M Mode Echo from the Parasternal Long Axis:
©Walter Rasmussen, R.D.C.S.
Above: A cursor is directed through the body of the left ventricle resulting in an M-Mode strip chart recording.
The Parasternal Long Axis
Because of the subsequent advances in ultrasound imaging, most of the routine M-Mode used in current echocardiography exams consists of just 3 views, and all can be crafted from the parasternal long axis view. Prior to the introduction of Doppler, M-Mode was a good way to assess some aspects of intracardiac hemodynamics. With M-Mode, an experienced sonographer can recognize aortic insufficiency by its effect upon the mitral valve leaflet motion. Mitral valve motion can also aid in the assessment of possible diastolic dysfunction however spectral Doppler of the mitral valve, pulmonary venous blood flow and mitral annular motion have largely replaced its use.
The utility of M-Mode is based-upon its ability to track fast motion with high resolution and to measure distances accurately. For the accurate measurement of diameters however, the ultrasound beam should be positioned as perpendicular as possible to the plane of the chamber or wall of interest and this can only be done in the parasternal long axis. Before recording any M-Mode, pan the transducer medially and laterally to verify that the center of ventricle is being recorded.
Manual measurement of the left ventricular and aortic root diameters from the frozen 2-D image is substituted for M-Mode measurements if the cursor cannot be positioned correctly.
In the M-Mode study of the left ventricle, the M-Mode cursor is placed to intersect the anterior right ventricular free wall, the right ventricle, the inter-ventricular septum, the left ventricle, the posterior wall and the posterior pericardium between the tips of the mitral valve leaflets and the posterior papillary muscle. The depth of the image should be set so that the relevant cardiac structures in the far field are at least a few centimeters within the M-Mode image border.
Ideally, the chordae tendineae should not be seen on the M-Mode recording however, in many cases they cannot be completely eliminated. It is important not to include a papillary muscle within the cursor position because the posterior wall will then appear hypertrophic. Adjust the image gain, dynamic range, and TGC tabs so that structures are clearly distinguished. Move the transducer higher or lower on the chest surface for better positioning if it is not possible to place the M-Mode cursor perpendicular to the structure.
LV-RV M-Mode Study Features:
Starting with the near field, the chest wall is recorded as horizontal lines, representing stationary tissue anterior to the heart. These reflections should be made visible on the recording by adjusting the TGC so that it is not overly bright, otherwise, the pericardium and right ventricular free wall thickness and motion can be obscured.
The next significant structure is the pericardium. With a normal amount of pericardial fluid, this structure will be recorded as white line of about 2mm representing the pericardial tissue with a clear fluid space posterior to it moving in-sync with the anterior right ventricular wall.
Adjust the TGC so that the line is not obscured by brighter tissue nearby. If there is an anterior effusion, there will be a thicker dark line, representing fluid, which changes diameter with the cardiac motion.
Right Ventricular Free Wall:
Following the anterior pericardial space is the right ventricular free wall (RVFW). Adjust the TGC so that the lines representing the wall are clearly seen, as it is easy for the RVFW to be obscured by the other strong reflections in the immediate area. Ideally, the RVFW thickness should be measurable at end-diastole and should be seen thickening during systole, but this is not always possible. Moving the focus to the level of the RVFW can help clarify the image if necessary however, structures in the far field may then loose some resolution.
Right Ventricular Chamber:
Adjustment of the Time-Gain Compensation for the right ventricular chamber.
Posterior to the RVFW is the right ventricular chamber. The RV chamber is often obscured with weak echoes since it is highly trabeculated and relatively close to the transducer. The RV chamber can be clarified with and adjustment of the corresponding TGC tabs as illustrated above.
The next significant structure posterior to the RV chamber is the inter-ventricular septum. M-Mode will record the septum as it thickens and thins during systole and diastole. M-Mode captures the motion much better than Two-Dimensional Echo because it has a higher sampling rate. RV volume or pressure overload, prior open-heart surgery, dilated and hypertrophic cardiomyopathies and abnormal electrical activation, are common causes of abnormal septal motion and are well delineated with M-mode.
