Goals While Imaging the Parasternal Long Axis Anatomy
©Walter Rasmussen, R.D.C.S.
A sonographer doesn’t just, “Get a view”. There are specific items which should be visualized as clear as possible in each view. Multiple versions of the parasternal long axis may be necessary in order to include as many of the listed anatomical elements as possible.
Anterior Pericardium and Pericardial Fluid: The visibility of the anterior portion of the pericardium depends upon the amount of pericardial fluid or epicardial fat present. Pericardial fluid is represented as a dark outline following epicardial surface and becomes progressively more visible in proportion to the volume of fluid present. Epicardial fat may mimic the appearance of pericardial fluid, but is usually distinguished by a somewhat granular texture. The actual pericardial tissue is depicted as a grey to white line which outlines the heart and the darker pericardial fluid.
If pericarditis is suspected it can be helpful to focus the ultrasound beam at the pericardium and zoom-in on the area. Once the gain and dynamic range are adjusted, assessment of pericardial thickness and the presence or absence of a sliding motion of the epicardial/pericardial interface is possible.
Right Ventricular Free Wall: The most anterior portion of the heart is the thin, right ventricular free wall and pericardium. Unfortunately, these delicate structures are subjected to the most intense level of ultrasound since they are so close to the transducer and without proper adjustment of the TGC slider tabs, this area is rendered so bright that they are difficult to see. Temporarily moving the focus to this area can greatly enhance right ventricular free wall image, when necessary.
Right Ventricular Chamber:The right ventricular chamber is seen as a dark, echo-free chamber immediately posterior to the anterior right ventricular wall. The parasternal window is not the best for imaging the right ventricle and is better seen from the apical four-chamber and subcostal four-chamber views however, its motion is a major component for diagnosing some serious conditions.
Interventricular Septum: Much anatomical and functional information can be obtained from the interventricular septum from the parasternal long axis and the echocardiographic image should be carefully adjusted with the TGC tabs to clearly delineate its right and left borders. This is a good initial point to set the focal zone for the parasternal view, which also aids in obtaining a good over-all long axis image. One of the primary goals of imaging in this view is for the accurate measurement of the septal wall thickness, motion and tissue characterization. This transducer perspective is one of the best for the detection of ventricular septal defects with color Doppler.
Careful positioning of the transducer to assure that the plane of the sector is placed through the center of the left ventricle, at the widest possible points between the inter-ventricular septum and the inferior/posterior wall allows for the most accurate assessment of the chamber diameter and wall motion.
Mitral Valve: In the parasternal long axis view, the mitral valve leaflets are viewed in profile and this is but a thin slice of the valve. As with all imaging of the heart, the clearest image of the valve is not always immediately obtained and since valvular pathology is of great interest in echocardiography, efforts should be made to make minor adjustments in transducer position, frequency and focus specifically to get the maximum resolution for each valve. Because the ultrasound beam is very narrow it is important to briefly pan medially and laterally across the valve in order to detect any possible abnormality such as a ruptured chord, prolapse, stenosis, calcification and vegetation. The anatomical exam of the mitral valve should be followed by Color Doppler, sweeping across the valve in the same manner in search of mitral regurgitation because it can occur at any point at the valve commissures or perforation of the valve leaflet.
If there is a mitral valve prosthesis or mitral annular ring, effort should be made to make sure that the hardware is seen as clearly as possible. If the mitral valve is highly reflective, decrease the TGC in the area of the prosthesis so that it can be clearly observed. In most cases, it should be possible to see the leaflets or disks in motion and adjustments to frequency and dynamic range can make all the difference in image quality. It is important to search for perivalvular leaks using color Doppler and panning side to side across the valve. Since a prosthesis will block some of the ultrasound, it is very difficult to see what is on the opposite side of it and this is problematic while using color Doppler in search of leaks. It can help to move to a higher intercostal space in search for prosthetic valve regurgitation in the parasternal view because the apical view provides no opportunity to see beyond the highly reflective valve. Direct the M-Mode cursor across the prosthesis to record the valve stent diameter and the valve excursion on a strip chart view.
Left Ventricular Outflow Tract: The left ventricular outflow tract is formed by the anterior mitral valve leaflet and the basal inter-ventricular septum. Two-dimensional imaging can reveal sub-aortic membrane, systolic anterior motion of the mitral valve, and localized basal to mid hypertrophy of the inter-ventricular septum. Color Doppler placed at the left ventricular outflow tract can help to detect outflow tract obstruction, aortic insufficiency and membranous ventricular-septal defect.
