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A Near-Field Clutter Artifact Mimicking Pulmonary Thrombus During Transesophageal Echocardiography

Liang, Yafen MD; Alvis, Bret MD; Rice, Mark J. MD; Shaw, Andrew D. MD; Deitte, Lori A. MD; Eagle, Susan MD

doi: 10.1213/ANE.0000000000001366
Perioperative Echocardiography and Cardiovascular Education: Echo Rounds

From the Departments of *Anesthesiology and Radiology, Vanderbilt University Medical Center, Nashville, Tennessee.

Accepted for publication March 7, 2016.

Funding: None.

The authors declare no conflicts of interest.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website.

Reprints will not be available from the authors.

Address correspondence to Susan Eagle, MD, Department of Anesthesiology, Vanderbilt University Medical Center, 1215 21st Ave N, Suite 5160, Nashville, TN 37232. Address e-mail to Susan.eagle@vanderbilt.edu.

A 23-year-old woman presented for pulmonary thrombectomy for chronic pulmonary embolism (PE). Her medical history included nonischemic dilated cardiomyopathy with left ventricular ejection fraction of 20%, endocarditis, right atrial thrombus, and end-stage renal disease secondary to granulomatosis with polyangiitis. A preoperative computed tomography angiography showed an abrupt cutoff of the right pulmonary artery (RPA) without significant pulmonary arterial (PA) flow indicating an occlusive thrombus. The main and left PAs were patent without filling defects.

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The patient was brought to the operating room. General endotracheal anesthesia was induced after invasive arterial blood pressure monitoring was established. The transesophageal echocardiogram (TEE) showed moderately depressed left ventricular function with estimated ejection fraction of 35%. The right ventricle was dilated with mildly depressed systolic function and moderate tricuspid regurgitation. The midesophageal ascending aorta short-axis view demonstrated a thrombus in the RPA occluding the vessel (Figure 1) with a persistent filling defect when echo contrast was injected (Figure 2). However, an additional “mass” was observed occupying approximately two-thirds of the RPA. This semicircular mass was located adjacent to the echo transducer. It was initially considered to be part of the thrombus; however, when echo contrast was injected, it easily traversed the mass both before (Figure 2; Supplemental Digital Content, Video 1, http://links.lww.com/AA/B433) and after thrombectomy (Figures 3 and 4; Supplemental Digital Content, Video 2, http://links.lww.com/AA/B434); therefore, this was proven to be an artifact. Subsequently, a 4.5 × 4 × 3.1-cm large chronic clot was found to totally occlude the right main PA. The clot also extended to the right superior, middle, and lower PA. The clots were evacuated successfully, and the patient had an uneventful recovery.

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PATIENT CONSENT STATEMENT

The patient gave written consent for publication of the case and echo images.

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DISCUSSION

PE is a common perioperative complication. Takahashi et al1 evaluated 60 patients undergoing elective lumbar spinal surgery with or without instrumentation and found that moderate to severe (grade 2 or 3) embolic events were seen in 80% of the instrumented patients. A grade 2 embolic event is defined as a cascade or shower of numerous fine emboli, whereas a grade 3 embolic event is defined as a serpentine macroembolus >5 mm among fine emboli. Moreover, PE is 1 of the leading causes for intraoperative cardiac arrest during noncardiac surgery, representing up to 47% of the total etiologies. Although computed tomographic pulmonary angiography is the definitive diagnostic study for PE, TEE is a valuable and generally a more available intraoperative tool for making this diagnosis in a hemodynamically unstable surgical patient.

The midesophageal ascending aorta short-axis and upper esophageal aortic arch short-axis views are considered the optimal windows to inspect the main PA and RPA for intraluminal thrombus,2 but direct visualization of a thrombus is not always possible.3 Therefore, the diagnosis of PE by echocardiography is usually suspected by indirect signs such as right ventricular dilation and hypokinesis, D-shaped interventricular septum, severe tricuspid regurgitation, and, more specifically, McConnell sign (right ventricular free wall akinesis with preserved apical motion).4 These indirect signs, although sensitive, are less specific for an acute PE. Patients with acute right coronary artery ischemia/infarction, air/fat embolism, and chronic pulmonary hypertension can have similar echocardiographic presentations to PE, but their treatment options are very different. What makes clinical diagnosis even more challenging is that artifact is frequently seen in the RPA, like in this case, at the level of the superior vena cava and can mimic a PA thrombus. The ability to differentiate this artifact from a central PA thrombus helps determine whether the patient is referred for a surgical thrombectomy or thrombolytic therapy.

Although the etiology of this artifact is not entirely clear, it most closely resembles near-field clutter artifact. This type of ultrasound artifact is created by high-amplitude reflections from the piezoelectric crystals of the transducer and affects the near field of the ultrasound beam.5 It typically appears as a diffuse haze and is most easily visualized in anechoic or hypoechoic regions such as heart chambers and blood vessels. The appearance of additional echoes in the near field can mask weaker echoes of true anatomic structures and is known to mimic a mass (or, in this case, a thrombus). A similar artifact has been described in the left ventricle mimicking a left ventricular thrombus during transthoracic echocardiography.6

There are several ways to differentiate near-field clutter artifact from true pathology. First, with gray-scale imaging, true structures usually have well-defined borders and attachments to nearby structures, do not typically cross anatomical borders, and can be visualized in multiple views. Artifacts typically have indistinct borders and do not persist on multiple views (such as midesophageal aortic valve long-axis view versus deep transgastric view to assess left ventricle thrombus). Second, with color Doppler imaging, artifacts usually do not produce interruption in the pattern of blood flow as would be seen with the presence of a true anatomic structure or thrombus. Finally, measures to help reduce or eliminate near-field clutter artifact include the use of (1) higher frequency transducers to improve near-field resolution; (2) sonographic contrast agents; and (3) harmonic imaging. In this particular case, the mass demonstrated a few gray-scale characteristics that made definitive diagnosis more challenging: it appeared to have a well-defined smooth border and to attach to the RPA wall. When color Doppler was applied, it was difficult to determine whether the color flow pattern was interrupted by the mass or not. However, with the injection of echo contrast, it was apparent that, unlike the true thrombus, the contrast traversed the mass, consistent with artifact.

