Introduction to Echocardiography, Part 1-2

Introduction

M-mode and 2D echocardiography are two commonly used imaging techniques in cardiac ultrasound (echocardiography). M-mode is not typically utilized as much during routine exams. However, knowing what and when to use it in conjunction with 2D and Doppler is helpful. M-mode and 2D ultrasound provide different perspectives of the heart and are used to assess its structure, function, and pathology. This course is an overview of M-mode and the basics of 2D ultrasound used in echocardiography.

Defining Ultrasound Probe Movements and Imaging Planes

Ultrasound imaging planes refer to the different orientations or positions in which an ultrasound probe is placed on the body in order to obtain specific views or images of the heart. These planes provide clinicians with different perspectives of the heart, aiding in the diagnosis and evaluation of various conditions and abnormalities. In ultrasound imaging, the most common planes include:

Figure 1- ASE Imaging planes heart diagram

Transverse (or short axis) Plane: The transverse plane provides a cross-sectional view of the heart. When scanning the heart, this will be 90 degrees rotation from the long axis of the heart. (see Figure 1)
Sagittal (or longitudinal) plane: In the sagittal plane, the ultrasound probe indicator is directed towards the patient's right shoulder. When scanning the heart, this will be 90 degrees rotation from the short axis of the heart. (see Figure 1)

The movements of the ultrasound probe refer to manipulating and adjusting the position of the probe in order to optimize images and obtain the necessary diagnostic information. Small manipulations of the probe can bring in structures clearer, so it is important, when scanning, to always try to optimize the image by using these movements.

The typical movements used during an echocardiogram are referred to as:

Sliding: Sliding refers to the movement of the ultrasound probe along the surface of the skin. By sliding the probe, the sonographer can explore different windows or obtain images of various different structures within that window.
Tilting: Tilting involves angling the ultrasound probe to change the orientation of the ultrasound beam. By tilting the probe, the examiner can adjust the angle of insonation and obtain different views of structures (example: PSAX scanning through the different levels of the LV requires tilting the probe).
Rotating: Rotating refers to the circular movement of the ultrasound probe around a fixed point. By rotating the probe, the examiner can examine structures from different angles and obtain a comprehensive evaluation. Rotating can also open up structures within a window or when going from PLAX to 90-degree rotation to PSAX.
Rocking: Rocking involves gently moving the ultrasound probe back and forth or side to side. This movement is particularly useful in certain examinations where it helps to visualize specific structures or features by bringing them closer to the center of the image. Rocking can improve the visibility of certain anatomical details and enhance the quality of the images.

Figure 2 Transducer probe movement

2D Echocardiography:

2D echocardiography, also known as two-dimensional echocardiography or B-mode imaging, is a non-invasive imaging technique that uses ultrasound to visualize the structures and function of the heart in real time. It provides detailed information about the cardiac chambers, valves, and surrounding structures. Here are some basic principles of physics used to create a 2D echocardiography image:

Ultrasound Waves:

Ultrasound is sound waves with frequencies above the upper limit of human hearing (>20,000 Hz) that is used in echocardiography to create images seen on the monitor. These waves are generated by a transducer and propagate through the body.
Transducer: The ultrasound transducer contains piezoelectric crystals that convert electrical energy into ultrasound waves and vice versa. The transducer emits ultrasound waves and receives the echoes reflected back from the tissues.
Wavelength and Frequency: The wavelength of ultrasound waves determines the spatial resolution of the image. Higher frequencies provide better resolution but have reduced tissue penetration. Lower frequencies have greater penetration but lower resolution.

Reflection and Echo Formation:

Reflection: When ultrasound waves encounter interfaces between different tissues, such as blood and heart muscle, they are partially reflected back. The amount of reflection depends on the acoustic impedance mismatch between the tissues.
Echoes: Reflected ultrasound waves are detected by the transducer and converted into electrical signals. These signals are processed to create an image displaying the intensity and timing of the echoes received from various structures.

Figure 3 How ultrasound signals can travel once they enter the body

Image Formation:

Beamforming: Ultrasound waves are emitted in a focused beam from the transducer. By adjusting the timing and amplitude of the electrical signals sent to individual elements of the transducer array, the beam can be steered and focused at specific depths and angles.
B-Mode Imaging: B-mode (brightness mode) imaging is the basic mode of 2D echocardiography. It displays the amplitude of the echoes as brightness levels, creating a grayscale image. Brighter areas correspond to stronger echoes, representing structures with higher acoustic reflectivity.
Sweeping and Scanning: To generate a 2D image, the transducer is moved or rotated over the patient's chest to acquire multiple scan lines from different angles. These scan lines and different views are combined to form a complete assessment of the heart. The ultrasound waves reflected back are received by the transducer and processed to create a real-time 2D image of the heart on the ultrasound monitor. Various views, including parasternal, apical, and subcostal, are used to visualize different cardiac structures.
2D echocardiography probe placement: The ultrasound probe is positioned on the patient's chest to obtain different 2D imaging views. Various views, including parasternal, apical, and subcostal, are used to visualize different cardiac structures.

