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Normal Mitral Valve Anatomy and Measurements


The complexity of the mitral valve is such that two-dimensional (2D) imaging does not adequately describe its anatomy and function. Indeed, the mitral valve apparatus is a dynamic three-dimensional (3D) structure composed of the saddle-shaped annulus; two asymmetric leaflets; multiple chordae tendineae of various lengths, thicknesses, and points of attachment; the left ventricular wall and the attached papillary muscles; and parts of the left atrium ( Figure 5-1 ). During normal function, this array of parts constantly shifts in a complex but defined pattern. Normal alignment of all aspects of this biologic machine is required to avoid dysfunction. Therefore, full understanding of the mitral valve anatomy is not ascertained without 3D imaging. Real-time 3D echocardiography (RT3DE) gives echocardiographers a rapid and easily accessible method of identifying the entire mitral valve apparatus in most patients. As a prominent structure in the posterior left heart, the mitral valve often can be displayed in 3D using either a transthoracic approach or a transesophageal approach. Furthermore, detailed volumetric and positional analysis of the various mitral valve components often is possible ( Table 5-1 ). RT3DE, however, is a novel modality, and its use in mastering the analysis of the intricate mitral valve requires a substantial time commitment. This chapter highlights the use of RT3DE in imaging the normal mitral valve, demonstrating usual transthoracic and transesophageal views for each of its components. It also provides an overview of typical 3D quantitative analyses of the size and position of the mitral valve structures.

Figure 5-1
The mitral valve apparatus as it sits in the left ventricle. It is composed of the annulus ( 1 ), the leaflets ( 2 ), chordae ( 3 ), papillary muscles and ventricular wall ( 4 ), and atrium ( 5 ). AL , anterolateral commissure; Ao , aorta; PM , posteromedial commissure.

Table 5-1
Key Normal Measurements Obtained by Real-Time Three-Dimensional Echocardiography for Each Mitral Valve Structure
Key Measurements Normal Values Reference
Mitral Valve Annulus
Anteroposterior dimension 30.8 ± 4.4 mm Sonne et al
24 ± 1 mm Kwan et al
Commissural dimension 35.1 ± 4.9 mm Sonne et al
28 ± 1 mm Kwan et al
Circumference 10.5 ± 1.4 mm Sonne et al
10.0 ± 0.8 mm Watanabe et al
Height 4.3 ± 2.1 mm Sonne et al
3.0 ± 1.0 mm Flachskampf et al
4.5 ± 1.1 mm Watanabe et al
Area/BSA (end diastole) 4.6 ± 1.3 cm 2 /m 2 Qin et al
5.9 ± 1.2 cm 2 /m 2 Flachskampf et al
Area/BSA (mid systole) 5.1 ± 0.8 cm 2 /m 2 Sonne et al
Area/BSA (end systole) 3.8 ± 1.1 cm 2 /m 2 Qin et al
Area change 23.8% ± 5.1% Flachskampf et al
15% ± 16% Qin et al
Linear systolic motion 16 ± 3 mm Qin et al
10 ± 3 mm Flachskampf et al
Mitral Valve Leaflets
Volume 4.5 ± 0.7 cm 3 Limbu et al
Maximum tenting height 5.3 ± 2.4 mm Sonne et al
3.1 ± 1.2 mm Watanabe et al
5 ± 0.2 mm Kalyanasundaram et al
Mean tenting height 1.9 ± 1.5 mm Sonne et al
0.7 ± 0.5 mm Watanabe et al
Tenting volume 1.5 ± 0.9 cm 3 Sonne et al
0.45 ± 0.29 cm 3 Watanabe et al
Papillary Muscles (Indexed to BSA)
AL papillary to annular distance 21.0 ± 5.8 mm/m 2 Sonne et al
PM papillary to annular distance 22.3 ± 5.6 mm/m 2 Sonne et al
Interpapillary muscle angle 17.6 ± 9.1 degrees/m 2 Sonne et al
Interpapillary distance 10.5 ± 3.3 mm/m 2 Sonne et al
Left Ventricle
End-systolic volume 43.7 ± 10.7 mL Chukwu et al
59 ± 18 mL Corsi et al
43 ± 18 mL Zeidan et al
47 ± 6 mL Nosir et al
End-diastolic volume 115 ± 22.6 mL Chukwu et al
143 ± 30 mL Corsi et al
108 ± 32 mL Zeidan et al
113 ± 16 mL Nosir et al
Left Atrium
End-diastolic volume 48.9 ± 25.1 cm 3 Azar et al
End-systolic volume 62.3 ± 17.2 cm 3 Azar et al
In some cases, values vary due to technique and studied population.
AL , anterolateral; BSA , body surface area; PM , posteromedial.

