Chest ImagingFree Access

Minimal Aortic Injury: Mechanisms, Imaging Manifestations, Natural History, and Management

Published Online:Oct 2 2020https://doi.org/10.1148/rg.2020200066

Abstract

Over the last 2 decades, increased depiction of minimal aortic injury (MAI) in the evaluation of patients who have sustained trauma has mirrored the increased utilization and improved resolution of multidetector CT. MAI represents a mild form of blunt traumatic aortic injury (BTAI) that usually resolves or stabilizes with pharmacologic management. The traditional imaging manifestation of MAI is a subcentimeter round, triangular, or linear aortic filling defect attached to an aortic wall, representing a small intimal flap or thrombus consistent with grade I injury according to the Society for Vascular Surgery (SVS). Small intramural hematoma (SVS grade II injury) without external aortic contour deformity is included in the MAI spectrum in several BTAI classifications on the basis of its favorable outcome. Although higher SVS grades of injury generally call for endovascular repair, there is growing literature supporting conservative management for small pseudoaneurysms (SVS grade III) and large intimal flaps (>1 cm, unclassified by the SVS), hinting toward possible future inclusion of these entities in the MAI spectrum. Injury progression of MAI is rare, with endovascular aortic repair reserved for these patients as well as patients for whom medical treatment cannot be implemented. No consensus on the predetermined frequency and duration of multidetector CT follow-up exists, but it is common practice to perform a repeat CT examination shortly after the initial diagnosis. The authors review the evolving definition, pathophysiology, and natural history of MAI, present the primary and secondary imaging findings and diagnostic pitfalls, and discuss the current management options for MAI.

Online DICOM image stacks are available for this article.

^©RSNA, 2020

SA-CME LEARNING OBJECTIVES

After completing this journal-based SA-CME activity, participants will be able to:

■ Identify direct signs, indirect signs, and mimics of MAI at CT.
■ Outline the pathophysiologic mechanisms, natural history, and current approach to the management of MAI.
■ Discuss controversies and the future direction of definitions of the MAI spectrum.

Introduction

Blunt traumatic aortic injury (BTAI) is the second leading cause of trauma-related mortality, exceeded only by fall-related death from traumatic brain injury, and accounts for 10%–20% of fatal outcomes (1–3). The last several decades have seen significant advances in multidetector CT technology, allowing rapid acquisition of high-resolution near-isotropic images. These advances, in combination with widespread availability and utilization of CT angiography, have led to marked improvement in imaging depiction of BTAI (sensitivity of 95%–100%, negative predictive value of 100%) (4–6). The spectrum of BTAI depicted at multidetector CT angiography encompasses a wide range of injuries, from small intraluminal defects to full-thickness circumferential ruptures.

BTAI is classified on the basis of the absence or presence of external aortic wall abnormality as minimal aortic injury (MAI) or significant aortic injury (SAI), respectively (Fig 1).

While the presence of SAI often dictates immediate or prompt invasive measures, MAI usually does not require immediate intervention (7–11). Recent literature estimates MAI incidence to be 10%–30% of all BTAI that manifests at imaging, with the diagnosis based solely on multidetector CT findings (11–13).

Different morphologic forms of BTAI at the aortic isthmus. Aortic injuries without outer aortic contour alteration are shown: A, small intimal flap (<1 cm); B, large intimal flap (>1 cm); C, small intimal flap with a small adherent thrombus; and D, intramural hematoma without contour deformity. While A and C are traditionally accepted MAI entities, the lesions in B and D have garnered strong support in favor of nonoperative management and are grouped together with traditional MAI entities in newer management-based classification systems. Aortic injuries with altered outer aortic contour include the following: E, intramural hematoma with contour deformity; F, contained rupture with small pseudoaneurysm (<1 cm, or occupying less than 50% of aortic circumference); G, large pseudoaneurysm (>1 cm, or occupying 50% or more of aortic circumference); and H, complete aortic rupture affecting all walls. It is important to note that even though E and F fall outside the confines of what is traditionally accepted as minimal, the trend toward empirical nonoperative management for these lesions indicates otherwise. — Figure 1. Different morphologic forms of BTAI at the aortic isthmus. Aortic injuries without outer aortic contour alteration are shown: A, small intimal flap (<1 cm); B, large intimal flap (>1 cm); C, small intimal flap with a small adherent thrombus; and D, intramural hematoma without contour deformity. While A and C are traditionally accepted MAI entities, the lesions in B and D have garnered strong support in favor of nonoperative management and are grouped together with traditional MAI entities in newer management-based classification systems. Aortic injuries with altered outer aortic contour include the following: E, intramural hematoma with contour deformity; F, contained rupture with small pseudoaneurysm (<1 cm, or occupying less than 50% of aortic circumference)*; G,* large pseudoaneurysm (>1 cm, or occupying 50% or more of aortic circumference); and H, complete aortic rupture affecting all walls. It is important to note that even though E and F fall outside the confines of what is traditionally accepted as minimal, the trend toward empirical nonoperative management for these lesions indicates otherwise.

