulcerations. Aortic dissection results from the disruption of the aortic media with bleeding within and along the aortic wall. The separation of the layers of the aorta from the intimal disruption results in the classic appearance of a thin linear filling defect, ‘flap’, between the true and false lumens. While TOE is equally reliable in ruling out thoracic dissection acutely,26 CT and MR angiography assess the extent and location of the dissection, the dimensions of the affected and unaffected native aorta, as well as associated complications; these include the progression of dissection, aneurysmal degeneration, as well as extension of the dissection into branch vessels and resultant end-organ perfusion deficits and ischaemia, myocardial ischaemia, acute aortic regurgitation, pericardial effusion, tamponade and aortic rupture. The true lumen is usually in continuity with the undissected portion of the aorta, while the false lumen is often partially thrombosed. Useful secondary findings of aortic dissection include intimal calcification displacement, and the delayed enhancement of the false lumen. Aortic dissection classification is commonly by the DeBakey and the Stanford systems,25 and the extent of dissection impacts on subsequent management strategies.27 In short, Stanford A dissection (figure 7) involves the ascending aorta, with (DeBakey type I) or without (Debakey type II) aortic arch and descending aorta propagation. Stanford B dissection does not involve the ascending aorta, while DeBakey type III dissection is defined as originating in the descending aorta, limited to above (DeBakey type IIIa) or extending below (DeBakey type IIIb) the diaphragm.

Figure 7

Multiplanar reformatted contrast enhanced CT images in a patient with an ascending aortic aneurysm who presented with a type A dissection.

Intramural haematoma may originate from the spontaneous haemorrhage of the medial vasa vasorum, or microscopic aortic intimal tears. It mimics the clinical features of classic dissection, except that one cannot identify either the false lumen blood flow or an intimal lesion on imaging. It may propagate both antegradely or retrogradely, similar to dissection. On CT (figure 8), it appears as a crescentic or concentric aortic wall thickening, with higher tissue density than unenhanced blood on non-contrast images, without enhancement on arterial phase images. Internal displacement of intimal calcification, if present, is a helpful finding.28 Penetrating atherosclerotic ulcers, most commonly found in the descending thoracic aorta,29 are ulcerations that penetrate the internal elastic lamina, permitting haematoma to form within the media, thereby leading to intramural haematoma, aortic dissection or aortic rupture. On CT or MRI, these appear as aortic luminal outpouching with overhanging edges.

Figure 8

(A) Precontrast CT image in a patient with an intramural haematoma in type A distribution. Notice the high attenuation rim surrounding the ascending aorta suggestive of blood within the aortic wall. (B) Contrast enhanced CT image at the same level demonstrating aortic wall thickening confirming absence of frank dissection.

Infectious and inflammatory aortopathies

Mycotic aortic aneurysm is most commonly saccular, although fusiform aneurysms are sometimes seen. On CT and MRI, signs of periaortic soft tissue mass, fluid, and/or stranding are often accompanied by rapid disease progression on successive imaging.

Takayasu arteritis (figure 9) and giant cell arteritis are inflammatory arteritis conditions that affect the younger and older populations, respectively. Diagnosis and follow-up of these conditions rely on the accurate tomographic assessment of the entire arterial tree, demonstrating the typical appearance of concentric aortic wall thickening, often extending to various branch vessels, leading to stenosis and/or aneurysmal degeneration. While both CT and MRI are adequate, MRI has the advantage over CT of aortic wall tissue characterisation that correlates with disease activity.30 ,31

Figure 9

MR images in a patient with Takayasu's arteritis. Coronal and sagittal black blood images (A) showing thickening of the walls of the aorta and pulmonary artery, and (B) T2 oedema weighted black blood images in the same configuration, confirming inflammation in the thickened walls.

Congenital anomalies

Aortic coarctation is characterised by a focal narrowing of the thoracic aorta most commonly affecting the isthmus. Both CT and MRI are suited for preprocedural assessment of aortic coarctation. Important data include the location, severity and length of the narrowing along with information on associated hypoplasia and/or interruption of the aortic arch and isthmus; aneurysmal degeneration proximal and distal to the narrowing; and the presence and extent of collaterals. MRI flow quantification of the aorta can also quantify the differential flow through the aortic coarctation versus collaterals. This information determines the type of surgical or percutaneous procedures most suited for the individual patient. Pseudocoarctation describes the phenomenon whereby the redundant aorta becomes kinked opposite the ligamentum arteriosum without a pressure gradient. Distinguishing this from a true aortic coarctation is important before corrective procedure.

An aberrant right subclavian artery is sometimes incidentally found on MRI and CT to arise as the fourth branch from the aorta, coursing behind the oesophagus, which can occasionally lead to dysphagia. When the aberrant artery enlarges as a Kommerell diverticulum, it may become prone to aneurysm formation, dissection and rupture.