Mitral Valve Annulus

3DE has increased the understanding of the anatomy of the mitral valve annulus more than that of any other structure in the heart. 3D reconstructions identified the mitral valve annulus as a hyperbolic paraboloid or a saddle-shaped structure with curved planes parallel to the anteroposterior axis opening upward and orthogonal curved planes that open downward ( Figure 5-2 ). This complex shape is not appreciated on 2D imaging, which tends to simplify the annulus as a planar ring. In fact, the anterior and posterior portions of the mitral valve annulus are about 5 mm higher than the medial and lateral commissural points. This height, along with the commissural diameter (the distance between the two low points) and the anteroposterior diameter (the distance between the two high points), can be easily measured with 3D imaging. The nonplanar shape of the mitral valve annulus is very important in reducing leaflet stress, which is increased when the annulus flattens. As such, the hyperbolic paraboloid annular shape with a height/commissural width ratio of 15% to 20% is preserved in many mammalian species, including humans.

Figure 5-2, The mitral value annulus as a hyperbolic paraboloid with anterior ( A ) and posterior ( P ) peaks that are higher than the plane containing the commissures. In this figure, the papillary muscles are in red. The top left image shows the annulus from the left atrial side. The top right image demonstrates the annulus from the left ventricular view.

RT3DE is fundamental to clarifying annular geometry. In fact, obtaining accurate annular dimensions is best done with 3D datasets because 2DE underestimates these values by 6% to 14%. The mitral valve annulus is best seen using the large-sector (3D full volume) and wide-sector focused (3D zoom) formats. In particular, computer remodeling from large-sector datasets can quickly measure and display these changes without requiring the time-consuming reconstructions from multiple 2D images. 3D volume data can be obtained from either transthoracic or transesophageal exams. These models are obtained offline by tracing the mitral valve annulus contours in relation to other structures (such as the papillary muscles and aortic valve) in several 3D views ( Figure 5-3 ). From these computer renditions, several annular dimensions, including circumference, anteroposterior (high-point) dimension, commissural (low-point) dimension, height, and annular area, can be measured ( Figure 5-4 ).

Figure 5-3, A computer rendition of the mitral value annulus ( bottom left ) obtained by identifying key annular structures on a three-dimensional full-volume dataset cropped to form several different views (two-chamber view at the top left, long-axis view on the top right, and a short-axis view on the bottom left ). A, anterior; AL, anterolateral commissure; AntPap, anterior papillary muscle; Ao, aorta; P, posterior; PM, posteromedial commissure; PostPap, posterior papillary muscle.

Figure 5-4, Computer renditions allow key measurements, including annular circumference ( A ); annular area ( B ); anterior leaflet area ( C ); posterior leaflet area ( D ); annular height, commissural distance, and anteroposterior distance ( E ); and aortic-mitral angle ( F ). A, anterior; AL, anterolateral; Ao, aortic valve; P, posterior; PM, posteromedial.

Recognizing the mitral valve annulus as a nonplanar structure is very important to understanding mitral valve function. For example, the complex shape of the mitral valve annulus has led to the overdiagnosis of mitral valve prolapse with 2DE. In certain 2D views, particularly the apical four-chamber view, which shows the mitral valve annulus in mediolateral directions, the leaflets appear to be on the atrial side of the mitral valve annulus, when, in fact, they are still inferior to the high anterior and posterior points. In 2DE imaging without fully realizing the nonplanarity of the mitral valve annulus, this normal anatomy can be mischaracterized as mitral valve prolapse.