Traditionally, MAI is defined as a subcentimeter intimal abnormality (thrombus or flap) without external contour deformity (14). Increasing experience with conservative management is expanding the boundaries of what can be called minimal on the basis of long-term stability or short-term resolution (11,15–17). Although the recommendations are not unanimous, localized intramural hematoma without external contour abnormality has now been accepted as part of several classifications as an additional manifestation of MAI (18–20). Both of these MAI patterns tend to have a good prognosis, with fewer than 10%–15% of cases showing progression at follow-up multidetector CT (7–12,19,21–25).

Advocacy for follow-up imaging is not unanimous, and no clear guidelines exist regarding the frequency and interval of follow-up (18). Moreover, recent reports on favorable outcomes after conservative management in cases of large intimal flaps (>1 cm) and small pseudoaneurysms (<1 cm) are fostering debate regarding optimal follow-up tactics, and some cases such as localized intramural hematoma are being considered by some surgeons as mild rather than significant aortic injury (18,25–28).

This review updates the reader on the current concepts of MAI diagnosis and management, with emphasis on imaging appearances, potential pitfalls, and natural history. It also provides insight into the evolving trends related to management and prognostication of MAI from the surgeon’s perspective.

Definition, Mechanisms of Injury, and Pathophysiology

MAI was first suggested by Gavant et al (29) in 1995 and further defined by Malhotra et al (30) in 2001 as “a small intimal flap with minimal to no periaortic hematoma.” The seminal paper from Azizzadeh et al (31) clearly defined an image-based grading system for BTAI on the basis of the anatomic extent of aortic wall involvement demonstrated at multidetector CT, as follows: intimal tear (grade I), intramural hematoma (grade II), pseudoaneurysm (grade III), and aortic rupture (grade IV). MAI was defined as a small intimal aortic abnormality with no external contour deformity (31). In 2011, this classification was adopted by the Society for Vascular Surgery (SVS) and began being used to help guide intervention (32).

The current definition of MAI is a “sub-centimeter intimo-medial abnormality with no external contour deformity” (18,24,25). This includes small intimal tear and intramural hematoma. While small intimal tear (SVS grade I) has always been considered an MAI lesion, the relatively recent inclusion of intramural hematoma (SVS grade II) is strongly supported by newer management-based classification systems (eg, the Vancouver and Harborview classifications) (18,20–22).

Overall, the majority of BTAIs occur in younger persons who have been involved in high-velocity motor vehicle collisions (3,33,34). Among scenarios in which BTAI has been reported, frontal collisions (60%–65%) are far more common than lateral impact collisions (21%–29%). These are followed by pedestrians being struck by motor vehicles, motorcycle collisions, and a fall from significant height, in decreasing order of frequency (3,33–36).

Although there are no clear pathophysiologic profile differences between MAI and SAI, a restrained frontal collision with use of seat belts and airbag deployment is most often linked to MAI (3,34,36). Conversely, SAI is more common with side-impact collisions and unrestrained passengers, which allow greater and quicker impact on the unsupported torso (3,36,37).

Blunt injury to the deeply situated aorta results from shear stress at rigid points of fixation or at the transition sites between mobile and immobile segments of the aorta. The aorta is tethered by multiple vascular and nonvascular structures such as the aortic arch branches, ligamentum arteriosum, heart, intercostal arteries, and diaphragmatic crura. The known sites of transition between mobile and fixed segments include the aortic root, aortic isthmus, and diaphragmatic aortic hiatus.

Four injury mechanisms or forces have been described that may result in BTAI: (a) stretch effect due to opposing vertical tension from arch branches cranially and the heart and intercostal branches caudally; (b) shear effect due to tug and torsion applied at the sites of anchorage such as the isthmus; (c) pinch effect from external osseous compression of the aorta between the sternum and the spine; and (d) thump effect, which involves pressurization of the aortic blood column (Fig 2). Although the precise contribution of different forces is unknown, the current general theory of BTAI indicates convergence of multiple simultaneous injury forces (38,39).

Figure 2. Illustration depicts the main injury forces in blunt aortic injury (including MAI) at the most common location, the aortic isthmus. These forces are often combined and described as follows: A, stretch effect with vertically distributed forces (arrows) by the cranial pull by the arch vessels with a hyperextended neck; B, shear force due to tugging (dashed arrows) at the site of aortic fixations; C, pinch effect due to osseous compression (blue arrow) of the aorta between the rigid sternocostal cage anteriorly and vertebral column posteriorly; and D, thump effect, which involves intravascular pressurization (double arrows) of the aortic blood column akin to a water hammer.

