Parasternal Short Axis View

Parasternal Short Axis View and heart valve imaging.

©Walter Rasmussen, R.D.C.S.

To obtain the Parasternal short axis view, the transducer is rotated approximately 90 degrees clockwise from the parasternal long axis until the aorta or left ventricle appears as circular as possible on the monitor.  It should be noted that the left ventricle is not always circular in shape in the short axis, but it is important to attempt to produce an image as close to circular as possible in order to accurately assess myocardial anatomy (e.g. LV aneurysm, hypokinesia, akinesia) and symmetry of contraction.  As in the parasternal long axis view, turning the patient steeper on their left side or having the patient exhale all of their residual air can produce considerable improvement in a faint or crude image, and as well, makes higher frequencies possible.

sax base

PSAX at the Base of the Heart

 
Once the short axis plane is approximated, the transducer is tilted until the aortic root and valve are located.  It may be necessary to move the transducer up or down the chest wall one intercostal space.  Fine-tune the image by incrementally rotating the transducer until the aortic root looks as close to circular as possible.  The focus should be set at the level of the aortic valve and the ultrasound plane should cut through the three valve cusps, demonstrating both diastolic coaptation and systolic opening.

The image obtained does not always reveal the complete anatomy pictured above, usually due to rib shadow.  When this happens, the imaging plane can be divided into however many areas needed to cover all of the elements of the target anatomy.  For example, if only the aortic and pulmonic valves are immediately visible, once they are recorded, the transducer can be moved and angled medially to visualize the tricuspid valve and right atrium.

Other structures that should be visible at the base of the heart are the pulmonic valve to the right (on the image) of the aortic valve and the tricuspid valve and right atrium, seen on the left.  All three valves will not necessarily be imaged well when the aortic valve is being featured and the sonographer may therefore need to investigate them individually, adjusting the transducer position,focus, gain, etc. for each valve and making individual clips of them.

After performing the basic two-dimensional imaging of the base of the heart, color Doppler is performed on the aortic valve, pulmonic valve, tricuspid valve and the inter-atrial septum.

Parasternal Short Axis View of the Aortic Valve

A normal aortic valve appears in the shape of a capitol Y when closed and a triangle when open.

sax d

Above: A zoomed image of the Normal aortic valve at the base of the heart during diastole. 

After imaging the aortic valve in the short axis, turn on color Doppler and adjust its size just enough to cover the aortic root.  Optimize the Color Doppler for a high frame rate by reducing the two-dimensional sector width and color Doppler box size.  When evaluating for aortic insufficiency with color Doppler, make sure that the short axis plane is positioned slightly on the ventricular side of the valve because in this view, aortic insufficiency cannot be detected from within the ascending aorta.

A perimembranous ventricular septal defect can sometimes be detected with Color Doppler in this view.  In the short axis, it is usually seen emerging from the anterior aortic root (11 o’clock position), and coded red flowing at high velocity into the right ventricular outflow tract.

vsd-sax

Above:  An arrow points to a perimembranous VSD jet in the parasternal short axis view at the base of the heart (aortic valve level).

Bicuspid Aortic Valve

A bicuspid aortic valve is one of the most common incidental findings in adult echocardiography.  A true bicuspid aortic valve (see below), clearly demonstrates only two cusps and two sinuses of Valsalva.   Other variations of bicuspid  aortic valve demonstrates three sinuses and fusion of two valve cusps with a residual raphe.

bicuspid diast bicuspid systole

True bicuspid aortic valve, systole (left) and diastole (right)  

If a bicuspid aortic valve is suspected, it is important to try to eliminate the possibility that improper rotation or angulation of the transducer is not causing a false diagnosis.  It is therefore important to make minute rotational adjustments to the transducer and observe whether or not there actually are three distinct cusps that open normally.  If the short axis aortic valve image is poorly resolved, the patient should be instructed to exhale all of the residual air in their lungs in combination with adjustments of the transducer position axis and angulation in order to obtain a higher resolution image.

Aortic Sclerosis

Sclerotic aortic valves will appear thickened and brighter than the surrounding tissue on an ultrasound image.  When an aortic valve appears sclerotic, it is important to briefly adjust the ultrasound frequency higher in order to see if the appearance of sclerosis is due to poor, frequency-related resolution.  A sclerotic aortic valve may or may not have some degree of restricted opening, while stenotic valves will demonstrate restricted opening and bright reflections caused by calcification or thickening of the valve leaflets.