RT3DE has also shown that annular area significantly depends on body size and height. The normal diastolic mitral valve area (which is a projection onto the least squares plane) is 5 to 6.5 cm 2 (indexed to body service area). The mitral valve annulus, however, is a dynamic structure whose complex motion throughout the cardiac cycle has not been fully ascertained. On the basis of early studies in 3DE from 2D renditions, it was believed that the mitral valve area decreases during systole by up to 24%, which is thought to occur as the anterior and lateral high points move closer together, increasing the height and eccentricity of the annulus. In addition to a decrease in annular area, there is a significant apical displacement of the annulus during systole, with an average annular motion of 16 ± 3 mm in normal hearts.

Recent investigations using RT3DE with volumetric reconstructions have further characterized this motion in both normal ventricles and those with cardiomyopathy. In disease states, the shape and dynamic function of the mitral valve can change dramatically. For example, in dilated cardiomyopathy, especially with functional mitral regurgitation, the annulus flattens, dilates, and becomes more circular. These annuli also demonstrate less dynamic variability. During systole, they may have less change in height and diameter, leading to a smaller change in the projected valve area. Most significantly, those with cardiomyopathy have less apical excursion during systole. In fact, the degree of annular displacement correlates well with ejection fraction; a linear motion less than 12 mm accurately identifies those with an ejection fraction less than 50%.

Mitral Valve Leaflets

Like the annulus, the attached mitral valve leaflets are 3D dynamic structures. The anterior leaflet covers two thirds of the orifice and is attached to the part of anterior annulus adjacent to the aortic valve near the right fibrous trigone, aortic-mitral fibrosa, and left fibrous trigone ( Figure 5-5 ). The posterior leaflet, although smaller, covering only one third of the mitral orifice, is semicircular in shape and borders most of the annulus from one commissure to the other. Figure 5-5 shows the mitral valve leaflets and related cardiac structures as seen from the left atrium (the surgeon's view) and the left ventricle. Each leaflet is composed of three scallops. Scallops A1 and P1 are the most lateral, located near the left atrial appendage; the middle scallops, A2 and P2, are the most posterior; and A3 and P3 are the most medial.

Figure 5-5, Mitral valve leaflets as viewed from the left atrium (the surgeon's view) on the left and as viewed from the left ventricle on the right . AL , anterior leaflet; AMC , anteromedial commissure; AMF , aortic-mitral fibrosa; APM , anterolateral papillary muscle; AV , aortic valve; LAA , left atrial appendage; LCC , left coronary cusp of aortic valve; LT , left trigone; LVOT , left ventricular outflow tract; NCC , noncoronary cusp of the aortic valve; PMC , posteromedial commissure; PL , posterior leaflet; PPM , posteromedial papillary muscle; RCC , right coronary cusp of the aortic valve; RT , right trigone.

With 3DE, all six scallops of the mitral valve leaflets can be clearly visualized from either the left atrium or the left ventricle. These views can be obtained offline from cropping 3D full-volume (large-sector) datasets, which can be obtained by either a transthoracic or a transesophageal approach ( Figure 5-6 and Video 5-1 ). During transesophageal exams, the 3D zoom (wide-sector focused) format also is helpful, especially to obtain real-time information on leaflet function during surgery or catheter procedures. In this case, the images are obtained by identifying the entire mitral valve on orthogonal biplane 2D images. The resulting 3D zoom display is rotated with the aortic valve in the 12 o'clock position to obtain the surgeon's view from the left atrium. In this view, normal systolic bulging of the central part of the anterior leaflet can be visualized. When this image is flipped 180 degrees, the leaflet scallops are displayed as viewed from the left ventricle ( Figure 5-7 ). In transthoracic exams, similarly detailed 3D images also can be obtained. Figure 5-8 demonstrates how clearly the mitral valve leaflets can be displayed from a 3D full-volume dataset acquired during a transthoracic exam. In this case, the image is rotated and the vertical axis is cropped from the atrial side to obtain the surgeon's view. By flipping the image 180 degrees and cropping from the ventricular side, the valve leaflets can be seen as viewed from the apex in relation to the commissures and papillary muscles.

