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A Review of the Use of Cardiac Computed Tomography for Evaluating the Mitral Valve before and after Mitral Valve Repair

The role of cardiac computed tomography (CT) for evaluating the mitral valve (MV) has been limited since echocardiography is the main method of evaluation. However, recent advances in cardiac CT have enable detailed evaluation of the anatomy and geometry of the MV. We describe assessments of the anatomy and coaptation geometric parameters of normal MVs, and also review repair of diseased/damaged MV. We also discuss pre- and post-surgical imaging of MV pathology using cardiac CT and various CT images. We found that cardiac CT could be used as an alternative imaging modality to echocardiography for pre-operative MV evaluation and to predict clinical outcomes following repair.


INTRODUCTION
Mitral regurgitation (MR) is a heart valve disorder wherein the mitral valve (MV) does not close adequately when the heart pumps blood. In such cases, MR or severe leakage from a damaged structure, will necessitate heart surgery to repair or replace the valve. The underlying mechanism for MV disease and surgical outcomes are not clear. Therefore, to know the normal anatomy and coaptation geometry of the MV is essential for understanding the changes in MV A Review of the Use of Cardiac Computed Tomography for Evaluating the Mitral Valve before and after Mitral Valve Repair parameters of normal MVs and surgical advances in MV reconstruction together with the development of new technologies, and discuss the clinical usefulness of the preand post-operative CT images for the surgical planning and follow-up of the MV.

MV Anatomy
The MV is a dual-flap valve that is critical for the unidirectional blood flow through the heart (5). The main components of the MV apparatus are the mitral annulus, MV leaflets, chordae tendineae, and papillary muscles (Figs. 1, 2) (5-7).

Mitral Annulus
The mitral annulus is a junctional zone that separates the left atrium from the left ventricle, and provides support for the MV leaflets. This dynamic structure changes shape during the cardiac cycle. When viewed in three dimensions, it is non-planar, saddle-shaped, and has a commissural kjronline.org diameter of the MV is larger than the anteroposterior diameter (6,(8)(9)(10). The aortic mitral leaflet (anterior) is in fibrous continuity with the aortic valve and the right and left fibrous trigones of the MV (11). Thus, the annulus region is fibrous and less prone to dilation. However, the posterior mitral annulus is largely muscular and more loosely connected to the surrounding tissue, allowing the posterior mitral annulus to move freely during myocardium contraction and relaxation (12). In cases of significant MR, the posterior region often dilates and is more prone to calcification (7).

MV Leaflets
The MV is composed of two leaflets known as the anterior and posterior leaflets; however, respective preferred terms are the aortic and mural leaflets (13). The mural leaflet is narrow, accounting for one-third of the annular circumference around the left atrioventricular junction. Furthermore, the mural leaflet has indentations and forms three scallops. Functional analysis of the MV leaflet by Carpentier classification revealed the following segments: the most lateral segment, P1, which is in continuity with the anterolateral commissure; the central segment, P2, which can vary significantly in size; and the most medial segment, P3, which is in continuity with the posteromedial commissure (14). The MV aortic leaflet accounts for twothirds of the annular circumference. Based on the number of neighboring mural leaflet regions, the aortic leaflet can be divided into three regions i.e., A1, A2, and A3. The aortic leaflet is also divided into three regions, A1, A2, and A3 based on the number of neighboring mural leaflet regions.

Chordae Tendineae
The heart strings or cord-like tendons known as the chordae tendineae are papillary muscles extending toward and inserted into the MV leaflets. Depending on where they attach, the chordae tendineae may be classified into three types: 1) primary, 2) secondary, and 3) marginal. Primary chords connect to the free edges of both leaflets in the rough zone. Secondary chords connect to the ventricular surface of both leaflets in the rough zone. The largest and thickest chords among the secondary chords of the aortic leaflet are the strut chords. The tertiary chords attach to the junction between the mural leaflet and the ventricular wall (7,15,16).

