The role of computed tomography in pre-procedural planning of cardiovascular surgery and intervention

The relationship of brachiocephalic vessels, aorta, right ventricle and pericardium to the midline sternum is assessed in axial or reconstructed sagittal or volume rendered images. Chamber enlargement, aneurysm, pectus excavatum deformity and adhesions from previous surgery or radiation bring these structures close to sternum. Ten millimetres is considered a safe distance from the sternal midline (Fig. 1). Close proximity or adhesion of these structures to the sternum (Fig. 2) increases the risk of injury during midline sternotomy. The point of proximity to midline sternum is often described in relation to the closest sternal wire. Knowledge of the anatomy helps the surgeon in planning the surgery, including altered approach, or a more superior extensive dissection prior to sternotomy or initiation of cardiopulmonary bypass before sternotomy.

Since patients with coronary bypass grafts, particularly those with LIMA (left internal mammary) grafts are highly dependent on the graft for cardiac function, injury to the graft can be potentially fatal [4]. Of re-operative surgeries, 5.3 % are complicated by internal mammary artery injury, which has mortality of 9–50 % [5]. The internal mammary grafts usually run anteriorly and inferiorly from subclavian origin, typically anastomosing to the LAD (Fig. 3). In an attempt to prevent sternal adhesion, the LIMA graft is frequently routed through the left pericardial slit, but may be medially displaced by adhesions. RIMA (right internal mammary) grafts and aorto-coronary grafts to the left coronary system typically cross the midline behind sternum to reach the distal anastomosis, making them prone to injury (Fig. 4). Saphenous venous grafts are usually anastomosed in an end to side fashion with the ascending aorta. Although they are typically at a safe distance from the sternum, RV enlargement, large aortic aneurysm or altered anatomy by mediastinal adhesions can bring them close to the sternum. In addition to injury, manipulation of saphenous grafts, which are prone to atherosclerosis, can result in embolic shower into the distal circulation, causing myocardial infarction. A bypass graft that crosses the midline and located <1 cm from the sternum or adherent to the sternum is at high risk of injury when the sternum is opened and the surgeon should be alerted about this finding. To avoid injury to grafts, alternative approaches may have to be used, such as lateral thoractomy.

During cardiopulmonary bypass, continuous antegrade cerebral perfusion is maintained through direct cannulation of the ascending aorta. However, extensive atherosclerotic calcification of the ascending aorta (porcelain aorta) precludes cannulation due to associated high risk of plaque dislodgement that may result in cerebrovascular accidents. There is an 8.7 % risk of peri-operative stroke in patients with calcification compared with those without [6], which is the major cause of post-operative morbidity and mortality in cardiac surgery [7]. The risk is lower in complete than partial cross clamping during surgery [6]. Calcification can be evaluated in axial images or sagittal reformatted images or volume rendered images (Fig. 5)

In the presence of extensive atherosclerotic disease of the ascending aorta, alternative sites are used for cannulation or surgical side grafts. Axillary or subclavian arteries are used [4] due to lower incidence of athero-emboli and lower risk of micro-emboli during bypass [8]. Assessment of the anatomy of these vessels, including variations, atherosclerosis and stenosis is essential prior to surgery. Axillary artery cannulation is performed by clamping the base of the innominate artery with antegrade cerebral perfusion through the right common carotid artery and perfusion of contralateral cerebral hemisphere through an intact Circle of Willis. Presence of a bovine variant (common origin of brachiocephalic artery and left common carotid artery) enables bilateral antegrade common carotid antegrade perfusion with a single cannulation, regardless of the integrity of Circle of Willis. A left-sided arch with aberrant right subclavian artery or a right-sided arch are not ideal for right axillary cannulation, due to perfusion of descending aorta in the first scenario and separate origin of right subclavian and right common carotid arteries in the second scenario [4].

