4.2.4 The echocardiographic examination
M-mode echocardiography can produce an infinite number of one-dimensional 'ice-pick' views of the heart; 2DE can produce an infinite number of image planes. There is therefore a need for standardisation of beam position to aid recognition of landmarks and facilitate communication between workers. In quantitative studies, regardless of the type of measurement, a standardised approach is needed so that accuracy and reproducibility are optimal. This will allow consistency of method both between different horses, in the same horse on different occasions and between different echocardiographers. Meticulous attention is required in order to obtain correctly oriented imaging planes in relation to internal landmarks. In clinical situations, planes of view can be lesion oriented, but if this approach is adopted exclusively, abnormalities may be missed. A standardised examination procedure should therefore be followed routinely.
The orientation of M-mode views is traditionally directed by recognition of the characteristic motion of valves and chamber walls and a knowledge of cardiac anatomy, imaged from a fixed transducer location. However, most machines allow the M-mode cursor to be positioned on a 2DE image in order to facilitate identification of landmarks for M-mode imaging. Measurements can therefore be made directly from the 2DE image, or from guided M-mode traces.
For optimal image quality, it is important that the gain and post-processing controls are suitably altered. The brightness of the echoes displayed on the screen will depend on the power setting (usually near 100%), the difference in the acoustic impedance of the two structures at the interface which is reflecting sound and the gain setting. The intensity of the ultrasound is reduced by tissues so that at greater depths less sound will be reflected from comparable interfaces. The gain setting can be adjusted to allow for reduced strength of signals from deeper structures by amplifying the echoes from structures further from the transducer. Post-processing controls allow the operator to alter the contrast of the image by altering the grey scale. For echocardiography, the settings are usually quite high because identification of the endocardial borders and the valves is more important than fine detail of myocardial texture. This is the opposite of the situation in ultrasonography of most soft tissue structures such as tendons or liver.
Imaging planes and transducer position
In comparison with man and small animals, the positions from which 2DE views of the adult equine heart can be obtained are limited. The principal limitation is that the apex of the heart rests on the sternum and this precludes a true apical view and prevents measurement of the length of the LV. Consequently, the only views of the heart obtainable in the adult are parasternal (sometimes called paracostal), with the long axis of the heart either perpendicular or oblique to the axis of the beam.
Transducer positions required to obtain specific views can be guided by internal cardiac landmarks or the position of the transducer on the chest wall. The transducer needs to be located in the correct position and then angled and rotated to different degrees to obtain the standardised image planes. The transducer location is defined as left or right parasternal, ventral or dorsal and cranial or caudal.
In order to be sure of the position of the ultrasound beam in relation to one's hand, it is helpful to place one's thumb on the guide mark on the transducer so that the plane of the beam runs parallel with the palm of the hand. With most machines, if the guide mark and the thumb are positioned dorsally, dorsal structures are displayed on the right of the screen. This is the conventional method of display.
Standardised imaging planes and transducer location
A number of standardised views for 2DE and M-mode imaging have evolved over the last decade. It is helpful to obtain these views before looking specifically at structures of special interest by angling, rotating or moving the transducer to image lesions. Angled views are usually required for Doppler echocardiography, so that the beam can be positioned as close as possible to parallel with blood flow. This is largely governed by the spectral Doppler signal rather than the exact anatomical position, although the 2DE image is helpful to guide positioning of the cursor and to identify areas when PWD is used to map out jets of regurgitant blood flow. The views described below and the transducer location required to obtain these images are summarised in Table 4.6.
Right long-axis views
Right long-axis view optimised for left ventricle and atrium reference view' The right parasternal long-axis view is optimised to image the ventricular inlets when the axial beam (the centre of the sector) transects the basal portion of the interventricular septum (IVS) and the chordae tendineae of the posterior mitral leaflet, with the long axis of the ventricle approximately perpendicular to the axial beam. The structures imaged in this plane are the RV, right atrium (RA), tricuspid valve (TV), IVS, interatrial septum, and the majority of the LV, mitral valve (MV), left atrium (LA) and LV free wall (Figure 4.9). The view is usually best obtained from the 4th intercostal space with the transducer positioned just dorsal to the olecranon, but the exact position should be adjusted to allow for individual variation. This view has been described as the 'reference' view and it acts as a guideline from which other views can be obtained. It helps to gain an initial impression of the overall motion and size of the chambers. Gross structural defects and valvular lesions can be seen.
Right long-axis view optimised for left ventricular outflow and aortic valve From the reference location the transducer can be moved slightly dorfor a view optimised for the aortic valve in long axis. In this view, the axial beam transects the base of the aortic valve, with the long axis of the LV outflow-tract perpendicular to the axial beam. The structures imaged in this plane are the RV, RA, TV, IVS, aortic valve and portions of the LV, MV, ascending aorta and LA (Figure 4.10). In small horses, or with machines with sufficient depth display, the pulmonary artery (PA) may be visible in cross-section, dorsal to the LA and deep to the aorta. This view shows the structure and motion of the aortic valve although abnormal motion is more easily seen on M-mode. The size of the RV, RA and aorta can be assessed. This view is usually the best in which to identify a VSD, positioned just below the aortic root. The transducer can be tilted slightly dorsally for Doppler assessment of the tricuspid valve.
