Echocardiography and Doppler Ultrasound

Chapter 4 Echocardiography and Doppler Ultrasound

Virginia Luis Fuentes

INTRODUCTION

Echocardiography has become the most important diagnostic technique for the diagnosis of canine and feline heart disease. The interaction between ultra-high-frequency sound waves and the heart allows the depiction of cardiac morphology, information on the movement of myocardium and valves, and blood flow within the heart. Echocardiography is complementary to physical examination, radiography and electrocardiography (ECG) and has replaced invasive techniques such as cardiac catheterization for all but a few specific indications.

TYPES OF IMAGING

Two-Dimensional Echocardiography

• A sector-shaped beam of ultrasound waves is reflected by the interfaces of cardiac tissue to provide a two-dimensional (2D) cross-sectional (tomographic) image (Figure 4-1).
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Figure 4-1 Right parasternal echocardiographic views. A, Long-axis four-chamber view optimized for left ventricular inlet. B, Long-axis view optimized for left ventricular outflow tract. C, Short-axis view at the papillary muscle level. D, Short-axis view at chordal level. E, Short-axis view at mitral valve lavel. F, Short-axis view at the heart base, optimized for left atrium and aortic valve. G, Short-axis view at the heart base, optimized for pulmonary artery. LA, Left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; Ao, aorta; R, right coronary sinus of Valsalva; L, left coronary sinus of Valsalva; NC, noncoronary sinus of Valsalva; PA, pulmonary artery; rPA, right pulmonary artery.

Applications

• Demonstrating cardiac morphology
• Increasingly important in quantification of chamber dimensions (as machines become capable of faster frame rates)

M-Mode Echocardiography

• M-mode uses a single narrow beam of ultrasound, but displays the resulting echoes as a distance-time graph (Figure 4-2). The time resolution is superior to that obtained with 2D, so that the frame is updated thousands of time per second rather than the 40 to 200 times per second obtained with 2D.
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Figure 4-2 M-mode echocardiograms of the left heart at the mitral valve chordal level (note that the 2D image is moved to the left of the screen to allow correct placement of the cursor) (A); mitral valve leaflet level (B); aortic valve level. (C). IVS, Interventricular septum; LV, left ventricle; LVFW, left ventricular free wall; RV, right ventricle; Ao, aorta; LA, left atrium; LAur, left auricle.

Applications

• Time-dependent measurements (chamber dimensions, wall motion)

Doppler Echocardiography

• Doppler echocardiography uses the Doppler principle: the frequency of a reflected sound wave depends on the direction and velocity of the reflector and the transmitted frequency (producing a Doppler shift).
• If the transmitted ultrasound frequency and the velocity of sound in soft tissue and blood are known, then the velocity of red blood cells can be calculated.

Key Point

The angle of the incident ultrasound beam is critical: the ultrasound beam must be parallel with flow (or less than 20° from the direction of flow) or the velocity will be underestimated.

There are several modes of Doppler echocardiography:

• Spectral Doppler, where the velocity of blood flow is calculated in a region of interest selected by moving a cursor (Figure 4-3)
• Color Doppler, where the blood flow is coded in red (toward the transducer) or blue (away from the transducer) and superimposed on the black-and-white 2D image (Figure 4-4)
• Tissue Doppler imaging (TDI), where the velocity of myocardial motion is displayed (Figure 4-5, E)
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Figure 4-3 Doppler echocardiographic studies. A, Mitral inflow obtained from the left caudal parasternal (apical) four-chamber view, with the cursor placed parallel with mitral inflow, and a small sample volume placed at the tips of the mitral valve leaflets when the mitral valve is open. The typical spectral Doppler waveform of mitral inflow displays an early diastolic (E) wave and an atrial contraction (A) wave of filling. B, Aortic flow obtained from the left caudal parasternal (apical) “five–chamber” view, showing left ventricular outflow tract and placement of pulsed wave sample volume in ascending aorta. The typical spectral Doppler waveform of aortic flow is displayed. C, Aortic flow obtained from the subcostal view, showing continuous wave cursor positioned in ascending aorta. The typical spectral Doppler waveform of aortic flow is displayed. D, Tricuspid valve flow obtained the left cranial parasternal view optimized for right ventricular inflow. The pulsed wave sample volume is placed at the tricuspid leaflets tips, with the probe in a cranial position. The spectral waveform is similar to that of mitral inflow, although sometimes an additional systolic forward flow wave is recorded. E, Pulmonary artery flow from the right parasternal short-axis view optimized for right ventricular outflow (left panel) with the pulsed wave sample volume positioned in the main pulmonary artery, and from the right cranial parasternal view optimized for the pulmonary artery (middle panel). The typical spectral Doppler waveform of pulmonary artery flow is displayed. LA, Left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; CdCV, caudal vena cava; Ao, aorta; RVOT, right ventricular outflow tract; PA, pulmonary artery.

