M-Mode Imaging

Introduction

M-mode echocardiography provides superior temporal resolution, and therefore subtle changes are more readily appreciated with m-mode than with two-dimensional or three-dimensional methods. M-mode methods may include more precise measurement of cardiac chambers (provided they are obtained on-axis), independent motion of valvular vegetations, early closure or early opening of valve structures with respect to timing in the cardiac cycle ( Fig. 2.1 ), identification of prosthetic valves and their function, assessment of paradoxical interventricular septal motion and dyssynchrony of the left ventricle, as well as fluttering of valve leaflets seen in association with valvular regurgitation ( Fig. 2.2 ). The exaggerated motion, as well as restricted motion, of various cardiac structures is readily appreciated with m-mode.

An m-mode echocardiogram provides one-dimensional information regarding a particular cardiac structure as it relates to time and distance, with time displayed on the horizontal axis and depth or distance displayed on the vertical axis. The strength of the reflected echo is represented as the brightness of structures appearing on the image display ( Fig. 2.3 ). The limitations of m-mode echocardiography relate to having to draw conclusions in one dimension about a three-dimensional structure. Furthermore, measurements are dependent on the identification of clearly defined borders, which may not be obtainable in technically challenging patients. With respect to m-mode’s derived ejection fractions, calculations may not be accurate when regional wall motion abnormalities are present.

Although m-mode echocardiography was described more than 50 years ago by Edler and Hertz, new concepts and technologies that take advantage of m-mode techniques continue to expand. For example, color m-mode echocardiography evolved in the 1990s to provide rapid evaluation of time-related events, such as diastolic mitral regurgitation, and has also been used to provide less load-dependent information regarding diastolic function. Color m-mode techniques have also been applied to the assessment of myocardial deformation or strain, in which a curved m-mode is traced along an area of interest of the myocardium and information is displayed in both parametric and graphic format, allowing sensitive evaluation of normal and abnormal patterns of ventricular contractility. M-mode images of the left ventricle are often displayed simultaneously during left ventricular strain analysis to improve interpretation of the curves with respect to the cardiac cycle. Although m-mode echocardiography has been around for a long time and the field of echocardiography has dramatically changed with numerous technological advances, m-mode recordings can still oftentimes provide additional and complementary information, resulting in a more accurate and complete echocardiographic assessment of the heart.

This chapter provides case examples of normal m-mode exams, as well as a diverse spectrum of abnormal m-mode exams illustrating classic cardiac anomalies. Each figure legend provides a “clinical pearl” highlighting important concepts involved in either technically obtaining or interpreting each m-mode image.

Normal M-Mode Measurements

Normal M-Mode Examination of the Aortic Root, Aortic Cusp Separation, and Left Atrial Dimension

Traditionally, m-mode measurements have been used to quantify aortic root size, aortic valve cusp separation, and left atrial dimensions. These measurements are obtained from the parasternal long-axis imaging plane. By convention, m-mode measurements are made leading edge to leading edge, which differs from two-dimensional measurements, which are made inner edge to inner edge. The m-mode cursor is placed perpendicular to the structure being measured. Fig. 2.4 illustrates the proper m-mode technique for measuring the aortic root, aortic valve cusps, and left atrium. The aortic root is measured in end-diastole, just before the onset of the QRS complex. The aortic valve cusp separation is measured in midsystole. The normal appearance of the aortic cusps during systole is that of an “open box,” which reflects holosystolic opening of the valve leaflets. By convention, the left atrium is measured during ventricular systole or atrial diastole, when the left atrium is maximally filled with blood.

Normal M-Mode Examination of the Left Ventricle

The m-mode examination of the left ventricle is also obtained from the parasternal long-axis imaging plane. By convention, left ventricular dimensions are made at end-diastole and end-systole, whereas measurements of left ventricular wall thicknesses, including the interventricular septum and posterior wall of the left ventricle, are usually measured only at end diastole. By convention, the m-mode cursor is placed perpendicular to the long axis of the left ventricle at the level of the mitral valve chordae. Fig. 2.5 illustrates the proper m-mode technique for measuring left ventricular internal dimensions at end systole and end diastole, as well as septal and posterior wall thicknesses in end diastole. In the absence of left ventricular regional wall motion abnormalities, m-mode recordings of the left ventricle have been shown to be an accurate method for calculating left ventricular ejection fraction via the method of Teicholtz. According to this method, the left ventricular dimension in diastole squared minus the left ventricular dimension in systole squared is divided by the left ventricular dimension in diastole squared.

Normal M-Mode Examination of the Mitral Valve

The normal m-mode recording of the mitral valve—like that of the aortic root, left atrium, and left ventricle—is also obtained from the parasternal long-axis imaging plane. The m-mode cursor is placed perpendicular to the long axis of the left ventricle at the level of the tips of the mitral leaflets. The anterior leaflet and posterior leaflet are noted to open fully in diastole and close completely during systole ( Fig. 2.6 ).

Normal M-Mode Examination of the Pulmonic Valve

The normal m-mode recording of the pulmonic valve is usually obtained from the parasternal short-axis view, but it can also be obtained from the right ventricular outflow tract view and main pulmonary artery and bifurcation view ( Fig. 2.7 ). Like the normal aortic valve, the normal pulmonic valve opens throughout systole and has the appearance of “an open box.” Normal m-mode letter designations for the pulmonic valve are as follows: a = atrial contraction, b = onset of ventricular systole, c = ventricular ejection, d = during ventricular ejection, and e = end of ventricular ejection.

Normal M-Mode Examination of the Tricuspid Valve

The normal m-mode recording of the tricuspid valve is obtained from the right ventricular inflow view ( Fig. 2.8 ). Usually, only the anterior leaflet of the tricuspid valve is transected by the m-mode cursor. M-mode letter designations for the tricuspid valve are as follows: D = onset of diastole, E = maximal opening of the leaflet, F = most posterior position of the leaflet, E–F slope = closing motion of the leaflet, A = leaflet reopening with atrial contraction, and C = leaflet closure following ventricular systole.

M-Mode Echocardiography in the Identification of Abnormal Cardiac Structure and Function