The isthmus of the descending thoracic aorta is the most commonly involved site of MAI, involved in approximately two-thirds of patients with BTAI who present to the hospital (2,6,9,11,40,41). A smaller proportion of cases involve the ascending aorta (8%–27%), aortic arch (8%–18%), distal descending aorta, (11%–21%), and abdominal aorta (7%–22%) (Fig 3) (2,6,9,11,40,41).

Aortic arch branches are the least common location for MAI. Instead, branch vessel injuries manifest more frequently with total luminal occlusion because of the smaller caliber and more muscular nature of these arteries (6,42).

Figure 3. Illustration with image examples (axial images with relevant multiplanar reconstructions) shows the incidence of MAI by location. MAI lesions (white arrows) are subdivided by location (orange arrows) into aortic root (aortic annulus to sinotubular junction); ascending aorta (between sinotubular junction and origin of brachiocephalic artery); aortic arch (segment with all arch vessels); and descending aorta, which includes an isthmic segment (2 cm caudal to the left subclavian artery origin) and a distal segment to the level of the hiatus. Among these, the isthmic segment is affected the most by MAI. Injuries to the aorta at the hiatus and abdominal aorta are less frequent but are commonly associated with lumbar spine injury (arrowhead).

In several autopsy series investigating BTAI, multiple simultaneous injuries were seen in 6%–18% of cases, and involvement of the abdominal aorta was seen in 5%–6% of cases (2,40,41). The injury morphologies and mechanisms affecting the abdominal aorta are overall quite similar to those seen in the thoracic aorta (43). Direct and indirect forces are implicated in the trauma. Direct force between a lap belt and the lumbar spine is thought to be a common mechanism, accounting for nearly half of the cases (44). Shear force, a major factor in thoracic aortic injury, has been shown to play a less important role in abdominal aortic injury and is exerted predominantly in the distal infrarenal aorta near the bifurcation (44,45).

Other than the aforementioned mechanisms of injury, patient factors can potentially influence the injury dynamics. Aortic position and configuration (eg, tortuosity or unfolding) and aortic wall compliance (eg, elasticity, compliance, or atherosclerosis) may contribute to different injury patterns. Most published mechanistic studies assume the transmission of forces in the healthy aorta of a young patient (38,39). It is plausible that the edges of calcific atherosclerotic plaques may act as points of shear vulnerability. However, no quality data exist to support this presumption. On the other hand, overall lower incidence (0.06%–7%) of aortic trauma in the pediatric population is presumably because of preserved thoracic elasticity, aortic configuration, and a lack of vascular atherosclerosis (46–48).

Imaging Evaluation of MAI

Imaging Modalities

Multidetector CT Angiography.—Multidetector CT angiography has become the standard for traumatic aortic evaluation, with sensitivity and specificity of 98%–100%, including in MAI (5). The examination is typically performed as part of a whole-body multidetector CT examination and includes concomitant evaluation for injuries in the abdomen and pelvis (49).

The conventional protocol includes two phases. Chest CT angiography is performed from the thoracic inlet to the caudal renal level. Next, additional abdominal and pelvic imaging is performed in the portovenous phase, starting just above the diaphragm and to the level of the proximal femurs.

Here, chest CT angiography performed with a section thickness of 3 mm or less is started by a bolus tracking and preceded by an injection of nonionic intravenous iodinated contrast material with a saline chaser. The rates of injection and volume of the contrast material and saline vary depending on the patient and vendor but generally are within 90–150 mL at 3–5 mL/sec for the contrast material bolus and approximately 30 mL at 4–5 mL/sec for the saline chaser. Imaging in the portovenous phase through the abdomen and pelvis is usually performed with a 70–75-second scan delay. Alternatively, a modified triphasic injection, single-pass, whole-body imaging protocol is described as well and is reported to provide better vascular and parenchymal opacification when compared with conventional two-phase trauma imaging (50).

Multiplanar reconstructions and maximum intensity projection images are vital and are highly recommended to help uncover a fair proportion of injuries that are not as apparent at axial imaging (5,51,52). Although not routinely used, electrocardiographic (ECG) gating at the end of diastole can aid in resolving pulsation and respiratory artifacts encountered in the initial assessment of aortic trauma at nongated multidetector CT and is typically used in imaging follow-up of MAI.

Invasive Imaging Modalities.—These include conventional percutaneous transcatheter angiography and transesophageal echocardiography (TEE). Both of these modalities are inferior to multidetector CT in sensitivity and specificity, especially in the evaluation of MAI because of the subtlety of the imaging findings. Use of TEE primarily for aortic injury screening is recommended only in preliminary intraoperative inspection for aortic injury when CT is delayed because of an emergent surgical exploration (49).

Imaging Findings

Overall, CT findings of BTAI can be subdivided into direct and indirect signs and associated injuries.