M-Mode measurements of septal wall thickness can be influenced by an improper TGC setting. Adjust the TGC so that both sides of the septum are clearly seen. Adjust the Dynamic Range or Compression so that weak echoes do not cloud the area. Position the focus at the level of the IVS for a better recording. Another technique to use when the IVS border is not well-resolved is lowering the frequency. This should be only a temporary adjustment because overall imaging is best done at the highest frequency for the conditions encountered.
It is easy to erroneously record the IVS as pathologically thickened in both M-Mode and 2-D Echo because there is sometimes a band of tissue on the right ventricular side, the moderator band, which lies adjacent and parallel to the septum, making it look thicker than it actually is. Before recording an M-Mode of the inter-ventricular septum, make sure that the depicted septal wall thickness is representative of the actual anatomy, otherwise, an erroneous diagnosis may result.
There are two conditions when it is impossible to record the inter-ventricular septum continuously throughout the cardiac cycle with M-Mode. Condition one is when the septum cannot be made perpendicular to the cursor and the recording looks broken-up. Condition two is when there is localized hypertrophy of the basal septum and portions of it will intersect the cursor line during systole but not in diastole, causing extreme changes in apparent septal diameter. Simultaneous two-dimensional imaging helps to explain the source of these distortions.
Left Ventricular Chamber:
The Left Ventricular chamber is the next structure encountered on the RV/LV study. M-Mode is very accurate for measuring distance and is valuable for the measurement of left ventricular diameter, provided the placement of the M-Mode cursor is perpendicular to the inter-ventricular septum and posterior wall. Take care to position the cursor at the widest diameter of the LV chamber and in the area between the papillary muscle and the mitral valve leaflet tips by observing the 2-D image while placing the cursor. Avoid recording the lateral or medial walls because it wall make the posterior wall look thicker and the left ventricular chamber smaller. The cursor should not be placed at the posterior wall papillary muscle because it will make the recording of the wall appear hypertrophic.
Left Ventricular Posterior Wall:
For a good overall RV/LV study, the left ventricular posterior wall is perhaps the key area to record clearly and correctly. Position the M-Mode cursor on a well-seen portion the posterior wall between the mitral valve leaflet tips and the posterior papillary muscle during diastole. The M-Mode should clearly demonstrate the wall movement during diastole and systole with little or no break-up of the recording.
Moving the focus closer to the level of the posteriorwall can improve the sharpness of that area, but can sacrifice detail in other areas. If the posterior wall is not visible on the two-dimensional image, it will not be possible to get a good M-Mode of the region.
Adjustment to a lower frequency can also help increase the reflections from the posterior wall, but should be only used as a temporary adjustment.
The epicardium/pericardium interface produces a much stronger linear echo and has high reflectivity because of the pericardial fluid. Since the pericardium is highly reflective, the TGC should be used to reduce the intensity of the pericardial reflections if they are overly bright. When there is more than a physiologic amount of pericardial fluid or when there is suspicion of pericarditis, it is customary to briefly decrease the overall gain and/or the dynamic range, in order to demonstrate a very high contrast view of the pericardial space for better assessment.
Mitral Valve Study:
When the M-Mode cursor is placed at the level of the mitral valve leaflet tips, its diastolic and systolic motion is recorded, yielding information about mitral valve anatomy and function as well as left ventricular diastolic function. The mitral valve opening pattern varies from individual to individual according to many physiologic variables such as: mitral pathology, left ventricular function, filling pressures, congenital disorders and aortic valve disease.