Aortic Stenosis

The distinguishing factors between aortic sclerosis and aortic stenosis is the degree of leaflet calcification and valve leaflet restriction.  When calcified valves are exceedingly bright, it is often necessary to decrease the gain, increase the frequency and decrease dynamic range or compression in order to obtain the clearest image of a calcified valve.   This makes it possible to more accurately observe the degree of calcification and the amount of leaflet restriction.

Aortic Stenosis severity cannot be judged solely on the basis of the two-dimensional image.  Continuous Wave Doppler of the aortic valve must also be performed and found to demonstrate an abnormally high velocity of blood flow across the valve from at least one of the following transducer positions: Apical, Suprasternal, Right Parasternal.

Short Axis view of Prosthetic Aortic Valve:

bio-aortic-diast bio aortic sys

Bio-prosthetic aortic valve, diastole and systole.  Stents (arrow) are visible.  

In the short axis, the bio-prosthetic valve will look tri-leafleted, smaller than a native valve and usually bordered by a rigid stent framework, although some newer stentless valves look similar to a native valve and are becoming more common.  It can be very difficult to see the valve leaflets of a bio-prosthetic valve and if they are not immediately visualized, the sonographer should work to try to image them by experimenting with the transducer frequency and having the patient exhale all of the residual air in their lungs.  Tilting disk valves are also common in the aortic position, and due to the materials they are constructed of, can appear very bright and thick on the image, thus requiring a decrease in the gain setting either by adjusting the overall gain or the individual TGC slide tab assigned to that area.  Adjustment of Dynamic Range, Compression or Reject can help to eliminate weak echoes if they obscure view of the mechanical valve.  Inspect the valve for motion, structure, stent integrity, calcification and vegetations.  Use color Doppler to assess for both valvular insufficiency and para-valvular leaks.

Short axis view of the Tricuspid valve:

In the short axis at the base of the heart, the tricuspid valve is seen slightly to the left of the aortic valve on the monitor.  As in the parasternal long axis, it may be necessary to move the transducer inferiorly and a little medially on the chest wall toward the sternum, in order to get the best view of the valve and Color Doppler blood flow.  Color Doppler is used to detect and help determine the degree of tricuspid regurgitation.  CW Doppler can be used to document the actual velocity of the TR jet if it was sub-optimal from the parasternal RVIT view.

This is also a good view to observe inferior vena cava flow into the right atrium with color Doppler.  Sometimes IVC flow is quite prominent on color Doppler and needs to be distinguished from a left to right atrial  septal shunt, usually seen emerging from the mid to upper portion of the interatrial septum in this view.

While evaluating for an ASD, adjust the scale downward to 30-40 cm/sec from the standard setting of 50 to 60cm/sec.   When making such an adjustment, observe the effect it has on the color Doppler display because too low of an adjustment can over-emphasize low velocity turbulence, making it difficult to distinguish the information that is being sought.  It is important to narrow the sector image and use a color box to just large enough to cover the area of interest in order to maintain a high frame rate.  This adjustment is necessary because ASD’s can be small, of low velocity and short duration.

Short Axis view of the Right Ventricular Outflow Tract:

In the short axis, at the level of the aortic valve, the right ventricular outflow tract arches just anterior to the aortic valve, ending at the pulmonic valve.  In normal patients, color Doppler of the outflow tract rarely reveals very much because the blood flow is largely horizontal to the position of the transducer, causing little or no Doppler shift.  Turbulence within the right ventricular outflow tract will occur in the presence of a membranous VSD as mentioned above, infundibular stenosis or severe pulmonic insufficiency and should be fully documented with color Doppler on the exam.

Short Axis view of the Pulmonic Valve and Pulmonic Valve Doppler

sax diagram with pwd pul val

From the short axis view of the aortic valve, it may be necessary to tilt the face of the transducer slightly toward the patient’s left side in order to see the pulmonic valve leaflets more clearly.  It can also be very helpful to actually move the transducer slightly laterally on the chest wall, so that the pulmonic valve is seen directly in front of the transducer.  If the pulmonic valve is not well visualized, instruct the patient to exhale completely, holding their breath out while images are composed and recorded.

If imaging is still difficult, color Doppler, can be very helpful in finding the pulmonic valve and main pulmonary artery which is seen as a wide, principally blue coded signal of medium brightness during systole just to the right of the aortic valve on the image.  If possible, angle the transducer so that the entire main pulmonary artery and its branches are visualized.