Direct Signs.—

The two most common imaging findings of an intimal tear in MAI are a rounded or triangular intraluminal filling defect (10 mm or less in 80% of cases) or a thin focal intimal flap (seen in 15% of cases) attached to a vessel wall without external contour abnormality (Fig 1, A–C; Fig 4) (6,12,35,53). These are most often found at the aortic isthmus and represent an SVS grade I injury (2,6,9,11,40,41).

Different imaging subtypes of MAI. The most common imaging manifestations of MAI are a small triangular or rounded wall-adherent thrombus (arrow in a and b) and a small intimal flap (arrow in c and d). These could be multifocal MAI (arrowhead in b). A small intramural hematoma with characteristic increased attenuation within the wall (arrows in e and f) without external contour abnormality is the least frequent imaging finding. (a, b) Wall-adherent thrombus (attached to a nonresolvable intimal flap) in a 39-year-old man who presented to the emergency department after a restrained high-velocity motor vehicle collision. Axial (a) and sagittal (b) CT angiograms show multifocal small triangular or rounded wall-adherent thrombi (arrow) within the distal isthmic or descending aorta. The lesions resolved at subsequent imaging before discharge. (A full DICOM image stack is available online.) (c, d) Intimal flap in a 48-year-old woman who presented to the emergency department after a restrained motor vehicle collision. Axial (c) and sagittal (d) CT angiograms show a single subcentimeter linear intimal flap (arrow) at the level of the aortic arch. The flap remained stable at two subsequent CT angiography follow-ups performed at 3 days and 4 weeks. No further follow-up images were available. (A full DICOM image stack is available online.) (e, f) Localized intramural hematoma (seen in addition to an intimal flap, not shown here) in a 55-year-old man involved in an all-terrain vehicle (ATV) accident. Axial (e) and sagittal (f) CT angiograms show a localized intramural hematoma (arrows). Complete resolution occurred after pharmacologic treatment. (A full DICOM image stack is available online.) — Figure 4a. Different imaging subtypes of MAI. The most common imaging manifestations of MAI are a small triangular or rounded wall-adherent thrombus (arrow in a and b) and a small intimal flap (arrow in c and d). These could be multifocal MAI (arrowhead in b). A small intramural hematoma with characteristic increased attenuation within the wall (arrows in e and f) without external contour abnormality is the least frequent imaging finding. **(a, b)** Wall-adherent thrombus (attached to a nonresolvable intimal flap) in a 39-year-old man who presented to the emergency department after a restrained high-velocity motor vehicle collision. Axial **(a)** and sagittal **(b)** CT angiograms show multifocal small triangular or rounded wall-adherent thrombi (arrow) within the distal isthmic or descending aorta. The lesions resolved at subsequent imaging before discharge. *( A full DICOM image stack is available online.)* **(c, d)** Intimal flap in a 48-year-old woman who presented to the emergency department after a restrained motor vehicle collision. Axial **(c)** and sagittal **(d)** CT angiograms show a single subcentimeter linear intimal flap (arrow) at the level of the aortic arch. The flap remained stable at two subsequent CT angiography follow-ups performed at 3 days and 4 weeks. No further follow-up images were available. *( A full DICOM image stack is available online.)* **(e, f)** Localized intramural hematoma (seen in addition to an intimal flap, not shown here) in a 55-year-old man involved in an all-terrain vehicle (ATV) accident. Axial **(e)** and sagittal **(f)** CT angiograms show a localized intramural hematoma (arrows). Complete resolution occurred after pharmacologic treatment. *( A full DICOM image stack is available online.)*

A small (1 cm or less in thickness) intramural hematoma is characterized by increased attenuation within the aortic wall due to hemorrhage from ruptured vasa vasorum or small intimomedial tears (Fig 1, D; Fig 4e). It represents SVS grade II injury but is considered to be in the MAI spectrum by multiple surgical groups and constitutes 5% of all MAI cases (22,27,35).

A nonenhanced CT examination is rarely performed in the setting of trauma, but intramural hematomas are most easily identified at nonenhanced CT as a crescentic or circumferential region of increased attenuation within the aortic wall (Fig 1). It is important to note that MAI may be multifocal and can be associated with SAI adjacent to or at a different aortic location (Figs 4a, 5).

Concurrent MAI and SAI. (a, b) Sagittal (a) and axial (b) CT angiograms show a large isthmic pseudoaneurysm (arrow in a), which is an SAI (SVS grade III injury), with concurrent small intimal flaps (white and black arrowheads) at the distal descending aorta, which is an MAI (SVS grade I injury). Note the large volume of periaortic and mediastinal hemorrhage, which in combination with aortic injuries helped guide the treatment toward urgent thoracic endovascular aortic repair (TEVAR). (c) Coronal CT angiogram shows a stent-graft covering both of these injuries. — Figure 5a. Concurrent MAI and SAI. **(a, b)** Sagittal **(a)** and axial **(b)** CT angiograms show a large isthmic pseudoaneurysm (arrow in a), which is an SAI (SVS grade III injury), with concurrent small intimal flaps (white and black arrowheads) at the distal descending aorta, which is an MAI (SVS grade I injury). Note the large volume of periaortic and mediastinal hemorrhage, which in combination with aortic injuries helped guide the treatment toward urgent thoracic endovascular aortic repair (TEVAR). **(c)** Coronal CT angiogram shows a stent-graft covering both of these injuries.