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Figure 4-4 Color flow echocardiographic studies of mitral regurgitation. A, Mild mitral regurgitation. The color jet occupies < 20% of the left atrial area. B, Moderate mitral regurgitation. The color mitral regurgitation jet occupies 20% to 40% of the left atrial area. C, Severe mitral regurgitation. The color jet occupies > 40% of the left atrial area (the jet is rebounding from the dorsal LA wall in red), the vena contracta is wide, and a proximal flow convergence region can be seen. D, Right parasternal long-axis view of a dog with severe mitral regurgitation, showing a large vena contracta width (black arrow). E, Left apical view of a dog with severe mitral regurgitation, showing a large proximal flow convergence region. The white arrows indicate the borders of the hemisphere where aliasing is occurring. The larger the hemisphere’s diameter for a particular aliasing velocity (in this case 69 cm/s), the more severe the mitral insufficiency.

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Figure 4-5 Echocardiographic evaluation of ventricular diastolic function. A, Normal transmitral flow pattern demonstrating an early filling wave (E) with higher velocity than the atrial contraction wave (A). B, Delayed relaxation transmitral flow pattern demonstrating reduced amplitude and prolonged duration of the E wave. C, Pseudonormal transmitral flow pattern demonstrating a normal E:A velocity ratio, but this is the result of the combined effects of delayed relaxation, increased LA pressures and increased LV stiffness. D, Restrictive transmitral flow pattern. High LA pressures result in increased E wave amplitude despite delayed relaxation. Decreased LV compliance (sometimes with atrial systolic dysfunction) results in a diminished A wave. E, Pulsed wave tissue Doppler (TDI) image of mitral annulus velocity, displaying a systolic wave (S), early diastolic wave (E’) and atrial wave (A’).

Spectral Doppler Echocardiography

• A graph of blood flow velocity against time is shown, usually with a simultaneous ECG display.
• Blood flow velocities toward the transducer are displayed as positive (above the baseline), and blood flow velocities away from the transducer are displayed as negative (below the baseline).
• The Doppler shift can also be represented by an audible sound, because the shift in frequency is usually quite small and within the audible range (around 10 kHz), so that most machines will show a visual spectral display with a simultaneous audible signal.
• Spectral Doppler is usually recorded with guidance from two-dimensional echocardiographic images (2D-Doppler, or Duplex Doppler). This allows a cursor to be superimposed over a 2D image, showing the angle of interrogation of the Doppler beam in two dimensions.
• Spectral Doppler can be further subdivided into pulsed wave, continuous wave, and high-pulse repetition frequency.

Pulsed Wave Doppler

• The ultrasound waves are transmitted as pulses of waves, with the transducer acting at different times as a receiver or transmitter of ultrasound waves, allowing the interrogation of blood flow velocities within a specific region of interest.
• This region of interest is represented as a sample volume on the cursor.
• Pulsed wave Doppler has limitations in the maximum velocities that it can display without ambiguity: high velocities result in “aliasing,” where blood flow will be displayed as both positive and negative velocities (i.e., signal wraps around the baseline).
• The velocity at which aliasing will occur depends on the Nyquist limit (half the pulse repetition frequency), with higher velocities without aliasing achieved at lower transducer frequencies, and reduced depth from the transducer.