There is wide variability in the size of the aortic root, which may be affected by aortic root remodelling in aortic stenosis [14]. Accurate measurement of the annulus, sinus and sinotubular junction is essential to select the appropriate sized device (Fig. 6a). The aortic annulus has an ellipsoid configuration, with larger coronal than sagittal diameters (Fig. 6b). Accurate measurement of the annulus is necessary to avoid mismatch between annulus and prosthesis. Placement of a large valve in a small annulus might result in rupture or AV block [15]. In contrast, placement of a small valve in a large aortic annulus has been shown to be associated with increased risk of post-procedural aortic regurgitation [16] and device dislocation [17]. For Edwards-Sapien valves, the annulus should be between 18 and 27 mm and for CoreValve device, it should be between 18 and 29 mm [12]. Measurement of the annulus is performed meticulously at the plane of virtual basal ring, which is exactly aligned with the three most caudal insertion points of the aortic cusp [18, 19]. Annulus is measured from double oblique multiplanar reconstructions with two orthogonal planes, which represent the short and long axis of the virtual basal ring [19]. The diameter can be derived as a mean of the minimal and maximal diameters or derived from circumference or area of the annulus. There are also specific measurements for the width and length of the sinotubular junction and the diameter of the ascending aorta (4 cm from the annulus) for a CoreValve device (SCCT 19).

LVOT diameter and septal thickness are also measured with CT (Fig. 6c). Device placement is difficult if the LVOT is small or the septum is thick. Position of the LV apex relative to the chest wall is required for apical approach. Presence of thrombus in the LV increases the risk of embolic complications, both in apical and femoral routes.

Leaflet morphology is assessed in short-axis projection of the aortic root. Aortic valve morphologies other than a normal trileaflet valve (e.g. bicuspid or rarely unicuspid or quadricuspid [Fig. 7]) are currently considered contraindications for TAVI. In contrast to the normal thin, uniform leaflets that open widely, the typically severely calcified and thickened leaflets in patients with severe aortic stenosis (AS) open in a restricted, star-shaped configuration with an aortic valve area of less than 0.8 cm². Calcification of the aortic valve can be classified as: grade 1—no calcification; grade 2—mild calcification (small isolated spots); grade 3—moderate calcification (multiple larger spots); grade 4—severe calcification (extensive calcification of all cusps) (Fig. 7). CT has high accuracy and reproducibility in detection and quantification of aortic calcification [20]. Higher calcification has been correlated with increased pressure gradients in echo, but it cannot be used to diagnose or rule out aortic stenosis [21, 22] Calcification is an independent risk predictor for disease progression and adverse clinical outcome [23, 24]. Heavy calcification may hamper the ability of the prosthesis to cross a native valve during percutaneous procedure. Higher calcification, especially in the intertrigonal area limits the expansion of prosthesis. This has been correlated with higher incidence of post-procedural aortic regurgitation due to decreased apposition of prosthesis with the aortic wall [25] and higher incidence of need for pacemaker implantation following procedure [15]. There is a strong correlation between valve calcification and coronary plaque [26]. Long leaflet with calcification increases risk of coronary occlusion during procedure. Calcific embolism from valve may cause stroke.

Valve opening area can be measured directly in CT by planimetry and shows good correlation with echocardiographic assessment both using planimetry and calculations using the continuity equation. A valve area of <1.0 cm² is considered severe. The area measured by CT is higher than that of echocardiography since the LVOT is ellipsoid and the echocardiography parasternal long axis projection measures the shortest diameter, resulting in underestimation of valve area and overestimation of stenosis. Contrast-enhanced CT acquired between 10 % and 30 % of R-R interval is used for measuring the valve area [26, 27, 28] (Fig. 8).

Leaflet opening pattern is assessed using 4D CT (Movie 1). In aortic stenosis, the leaflets are thickened and calcified with decreased valvular excursion. Incomplete diastolic leaflet coaptation is seen in aortic regurgitation. LV hypertrophy and post stenotic dilation are also seen.