Figure 5-6, Demonstration of the mitral valve leaflets obtained from a three-dimensional full-volume data-set. Multiplane reconstructions are shown in A through C. The on-face view of the leaflets from the left atrium ( LA ) is shown in D. LV, left ventricle; LVOT, left ventricular outflow tract; A2, P1, P2, and P3, scallops of the mitral valve leaflets.

Figure 5-7, The mitral valve ( MV ) leaflets from three-dimensional wide-sector datasets obtained during a transesophageal exam. The leaflets are viewed from the left atrium ( LA ) on the left and from the left ventricle ( LV ) on the right . Red asterisk , anterolateral commissure; green asterisk , posteromedial commissure; AL , anterior leaflet; AV , aortic valve; LVOT , left ventricular outflow tract; PL , posterior leaflet.

Figure 5-8, The mitral valve ( MV ) leaflets displayed from a cropped three-dimensional full-volume dataset obtained during a transthoracic exam. The leaflets are viewed from the left atrium ( LA ) on the left and from the left ventricle ( LV ) on the right . Red asterisk , anterolateral commissure; green asterisk , posteromedial commissure; AL , anterior leaflet; AV , aortic valve; LVOT , left ventricular outflow tract; PL , posterior leaflet.

The commissures of the mitral valve are easily identified by RT3DE. The anterolateral commissure is located next to the left atrial appendage and the left fibrous trigone and represents the point of fusion between A1 and P1. The posteromedial commissure is located next to the right fibrous trigone and represents the point of fusion between A3 and P3. A short-axis view of the mitral valve using the narrow-sector format can clearly display both commissures from the atrium and the ventricle by either the transesophageal or the transthoracic approach (see Figures 5-7 and 5-8 ). In the transesophageal exam, the mid-esophageal view allows ideal visualization of both commissures. The anatomy of the commissures is of particular importance in patients with mitral valve stenosis before balloon valvuloplasty. In fact, commissural splitting is believed to be the primary mechanism of increasing valve area during balloon valvuloplasty. Thus, the degree of noncalcified commissural fusion, as assessed by RT3DE, correlates with successful valvuloplasty and may be a better predictor of procedural success compared with traditional 2D scoring algorithms. Heavily calcified commissures are unlikely to split during the procedure and should be identified before referral for valvuloplasty.

In the evaluation of mitral stenosis, RT3DE can improve assessments of leaflet opening. In 2D, calculating valve area by planimetry is unreliable because it is difficult to determine if the traced plane intersects the valve at its smallest dimensions. This limitation, however, is overcome with 3DE imaging because the entire valve can be imaged and the short-axis plane can be cropped at the smallest area. As demonstrated in several studies, this method may be superior to estimating true mitral valve area when compared with 2D measurements by pressure half-time, proximal isovelocity surface area, and planimetry. Some have suggested that 3D planimetry may be more accurate than even invasive measurements of mitral valve area.

With RT3DE, it is also possible to ascertain mitral valve leaflet volume. As rheumatic disease progresses, the leaflets thicken. The degree of thickening correlates with complications such as atrial fibrillation and death. Leaflet volume also can be assessed with RT3DE, which may have as much prognostic value as mitral valve area.

RT3DE allows full assessment of the mitral valve leaflet motion in the majority of cases. Even during transthoracic exams, all three scallops of the anterior and posterior leaflets are completely visualized in up to 84% and 77% of cases, respectively. This technology is of particular use in identifying the exact location of leaflet pathology. On transesophageal echocardiography (TEE), it is possible to see each individual scallop with 2D imaging alone, but this process is cumbersome because the entire leaflets cannot be seen in the same view. To reconstruct all six scallops, an excellent working memory of mitral valve anatomy is required, and it is necessary to rotate through the various views to identify and locate pathology. With 3D imaging, however, all six scallops, along with their relationships to other structures, can be visualized at once ( Figure 5-9 ). For example, a fibroelastoma is shown in Figure 5-10 . In the 2D transesophageal views, this tumor is easily seen on the posterior leaflet, but the 3D zoom view demonstrates its exact location on P1 very close to the anterolateral commissure. In addition, mitral valve prolapse can be easily identified in 3D with the wide-sector focused view. As seen from the left atrium, the prolapsed segment is a bright bulge. From the ventricle, it is a spoon-shaped depression. Although 2DE may diagnose the presence of flail or prolapse in general, 3DE often is needed to identify the specific scallops involved. In fact, myxomatous mitral valve disease highlights the usefulness of 3D techniques. Several studies have compared the accuracy of the various echo modalities in identifying the location of mitral valve prolapse ( Table 5-2 ). 3DE also can help visualize and quantify the location and size of the orifice regurgitant area that leads to mitral regurgitation. Color Doppler can be added to full-volume acquisitions. This increased level of visualization shows the regurgitation as a column or plane. Especially in eccentric mitral regurgitation, 3D allows visualization beyond a 2D “jet,” thus better characterizing its severity. In planning mitral valve surgery, the increased level of detail proved by RT3DE is helpful to the surgeon and may increase repair rates.