While imaging Color Doppler of the pulmonic valve, the color box can be lengthened in order to include the entire length of the main pulmonary artery to it’s bifurcation.  This action makes a quick assessment for the presence of a patent ductus arteriosus (PDA) possible.   A PDA is identified by a narrow, streaming, principally red encoded color Doppler signal which emerges just proximal to the bifurcation of the pulmonary artery throughout systole and diastole.

Pulmonic Regurgitation

The American Society of Echocardiography states:

“Although they are quantitative, Doppler techniques may be subject to acquisition and observer variability.  Measurement of color flow jets (and spectral Doppler) are operator and machine dependent, requiring careful attention to uniform image acquisition.” 1

The color Doppler box is placed for detection of pulmonic regurgitation when present, within the right ventricular outflow tract (RVOT) and directly across the pulmonic valve on the image.  Pulmonic regurgitation is seen as one or more small red-coded jets emerging from the right ventricular outflow tract side of the pulmonic valve during diastole.  As the degree of pulmonic regurgitation increases, the area and length of the jet will increase with greater turbulence demonstrated within the RVOT.

Once the best possible Color Doppler signal across the pulmonic valve has been obtained, place the CW cursor across the valve and record forward flow, which is displayed below the baseline and if present, pulmonic regurgitation will be seen above the baseline at 100 mm/sec chart speed.  Adjust the scale so that the velocity profiles both above and below the baseline fill the graph, thus allowing for accurate measurement and analysis.

PW Doppler of the Pulmonic Valve

Serial acquisition and measurement of Doppler signals are operator and machine dependent, requiring careful attention to uniform image acquisition and cursor placement.

Goal: Obtain the highest velocity systolic flow Pulsed-Wave Spectral Doppler Signal at 100 mm/sec chart speed from within the main pulmonary artery at the tips of the pulmonic valve leaflets while maintaining the presence of the opening and/or closing valve spikes.

sax pw doppler

Above: PW Doppler recording at the pulmonic valve

sax diagram with pwd pul val

Obtain the parasternal short axis view at the base of the heart.  It may be necessary to tilt the face of the transducer slightly toward the patient’s left side in order to see the pulmonic valve leaflets more clearly.  It can also be very helpful to actually move the transducer slightly laterally on the chest wall, so that the pulmonic valve is seen directly in front of the transducer.  If the pulmonic valve is not well visualized, instruct the patient to exhale completely, holding their breath out while images are composed and recorded.

Goal: Obtain the highest velocity systolic flow Pulsed-Wave Spectral Doppler Signal at 100 mm/sec chart speed from within the main pulmonary artery at the tips of the pulmonic valve leaflets while maintaining the presence of the opening and/or closing valve spikes.

  1. Set to PW Doppler sample volume size to 5mm.  Place Color Doppler set at a scale of 50-60 cm/sec on the pulmonary artery side of the pulmonic valve.  Place the sample volume within the brightest blue portion of the color Doppler map within a few millimeters of the pulmonic valve on the pulmonary artery side of the valve, thus avoiding eddy currents which are displayed as yellow or red. Record the spectral Doppler signal at 100 mm/sec chart speed.
  2. Move the sample volume a few millimeters away from the pulmonic valve leaflet tips further into the the main pulmonary artery, positioning it within the brightest blue portion of the color Doppler map, observing whether the spectral Doppler velocity increases or decreases.  Record the spectral Doppler image which demonstrates the highest velocity at 100 mm/sec chart speed.
  3. Change the position of the spectral Doppler sample volume slightly to the left and right of mid-line within the main pulmonary artery while maintaining close proximity to the valve as noted above.   Search for and record the highest velocity encountered at 100 mm/sec chart speed.

Spectral Doppler Scale and baseline:  If necessary, reduce the scale and move the baseline up so that the systolic velocity profile (below the baseline) can be seen and measured in the greatest detail.  Unless severe pulmonic regurgitation is present, it should not be detected while performing pulsed Doppler of the pulmonic valve.

When the main pulmonary artery is dilated, the systolic pulmonic valve flow is best obtained by observing the color Doppler and placing the sample volume within the brightest blue portion of the map on the pulmonary artery side of the valve, thus avoiding slower velocities and eddy currents which are displayed as yellow or red.

