Double aorta artifact on ultrasound - Identification, mechanism, and correction

INTRODUCTION

The word artifact has its origin from Latin and is defined as an object that is made by a person, especially something of historical or cultural interest (Latin arte “by or using art” + factum “something made”). Artifact in medical imaging bears a similar meaning, albeit with a few changes, and is defined as part of an image that does not correctly represent the anatomic structures being evaluated.^[1] Artifacts in imaging are inherent to every modality and can be assuaged but never completely eradicated. A radiologist should be aware of the artifacts that can be encountered in imaging to avoid errors in judgment and reporting.

In ultrasound (USG), Image production of stationary and moving tissues alike depends on the physical characteristics of the USG pulse, the acoustic property of the tissue through which the USG pulse propagates, its interaction with tissues, echo detection, and processing of the echo. Recording of the echoes gives information about the tissues, which is used to produce the image. During this processing of echoes, there are various assumptions made, deviations from which create artifacts.^[2,3] In this review article, we aim to briefly describe the minutiae of USG image production and the assumptions made in processing the echoes; double aorta artifact - its appearance, relevant physics behind it, anomalies which can mimic this artifact, and its mitigation.

USG IMAGE PRODUCTION – PHYSICS AND ASSUMPTIONS

USG uses a short pulse of mechanical energy traveling at the speed of sound, which is delivered to the tissues. As the pulse encounters tissues with different acoustic properties, a fraction of it is reflected as an echo that returns to the source. These echoes are collected over time, and their amplitude is recorded, providing information about the tissues along the path of travel. This process is repeated a number of times to produce a grey-scale image. A transducer is used to generate and direct the USG pulses and detect the returning echoes. The depth of any echo-producing structure can thus be determined from the time the returning echo takes to reach the transducer, and the amplitude of the echo is encoded as a grey-scale value. USG image production, hence, depends on the physical properties of the USG pulse, its propagation in tissues, interaction with reflective interfaces, echo-detection, and processing.^[4]

USG processing and display make various physical assumptions to ascribe the location and intensity of each received echo. These assumptions are critical to understanding artifact production. These assumptions are enumerated as follows:^[2,5,6]

• The echoes detected originated from within the main USG beam

• An echo returns to the transducer after a single reflection

• The depth of an object is directly related to the amount of time for a USG pulse to return to the transducer as an echo

• The speed of sound in human tissue is constant

• The sound beam and its echo travel in a straight path

• The acoustic energy in a USG field is uniformly attenuated.

Since these are assumptions, they are not always observed, and hence, the artifacts that we encounter, occur. Artifacts can arise due to inherent errors of USG beam characteristics and due to deviations from the assumptions mentioned. These deviations can be the presence of multiple echo paths, multiple reflections of a single pulse, different speeds of sound in different media such as air, fluid, fat, soft tissue, and bone, and due to inconsistent attenuation of the USG beam, due to the heterogeneous nature of the medium.^[2]

DOUBLE AORTA ARTIFACT

The double aorta artifact, like other artifacts, occurs due to deviation from the assumptions in the USG echo processing. It occurs due to the different velocities of the USG pulse in different human tissue types and refraction at the border of two different tissues^[7-9] in such a way that the two aortic lumens are visible on the image as shown in Figure 1a. This can be mistaken for rare anomalies such as a double aorta or aortic dissection.

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Figure 1:

(a) Ultrasound image of the upper abdomen in the epigastric region in axial plane, in a young adult, shows two rounded vascular structures in the midline (red arrows) representing the two aortic lumens. Oval shaped vascular structure to the right of midline (blue arrow) representing the Inferior vena cava. (b) Colour doppler shows consistent flows in both midline vascular structures. The blue and red patch is the flow as seen on colour doppler. Note that the anterior margin of the vertebral body located behind the aorta is also duplicated, confirming the artifact rather than anomalous duplication (yellow asterix) as illustrated in Figure 1a and b.

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APPEARANCE

The double aorta artifact is noted in the young athletic population, with a few documented case reports of this artifact seen in the pediatric population.^[10] The anatomic requirement for this artifact is the presence of lens or oval-shaped rectus abdominis muscles with fat beneath and between the muscles arranged in such a way that it gives the adjacent fat tissue a prismatic shape.^[8] The rectus muscle thickness also plays an important role. Though no absolute cut-off has been defined in literature, the configuration required for this artifact appears with thicker muscles than thinner muscles.^[7,8] The resultant artifact appears on USG of the upper abdomen as two separate abdominal aortic lumens [Figure 1a]. The aortic duplication is then usually observed along a short course in the upper abdomen. The two lumens are separated by a septum or an apparent wall between the two lumens. Doppler examination of both lumens shows normal and similar flow as shown in Figure 1b. Partial duplication may also be observed, as shown in Figure 2.^[11]

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Figure 2:

Ultrasound image of the upper abdomen in the epigastric region in the axial plane in another young adult shows partial duplication (represented by yellow stars) of the midline aorta (red star).