Several surgical groups advocate for inclusion of small pseudoaneurysm in the spectrum of mild nonoperative (similar to MAI) group of BTAI on the basis of its favorable outcome and survival (Fig 1, F; 4c) (26–28). A small pseudoaneurysm is usually defined as a contour abnormality occupying less than 50% of the aortic circumference (<1 cm in length) and occurring as a result of a partial tear of the aortic wall beyond the intima. These are commonly observed at the anteromedial aspect of the aortic isthmus. If they progress, aortic pseudoaneurysms have a tendency to enlarge in a transverse direction, and those at the isthmus are often eccentric due to flow-associated dynamics (Fig 1, F, H).

Indirect Signs.—Indirect signs associated with BTAI are most often seen with SAI rather than MAI. The most common indirect signs are periaortic and mediastinal hematoma, such that both were initially proposed as mandatory exclusion criteria for the diagnosis of MAI (30,52,54). However, Forman et al (55) found frequent occurrence of periaortic hematoma in MAI (in 79% of cases), and thus it is now accepted that surrounding hematoma can be present in the setting of solitary MAI or in conjunction with SAI (Fig 5).

Two imaging findings that help differentiate hemorrhage from an alternate source such as small mediastinal veins, smaller branch arteries, or aortic vasa vasorum are (a) preserved fat plane between the hemorrhage and an intact aortic wall and (b) hemorrhage without significant mass effect (9,54,56). However, hemorrhage from aortic arch branches and bronchial and intercostal arteries might be large enough to cause mass effect on adjacent structures.

Associated Injuries.—The presence of certain injuries within the thorax is often representative of high-velocity impact, raising the suspicion for deep vascular injuries. Such injuries include fractures of the sternum, first three ribs, scapula, and thoracic spine, which are seen with MAI and SAI (3,14,43,44,55,56). While some studies describe a higher incidence of traumatic brain injury and long bone fractures with SAI and a higher association of isolated rib and spinal fractures with MAI, other studies find no particular injury pattern predictive of the degree of aortic injury (22,24,57). Although similar all-cause mortality is reported for SAI and MAI, this is confounded significantly by the presence of multiple additional severe injuries in these patients (24).

Imaging Pitfalls

The imaging interpretation of MAI can be challenged by a number of pitfalls (51,58). These can be conceptually separated into technical, morphologic, and pathologic mimics (Figs 6–8).

Technical mimics of MAI. (a) Axial multidetector CT image shows motion artifact (arrows) due to cardiac or respiratory motion, which is often obvious due to extension of the artifact beyond the aortic walls or blurred margins of pulmonary vessels or interstitium (not shown). (b) Axial multidetector CT image shows streak artifact (red arrow) from attenuating intravenous contrast material within the left brachiocephalic vein. Note the typical attachment of the ligamentum arteriosum at the medial side of the aortic isthmus (white arrow). (c) Axial arterial phase multidetector CT image demonstrates mixing artifact (arrow). (d) Axial portal venous phase multidetector CT image obtained through the abdomen during the same examination shows that the mixing artifact has been completely resolved. — Figure 6a. Technical mimics of MAI. **(a)** Axial multidetector CT image shows motion artifact (arrows) due to cardiac or respiratory motion, which is often obvious due to extension of the artifact beyond the aortic walls or blurred margins of pulmonary vessels or interstitium (not shown). **(b)** Axial multidetector CT image shows streak artifact (red arrow) from attenuating intravenous contrast material within the left brachiocephalic vein. Note the typical attachment of the ligamentum arteriosum at the medial side of the aortic isthmus (white arrow). **(c)** Axial arterial phase multidetector CT image demonstrates mixing artifact (arrow). **(d)** Axial portal venous phase multidetector CT image obtained through the abdomen during the same examination shows that the mixing artifact has been completely resolved.