Post-aortic surgery tomographic evaluation

Complications following thoracic aortic surgery are rare, occurring in approximately 2% of cases, although they are not always clinically apparent. As a result, regular surveillance imaging in asymptomatic patients is useful in demonstrating postprocedural complications such as aneurysmal degeneration, dehiscence, and pseudoaneurysm, as well as progression of disease in the native untreated aorta.32 Table 3 describes the recommended follow-up of patients with prior aortopathies and prior surgeries. Actual surgical techniques depend on a multitude of individual factors, including the anatomic extent of disease, age, anticoagulation requirement, aortic tissue and valve characteristics, and previous surgeries. Prior knowledge of the operative details is essential for the accurate tomographic assessment of postoperative patients. Table 4 describes the potential surgical options for various aortopathies and tomographic imaging findings to watch out for.

Table 3

Suggested tomographic follow-up of aortic pathologies after repair or treatment

Table 4

Potential surgical options for various aortopathies and pertinent findings on tomographic imaging

Normal postoperative appearance following thoracic aortic surgery

Table 5 outlines the important CT imaging features of various aortic valve replacements (AVR). Of note, mechanical AVRs are readily visualised on CT. With retrospectively gated cine images, mechanical valve dysfunction is readily diagnosed by measuring valve leaflet opening angle. MRI assessment of prosthetic valve function is often hindered by significant metal related susceptibility artefacts. Echocardiography, either transthoracic or transoesophageal, may be hindered by acoustic shadowing.

Table 5

Aortic valve replacements (AVR) and CT imaging features

Surgical aortic grafts are made of either porcine tissue or, more commonly, synthetic material such as Dacron. Both can be identified on contrast enhanced CT or MRI by their smooth appearance. Synthetic grafts often have slightly higher attenuation values than native aortic tissue on non-contrast CT. Figures 10 and 11 outline examples of Bentall's procedure and David's procedure, respectively. Aortic arch surgery is often complicated and correlation with the operative report is essential. Proximal arch branch vessels may be preserved, replaced with prosthetic side grafts, ligated, or occluded by stent grafts. Assessment of the patency of these additional structures, including surgical side grafts, is essential. Figure 12 outlines an example of the elephant trunk procedure.

Figure 10

Multiplanar reformatted contrast enhanced CT image (A) and volume rendered image (B) in a patient with ascending aortic aneurysm and a penetrating ulceration in the ascending aorta (arrow). Patient underwent Bentall procedure with a metallic aortic valve and corresponding images are shown (C, D) without evidence of complications.

Figure 11

Short axis multiplanar reformatted image of a patient status post-David procedure with reimplantation of the native aortic valve.

Figure 12

Multiplanar reformatted contrast enhanced (A) and volume rendered (B) CT images of a patient with an extensive thoracic aneurysm who subsequently underwent surgery. (C, D) Corresponding images, without evidence of complications, after completion of stage I of the elephant trunk procedure.

Various normal postoperative imaging features are worth mentioning.

  • High attenuation materials are often seen outside the aortic wall, which represents strips of felt used by the surgeons to buttress aortic anastomoses. Pledgets may also be seen at sites of intraoperative cannula placement. These occasionally resemble contrast leak outside the aortic wall on CT. Their location and comparison with non-contrast images as well as prior studies are often helpful in differentiation.

  • Rarely, aortic grafts may be covered by a strip of bovine pericardium or omentum, in order to separate the grafts from adjacent structures and prevent fistula formation. Bovine pericardium usually has lower CT attenuation than soft tissue, whereas omentum shows fat attenuation.

  • Occasionally, an aortic graft may be slightly kinked, thereby simulating a dissection flap on axial images. Multiplanar reformatted images are useful in differentiating the two.

Complications following aortic surgery: tomographic appearance

A small to moderate amount of low attenuation mediastinal material that likely represents seroma, healing haematoma or fibrosis is commonly seen in the immediate or recent postoperative scans. However, appearance of new fluid collection, persistence or an increase in existing collection on follow-up scans should raise a suspicion for infection or abscess surrounding a graft. On CT, it typically has a lower attenuation than blood but higher than simple fluid (figure 13). The presence of gas bubbles further heightens the suspicion of an infection or a concomitant bronchial–oesophageal fistula. Various MRI sequences can be useful in differentiating simple fluid, blood or abscess.

Figure 13

Axial and sagittal contrast enhanced CT image in a patient with a remote history of ascending aortic grafting who presents with fever and leucocytosis. Notice the rim of fluid collection in the mediastinum, which under the clinical circumstances was thought to be most consistent with an aortic graft infection.