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Hence, the pre-requisite for the artifact formation is the specific arrangement of the rectus abdominus muscle and the fat beneath and between them. There have been studies reported in the literature that studied such an arrangement and found that when such a configuration is present, 40% is noted in the upper abdomen in the epigastric region and 36% in the lower abdomen and pelvic region.^[8] Such an anatomic configuration is, however, not observed in 60–95% of individuals. Consequently, individuals with small rectus abdominis muscles, the presence of a thick connective tissue band between the rectus abdominus muscle or excessive fat beneath the muscle, prevents the formation of the prismatic shape required for appropriate USG wave refraction and formation of the artifact.^[8,9]

In addition to the aortic lumen, the branches of the aorta, such as the coeliac and the superior mesenteric, may also be seen arising from the anterior surfaces of both lumens as shown in Figure 3.^[10] A partially duplicated image of the anterior margin of the vertebral body located dorsal to the aorta may also be seen, as shown in Figures 1-3.

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Figure 3:

Colour doppler image at the level of the upper abdomen in the epigastric region in axial plane, of another young adult shows consistent flows in both aortic lumens. The symmetrical flow in the coeliac arteries just beyond their origin from the anterior wall of the abdominal aorta is noted. Note the duplicated anterior margin of the vertebral body, confirming the artifact rather than anomalous duplication (yellow asterix). The red and blue colour is the flow as seen on colour doppler.

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HOW IS IT FORMED - THE PHYSICS BEHIND IT

USG image formation depends on the physical characteristics of the USG pulse, its propagation in matter, interaction with reflective interfaces, echo detection, and processing, with many assumptions made during the processing of the echo. Two such assumptions were that the speed of sound within the human tissue is constant, 1540 m/s, and that the echo travels in a straight path. However, the USG beam in its path encounters soft tissue, fat, fluid, air, and bone, each having different density and elastic properties and hence different speed of sound within them. The speed of sound in fat is 1450 m/s, in muscles is 1580–1600 m/s, in the rest of the soft tissues is 1540 m/s, and in bone is 4080 m/s.^[4,7-10]

Snell’s law governs the law of refraction. It was described for light rays, giving the relationship between the path taken by a ray of light in crossing the boundary between two contacting surfaces with different refractive index. This law was discovered in 1621 by the Dutch astronomer and mathematician Willebrørd Snell.^[12] Similar to light waves, when a USG wave passes from one material to another having a different refractive index, at an oblique angle, both reflected and refracted waves are produced, refraction being the change in the direction of the USG wave, which is governed by Snell’s law [Figure 4] and is given by the following equation:

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Figure 4:

Refraction of an ultrasound pulse as governed by Snell’s law when the ultrasound pulse moves from a medium with index of refraction n1 to a medium with index of refraction n2.

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sin θ1 = n2

sin θ2 = n1

θ1 = angle of incidence

θ2 = angle of refraction

n1 = index of refraction in medium 1

n2 = index of refraction in medium 2

In the above formula, the index of refraction in a medium is equal to c/v, where c is the speed of light in a vacuum and v is the speed of light in the medium.^[7,13,14]

A double aorta artifact on USG occurs due to the prismatic shape of the fat beneath and between the lens-shaped rectus abdominis muscles. The base of the prism is made by the upper edge of the liver, and the other two sides are made by the border of the rectus muscle and fat. The USG beam is refracted at the interface of the rectus abdominis muscles and the triangular shape of fat tissue between them, as shown in Figure 5a.^[3,15] Subsequently, the USG beam reaches the aorta through soft tissues from both sides. Produced echoes undergo identical refraction on their way back to the transducer. The image produced shows two aortic lumens because of the assumption that the echo travels in a straight path. The two lumens are observed to the left and right of the original location of the medially located aortic lumen, as shown in Figures 5a and b.^[8,10,15] Similarly, not only the aorta, but also other midline structures located in the way of USG beam undergoing refraction, appear duplicated, such as the anterior margin of the vertebral body dorsal to the aorta, as shown in Figures 1-3.^[11]

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Figure 5:

(a) Diagrammatic representation, (b) Extrapolation of the diagrammatic representation on an ultrasound image illustrating the physics of double aorta artifact production due to refraction of ultrasound beams at the interface of the rectus abdominis and the fat beneath and underneath the rectus abdominis. Ultrasound beam (magenta arrows) sent by the transducer (magenta circle) undergoes refraction at the junction of the rectus abdominis muscle and fat. The refracted ultrasound beam (red arrows) reaches the midline aorta (red circle) through soft tissues. Produced echoes take the same path on their way back to the transducer (magenta circle), but are, however, assumed to be coming along a straight line path as before refraction (blue broken lines), and the image shows duplicated aortic lumens.