The distance between the aortic annulus and inferior aspects of the coronary ostia (Fig. 9a) should be measured carefully, since there is wide variation between patients. This ranges from 7.1 to 22.7 mm for left and 9.2–26.3 mm for the right [18]. In addition, the length of the leaflets is also measured from the tip to the base (Fig. 9b). An annulus-coronary ostial distance of >10 mm is considered safe to avoid interference of the ostia with the calcified leaflet tips and the prosthesis stent struts after deployment. There is a higher risk of coronary occlusion if: the distance between the annulus and ostium is <10 mm; long leaflet; heavily calcified leaflet; shallow sinus; narrow sinotubular junction; a balloon expandable valve [19].

Three-dimensional analysis of the relationship of the aortic root with the body axis is essential for planning of interventional access planes [29]. Optimal angiographic views perpendicular to the aortic valve plane is vital for exact placement of the device in the catheter lab. Using CT, 2D angiographic projections that are orthogonal to the aortic valve area can be predicted, which simplifies the percutaneous implantation procedure, with lower angiographic runs, and thus radiation and contrast doses. Various cranial and caudal reconstructions are performed in various left anterior oblique (LAO) angulations to determine the best projection orthogonal to the axis of the aortic root [29] (Fig. 10). If this plane is used during catheterisation, a view exactly orthogonal onto the annulus is obtained.

The course, tortuosity, calibre and calcification of the femoral, iliac and lower abdominal aorta are assessed using CT.

Although echocardiography is the modality of choice in comprehensive assessment of mitral valve morphology and function, contrast enhanced CT can provide information on morphology. From a short axis image at the level of mitral commissures, three anteroposterior planes perpendicular to the mitral valve are used to assess geometry of anterolateral (A1-P1), central (A2-P2) and posteromedial (A3-P3) parts of mitral leaflets. The leaflet angles and mitral valve tenting height are measured in all the three planes. Leaflet/annulus thickening (Fig. 12a, b) and calcification (Fig. 12c) can be assessed [32].

Robotic mitral valve repair is not performed in patients with extensive mitral annular calcification((Fig. 12c).

Mitral annulus dimensions are helpful in determining the optimal size of annuloplasty ring. Using two-chamber and four-chamber views, a short-axis image at the level of the mitral annulus is reconstructed. The anteroposterior and intercommissural diameters are then measured (Fig. 13). Planimetry is used to measure the mitral annulus area [32].

In robotic repair, cardiopulmonary bypass is established by retrograde perfusion through femoral vessels. The femoral and iliac vessels should be of sufficient calibre to allow the deployment device.

Absence of significant atherosclerotic disease is essential for reducing risk of cerebral embolism or retrograde dissection [1].

The distance from the sinotubular junction to the origin of the aortic arch branch vessels is measured by coronal reformatted CT images to determine the ideal size of endo-aortic balloon for establishing cardiopulmonary bypass (Fig. 14).

The distance between coronary sinus (CS)/great cardiac vein (GCV) and mitral valve annulus (MVA) is variable along the length of the CS (Fig. 15). In the majority (90 %), the CS is located more superiorly to the mitral valve annulus and runs posterior to the left atrium than the MVA [35]. This distance may be further increased in patients with severe chronic mitral regurgitation. If there is a large distance between the CS and MVA and the CS is located posterior to the left atrium, percutaneous annuloplasty is not useful. This distance may also vary with the phases of the cardiac cycle [35].

The distance, crossing point, length of crossing of the left circumflex artery (LCX) between coronary sinus- great cardiac vein and mitral valve annulus is measured using CT. If the LCX runs close and crosses between the CS-GCV and mitral annulus, there is high risk of compression of LCX due to the percutaneous mitral annuloplasty. In 68 % of patients, the LCX may run inferior to the CS, as a result of which it is located between the CS and mitral valve annulus, with an overlapping segment >30 mm in 17.58 % of these patients. If the LCX is coursing between the CS and MVA for a long distance, a percutaneous procedure is not possible since it might cause LCX compression [35].