Continuous Wave Doppler

• In contrast, with continuous wave (CW) Doppler, ultrasound waves can be transmitted and received simultaneously. This allows much higher velocities to be displayed, but it is not possible to draw any conclusions about the depth along the cursor from where these velocities are originating (i.e., there is range ambiguity).

High-Pulse Repetition Frequency Doppler

• High-Pulse repetition frequency Doppler is a form of pulsed Doppler that shares some similarities with CW Doppler. Frequent pulses of ultrasound waves are produced so that a number of sample volumes will be superimposed on the 2D image. This not only allows the display of higher velocities without aliasing, but also increases the number of possible sites from which the velocities are being recorded.

Color Flow Doppler

• Color flow Doppler represents the velocity and direction of blood flow in color, superimposed on a black-and-white 2D image. In effect, the color is displayed within a very large sample volume superimposed on the 2D image. Blood flow away from the transducer is shown in blue, and blood flow toward the transducer is displayed in red (“BART:” blue away, red toward). Disturbed or turbulent flow may be displayed in green or yellow. Aliasing may also occur in color flow Doppler.

Tissue Doppler Imaging

• More sophisticated machines may include facilities for recording the velocity of myocardial motion. The signals reflected by the moving myocardium are high amplitude, but low velocity. These myocardial velocities can be displayed in spectral format, as a color display, or as color M-mode. It is believed that TDI indices may have advantages over conventional echocardiographic indices, because they may be less influenced by loading conditions.

Applications

• Doppler echocardiography is most often used to characterize abnormal direction or velocity of blood flow, or to indicate the origin of turbulent blood flow. This is invaluable in valve disease or congenital heart disease, but Doppler echocardiography may also be used to estimate flow volumes, to assess systolic and diastolic function, and to obtain information about intracardiac pressures.
• Abnormal blood flow direction may be noted with conditions such as valvular insufficiency.
• Abnormal blood flow velocity may be a clinically useful finding, as the velocity of blood flow across an orifice is chiefly determined by the difference in pressure. If there is a large difference in the pressures in the chambers on each side of a valve, then the velocity of flow across the valve will be high. For example, during systole the pressure in the left ventricle is very high, and the pressure in the left atrium is very low. If the mitral valve is incompetent and regurgitation occurs, then the velocity of the regurgitant jet traveling from the left ventricle to the left atrium will be very high, reflecting the large difference in pressures. Conversely, for much of diastole, the pressure in the left atrium is only slightly higher than the pressure in the left ventricle, so the velocity of the early filling (E) wave is fairly low. This velocity information can therefore be used to derive information about intracardiac pressures.
• Turbulent blood flow is relatively uncommon in the normal heart, but when present, is most likely to occur where velocity is highest (i.e., within the aorta). Even in a normal heart, vigorous ejection into the aorta can sometimes result in signal aliasing on color Doppler. Blood flow will be turbulent whenever velocity is high, and valvular insufficiency or stenosis almost inevitably results in turbulent blood flow. Turbulent blood flow may be displayed in a different color (green/yellow) or color distribution (mosaic pattern); this is termed variance. In most cases, turbulent flow will be present when a murmur is audible, making Doppler echocardiography particularly useful in conditions where a murmur is present.
• Estimation of flow volumes is possible using Doppler echocardiography combined with 2D measurements. Flow is the product of cross-sectional area multiplied by the area under the spectral Doppler curve (velocity time integral).

Related posts:

  1. Emergency Management and Critical Care
  2. Anesthesia of the Cardiac Patient
  3. Cardiac Surgery
  4. Radiology of the Heart

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Aug 15, 2016 | Posted by in SMALL ANIMAL | Comments Off on Echocardiography and Doppler Ultrasound

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