Left Ventricular Posterior Wall: It is important to include as much of the inter-ventricular septum and posterior wall, from base to apex as possible in the parasternal long axis view. If the initial transducer position only includes the base and mid-ventricle, moving the transducer lower on the chest for observation and recording of the mid to apical zone is often possible.
Posterior Pericardium: The posterior pericardium acts like a convex mirror and therefore reflects a higher intensity of ultrasound energy. For this reason, the TGC should be adjusted to compensate for this higher reflectivity. The goal is to clearly distinguish myocardial tissue texture from the pericardium, making measurements and assessment of pericardial fluid easier.
Aortic Valve: From the parasternal long axis, observe the aortic root and valve. In most cases, the sinuses of Valsalva should be clearly visible, as well as the valve leaflets. In some patients, the aortic root and ascending aorta are not well-seen and a remedy for this is to ask the patient to exhale all of the residual air in their lungs for several seconds while a better image is obtained. If the respiratory maneuver is not effective, slide the transducer superiorly, usually about one intercostal space, and repeat the exhalation.
If a prosthetic valve is in place, use the TGC to adjust the image brightness so that the valve components can be clearly seen. If the valve leaflets are not visible, it is often beneficial to raise or lower the frequency in order to better visualize them. Adjustment of dynamic range, compression and reject can also help.
- Use the zoom function in order to obtain a close-up view of the valve.
- Use color Doppler to check for aortic insufficiency and perivalvular leaks at the aortic annulus.
- Record an M-mode of the prosthetic aortic valve for the purpose of demonstrating aortic root diameter and valve motion.
Ascending Aorta: In most patients, the ascending aorta needs to be better visualized by moving the transducer up one intercostal space while in the parasternal long axis view and making minor adjustments to transducer axis. If it is not immediately visible, have the patient exhale deeply while searching for its walls. This portion of the exam will screen the patient for ascending aortic aneurysm or aortic dissection. Color Doppler, set to a low Nyquist such as .35 meters per second, can help to determine whether a false lumen is present when aortic dissection is suspected. The diameter of the ascending aorta can be measured in freeze-frame on the two-dimensional image.
A dilated ascending aorta can be further investigated by turning the transducer 90 degrees clockwise for the short axis cross-section of the aorta. Color Doppler can again be used to search for evidence of dissection, as above.
Pulmonic Valve in the Parasternal Long Axis: The pulmonic valve and main pulmonary artery are often visible at the same intercostal space as the ascending aorta in the parasternal long axis imaging plane. From the ascending aorta view, tilt the transducer slightly laterally in order to achieve the view. This view images the valve approximately 90 degrees from the standard short axis view of the pulmonic valve, described later in this guide. This view often gives a clearer image of the pulmonic valve root and main pulmonary artery diameters. Color Doppler in this view is useful for obtaining complete characterization of pulmonic insufficiency when combined with color Doppler images made from the short axis view.
M-Mode is an imaging mode in which one scanning line is isolated from the two-dimensional image and the movement of tissue that it intersects is charted on graph paper.
Most of the routine M-Mode in current echocardiography 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 and an experienced sonographer can easily recognize aortic insufficiency without the use of color Doppler by its effect on mitral valve motion. M-Mode of mitral valve motion can help in the assessment of diastolic function.
The utility of M-Mode is based-upon its ability to track fast motion with high resolution and accurate measurement of distances. For precise measurement of diameters however, the sonographer needs to position the ultrasound beam as perpendicular as possible to the plane of the chamber or wall of interest. It is therefore recommended to perform M-Mode from the PLAX rather than the short axis because the sonographer can then be certain that the cursor is perpendicular to the structure. Before recording any M-Mode, pan the transducer medially and laterally to verify that the center of the structure is being recorded.
Review of Parasternal Long Axis View:
- 2D of standard view -Left ventricle (LV), Mitral Valve (MV), Aortic valve (AV) and left atrium (LA)
- Color flow Doppler of AV, MV, and inter-ventricular septum
- M-mode of Aortic root and left atrium (LA)
- Specific focus and multiple view of LVOT for any TAVR patient
- M-mode of MV
- M-mode of IVSd, LVd, PWd, and LVs
- 2D measurement of IVSd, LVd, PWd, and LVs if M-Mode is not obtained/off axis (if physician indicated)
- Parasternal view of the PV
- 2D view of the PV from the RV outflow view
- Color flow over the PV
- CW of the PV(100mm/sec sweep speed)
- PW if needed to calculate PVA(100mm/sec sweep speed)
- RVOT diameter if accurate
- Ascending Aorta
- 2D view and measurement of the ascending aorta