In patients with severe calcification of the mitral annulus, surgical approach is preferred over percutaneous approach [35].

Percutaneous closure is performed for ostium secundum type of ASD and patent foramen ovale, but is not appropriate for sinus venosus or coronary sinus defects.

The length of rims from the edge of ASD to the tricuspid valve (anteroinferior rim), aortic valve (anterosuperior rim), SVC (posterosuperior rim) and IVC (posteroinferior rim) are measured using CT (Fig. 16) [36]. Rim deficiency of <3 mm is a contraindication for percutaneous closure (Fig. 17) [37]. Anterosuperior rim deficiency is seen in 28–54 % of patients [36]. With a large ASD, a posteroinferior rim of at least 10 mm is an important predictive factor for successful percutaneous closure. For PFOs, the minimum distance between the device and SVC or aorta should be 9 mm, and ideally the device should not extend across more than 90 % of atrial septum [38]

Small atria, floppy septum and thin septum are relative contraindications for percutaneous closure.

Presence of associated cardiovascular anomalies and thrombi in the atria or ventricle are contraindications for percutaneous closure.

Muscular and certain type of perimembranous defects are suitable for percutaneous closure. Inlet and outlet defects are difficult to be closed with catheter devices.

The size and shape of the defect are measured in the short and long axes in end diastolic images. En face views may be used to measure the area. There is a 20 % change in defect size between systole and diastole. Most of the defects are oval on the right and left ventricular aspects with variable size.

Proximity to the aortic, pulmonary and tricuspid valves and the atrioventricular conduction bundle determines the suitability of a defect for percutaneous closure. Muscular defects are usually at a safe distance from these structures making them ideally suited for catheter device closure. Doubly committed and juxta-arterial defects are bordered by aortic and pulmonary valves, making them unsuitable for catheter closure. Perimembranous outlet defect is closest to the aortic valve and a confluent defect that extends to both inlet and outlet portions is close to both the atrioventricular and aortic valves [40]. However, device closure can be done in some perimembranous defects that are not complicated by septal malalignment or valvular prolapse with careful evaluation of defect size, shape, and distance from vital structures and optimal device selection. The area of fibrous continuity between tricuspid, mitral and aortic valve is crucial to avoid atrioventricular dissociation [41].

The most commonly seen pattern (seen in 57–80 %) of pulmonary venous drainage is four veins (right superior, right inferior, left superior, left inferior) draining into the left atrium, with the right middle vein opening into the right superior vein. Various variations have been described in pulmonary venous drainage, which are related to the embryologic development. In the conjoined pattern (2.4–25 %), which is more common on the left, the superior and inferior pulmonary veins unite before the left atrium to result in a single ostium that is larger than normal [42]. Due to ostial size larger than that of normal catheters, segmental isolation may not be possible [30]. Accessory veins may be seen (1.6–19 % of patients) and have smaller ostia than normal [43]. The right middle lobe (4–27 %) and superior segment of right lower lobe are the most common accessory veins (Fig. 18a) [42]. Recognition of these accessory veins is essential to avoid injury that results in pulmonary vein stenosis and to ensure that all veins are ablated. In the presence of an accessory right middle lobe vein, the figure of eight ablation might not be feasible due to inadequate tissue between the veins to support an ablation catheter. Accessory top veins draining in the roof of the left atrium are also at risk of injury. Veins crossing midline posterior of left atrium are at risk both from ablation of ostia and ablation of the posterior wall of the left atrium. Anomalous pulmonary venous drainage refers to pulmonary veins opening into the systemic veins or into the right atrium and may be associated with sinus venosus defect [43].

Early branching is defined as branching within 1 cm of the pulmonary vein bifurcation or 5 mm from the ostium of the main pulmonary vein. Early branching is seen in up 41 % of patients and is more common on the right side. Early branching veins are more prone to pulmonary venous stenosis [44].