On some patients the velocity profile will vary with respiration. Experiment with the sample volume size if the velocity profile is faint or ragged-looking.  Mid-systolic notching of the pulmonic valve blood flow velocity profile is not uncommon and does not reflect an error in technique.

Prosthetic pulmonic valves are typically bio-prosthetic and should be evaluated for both valvular and paravalvular leak, in addition to calculation of pressure gradients with spectral Doppler recordings.

sax mv

Short Axis at the level of the Mitral Valve: 

From the short axis at the base of the heart, tilt the transducer face inferiorly until the characteristic fish mouth view of the mitral valve is attained.  Twist the transducer clockwise or counter-clockwise until the valve appears as centered as possible within the left ventricular chamber.  At this level, the left ventricular walls should also be clearly visible, looking roughly circular, surrounding the mitral valve.  In this view, the right ventricle comes in to view as the most anterior structure and the TGC should be adjusted so that it is not overly bright.

When imaging the mitral valve in the short axis, it should be made to look as centered as possible within the left ventricular chamber and appear symmetric across the commissures as it opens and closes.  Exceptions to this are seen in cardiomyopathy where the valve will appear displaced posteriorly, when the transducer must be placed at a low intercostal space, and a cleft mitral valve.

The image of the mitral valve in this view should visualize the valve leaflets at their very tips.  If possible, try to avoid the chordae tendineae by recording the image where the anterior and posterior leaflets co-apt completely and they appear relatively un-broken on the image. When evaluating a stenotic valve, it is important to record the valve at its most restricted point of opening during diastole; otherwise the measurements for tracing the mitral valve area will be erroneously large.  It is however, sometimes impossible to accurately record the valve orifice when mitral stenosis is present.

If, due to the limitations of the current transducer placement, tilting the transducer does not bring the mitral valve into complete view or the left ventricular walls are not circular in shape, move the transducer up or down the chest one intercostal space and re-adjust its tilt and rotation to compensate.  A sustained exhalation can greatly improve the image if it becomes faint while moving the transducer to a better intercostal space.

Color Doppler of the mitral valve in the short axis is primarily used for detection of mitral insufficiency and requires visualization of the mitral valve with the imaging plane positioned slightly on the atrial side of the commissures.  Because insufficiency may originate from only a small portion of the valve, it is important to search for it by slightly tilting and rotating the transducer.

sax papSAX of the left ventricle at the level of the Papillary Muscles:

From the short axis of the mitral valve, tilt the face of the transducer inferiorly while watching the transition to chordae tendineae and then, the papillary muscles.  The slice of the heart at the papillary muscle level should look as circular as possible with two distinct bulges at about the 4 o’clock and 8 o’clock positions.  In this view the papillary muscles can vary from very prominent, to almost undetectable and their position on the left ventricular wall is not exactly the same in every patient.

If the image looks elliptical, it is likely that the transducer position is too low and not transecting the heart in the true minor axis.  If possible, try to eliminate the distorted view, by moving the transducer up an intercostal space.  An abnormal shape of the left ventricular wall in the short axis can indicate some important pathological findings such as elevated right ventricular pressure or a left ventricular aneurysm.  The perceived motion of the walls can also be affected by improper transducer rotation and can lead to the false appearance of, or the inability to see, a wall motion abnormality.

If the image is faint, it may also be necessary to move the transducer more medially or  have the patient exhale in order to shrink-back the lungs.  Often, the antero-lateral wall is the most difficult surface to image because it is obscured by lung tissue or a rib.  If the image is still faint, lowering the frequency of the transducer can also be very helpful for delineation of the endocardial border and myocardial thickness.

The right ventricular chamber is an important feature on the parasternal short axis at the papillary muscle level and the gain and TGC should be adjusted so that it is also imaged well.

Some sonographers prefer to obtain the M-Mode of the left ventricle from this view because it is easier to see correct placement of the cursor at the inferior wall however, if the view is obtained from a low interspace, the derived diameter and wall motion will be exaggerated.

SAX at the Apex of the heart:

Tilting the transducer even more inferiorly from the short axis at the level of the papillary muscles can bring the short axis of the left ventricular apex into view.  This view is useful for finding apical wall motion abnormalities, apical aneurysm and thrombi.  This view can be difficult to obtain in a large percentage of patients however; working with the patient’s breath often makes its acquisition much easier.  Sometimes it is easier to attain this view by starting from an apical view and sliding the transducer up the chest while tilting the transducer face more posteriorly to compensate.

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