Clinical Applications: 2D echocardiography provides comprehensive information about the structure and function of the heart. It allows for the assessment of:

Chamber size and morphology: 2D imaging provides information about the size and shape of the heart chambers, aiding in the evaluation of chamber dilation, hypertrophy, or congenital abnormalities.
Valve function: The opening and closing of heart valves can be visualized in 2D, allowing for assessment of valve morphology, regurgitation, and/or stenosis.
Wall motion abnormalities: 2D echocardiography can detect regional wall motion abnormalities, helping in the diagnosis of conditions such as myocardial infarction or cardiomyopathy.
Pericardium: Assessment for the presence and size of a pericardial effusion (accumulation of fluid around the heart).

Echocardiogram Imaging Views

During an echocardiogram, there are several standard imaging planes used to obtain different views of the heart. Here are the commonly used imaging planes:

Parasternal Long Axis (PLAX) View: This is typically the first imaging view used during an echo exam. It provides a longitudinal view of the heart, with the transducer placed at the left parasternal border. It allows visualization of the left ventricle, mitral valve, aortic valve, wall thickness, and the ascending aorta.
Parasternal Short Axis (PSAX) View: This plane is obtained by rotating the transducer 90 degrees from the PLAX view. It provides a cross-sectional view of the heart, starting at the apex of the LV, panning through to the AV level. It helps evaluate the ventricular function, assess myocardial wall thickness, and visualize the mitral, aortic, pulmonary, and tricuspid valves.
Parasternal RV inflow view: From the PLAX, tilt the face of the probe inferiorly toward the right hip. This allows visualization of the right atrium, right ventricle, and tricuspid valve.
Apical Four-Chamber View: In this view, the transducer is placed at the cardiac apex to obtain a four-chamber view of the heart. It allows visualization of the atria, ventricles, atrioventricular valves (mitral and tricuspid), and interventricular septum.
Apical Two-Chamber View: This plane is obtained by rotating the transducer from the four-chamber view to visualize the left ventricle, left atrium, and mitral valve.
Apical Three-Chamber View: This view is obtained by further tilting the transducer from the apical two-chamber view to visualize the left ventricle, left atrium, and the aortic valve.
Subcostal Four-Chamber View: In this view, the transducer is placed just below the sternum in the subcostal area and angled towards the left shoulder. It provides visualization of the interatrial septum, inferior vena cava, right and left atrium, and ventricles. This view can be useful in patients with suboptimal parasternal and apical windows.

These are just some of the standard imaging planes used in echocardiography. There are, however, other additional views or modifications depending on the specific clinical situations and information required by the physician performing or interpreting the echocardiogram.

M-Mode in Echocardiography

M-Mode in ultrasound stands for “motion mode,” which is a diagram that shows how the position of cardiac structures moves during the cardiac cycle. M-mode imaging has superior temporal resolution than 2D imaging. Typically, Doppler imaging is used during an echocardiogram which records the reflected blood velocity, whereas M-mode of cardiac structures reflects the volumetric blood flow. It is a one-dimensional imaging technique that provides a single, thin slice of the heart in motion. In M-mode, the ultrasound beam is focused on a specific location within the heart, and the motion of structures in that location is recorded over time. To obtain a m-mode image, the cursor may be aligned through the 2D image, and that specific line will be displayed with movement of the specific cardiac structure.

The M-mode displays time along the horizontal axis and depth along the vertical axis to depict movement of the cardiac valves and wall motion (Figure 4). It also allows measurements of cardiac dimensions and motion patterns in relation to the ECG rhythm.

Figure 4. This M-mode drawing shows an example of how structures are displayed on an image.

Procedure:

M-mode is traditionally used from the parasternal view but can also be used in other windows, such as RV lateral wall movement (TAPSE).
Probe Placement: The ultrasound probe is positioned on the patient's chest to obtain the desired M-mode view. The location depends on the specific structure or area of interest, such as the left ventricle or heart valves.
M-Mode Line Placement: A single M-mode line is placed perpendicular to the structure of interest. For example, the M-mode line is positioned across the left ventricle at the level of the mitral valve to assess its motion.
Image Acquisition: Once the M-mode line is correctly positioned, the ultrasound machine records the motion of the structures along that line. The resulting image shows the movement of the structures over time, with time displayed on the x-axis and depth (distance from the transducer) displayed on the y-axis.

