Thoracic Imaging of the Coughing and Dyspneic Patient: Echocardiography and Doppler Echocardiographic Imaging
Philip R. Fox, DVM, DACVIM, DECVIM-CA (Cardiology), DACVECC
Echocardiography plays an integral role in cardiovascular evaluation, providing accurate, noninvasive evaluation of cardiac structure, function and blood flow dynamics. Importantly, echocardiography is a diagnostic test which should complement - but not replace - the survey thoracic radiograph.
Types of Echocardiography
Standard two-dimensional and M-mode echocardiography evaluate cardiac chamber anatomy and motion. The best images result when structures are perpendicular to the ultrasound beam. Doppler echocardiography (spectral and color flow) evaluates cardiovascular blood flow by assessing relative change in returned ultrasound frequency compared with the transmitted frequency. Information is most accurate when Doppler echo sound waves are directed parallel to the target RBCs.
A. M-Mode Echocardiography
M (motion)-mode echocardiography was the first widely used form of diagnostic cardiac ultrasound.
Advantages. The sharp axial resolution and high sampling frequency of M-mode echo compared to slower 2-D scanning rate allows small, rapidly moving structures to be discerned and accurately correlated with timed relative to the ECG.
Disadvantages. The limited image sector associated with a single, "ice pick," one-dimensional beam focuses only on a very small portion of the heart.
B. Two-Dimensional Echocardiography
Advantages. Utilizing an ultrasonic beam moving in a sector, 2-D echo creates a pie- or fan-shaped image, displaying anatomic and functional characteristics which are more anatomically intuitive than M-mode imaging.
Disadvantages. Compared with M-mode echocardiography, 2-D echo image acuity is less precise and frame rate is slower.
C. Three (or 'Four') Dimensional Echocardiography
Advantages. These techniques allow the heart and various structures to be reconstructed as solid objects. Such processing facilitates the ability to quantify structure size and shape, assess function, and evaluate related spatial and dynamic relationships.
Disadvantages. Current costs are high. Technology is limited to low- or mid-frequency probes, making use on small dogs and cats impractical. Patient movement during 3D image acquisition reduces acuity.
D. Transesophageal Echocardiography (TEE)
TEE uses a 2-D transducer at the end of a flexible endoscope placed in the esophagus to provide high-quality images compared with other approaches. TEE provides markedly improved imaging. Disadvantages include the high cost of the specific transducers, the need to anesthetize the animal, and increased time required for examination.
E. Doppler Echocardiography
Most cardiac disorders affect blood flow velocity or direction. Doppler echocardiography permits evaluation of these characteristics within the heart and great vessels.
The Doppler shift is greatly influenced by transducer frequency. The higher the transducer frequency (e.g., 7.5 MHz vs. 2.5 MHz), the lower the velocity of blood flow which can be measured.
Direction of Blood Flow
Sound waves which strike RBCs moving toward the transducer are reflected off the RBCs at a higher frequency. This is displayed as a spectral recording above the baseline as a positive Doppler shift. Conversely, sound waves which strike RBCs moving away from the transducer are reflected back at a lower frequency. This is displayed as a spectral recording below the baseline as a negative Doppler shift.
Accuracy of Doppler Velocity Determination
The intercept angle, theta, is a key factor influencing the accuracy of gradients determined by Doppler echocardiography. It represents the angle between the ultrasound beam and the moving red blood cells (RBCs). When Doppler echo beam alignment is parallel to moving RBCs, blood velocity is most accurately measured. In contrast, 2-D and M-mode echocardiography require the beam to be perpendicular to tissue interfaces for ideal imaging. If the intercept angle is wide, there will be a greater reduction in measured blood flow velocity compared with true velocity. Practically speaking, angles > 25° generally yield unacceptable quantitative estimates of velocity.
Estimation of Pressure Gradients
The gradient (i.e., pressure drop) across an obstruction may be calculated by the simplified Bernoulli equation, which approximates the pressure gradient across an obstruction (e.g., across a stenotic valve). Thus, the pressure gradient = 4 x velocity2.
F. Spectral Doppler Echocardiography
Pulsed-wave Doppler. Pulsed-wave (PW) Doppler echo uses the same transducer to alternate between sending and receiving sound waves.
Advantages. This provides Doppler shift data selectively along the ultrasound beam at any given range (known as depth discrimination or range resolution).
Disadvantages. PW Doppler echo has limited ability to measure high blood flow velocities as occur frequently with acquired or congenital valvular diseases.