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Two inferences can be logically derived from the above. One is that the greater the refraction, the greater will be the distance of the apparent aortic lumens from each other.^[3] Furthermore, the greater the distance from the probe, the greater will be the distance between the two apparent lumens. This is the reason that this artifact is more commonly seen when using a lower frequency curvilinear probe because of higher depth penetration with lower frequencies.^[16]

WHAT ELSE CAN CAUSE TWO AORTIC LUMENS

Two aortic lumens can definitely cause a diagnostic dilemma; hence, knowledge of the conditions where such an appearance can be seen is essential. Appearance of a double aortic lumen apart from the artifact described above can be mimicked by the presence of two separate aortas, an aortic aneurysm, aortic dissection, or duplication of the inferior vena cava. The presence of two separate abdominal aortas is a very rare anomaly with hardly any cases reported in the literature.^[17] One study described two cases of double aortas on post-mortem.^[11] Another case report has described a case of double abdominal Aorta with a double Inferior Vena Cava.^[18]

Aortic aneurysm may give the appearance of two lumens but can be easily differentiated due to asymmetrical size, involving a limited length of aortic wall, and Doppler characteristics at the level of the aneurysm in the form of bidirectional and swirling flow (called the Yin-Yang sign).^[19]

In aortic dissection, two asymmetric lumens are visualized, extending for a varying length, with a septum separating them. The false lumen is more commonly larger in size than the true lumen and is typically located left postero-laterally. On color Doppler, asymmetrical flow will be seen in the two lumens, with typically reduced flow in the false lumen. In some cases, no flow may be seen in the false lumen due to thrombus formation.^[20,21]

Duplication of the inferior vena cava, though much more common than aortic duplication, with a prevalence of 0.2–0.3%^[22,23], may be easily differentiated by the Doppler characteristic of venous flow within. Any vessel in the paraaortic region that attains a sufficient caliber to match the aorta may give this appearance, such as a dilated left testicular vein or a dilated inferior mesenteric vein.^[11]

Due to the rarity of the above-mentioned mimickers of double aorta artifact, if consistent symmetrical flow is detected in two adjacent para-midline abdominal vessels, one must consider the more frequent double aorta artifact than the rarer anomalies.

GETTING THE NORMAL IMAGE – HOW DO WE DO IT

Since this artifact can be identified with both the grey scale and with Doppler, the remedial measures are not difficult to implement. The probe can be moved laterally to the right or left from the midline or rotated 90° to the sagittal position^[8] to remove the duplication artifact, since the USG beam no longer passes through the prismatic-shaped fat. If, however, the image of a single abdominal aorta still cannot be obtained, other probe positions, enabling its assessment, should be applied, such as the left or right lateral decubitus position, where the right lobe of the liver and the kidney, respectively, form an acoustic window for the aorta.^[24,25] In these positions, the effect of the rectus muscle and fat is avoided.

In some cases, the presence of bowel gas may limit the artifact correction technique of moving the probe to para-median positions. Identifying the symmetrical nature of branches originating from the aorta, duplication of the anterior vertebral body margin as described above can help differentiate artifact from abnormality. Doppler examination will usually confirm the duplication due to artifact with symmetrical flow in both the apparent aortic lumens.

WHAT ELSE CAN BE SEEN AS DUPLICATE STRUCTURES

This duplication artifact occurrence was first documented in 1984 as the false appearance of two intrauterine devices (IUDs) instead of one. Since it was with a Copper-7 IUD, the artifact was called the “Copper-14” artifact.^[9] The duplication artifact seen in the pelvis is due to the presence of a similar anatomic acoustic prism in the lower anterior abdominal wall. This can cause a single gestational sac to appear double and be misinterpreted as a multiple pregnancy in early pregnancy.^[9,26]

Duplication artifact has also been documented to be seen in echocardiography, with inducers of this artifact being the costal cartilage, fascial structures, fat, pleural surface, and pericardial surface. On the parasternal short-axis view of the aortic valve and left ventricle, and subcostal view of the pulmonary outflow tract, they are seen depicting duplications of the aortic, mitral, or pulmonary valve.^[15,27,28] Duplication artifact has also been documented in carotid arteries, causing problems in diagnosing stenosis and plaques.^[29]

CONCLUSION

Imaging artifacts in USG are unavoidable and can occur due to physical properties of the USG or its echo, due to the limitations of the display and processing paraphernalia, and due to violations of the underlying assumptions made in forming the image. These can be explained by underlying physical principles, understanding of which is necessary to optimise the technique to diminish artifacts. Recognizing these artifacts is necessary to avoid misdiagnosis and unnecessary investigations for the patient. The double aorta artifact due to refraction through the abdominal muscles and fat is one such artifact that has not only been proven by phantom models but also experienced by many in clinical practice. It remains pertinent for radiologists to proactively maintain awareness of this artifact while scanning to reduce unwarranted investigations for the patient, saving time for both - the most precious commodity!