Clinical Applications: M-mode echocardiography is particularly useful for assessing the motion and dimensions of specific cardiac structures. It can provide information about:

Ventricular wall motion: M-mode can evaluate the motion of the ventricular walls and detect abnormalities such as regional wall motion abnormalities, ventricular hypertrophy, and/or ventricular dilation.
Valve motion: M-mode allows for assessment of the opening and closing movements of heart valves, such as the mitral valve or aortic valve, aiding in the evaluation of valvular pathology.
Interventricular septal motion: M-mode can assess the motion of the interventricular septum, aiding in the diagnosis of conditions such as septal defects, ventricular dyssynchrony, or arrhythmias.

M-mode Explanation:

Figure 5: Example of m-mode through the AV and LA

Figure 5 shows a M-mode tracing of the aortic valve to assess its motion and function.

Valve Opening: During systole, the cusps of the aortic valve should open, creating an upward motion on the M-mode image.
Valve Closure: During diastole, the cusps of the aortic valve should close, resulting in a downward motion on the M-mode image.

Measurements: Using the M-mode image, you can obtain several measurements related to the aortic valve and left atrium:

Aortic Valve Excursion: Measure the maximum excursion or opening of the aortic valve cusps during systole. This provides information about the mobility and function of the valve.
Valve Closure Time: Measure the duration of valve closure during diastole. Prolonged closure time may indicate aortic valve stenosis or other pathologies.
Left atrium can be measured if the angle of the tracing is not off-axis.
Color m-mode: In parasternal short axis AV or MV level, color m-mode can help demonstrate the severity of regurgitation.

Figure 6: Example of m-mode through the MV

To obtain the m-mode of the mitral valve:

From the parasternal window, position the M-mode cursor perpendicular to the mitral valve leaflets. The cursor line should pass through the tips of both the anterior and posterior leaflets.

Measurements: Using the M-mode image, you can obtain several measurements related to the mitral valve (figure 6):

EPSS: MV E-point septum separation is a measurement used as an indicator of global LV function. Any EPSS greater than 1 cm is considered to be abnormal. In the presence of mitral stenosis or aortic regurgitation, the EPSS is distorted by factors other than flow passing through the valve and cannot be used to measure EPSS
E-F Slope: Measure the slope of the early rapid filling (E-wave) and late filling (A-wave) phases of the mitral valve. This provides information about diastolic function.
Valve Excursion: Measure the maximum excursion or opening of the mitral valve leaflets during diastole. This provides information about the mobility and function of the valve.
Closure Time: Measure the duration of mitral valve closure during systole. Prolonged closure time may indicate mitral valve stenosis or other pathologies.
Mitral Valve Prolapse (MVP): The degree of MVP, along with the affected leaflets, can be assessed using m-mode. (see Figure 7)

Figure 7 MVP

Color m-mode: Color m-mode can be helpful in assessing the timing and severity of valvular pathology (see Figure 8).

Figure 8 Color m-mode through AV showing severe aortic regurgitation.

Figure 9 Example of m-mode through the LV and RV

M-mode through the left ventricle (LV) and right ventricle (RV), allows for the visualization of the movement of the ventricular walls and chambers and the measurement of various parameters related to its function.

From a parasternal long-axis view, position the M-mode cursor perpendicular to the LV and RV. The cursor line should pass through the middle of the LV cavity, crossing the interventricular septum and the posterior wall.
From apical four-chamber, position the M-mode cursor through the lateral wall of the RV (TAPSE).

Measurements using M-mode:

Using M-mode, you can obtain several measurements related to the LV and RV and valvular function:

RV TAPSE (Tricuspid Annular Plane Systolic Excursion):
- TAPSE is a method used to assess the RV longitudinal function, < 17 mm indicates RV systolic dysfunction.
- TAPSE is a distance measurement, not a slope or time measurement (Figure 10).
- From an apical 4-chamber, align the m-mode cursor parallel with the RV free wall as it meets the TV annulus.
  - Measure vertically, leading edge to leading edge.
  - Measurement is angle-dependent, so try to swing the RV toward the center of the screen.

Figure 10 TAPSE

If the images are difficult to visualize, you can always try to add tissue Doppler to the m-mode TAPSE image to help visualize the timing of where to measure (figure 11).

Figure 11 TAPSE with tissue Doppler

Diastolic Function: From an apical 4-chamber, position the m-mode cursor through the mitral valve and turn on color. This can demonstrate and help assess diastolic dysfunction. This is typically called m-mode color flow propagation (figure 12).

Figure 12 Color m-mode for diastolic function assessment.