Figure 5-9, Two-dimensional (2D) imaging ( left ) shows only parts of the mitral valve leaflets in any given view. To mentally reconstruct the exact location of leaflet pathology during a 2D exam, one must have an excellent working knowledge of mitral valve anatomy. With three-dimensional imaging ( right ), the entirety of both leaflets can be seen in one view. ALC, anterolateral commissure; AV, aortic valve; LAA, left atrial appendage; LCC, left coronary cusp; NCC, noncoronary cusp; PMC, posteromedial commissure; TEE, transesophageal echocardiography; A1, A2, A3, P1, P2, and P3, scallops of the mitral valve leaflets.

Figure 5-10, A mitral valve fibroelastoma clearly seen on P1 close to the anterolateral commissure ( red asterisk ) in this three-dimensional wide-sector view. A and B show the tumor from the surgeon's view in systole and diastole. It is also seen prolapsing into the ventricle from the left atrial view in C. In D, the tumor is seen in diastole as viewed from the apex. Green asterisk , posteromedial commissure. Ao, aortic valve; AL, anterior leaflet; LAA, left atrial appendage; LV, left ventricle; PL, posterior leaflet.

Table 5-2
Comparison of the Accuracy of Each Echo Modality in Identifying the Correct Location of Mitral Valve Prolapse (Validated by Surgical Inspection)
N 2DTTE (%) 2DTEE (%) 3DTTE (%) 3DTEE (%)
Grewal et al 42 — 90 — 98
Pepi et al 112 77 87 90 —
Manda et al 18 — 50 — 77
Muller et al 74 — 86-97 — 97-100 *
Fabricius et al 42 — 97 — 91
Sharma et al 39 82 96 — 94
Hirata et al 42 97 100 — 100
Agricola et al 59 — 92 94 —
Garcia-Orta et al 54 67-94 89-100 * — —
2DTTE, two-dimensional transthoracic echocardiograpy; 2DTEE, two-dimensional transesophageal echocardiograpy; 3DTTE, three-dimensional transthoracic echocardiograpy; 3DTEE, three-dimensional transesophageal echocardiography.

* Accuracy rates reported on each segment; entire range is given here.

In addition to prolapse, RT3DE has been helpful in quantifying leaflet tethering associated with functional mitral regurgitation. Normally, the mitral valve leaflets are nearly level with the annulus (with a mean tenting height of 1.9 ± 1.5 mm). Figure 5-11 demonstrates mitral valve tenting in a normal mitral valve. In ischemic mitral regurgitation, the mitral valve leaflets may be pulled toward the ventricle by increased tension of the chordae. In extreme cases, with bileaflet restriction, the valve can form a funnel shape, allowing free central regurgitation. Several laboratories have developed methods of quantifying leaflet tethering using datasets from RT3DE. With these models, information on the mechanism of ischemic mitral regurgitation is ascertained. For example, Kwan and colleagues showed that an inferior infarct caused asymmetric tethering on the medial side of the valve.

Figure 5-11, Mitral valve tenting. The tenting volume is shown in two-dimensions in A and is highlighted by the corresponding three-dimensional view ( B ). The tenting volume is calculated and shown in relation to the annulus in C and D. L, lateral; LA, left atrium; LV, left ventricle; S, septal.

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