Anatomic mimics of MAI. (a, b) Sagittal multidetector CT images show ductus diverticulum without calcifications (arrow in a) and with small wall calcifications (arrows in b) with typical smooth convexity and congruity with the native aortic wall. (c) Sagittal multidetector CT image shows aortic spindle (arrows) with typical smooth and symmetric increase in the caliber of the aorta just below the isthmus. (d) Axial multidetector CT image depicts vascular infundibulum with its typical conical shape and a vessel emanating from its apex (arrow). — Figure 7a. Anatomic mimics of MAI. **(a, b)** Sagittal multidetector CT images show ductus diverticulum without calcifications (arrow in a) and with small wall calcifications (arrows in b) with typical smooth convexity and congruity with the native aortic wall. **(c)** Sagittal multidetector CT image shows aortic spindle (arrows) with typical smooth and symmetric increase in the caliber of the aorta just below the isthmus. **(d)** Axial multidetector CT image depicts vascular infundibulum with its typical conical shape and a vessel emanating from its apex (arrow).

Pathologic mimics of MAI. (a) Axial multidetector CT image obtained in an elderly patient demonstrates a noncalcified linear atheroma (arrow) in a frequently seen location adjacent to the wall calcifications. (b, c) Axial (b) and coronal (c) multidetector CT images obtained in an elderly patient show a round noncalcified atheroma (arrow) with diffuse circumferential atherosclerosis. — Figure 8a. Pathologic mimics of MAI. **(a)** Axial multidetector CT image obtained in an elderly patient demonstrates a noncalcified linear atheroma (arrow) in a frequently seen location adjacent to the wall calcifications. **(b, c)** Axial **(b)** and coronal **(c)** multidetector CT images obtained in an elderly patient show a round noncalcified atheroma (arrow) with diffuse circumferential atherosclerosis.

Technical Mimics.—Technical artifacts such as motion artifacts (respiratory motion and cardiac pulsation) and streak artifacts from injected intravenous contrast material can mask or simulate minimal intimal or small contour abnormalities (Fig 6a, 6b). ECG leads or catheters can also cause streak artifact. Mitigation of these artifacts can be achieved with optimal breath hold, cardiac gating, and repeat delayed imaging or repeated injection from the contralateral extremity, respectively (Fig 9).

Troubleshooting cardiac motion with ECG-gated CT angiography of the aorta. CT angiography of the aorta can be performed with prospective or retrospective gating (the preferred method) in cases of significant artifact at the ascending aorta. This approach allows resolution of cardiac motion–produced blur or margin unsharpness (arrows in a). Axial CT angiograms obtained before (a) and after (b) gating demonstrate the resolution of cardiac motion artifact. — Figure 9a. Troubleshooting cardiac motion with ECG-gated CT angiography of the aorta. CT angiography of the aorta can be performed with prospective or retrospective gating (the preferred method) in cases of significant artifact at the ascending aorta. This approach allows resolution of cardiac motion–produced blur or margin unsharpness (arrows in a). Axial CT angiograms obtained before **(a)** and after **(b)** gating demonstrate the resolution of cardiac motion artifact.

Anatomic Mimics.—Three anatomic variants of the aortic contour at or around the ligamentum insertion can also be mistaken for small pseudoaneurysms: ductus diverticulum, aortic spindle, and branch vascular infundibula (at the bronchial, intercostal, and spinal arteries). A ductus diverticulum or ductus bump is a congenital variant appearing as a smooth convex bulge along the aortic undersurface at the point of ligamentum arteriosum attachment (Fig 7a). Frequently, a focal calcification at the ductus diverticulum origin is present, providing a helpful clue (Fig 7b).

Aortic spindle is a mild fusiform dilatation of the aorta just distal to the isthmus (Fig 7c). Ductus diverticulum and aortic spindle result in smooth contour variations, forming obtuse margins with the normal aorta.

A vascular infundibulum appears more conical and has a small branch vessel emanating from its apex (Fig 7d). The third intercostal artery, also called the superior or highest intercostal artery, is often more prominent on the right compared with on the left, and its infundibula may be mistaken for a ductus remnant or isthmic injury (58).

Pathologic Mimics.—Noncalcified and partially calcified atheroma can also mimic MAI entities, specifically small intimal flap, small thrombus, or even controversial intramural hematoma, at multidetector CT evaluation of acute trauma (Fig 8). They less commonly present a diagnostic challenge with diffuse atheromatous aortic changes, but a lack of prior imaging in the setting of trauma can be challenging for differentiation in nondiffuse atherosclerosis. Requesting prior cross-sectional studies along with follow-up imaging and correlation for the signs of other injuries might aid management.

Natural History of MAI

Significantly increased depiction of MAI owing to improved CT technology and increased utilization has allowed studies of larger cohorts to be conducted over the last several years. Investigators report spontaneous healing of MAI within the first 4–8 weeks, with only 10%–15% of cases persisting or progressing during the follow-up interval (Fig 10) (7–12,19,21–25). Variation in the reported range of resolution (55%–85%) is largely due to short- to midterm follow-up data variation (7–12,19,21–25).