To start, obtain a good parasternal long axis image. Activate the cursor and move it to the mitral leaflet tips. SLide the transducer on the chest wall so that the cursor is as parallel as possible to the mitral valve annular plane so that the complete and widest excursion of the anterior and posterior leaflets will be recorded. Maintaining the cursor position throughout the cardiac cycle can be a little difficult to achieve when there is normal left ventricular contractility because the annulus and valve shift position slightly with the cardiac cycle.
The key to getting a good M-Mode recording is to focus attention exclusively on the M-Mode image and manipulate the transducer until the mitral valve leaflets are recorded as a single line, throughout as much of systole as possible.
If the systolic portion is recorded properly, the diastolic portion will also be correct, demonstrating a continuous and ideally, unbroken tracing of the mitral leaflet motion.
In the case of a stenotic valve, observe the 2-D image and place the cursor across the leaflets at the narrowest point during diastole. Many Sonographers have been trained to record the mitral valve from the parasternal short axis, however it is more difficult to determine if the transducer is in the best position for maximum leaflet excursion.
In the case of a bio-prosthetic mitral valve, position the M-Mode cursor at the valve leaflet tips, as is done for a native mitral valve. If the valve apparatus appears brighter than surrounding structures, decrease the Gain and/or Dynamic Range until it is clearly seen on the recording.
Current mechanical valves typically contain tilting single or pair of disks, while some patients who had valves replaced in the 1960’s to early 2000’s might still have a ball-cage valve. Adjust the 2-D image gain and dynamic range until the apparatus is clearly seen and then direct the M-Mode cursor through the maximum excursion of the valve’s moving elements.
Aortic Root/Left Atrium M-Mode Study:
From the parasternal long axis view, the M-Mode cursor is positioned to transect the anterior aortic root, aortic valve, the posterior aortic root, the left atrial chamber and the posterior left atrial wall.
The aortic root tracings should appear parallel to eachother.
Attainment of a good M-Mode recording of the aortic root is dependent upon the M-mode cursor being placed is as perpendicular as possible to the aorta in order to assure that the diameter of the vessel is not exaggerated. This is usually a different transducer position from the one used for recording 2-D and M-Mode of the mitral valve and left ventricle. In addition to the aortic root, the left atrium, immediately posterior to the aortic root is also sampled by the M-Mode cursor but it is often the case that the cursor does not transect the minor axis of the left atrium as would be ideal, and so, a frozen two-dimensional image is preferable for measurement.
Due to its high reflectivity, the posterior pericardium commonly causes a bright artifact that extends into the left atrium on both 2-D and M-mode echo, making it difficult to properly distinguish the posterior wall of the left atrium, especially on M-Mode. This is caused by sound waves at the edge of the ultrasound beam which although normally of too low intensity to be recorded, are reflected back by the highly reflective pericardium. The TGC can be helpful in eliminating much of this glare, but it is important to recognize the artifact and to not mistake it for the posterior left atrial wall.
The anterior and posterior walls of the aortic root should appear parallel to each other on the M-Mode tracing, bounding upward during systole and downward during diastole. If the lines break-up, or if the lines do not remain parallel throughout the cardiac cycle, it is due to malposition of the cursor, causing it to cross the LVOT during a portion of the cardiac cycle. Observe carefully where the cursor is located at each stage of the cardiac cycle. It should be as stationary as possible across the widest diameter of the aortic root throughout the cardiac cycle.
Another anatomical element recorded in this view is the opening and closing pattern of the aortic valve. It is often the case that the best placement of the cursor for the aortic root is not the best placement for recording the maximal opening of the aortic valve therefore, slight adjustment of the cursor and transducer angle may be needed for this purpose. Aortic valve opening diameter and the appearance of sclerosis or calcification aids in the diagnosis of valvular disease and other hemodynamic abnormalities.
For more accurate measurements, adjust the Gain and Dynamic Range to make the M-Mode recording of the aortic root and valve free of unnecessary low-intensity echoes. The goal is to record an image that is not over-gained and does not exaggerate the thickness of the lumen. See example above.