Continuous-wave Doppler echocardiography uses separate transmitting and receiving transducer crystals to enable ultrasound waves to be continuously transmitted and received.
Advantages. CW Doppler echo accurately measures high blood velocities.
Disadvantages. CW Doppler echo is unable to selectivity sample at a given location and lacks depth discrimination. The CW beam contains Doppler shift information all along the course of its ultrasound beam.
G. Color-Flow Doppler Echocardiography
Color-flow (CF) Doppler echocardiography synthesizes the anatomic image of the two-dimensional or M-mode echocardiogram with Doppler blood flow characteristics to create a spatially correct, dynamic image. By standard convention, blood flowing towards the transducer is coded red, and blood flowing away from the transducer is coded blue. Blood flow velocity is indicated by the intensity of the color. Slowly moving blood is colored darker; faster moving blood is colored more brightly.
H. Doppler Echocardiography and TDI in Assessment of Diastolic Function
Noninvasive assessment of diastolic filling by Doppler echocardiography provides important information about left ventricular (LV) status in certain patients. Diastolic dysfunction is common in heart disease, particularly in feline cardiomyopathy. Thus, Doppler echocardiography is commonly employed to assess LV diastolic filling via analysis of mitral inflow velocity profiles. Because mitral inflow is affected by various factors which limit its application in some cases, mitral flow velocity curves are often analyzed in conjunction with other Doppler-derived indices and parameters include pulmonary venous velocity curves, color M-mode, and tissue Doppler imaging (TDI) (including early mitral annular diastolic velocity [Em], radial and longitudinal velocities, and other parameters).
The Two-Dimensional and M-Mode Echocardiographic Examination
Standard Imaging Planes for Two-Dimensional Echocardiography
Standard imaging planes (also called views) are designated based upon 1) transducer location (also called "windows"), 2) spatial orientation of imaging plane, and 3) recorded structures. For example, right parasternal long axis describes a view recorded with the transducer positioned on the right parasternal location and the imaging plane oriented parallel to the LV long axis.
Right Parasternal Location
Two principal imaging planes are: 1) long-axis views, and 2) short-axis views. M-mode echocardiogram is derived from either the long-axis or short-axis views. When recording the M-mode from short-axis views, one must transect the heart in the true minor axis, avoiding angled/oblique views.
Long-axis views. Two views: 1) a four-chamber view with the ventricles (cardiac apex) displayed to the left and atria (cardiac base) displayed to the right, and 2) a second view obtained by slight clockwise transducer rotation showing the LV outflow tract, Ao valve and root.
Short-axis views. Obtained by rotating the transducer (and beam plane) 90° from long-axis views, then angling the beam from apex (ventral) to base (dorsal) to obtain a series of progressive views at LV apex, papillary muscles, chordae tendineae, mitral valve, and Ao valve, respectively.
Left Cranial Parasternal Location
Is located between the left 3rd and 4th intercostal spaces between the sternum and costochondral junctions.
Long-axis views. A series of views may be obtained with the beam plane oriented approximately parallel with the long axis of the body and heart.
Short-axis views. A series of short axis orientations is obtained by rotating the transducer beam 90° from the long-axis view.
Left Caudal (Apical) Location
This location is close to the sternum between the 5th–7th ICS.
Left apical four- and five-chamber views. A four-chamber view of the heart oriented vertically may be obtained with the left heart appearing to the right and right heart to the left, and the ventricles in the near field. A left ventricular outflow region may be brought into view by tilting the beam slightly cranial from the four-chamber view. A five-chamber view is denoted when all four cardiac chambers, both atrioventricular valves, and the aortic valve appear in one plane.
Left apical two-chamber views. When the beam plane is nearly perpendicular to the long axis of the body and parallel to the long axis of the heart, a two-chamber long axis view is obtained of the left atrium, mitral valve, and left ventricle.
Standard Imaging Planes for M-Mode Echocardiography
M-mode echocardiograms are derived from 2-D images which guide proper spatial and anatomic orientation. Most M-mode data are derived from the right parasternal long axis view of the LV, although the short axis views often allow better anatomic alignment. Standard recordings are made 1) through the aorta and LA, 2) left ventricle at the mitral valve level, and 3) LV at the level of chordae tendineae.
The Doppler Echocardiographic Examination
Standard Imaging Planes for Doppler Echocardiography
Transducer locations and imaging planes described for 2-D imaging are used for Doppler echo studies. Views which provide optimal parallel beam orientation are selected. Two additional locations may be useful - subcostal and suprasternal.