MAI in a 55-year-old woman who was an unrestrained driver in a motor vehicle collision and presented with stable hemodynamic parameters. (a, b) Sagittal (a) and axial (b) CT angiograms show multifocal MAI (arrows) in the descending aorta. (c, d) Sagittal (c) and axial (d) multidetector CT angiograms obtained 4 weeks later demonstrate complete resolution of the imaging findings (arrows). — Figure 10a. MAI in a 55-year-old woman who was an unrestrained driver in a motor vehicle collision and presented with stable hemodynamic parameters. **(a, b)** Sagittal **(a)** and axial **(b)** CT angiograms show multifocal MAI (arrows) in the descending aorta. **(c, d)** Sagittal **(c)** and axial **(d)** multidetector CT angiograms obtained 4 weeks later demonstrate complete resolution of the imaging findings (arrows).

In addition, a recent systematic review of 74 studies of BTAI reported a progression rate of 3.4%, with a need to perform salvage therapy in only one out of 146 patients with MAI (10). Progression (defined as an increasing grade of the aortic injury) was found to be associated with other injuries, an overall higher degree of multitrauma, advanced age, and persistent symptoms such as chest pain or unstable blood pressure (10).

The location of MAI may play a role in the difference in resolution and the course. The vast majority of data collected about MAI are focused on thoracic aortic injury, with the most frequent location of MAI at the aortic isthmus. Although nonisthmic MAI is believed to manifest similarly, data specifically pertaining to the natural history of these injuries are limited.

A recent study by Sabra et al (59) showed that BTAI or MAI in the distal descending aorta tends to be a lower grade and requires intervention less frequently. These injuries were associated with higher mortality because of higher overall trauma severity, specifically when there was involvement of the spine (59). At the same time, Shalhub et al (45) found that six out of 28 abdominal MAIs (21.4%) involved the perirenal abdominal aorta, and all but one resolved completely at 72-hour multidetector CT follow-up (45).

To our knowledge, the interesting concept of downstream emboli from MAI injuries has been examined only by Gunn et al (35) (Fig 11). It is plausible that an intimal breach in a turbulent flow environment of the aorta can pose a risk for dislodgement of intimal thrombi, which can embolize to downstream capillary beds. The report by Gunn et al found new renal or splenic infarcts arising in four out of 23 cases of MAI (17%) (35). The full impact of this new area of evaluation of MAI on overall mortality is yet to be defined.

Downstream emboli with MAI in a 14-year-old girl who was an unrestrained driver in a motor vehicle collision. (a, b) Sagittal (a) and axial (b) multidetector CT angiograms show an isthmic MAI manifesting as an intraluminal thrombus (arrow). (c) Coronal multidetector CT angiogram obtained through the abdomen shows multiple small left renal infarcts (arrowheads). The results of laboratory tests showed no associated renal dysfunction. — Figure 11a. Downstream emboli with MAI in a 14-year-old girl who was an unrestrained driver in a motor vehicle collision. **(a, b)** Sagittal **(a)** and axial **(b)** multidetector CT angiograms show an isthmic MAI manifesting as an intraluminal thrombus (arrow). **(c)** Coronal multidetector CT angiogram obtained through the abdomen shows multiple small left renal infarcts (arrowheads). The results of laboratory tests showed no associated renal dysfunction.

Current and Evolving Management of MAI

In 2011, the SVS designated a small intimal tear as grade I (minimal injury) and recommended “expectant management with serial imaging” (32). Intramural hematoma and pseudoaneurysm were considered surgical entities, representing grade II and III injuries, respectively (Fig 12).

Figure 12. Classifications for BTAI that include MAI. The SVS classification (31) was introduced in 2009 and is based on the morphologic severity of aortic wall disruption. It remains the most widely used classification system. The Vancouver classification (20) (introduced in 2012) and the Harborview classification (18) (introduced in 2016) have incorporated additional details, such as size and stability, on nonoperative treatment of small pseudoaneurysm and intramural hematoma without external contour deformity, allowing improved patient selection for optimal treatment. The Vancouver classification has better reproducibility, and the Harborview classification is tailored to help guide management. NA = not applicable.

While the classification scheme reflected progression of injury through the aortic wall, it did not address heterogeneity of the lesions by their size, morphology, and location, and it did not help guide treatment. Moreover, multiple definitions of BTAI were proposed with variable inclusion of injury morphology and arbitrary size limits based on surgeon and institutional thresholds (8,55). At least five different definitions of MAI exist in the contemporary literature, complicating detailed comparison of management and outcomes (8,11,30,35,55,60).

More recently, data from nonoperative management of grade II injuries (intramural hematoma) that could not be repaired emergently because of concomitant severe injuries have shown favorable spontaneous resolution with medical therapy, in turn leading to its wide acceptance with its inclusion in the MAI spectrum (18–20). Also, the SVS simplified their grading system in the most recent revision by combining grade I and grade II injuries as mild and leaving grade III (pseudoaneurysm) and grade IV (rupture) injuries separate as moderate and severe, respectively (18,19).