Wall Motion: The M-mode image will display the movement of the LV and RV walls during the cardiac cycle. This can be especially helpful in pericardial effusion with tamponade (figure 14) and constrictive pericarditis (figure 15) as it allows assessment of RV chamber collapse during respiration. It can also assess timing changes in cases of abnormal rhythm (figure 13) or desynchrony.

Figure 13 Example of abnormal septal motion in a patient with LBBB.

Figure 14 Example of RV diastolic collapse in tamponade patient.

Figure 15 Example of m-mode in constrictive pericarditis.

Dimensions: The M-mode image allows for the measurement of left ventricular dimensions, including the end-diastolic diameter (LVEDD) and end-systolic diameter (LVESD). These measurements provide information about the size and function of the LV.

Other M-mode uses:

Inferior Vena Cava (IVC): IVC looking for respiratory change (figure 16)
- M-mode of the IVC allows the visualization of its diameter changes over time during the respiratory cycle. Normally, the IVC diameter decreases with inspiration and increases with expiration. A greater respiratory variation suggests normal compliance and fluid responsiveness, while a diminished variation and dilation may indicate volume depletion or right heart dysfunction. It is also used when assessing cardiac tamponade when a pericardial effusion is present.

Figure 16 Example of a patient in tamponade with a dilated IVC without collapse

Systolic Anterior Motion (SAM) of the Mitral Valve: In the presence of SAM, the anterior mitral valve leaflet moves anteriorly toward the left ventricular outflow tract during systole instead of closing in the normal manner. This can result in obstruction of blood flow and potential complications, such as mitral valve regurgitation or left ventricular outflow tract obstruction. To assess with m-mode, use the parasternal long-axis view and position the M-mode cursor perpendicular to the mitral valve leaflets, passing through the tips of the anterior leaflet, interventricular septum, and outflow tract.
- Interpreting the M-mode image for SAM can help assess the presence and severity of an obstruction and septal hypertrophy. However, it is important to note that SAM is typically confirmed with 2D echocardiography or color flow imaging, which provides additional information about the direction and severity of flow obstruction.

Figure 17 Example of m-mode showing SAM

Pulmonary Valve: The optimal view is generally a high-parasternal short-axis view with a slight angulation of the beam toward the left shoulder.
At the beginning of diastole, the pulmonary valve is displaced downward and is represented anteriorly on the ultrasound recording. All elevations of the pulmonary valve in the stream of flow are represented as posterior movements on the echo. Likewise, downward movements are represented by anterior cusp positions on the trace, see (Figure 18).

Figure 18 Example of relationship between the M-mode recording of the pulmonic valve and the pressures in the right ventricle and pulmonary artery.

Figure 19 M-mode patterns of the normal and abnormal pulmonic valve

Technical considerations in M-mode measurements.

It is important to note that acquiring and interpreting M-mode images and measurements should be compared to the findings of 2D echocardiographic, color, or Doppler images and clinical data for a comprehensive assessment.
If M-mode angles are oblique, measurements should be made using 2D instead.

Conclusion

2D Echocardiography key points:

2D echocardiography is a non-invasive imaging technique that uses ultrasound waves to create a real-time, two-dimensional image of the heart.
It provides detailed visualization of the cardiac structures, including the chambers, valves, and surrounding tissues.
The ultrasound waves are emitted from a transducer and reflected back from the tissues, creating echoes that are detected and processed to generate the image.
The transducer is moved or rotated over the chest to obtain multiple scan lines from different angles, which are combined to form a complete image of the heart.
2D echocardiography provides information about cardiac anatomy, function, wall motion abnormalities, valve morphology, and the presence of any structural or congenital heart diseases.

M-Mode Echocardiography key points:

M-Mode (motion mode) echocardiography is a one-dimensional imaging technique that provides a time-motion display of cardiac structures.
- It uses a single scan line to generate a motion profile of the heart structures over time.
M-Mode is commonly used to assess the motion and function of specific structures such as the mitral valve, aortic valve, left ventricle, or right ventricle.
- It allows for the measurement of various parameters related to the motion, opening, and closing of the structures being examined.
M-Mode provides information about wall motion abnormalities, ventricular function, valve motion, and the timing of different cardiac structures.
M-Mode is often used in combination with 2D echocardiography to provide additional insights into cardiac function and pathology.

Both 2D and M-Mode echocardiography play essential roles in the evaluation of cardiac anatomy, function, and detection of abnormal pathology. They are complementary imaging modalities that provide valuable information for the diagnosis, management, and follow-up of various cardiovascular conditions. The information obtained from these techniques helps clinicians make informed decisions regarding treatment and patient care.