Papillary Muscles
The papillary muscles located along the mid to apical segments of the left ventricle are usually called the anterolateral and posteromedial muscles. The papillary muscles are labeled this way based on their respective projected relationships to the mitral commissures (17).

MV Coaptation Geometry
Imaging enables the morphological and functional assessments of the MV apparatus. For instance, measuring the MV annulus and leaflet geometry is possible by using various 3D or 4D-imaging reconstruction methods for cardiac CT. The anteroposterior (septo-lateral distance) and

Carpentier Surgical Classification of MV Pathology
Mitral regurgitation can be classified into three main pathological types based on leaflet motion: normal leaflet motion (type I), leaflet prolapsed or excessive leaflet motion (type II), and restricted leaflet motion (type III). Type III is subdivided into types IIIa (leaflet restriction during the diastole and systole phases) and IIIb (leaflet restriction during the systole phase) (

MV Repair
Mitral valve diseases have been successfully treated for the last four decades by replacing the native MV with prosthetics. However, using a prosthetic MV has some disadvantages. For instance, infective endocarditis, embolization, coagulopathy disorders and cerebral vascular disease may occur. Thus, some surgeons attempt to repair the damaged native MV to avoid MV replacement complications. The MV annuloplasty technique has progressively improved since its initial introduction by Prof. Alain Carpentier in 1969. Currently, it is the gold standard for treating MV dysfunction. Also, MV repair is commonly associated with improved mid-term and long-term survival compared with MV replacement (23)(24)(25).

Indications
Based on the American College of Cardiology/American Heart Association guidelines (23), MV surgery (class I) is recommended for symptomatic patients with chronic severe primary MR and left ventricular (LV) ejection fraction (LVEF) > 30%, and asymptomatic patients with chronic severe primary MR and LV dysfunction (LVEF 30-60% and/ or an LV end-systolic diameter ≥ 40 mm). MV repair, which is preferable to MV replacement, is recommended for the surgical treatment of patients with chronic severe primary MR that is limited to the posterior leaflet and involves the anterior leaflet or both leaflets, and when the pre-operative tests indicate that a successful and durable repair can be achieved.

Artificial Neochordae
Artificial neochordae can be constructed by coupling the mitral leaflet to the corresponding papillary muscle of damaged or ruptured chordae with the aid of polytetrafluoroethylene sutures (Fig. 3A) (26,27).

Quadrangular and Triangular Resection of the Posterior Leaflet
During this procedure, the prolapsed leaflet is excised from the leaflet margin to the annulus; thus, a quadrangular section of the leaflet tissue is removed. Plication sutures are placed along the posterior annulus in the resected area. Subsequently, the resection gap margins are approximated with interrupted polypropylene sutures to restore leaflet continuity ( Fig. 3B) (28).

Pericardial Patch Leaflet Augmentation
Diastolic restriction of the leaflet in type IIIa regurgitation cases can be corrected with leaflet augmentation. To perform this procedure, a transverse incision is made 5 mm from the posterior annulus. An elliptical patch is fashioned from the pericardium, and

Illustrative image (A) and CT image (B)
showing Cosgrove-Edwards annuloplasty system (black arrows) that partially surrounds mitral annulus.

A B
Korean J Radiol 18 (5), Sep/Oct 2017 kjronline.org then it is sutured to the incised site to increase the leaflet height ( Fig. 3C) (29).

Commissuroplasty
This method of repair involves surgical resection of the prolapsed area and sliding plasty of the adjacent paracommissural segments to reconstruct the prolapsed MV (30).