Right long/oblique-axis view optimised for pulmonary valve From the reference location, with the plane of the beam in long axis, the transducer can be angled cranially for a view which is often described as a right ventricular inflow/ outflow view. A wide range of angulations produce similar views with different degrees of obliquity. The axial beam transects the right coronary artery, with the aorta in an oblique section. The structures imaged in this plane are the RA, TV, RV, PV, PA, aorta, and a portion of the LA (Figure 4.11). This view allows an overall impression of RV size to be made and the structure of the TV and PV to be assessed. The size of the PA can be measured.
Right short-axis views These 2DE views are principally used to guide the position of the M-mode cursor so that LV dimensions can be measured and the motion of the MV and IVS can be assessed. The transducer is rotated through approximately 90o from the long-axis position to produce short-axis views. The author finds anti-clockwise rotation more comfortable than clockwise rotation, but it results in cranial structures being displayed on the left side of the screen which breaks convention. So long as the same method is always used, the cranial and caudal aspects of the LV should not be confused. The cross-sectional views can be obtained at various levels of the LV.
Right short-axis view, chordal level Measurements of LV diameter are
conventionally made at the chordal level. At this level, the papillary muscles are no longer visible indenting the LV lumen, but chordae tendineae are clearly seen (Figure 4.12). Neither the characteristic 'fish-mouth' appearance of the MV in 2DE or 'M'-shape excursion on M-mode or the LV outflow-tract come into view. The position and angulation of the transducer should be adjusted to try to make the LV lumen appear as close to circular as possible to ensure that the true short-axis is obtained. The M-mode cursor can then be placed across the maximum diameter of the LV, bisecting the IVS. Consistent positioning of the transducer and M-mode cursor is essential if accurate measurements are to be made. The Mtrace at this level has a characteristic appearance and is useful for assessing the motion of the IVS (Figure 4.13). M-mode is preferable to 2DE for measurement of LV diameter because it is easier to identify end-systole and endfrom the M-mode trace than to freeze the 2DE image at exactly the right time.
Many papers quote values for thickness of the RV free-wall and RV diameter made from Mmode recordings at this level. However, unless these are grossly abnormal, they are of little value because there are no suitable landmarks for accurate measurements. In addition, the surface of the RV is irregular and trabeculated and the measurements made from M-mode cannot allow for this.
Right short-axis view, mitral valve level At the mitral valve level, a characteristic 'fish-mouth' appearance is seen on 2DE. The 'M' shape excursion of the valve seen on M-mode corresponds with the opening and closing of the mitral valve in early and late diastole. The points of motion of the valve on the Mtrace have been given letters to identify each phase of the cycle (Figure 4.14). The relative size of the waves of motion and their timing provides inforabout blood flow. High frequency motion such as that associated with aortic regurgitation is particularly well seen in this view. The presence or absence of characteristic motion is helpful to identify some arrhythmias.
Right short-axis view, aortic valve level The aortic valve level is the level at which the aortic valve is seen in circular cross-section, with the valve leaflets creating a 'Mercedes-Benz' sign in diastole (Figure 4.15). An M-mode recording is made by placing the cursor across the middle of the valve (Figure 4.16). Mtraces may show high frequency motion. Lesions on the valve are often more easily seen on 2DE images in this view than in the long-axis view. In some animals with a VSD, the defect is more easily seen just below this level in short-axis rather than in the long-axis view.
The ratio of the aortic root and left atrial diameter has often been measured from an M-mode trace recorded at this level, However, this is fraught with error because the angle at which the cursor bisects the LA is very variable. In fact, the cursor actually crosses the left atrial appendage, rather than the body of the atrium itself. Left atrial diameter is best measured from the left long-axis paraview in 2DE (see below). M-mode aortic root LA ratios are only of any value if the atrium is very dilated.
Left long-axis views In all but the largest animals or those with very dilated ventricles, with suitable equipment, the whole of the heart can be seen from a right parasternal position. However, examination from the left side allows the heart to be examined in a different plane than from the right side and may provide additional information. For example, rupture of the MV chordae tenoften involves the right commissural cusp, and this is more easily seen in long axis from the left parasternal view than from the right. Examination of the LA, and measurement of its diameter is also routinely performed from the left parasternal long-axis view. The transducer is placed in a slightly more dorsal position than for the view centred on the LV, with the axial beam transecting the base of the MV (Figure 4.17). The size of the LA is maximised arid the right atrium appears as a small round chamber directly deep to the MV annulus precluding measurement of LV diameter. The measurement callipers are p051-tioned at the base of the valve, rather than more dorsally where there is a risk that they will be placed into a pulmonary vein.
Left short-axis views Short-axis views can be used to measure the size of the LV; however, this is in a plane which is perpendicular from that measured from the right parasternal location. The M-mode cursor, should be placed through the junction of the LV free-wall and the septum (Figure 4.18). This view can be useful for assessing LV size and the structure and motion of the MV.
In animals with thick chest walls or a dilated heart, or when using equipment with limited depth display, the LV free-wall may be beyond the range of display from a right parasternal view, precluding measurement of LV diameter. Measure from the left parasternal view offers an alternative. However, it should be remembered that this is not exactly the same measurement as from the right parasternal view, and values are slightly different.