Another type of artifact that can exist in the RPA is reverberation artifact, generated by a PA catheter balloon tip. With our patient, however, the PA catheter insertion was guided by TEE and confirmed to be in the main PA, remote from the artifact in the RPA. Of note, the echocardiographic findings of chronic versus acute PE can be very different. There are a few ways to differentiate them.7 First, McConnell sign is diagnostic for acute PE because it is observed in the setting of acute but not chronic pulmonary hypertension associated with chronic PE. Second, interrogation of the flow velocity curve in the right ventricular outflow tract can help differentiate acute from chronic PE. In acute PE, forward pressure waves are reflected by intraluminar emboli lodged in PAs and therefore return very early despite an only mild to moderate increase of PA pressure. In fact, an echocardiographic “60/60 sign” (pulmonary ejection acceleration time in the right ventricular outflow tract below 60 milliseconds in the presence of tricuspid insufficiency pressure gradient above 30 but below 60 mm Hg) has been proven to be highly specific for the diagnosis of acute PE. Third, patients with chronic PE more likely present with a hypertrophic right ventricle because of chronic pressure overload, less so with acute PE. Depending on the type of chronic PE, the controversial small-vessel versus the proximal massive-unresolved PE, direct visualization of the thrombus may or may not be possible. We believe our case was a chronic proximal massive-unresolved pulmonary thromboembolism instead of an acute PE, but the application of echocardiographic technique to differentiate artifact from true thrombus should still apply in this situation.

In conclusion, near-field clutter artifact can mimic a pulmonary thrombus in the RPA during TEE examination. Understanding the mechanism of artifact formation and methods to differentiate this artifact from true thrombus is critical to avoid erroneous diagnosis and treatment.

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Clinician’s Key Teaching Points

By Nikolaos J. Skubas, MD, Kent H. Rehfeldt, MD, Martin J. London, MD

  • Chronic pulmonary thromboembolism obstructs blood flow within the pulmonary arteries and with time may lead to right ventricular hypertrophy. Acute pulmonary embolism is a leading cause of intraoperative cardiac arrest during noncardiac surgery.
  • A mass located in the main or right pulmonary artery can be imaged in the midesophageal ascending aorta and upper esophageal aortic arch short-axis views. Well-defined borders, attachment to, and containment by, nearby anatomic structures, and visualization in multiple imaging planes help to differentiate thrombus from artifact. However, in most cases of pulmonary embolism, a thrombus is not visible and the echocardiographer instead looks for indirect signs such as a dilated and hypokinetic right ventricle, flattening of the interventricular septum, and tricuspid regurgitation.
  • In this case of a patient presenting for pulmonary thrombectomy, an occlusive thrombus was imaged within the right pulmonary artery. An additional mass, adherent to the posterior wall of the artery and adjacent to the transesophageal echocardiogram probe, was imaged in the midesophageal ascending aorta short-axis view. Injection of IV contrast material demonstrated a filling defect at the site of the former mass, but it traversed the latter mass, confirming it was a near-field clutter artifact.
  • Near-field clutter artifact, created by high-amplitude reflections from piezoelectric crystals, typically appears as a diffuse haze within several centimeters of the transducer, which may simulate a thrombus or other mass. The passage of color Doppler flow and echo contrast material through, rather than around, the hazy area help confirm a diagnosis of artifact. The use of a higher imaging frequency or harmonic imaging may avoid this potentially confusing artifact.
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DISCLOSURES

Name: Yafen Liang, MD.

Contribution: This author helped design the study and prepare the manuscript.

Name: Lori A. Deitte, MD.

Contribution: This author helped prepare the manuscript.

Name: Mark J. Rice, MD.

Contribution: This author helped prepare the manuscript.

Name: Andrew D. Shaw, MD.

Contribution: This author helped prepare the manuscript.

Name: Bret Alvis, MD.

Contribution: This author helped prepare the manuscript.

Name: Susan Eagle, MD.

Contribution: This author helped design the study and prepare the manuscript.

This manuscript was handled by: Martin J. London, MD.

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REFERENCES

1. Takahashi S, Kitagawa H, Ishii T. Intraoperative pulmonary embolism during spinal instrumentation surgery. A prospective study using transesophageal echocardiography. J Bone Joint Surg Br 2003;85:90–4.
2. Jerath A, Roscoe A, Vegas A. Normal upper esophageal transesophageal echocardiography views. Anesth Analg 2012;115:507–10.
3. Rosenberger P, Shernan SK, Body SC, Eltzschig HK. Utility of intraoperative transesophageal echocardiography for diagnosis of pulmonary embolism. Anesth Analg 2004;99:12–6.
4. Lau G, Ther G, Swanevelder J. Echo rounds: McConnell’s sign in acute pulmonary embolism. Anesth Analg 2013;116:982–5.
5. Pamnani A, Skubas NJ. Imaging artifacts during transesophageal echocardiography. Anesth Analg 2014;118:516–20.
6. Armstrong WF, Ryan T. Physics and instrumentation. Feigenbaum Echocardiography. 2009:7th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 27.
7. Torbicki A. Echocardiographic diagnosis of pulmonary embolism: a rise and fall of McConnell sign? Eur J Echocardiogr 2005;6:2–3.

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