Mitral Annuloplasty
Mitral annuloplasty is a key procedure for MV repair. Annuloplasty rings have many types: complete or incomplete, rigid or flexible, and planar or saddle-shaped. Complete rings (e.g., the Carpentier-Edwards Physio annuloplasty ring) are sutured to the anterior and posterior annuli (Fig. 4), whereas partial rings (e.g., a Cosgrove-Edwards annuloplasty system) are sutured at the right and left trigones and at the posterior annulus, including the anterolateral and posteromedial commissures (Fig. 5). Ring annuloplasty restores the normal physiological size and shape of the annulus, and it improves leaflet coaptation. The procedure also improves the durability of the repair by preventing delayed annular dilation (14,22,31). Posterior mitral annuloplasty strips (Mitra-Lift® Strip, Scien-City, Inc., Seoul, Korea) are flat Dacron strips that are placed only along the posterior annulus and spare the anterior half of both commissures and the anterior annulus. The middle portion of the posterior annulus is lifted because of its curvilinear profile (Fig. 6) (32, 33).

Pre-Operative and Post-Operative Imaging of the MV CT Imaging Techniques
At our institution, cardiac CT before MV surgery is mainly performed for MV and coronary artery evaluations using a second-generation dual-source CT scanner (Definition Flash, Siemens, Erlangen, Germany). We used a retrospective electrocardiographically-gated spiral scanning mode and electrocardiography-based current modulation to reconstruct images. A full-strength tube current is used during 30% to 80% of the R-R interval (the interval between two R peaks in the electrocardiogram [the time between two heartbeats]). For this protocol, the peak tube voltage is set as low as possible, at approximately 80-100 kVp depending on the size of the patient's body, to reduce radiation exposure. If the patient's heart rate is higher than 75 beats per minute at 1 hour before the CT examination, 2.5-mg bisoprolol (Conbloc, Elyson Pharmaceutical, Co., Ltd., Seoul, Korea) is sublingually administered. The protocol for contrast material injection is the same protocol used for CT angiography to evaluate the coronary arteries. Depending on the patient's weight, a 70-90 mL bolus of an intravenous non-ionic contrast medium (Xenetix, Guerbet, Roissy, France) is injected at a rate of 4.0-5.0 mL/s rate using a power injector (Stellant D, Medrad, Indianola, PA, kjronline.org USA), followed by a 40-mL saline flush with the bolustracking technique (ascending aorta; attenuation threshold, 100 Hounsfield unit; scan delay, 7 seconds). Acquisition parameters are as follows: section thickness, 0.6 to 0.75 mm; tube voltage range, 80 to 120 kV; tube current-time product, 185 to 380 mAs; collimation, 128 x 0.6 mm; and gantry rotation time, 280 ms. After transferring multiphase source data to an external workstation (AquariusNet, TeraRecon, Foster City, CA, USA), post-processing is performed for image reconstruction.

Imaging Reconstruction
After reviewing the multiphase CT data, we selected the best cardiac cycle phase for the MV evaluation. Koo et al. (34) reported that the best quality images were obtained during the phases at 25% to 35% of the cardiac cycles in patients with MV prolapse. To more accurately assess the MV anatomy and geometry using CT, the reconstructed shortaxis view should be generated after precisely selecting the mitral annulus plane in four-and two-chamber views. Koo et al. (34) introduced the en face view of the MV, i.e., the surgeon's view. By setting the images on the en face view, either perpendicular or parallel to the coaptation line, the MV sagittal or coronal views, respectively, can be obtained by using a multiplanar reformatting display technique ( Fig. 1). Furthermore, methods that render 3D cardiac cine reconstruction of the images may be helpful.