Slow anastomotic leaks have an appearance similar to that of fluid collection around the grafts. These findings have to be considered in the right clinical context. On the other hand, surgical graft dehiscence with a relatively active leak will appear on CT as a high density fluid collection surrounding the graft (figure 14). Dehiscence may be seen at any time following surgery, even many years afterward. This is an urgent finding often with haemodynamic instability, requiring a rapid diagnosis and management.

Figure 14

Contrast enhanced CT image in a patient with supracoronary ascending aortic grafting. The arrow denotes a small pseudoaneurysm and the graft anastomotic site.

Post-endovascular stent-graft evaluation

CT surveillance is routinely performed after percutaneous endovascular stent-graft implantation.25 CT assesses the dimension of the residual aneurysmal sac and/or dissection, as well as side branch vessel patency, which often indicates the presence of end-organ ischaemia. Other procedural complications such as stent migration, pseudoaneurysm formation, and stent malapposition to the vessel wall are also important.

Endoleak is commonly observed after endovascular stent-graft implantation and describes the persistence of blood flow in the aortic aneurysm sac outside the stent graft25 (figure 15). The persistence of blood flow results in increased aneurysmal sac pressure that leads to potential aneurysm growth. Most commonly, endoleak relates to the persistent blood flow into the aneurysmal sac via small arterial aortic branches that are excluded by the stent-graft (type II endoleak). Side branch embolisation may be needed if this leads to aneurysmal sac growth on follow-up. Other endoleaks occur at the attachment sites (type I), or result from mechanical endograft fabric disruption or mechanical stent separation (type III), graft porosity (type IV) and endotension (type V).33 Endoleaks are best evaluated by CT, using non-enhanced, arterial and venous phase images. While arterial phase images usually demonstrate endoleak best, subtle leaks are sometimes apparent only on delayed venous phase images as an area of increased attenuation within the aneurysmal sac. Non-enhanced images are helpful in distinguishing endoleak from small focal hyperattenuating areas of vessel calcification.

Figure 15

Post-contrast CT images in a patient with thoracoabdominal aorta who underwent endovascular stent grafting. Axial (A) and sagittal (B) images reveal a stent within the aneurysmal sac. However, there is high attenuation fluid within the excluded aneurysm sac which is suggestive of an endoleak.

Conclusions

With the rapid expansion of complicated surgical and percutaneous cardiovascular procedures, novel tomographic imaging approaches are increasing in importance, both preoperatively in the context of surgical planning, as well as postoperatively in the assessment of complications. Knowledge of the various techniques and imaging features are important for all practising cardiologists, including their potential limitations such as radiation and contrast nephrotoxicity for CT imaging, and nephrogenic systemic fibrosis for contrast MRI. Furthermore, the importance of practising cost effectiveness medicine should prompt research efforts into establishing the incremental clinical utility of CT and MRI imaging in the above mentioned clinical scenarios.

Role of tomographic imaging in cardiovascular surgery: key points

  • Both MRI and CT are excellent imaging modalities for aortic diseases. Three-dimensional image analysis methods allow the assessment of the aorta in an unlimited number of reconstructed planes and orientations.

  • In cardiac surgery

  • Preoperative imaging assesses the extent of ascending aortic calcification that affects bypass cannulation, as well as the proximity of mediastinal structures to the sternum in redo surgeries.

  • MRI and CT are often useful adjuncts to echocardiography in the assessment of myocardial viability, pericardium, valvular heart disease, and cardiac masses.

  • In vascular surgery

  • Preprocedural planning of patients with aortopathies is routinely performed with CT and/or MRI. Specific imaging modes with or without ECG gating are suited for specific clinical indications.

  • In postprocedural imaging, normal postoperative findings such as high attenuation materials outside the aortic wall should be distinguished from complications such as anastomotic leaks.

  • Endoleak is commonly observed after endovascular stent-graft implantation.

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References

  1. Summary article on the use of tomographic imaging in minimally invasive valve surgery and transcatheter cardiovascular interventions.
  2. Summary article on non-coronary cardiac CT imaging, covering topics beyond the scope of this article.
  3. Excellent article on the imaging assessment of cardiac masses, highlighting the importance of multimodality imaging with echocardiography, CT and MRI.
  4. Summary article providing more details on the use of CT in stent-graft assessment.
  5. Summary article on acute aortic syndromes and their management.
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Footnotes

  • Contributors ACYT, MYD: designed the framework of the article, wrote the initial draft and made appropriate revisions. PS: designed the framework of the article and made appropriate revisions.

  • Competing interests In compliance with EBAC/EACCME guidelines, all authors participating in Education in Heart have disclosed potential conflicts of interest that might cause a bias in the article. The authors have no competing interests.

  • Provenance and peer review Commissioned; externally peer reviewed.

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