Reverberation Artifact

Apical 4-Chamber View resolution of tissue interfaces, in particular, the endocardial border. Harmonic imaging is an option on many modern ultrasound machines (Fig. 5; please see companion DVD for corresponding video). However, image quality is not always improved by harmonic imaging, and in some patients, fundamental imaging provides better overall image quality.

Obtaining Images and Image Quality

Echocardiography is dependent on the operator applying the transducer to the chest of the patient and obtaining images in real-time. The quality of the images, therefore, is dependent on the skill of the operator, as well as the body habitus of the patient. In addition, the ability to obtain the quality images can often be hampered by aspects of the patient's medical care that cannot be controlled, such as the patient being on a ventilator, or having bandages following surgery that interfere with scanning. Well-trained sonographers and echocardiographers learn to work around many of the inherent limitations imposed by the need to scan very sick patients. Obesity and chronic obstructive pulmonary disease are probably the two patient characteristics that affect image quality most.

Ultrasound Artifacts

Imaging artifacts suggesting the appearance of structures that are not actually present are common in ultrasonic imaging. Artifacts include both the apparent presence of structures that do not exist, or the obscuring of structures that do exist. Artifacts in the aorta and the left atrial appendage frequently pose important diagnostic and decision-making challenges (Chapters 16, 17, and 19). To identify artifacts, the experienced echocar-diographer must understand the reasons for artifact appearance in ultrasonic images. Artifacts can be caused by many of the same problems that result in poor image quality, including body habitus and the location of ribs. Indeed, rib artifact remains one of the most prominent artifacts seen in echocardiography studies (Fig. 6; please see companion DVD for corresponding video). Rib artifacts can often be distinguished from actual structures because the artifact will remain in one place relative to the transducer while the beating heart moves separately from the artifact. Often, however, a rib artifact will move with respiration owing to movement of the thorax with breathing. It is, therefore, sometimes necessary to distinguish respiratory movement from cardiac movement in order to distinguish artifacts from real structures.

Reverberation artifacts are caused by reflections that occur internally within the imaging region. Calcifications frequently cause ultrasound signals to "ping-pong" within the interrogated structure before returning to the transducer (Fig. 7). Not accounting for the internal reverberation, the ultrasound machine interprets the extra time for the ultrasound to return as an indication that the reflections occur at a further depth, resulting in a ghost reflection usually at a distance that is a multiple of the original reflection.

Acoustic shadowing » (Rtb artifagj^

Ultrasound Reverberation Artifact

Reverberation artifact

Imaging Dropout

Parasternal short axis view of left ventricle (apical region)

Fig. 6. Three types of artifacts are visible in this parastenal short axis at the mid ventricular level in a 50-yr-old patient with a left ventricular assist device. (Please see companion DVD for corresponding video.) Reverberation artifacts (multiple arrows) generated by the cannula of the device are seen along with acoustic shadowing from the ribs, and dropout owing to loss of lateral resolution.

Another type of artifact originates from the fact that ultrasound beams can be wider than the scanline representation on the image. Thus, an ultrasound beam may reflect from a structure slightly off the true axis of the beam, causing a loss in lateral resolution. Mirror image artifacts are frequently seen in the aorta on transesphageal echocardiography (Fig. 8; please see companion DVD for corresponding video).

principles of doppler ultrasound

Doppler ultrasound relies on the Doppler principle to determine the velocity of moving fluids or tissues. The Doppler principle states that the frequency of a sound (or any wave) will shift (higher or lower) when it is emitted from, or reflected off, a moving object. This occurs because sound waves emitted from a moving source (or reflected off a moving source) are either compressed or expanded depending on the direction of the movement. This is the same principle responsible for the changing frequency of an ambulance siren as it travels toward or away from an observer (Fig. 9).

In diagnostic ultrasonography, waves are emitted from the transducer at a particular frequency and reflected off moving red blood cells within the heart or blood vessels. If the flow of blood is moving toward the transducer, the sound waves will be compressed (and the frequency of the returning ultrasound will be slightly higher than the emitted ultrasound). The opposite is true for blood flow moving away from the transducer (Fig. 9). The difference between the emitted frequency and the returning frequency is called the Doppler shift. Because the ultrasound machine emits sound at a particular known frequency, the difference between the original ultrasound frequency and the returning ultrasound frequency can be easily determined. This difference in frequency is directly related to the velocity of the structures reflecting the sound (the red blood cells) and, therefore, is related to the velocity of blood flow. This relationship is described by the following equation:

Reflective structure r

Transducer

"Ping-pong" effect of interrogating reflective structure (e.g rib or calcification) on ultrasoundwaves

Reverberation Artifact

Fig. 7. Illustration of the generation of a reverberation artifact. In this case, a reflective structure, such as an area of calcification, causes an "internal" reverberation. The additional "back-and-forth" trip causes the machinery to place an artifactual distal to the original image, but spaced a multiple away from the original distance between the transducer and the reflective structure.

Reverberation artifact

(arrows)

Fig. 7. Illustration of the generation of a reverberation artifact. In this case, a reflective structure, such as an area of calcification, causes an "internal" reverberation. The additional "back-and-forth" trip causes the machinery to place an artifactual distal to the original image, but spaced a multiple away from the original distance between the transducer and the reflective structure.

Mirror Image Artifact UltrasoundMirror Image Artifact Ultrasound
Fig. 8. Mirror artifacts are commonly seen in the aorta on transesphageal echocardiography as shown in these still frame images. (Please see companion DVD for corresponding video.)

where v = the velocity of blood flow, c = the propagation velocity of sound through the tissue (1540 m/s), Fs = the shifted (returned) ultrasound velocity, Ft = the original emitted ultrasound frequency, and 0 = the angle of incidence between the ultrasound beam and the blood flow.

Pulsed- and Continuous-Wave Doppler

Two modes of Doppler ultrasound are typically employed in standard diagnostic ultrasonography— pulsed-wave (PW) Doppler and continuous-wave (CW) Doppler (Figs. 10 and 11). PW Doppler requires individual

Doppler Principle

Fig. 9. The Doppler principle: when the sound emitting source (in the illustration, the ambulance ), is moving toward the listener, the wavelength of the sound waves shorten (or the frequency increases); when the sound emitting source is moving away from the listener, the sound waves will lengthen (the frequency will decrease). Ultrasound emitted from the transducer bounces off moving red blood cells (Bottom), and returns to the transducer. The difference between the emitted frequency and the returning frequency is the Doppler shift.

Fig. 9. The Doppler principle: when the sound emitting source (in the illustration, the ambulance ), is moving toward the listener, the wavelength of the sound waves shorten (or the frequency increases); when the sound emitting source is moving away from the listener, the sound waves will lengthen (the frequency will decrease). Ultrasound emitted from the transducer bounces off moving red blood cells (Bottom), and returns to the transducer. The difference between the emitted frequency and the returning frequency is the Doppler shift.

Pulsed Wave Doppler (Aortic Valve)

Apical

5-Chamber

View k Jt

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Responses

  • leah
    What type of artifact is rib on the echocardiography?
    2 years ago

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