MV CT Imaging
In MV repair, the operative field finding is important in deciding on the surgery type. However, evaluating MV function in the operative field without ventricular dynamic compression has limitations during cardiac arrest. In cardiac surgeries, cardiopulmonary bypass and aortic cross-clamping times are important risk factors for survival and risks of complications (35). Therefore, pre-operative findings of MV structure and dynamic movement are important. Preoperative CT can provide the structural details, such as calcification, valvular thickening, subvalvular structure, such as papillary muscle and ventricular cavity, and annular stenosis with fibrosis. Blood flow can be evaluated more precisely in echocardiography. However, the evaluation of the entire mitral valvular and subvalvular structures and dynamic movement has a limitation in echocardiography. Therefore, cardiac CT can be an additional important evaluation tool in deciding for the best surgical method (plasty or replacement).
Multidetector row CT enables accurate diagnosis of MV prolapse (Fig. 7) (34,36,37). Also, Delgado et al. (3) demonstrated that multislice CT allows a comprehensive This study showed that patients with heart failure with moderate to severe functional MR had a more pronounced tethering at the central and posteromedial levels of mitral leaflets. These CT findings can provide important inferences for MV repair in patients with heart failure with severe functional MR. Recent advances in 3D imaging techniques have allowed for a better understanding of pre-operative and post-operative changes in the MV anatomy and geometry (8,9,18,38). By using 3D echocardiography, Mahmood et al. (18) showed that the MV geometry might vary depending on the nature of the implanted prosthetic device. Although follow-up imaging after MV repair is primarily achieved in routine clinical practice by using echocardiography, CT is preferred for post-operative evaluations since the spatial resolution is better and susceptibility to metallic artifacts is decreased (Figs. 8-11 (Table 2) (40,41). CT has a latent risk from ionizing radiation (38). Also, radiation doses will be increased to obtain a systolic phase to assess the MV (34). The inferior temporal resolution requires medication to control high or irregular heartbeats compared with echocardiography (41).
Because iodinated contrast material injection is required to evaluate the cardiac structures, including cardiac valves, cardiac CT is contraindicated in some patients. Furthermore, hemodynamic information through the MV cannot be evaluated in cardiac CT (38,42). Despite these disadvantages, evaluation of the LV geometry in cases . The normal MV annulus is a non-planar, saddle-shaped structure (6,(8)(9)(10). Thus, the non-planarity of the annulus is considered to contribute to reducing leaflet stress (45,46). The goals of MV repair are to preserve or restore full leaflet motion, create a large surface of coaptation, and remodel and stabilize the entire annulus (22). Therefore, to restore non-planarity after repair, the introduction of specific surgical repair techniques and prostheses is warranted (47). Consequently, the detailed assessment of the MV geometry before and after MV repair by using cardiac CT may help surgeons select better options that consequently facilitate a good clinical outcome.

CONCLUSION
Cardiac CT enables clinicians to perform a detailed assessment of the MV anatomy and geometry before and after surgery, regardless of the surgical method. Therefore, cardiac CT can be used as an alternative imaging modality to echocardiography for evaluating the causes of MV disease pre-operatively and predicting the prognosis with the MV geometry measurement after MV repair.

Supplementary Movie Legends
Movie 1. This video shows CT images of well-functioning mitral valve (MV) after MV repair using a 3D volumerendered cine image. The post-operative change in the annular and leaflet geometry is well demonstrated in this video.
Movie 2. This video shows CT images obtained from the en face view of mitral valve (MV) after MV repair (posterior annuloplasty with ML strip) in a patient with rheumatic mitral stenosis and regurgitation. The reduction in thickening and coaptation defect of the MV after MV repair is better demonstrated in this video than was shown in Figure 9C.
Movie 3. This video shows CT images of well-functioning mechanical aortic valve and mitral valve at the same time after aortic valve replacement and Cosgrove-Edwards annuloplasty in a patient with bicuspid aortic valve and functional mitral regurgitation.
Movie 4. It is difficult to accurately determine whether the coaptation defect remained in the multiplanar reformatted images of the cardiac CT ( Figure 10C), but it is evident in the 3D volume-rendered cine image obtained from a en face view of the mitral valve.
Movie 5. This video shows well-functioning mitral valve (MV) without coaptation defect after MV repair (posterior annuloplasty with ML strip) as shown in Figure 11C. Using a 3D volume-rendered cine image, the artificial neochordae