The superior pulmonary veins enter the left atrium at a typical angle of 45–60 ° and inferior pulmonary veins enter at an angle of 30–45° to the horizontal plane [44]. Knowledge of this orientation helps in determining the proper orientation of catheters during the procedure, which shortens procedure time. In addition, since the ridge of atrial tissue separating the right superior and inferior veins is narrow, an unstable position of a catheter in this area potentially can result in application of energy within one of the veins [45].

The diameter and area of the individual pulmonary veins are measured in orthogonal short-axis or endocardial views. The ostial size varies with the phase of the cardiac cycle and should ideally be measured in the same phase each time. Patients with atrial fibrillation typically have larger ostia. Knowledge of size helps in selecting the optimal catheter. Small ostia (<10 mm) are prone to stenosis (Fig. 18b) and some operators avoid ablation in these small veins.

Presence of thrombus, either in the left atrium or left atrial appendage is a contraindication to pulmonary ablation. Thrombus is seen as a filling defect in contrast-enhanced CT scans (Fig. 18c). A filling defect can also be secondary to slow flow with admixture of contrast. Thrombus, however, has lower density than slow flow and is persistent on delayed phase imaging [46]. In indeterminate cases, a trans-oesophageal echocardiogram is performed.

Assessment of the left atrial (LA) size is an important factor in patients with atrial fibrillation [47]. Left atrial size is an important predictor of cardiovascular outcomes such as atrial fibrillation, congestive cardiac failure, stroke and cardiovascular death [48]. LA enlargement is an indicator of the duration and severity of diastolic dysfunction [49]. It is a risk factor for stroke, and atrial fibrillation before and after therapy. Increased LA volume is an independent strong predictor of atrial fibrillation, and atrial fibrillation is an independent predictor of LA size. The LA can be measured by diameter, area or volume. Diameter and area are measured in four-chamber reconstructed image. Volume can be estimated by area-length biplane method, prolate ellipsoid method or modified Simpson’s method [49]

A thin layer of fat is present between the thin posterior wall of the left atrium and anterior oesophagus, which insulates the oesophagus from thermal injury of ablation. The fat layer is occasionally absent, particularly at the level of the mid posterior wall. While in 62 %, oesophagus is immediately posterior to the left pulmonary veins, in 15 % it is behind the right pulmonary veins [50]. Knowledge of the relationship between posterior wall of the left atrium/pulmonary veins and the oesophagus is essential, since there is a potential risk of atrio-esophageal fistula if the oesophagus is close to the pulmonary venous ostium. However, it should be noted that the anatomical relationship between the oesophagus and left atrium may vary from the time of CT scan to the procedure [50].

A patent coronary vein is essential for transcatheter placement of an electrical lead in the lateral wall of LV. Coronary sinus and tributaries can be absent as normal variation. While the great and middle cardiac veins are present in all patients (Fig. 19a), lateral and posterior cardiac veins (which are targets of cardiac resynchronisation therapy [CRT]) are congenitally absent in 1–3 % [51]. Coronary sinus tributaries can also be absent due to thrombosis following myocardial infarction. A CT study showed that the left marginal vein and posterolateral vein was absent in 75 % of patients with ischaemic cardiomyopathy and impaired ejection fraction, compared with normal controls and those with coronary artery disease (Fig. 19b). There was correlation between extent and geography of myocardial infarction (MI) and absence of coronary sinus [51]. CT venography is helpful in mapping the venous anatomy prior to the procedure. The acquisition parameters are the same as CT coronary angiography except for acquisition in the venous phase. If coronary sinus and tributaries are absent, epicardial lead placement through surgical approach is required [52].

Another cause for failure of CRT is the presence of extensive scar in the LV, which causes electrical capture and mechanical contraction. MRI is the ideal method for detecting scar, but it can also be detected in delayed phase CT scan (Fig. 20). A previous infarct can also be detected as wall thinning, hypodensity, and calcification of the LV wall in CT. It has also been shown that there is more absence of coronary sinus tributaries in patients with extensive scar in inferolateral wall [53].

Accurate measurement (length, diameter) of the primary pathology such as aneurysm, dissection, rupture and congenital abnormality is required for selecting the appropriate sized device. In patients with an aneurysm, 10–15 % over sizing of stent grafts is ideally preferred to prevent stent migration and optimal seal [57]. It is important to consider measurements of the lumen and overall vessel size. In patient with aneurysm and adherent wall thrombus, the vessels size is significantly larger than the lumen size. The risk of rupture correlates with overall vessel size, which therefore determines surgical or interventional indication. Luminal size determines the need for intervention in patients with obstructive disease [57].

Landing zone refers to the segment of aorta proximal and distal to the primary pathology where the limbs of the stent are positioned. Suitable proximal and distal sealing zones are critical for optimal close fitting of the stent within the aorta and exclusion of flow outside the stented lumen (Fig. 21). For optimal graft attachment, the maximum diameter of the aorta in the landing zone should be 40–42 mm, both proximally and distally [57].

Presence of circumferential thrombus or atheroma in the landing zone increases the risk of its dislodgment upon deployment of the stent graft, which increases the risk of cerebrovascular accidents [58].

Ideally, there should be at least 15-mm distance between the primary pathology and the origin of major branch vessel, such as the left subclavian artery. Involvement of the major branch vessel may preclude the use of endovascular stent or require pre-procedural “de-branching”, e.g. with a left carotid to subclavian bypass graft and coil occlusion of the native left subclavian artery ostium (Fig. 21). If the stent has to be placed through branch vessels, fenestrated grafts with openings in the graft fabric to accommodate the ostia of branch vessels may be used. These orifices can then be fixed by implanting bare or covered stents across the graft/ostial interfaces. Side or bypass grafts can also be used. Covered grafts can be used over the origin of the left subclavian arteries if patency of both the vertebral arteries is demonstrated. Hence precise analysis of the branches and angle of the branches with respect to each other is critical.

The femoral and iliac arteries should be >9 mm to enable access to the deployment device. Severe aorto-iliac tortuosity hinders endovascular device placement [3]

Acute angulation of the aorta at the level of the diaphragm might potentially hinder stent placement. Sagittal or coronal reformatted images are the most useful in the assessment of angulation

Calibre of the RV-PA conduit- The Melody valve (Medtronic) is a trileaflet bovine jugular venous valve sutured inside a platinum-iridium balloon expandable stent. The delivery system is a balloon-in-balloon catheter that has diameter of 18–22 mm. CT is useful in the measurement of dimensions of the RV-PA conduit, which has to be at least 14 mm and not more than 22 mm [60]. Placement of the valve in smaller conduits will result in residual gradients, while placement in larger conduits are associated with failure of leaflet coaptation.

Length of the conduit available away from pulmonary artery bifurcation- The stent system has a length of 34 mm, which will shorten by 26 %, i.e. 24.6 mm with 22 mm balloon inflation and shortens by 13.5 %, i.e. 28.8 mm at 18-mm balloon inflation [59]. CT can estimate this measurement and help in selecting the appropriate patients.

RVOT/pulmonary morphology- CT with MPR and 3D reconstructions is used in determining the RVOT morphology, which is another factor in determining suitability for PPVI. Type I, the aneurysmal (Pyramidal) shape is the most common shape, where the RVOT funnels towards pulmonary bifurcation. This is seen in patients with trans-annular patch and is not suitable for PPI due to high risk for device dislodgment. Type 2 (14 %) has a cylindrical shape with a constant diameter and is suitable for PPVI. Type 3 (3 %) has an inverted pyramidal shape. Type 4 (17 %) is fusiform, while type 6 (13 %) is narrow and tubular. [61]. The best site for anchorage can also be determined by CT.