However, this change still does not address small pseudoaneurysms (usually less than 1 cm) that also are shown to be stable during follow-up and are unlikely to be linked with lesion-specific mortality (26–28). Currently the treatment algorithm suggests repair of all pseudoaneurysms regardless of size, given the potential for devastating complications resulting from expansion and rupture.

While most specialists treating aortic injury tend to follow the SVS guidelines, two other grading systems have been proposed. The Vancouver classification is aimed at improving the modest reproducibility of the SVS classification (20). The Harborview classification is aimed at aligning the injury grades with the current management algorithms (18). The management-focused approach of the Harborview classification system has made it the second most commonly used system (Fig 12) (24). Despite conceptual controversies concerning all three classification systems, MAI represents the lowest aortic injury grade with the most favorable prognosis.

The current recommendation for MAI is nonoperative management (7–12,19,21–25). The SVS guidelines for BTAI recommend expectant management with serial imaging for grade I injuries, while recommending that grade II–IV injuries be repaired operatively. These recommendations are based on the initial study by Azizzadeh et al (31), which demonstrated that most of these intimal injuries heal spontaneously (32).

In a recent retrospective study by Paul et al (23), 13 patients with MAI had zero aortic-related complications with nonoperative management, with an average of 1.6 follow-up CT angiographic studies per patient. Osgood et al (10) evaluated results of nonoperative management for grade I and grade II injuries with a mean follow-up of 74 days, with 55% of cases resolving and 40% of cases demonstrating stability. A contemporary study by Spencer et al (22) with 11-year follow-up of 71 patients with BTAI identified 30 patients with grades I and II injuries. Two out of 16 patients randomized to the nonoperative management group showed progression of aortic injuries at follow-up CT, with none requiring operative intervention (22).

The nonoperative management strategy for MAI primarily consists of blood pressure and heart rate control. Effective and uninterrupted therapy has shown to drastically decrease the risk of progression from 12% to 1.5% (61). Patients are generally admitted to an intensive care or telemetry unit, where intravenous infusion of β-blocking agents, calcium channel blockers, or vasodilators (or a combination thereof) is administered to achieve a heart rate goal of less than 70–80 beats per minute and a systolic blood pressure goal of less than 110–120 mm Hg. A transition to oral agents is done as soon as possible. Antiplatelet therapy is also recommended for intimal injuries, especially those associated with thrombus. However, this must be done with consideration for the risk of bleeding from other associated injuries (10).

Repeat CT angiography is the key to evaluation of nonoperative management failure in MAI. However, there is no consensus on the timing of the repeat imaging, with most studies demonstrating an average of 1.6–2.7 follow-up CT angiographic examinations (10,18,21,23,35). Some advocate against imaging follow-up of MAI for grades I and II, while others suggest follow-up at 6 months, 12 months, and then annually for persistent but stable cases (8,9,17,25,62). Our practice is to repeat ECG-gated CT angiography at 48–72 hours postinjury and then again at 1 month. Should the imaging or clinical picture of the patient demonstrate progression of the grade of aortic injury, operative management may then be needed.

Treatment of MAI from the Surgical Perspective

Very few scenarios exist in which surgical intervention is required for MAI. An example is when the patient has concomitant injuries that prevent lowering of the heart rate and blood pressure, such as severe cerebral or spinal injuries that require increasing cerebral or spinal perfusion pressures, respectively (16,63). In these cases, a coordinated discussion between associated specialists should occur to determine the best course of action. A minimally invasive thoracic endovascular aortic repair (TEVAR) can be performed by placing a stent-graft to cover the intimal injury and allow the elevated heart rate and blood pressure typically required to improve neurologic perfusion pressures (Fig 13).

TEVAR management of MAI and small pseudoaneurysm. (a–c) Axial CT angiogram (a) in a 42-year-old man involved in a motor vehicle collision shows a small intimal flap (arrow). (b) Axial CT angiogram obtained 3 weeks later shows the small intimal flap (arrow) to be stable. However, the patient reported persistent chest pain, which prompted TEVAR stent-graft. (c) Axial CT angiogram shows the TEVAR stent-graft (arrowheads). The patient had complete resolution of symptoms after surgery. (d, e) Sagittal CT angiogram (d) and volume-rendered reconstruction (e) in a 37-year-old man show a small (less than 1 cm) pseudoaneurysm (arrow) that was associated with a small mediastinal hematoma (not shown) but was otherwise asymptomatic. The pseudoaneurysm underwent semielective TEVAR repair (based on SVS guidelines) on day 8. (f, g) Sagittal CT angiogram (f) and volume-rendered reconstruction (g) depict the TEVAR stent-graft (arrowheads) that was placed without intraoperative or postsurgical complications. — Figure 13a. TEVAR management of MAI and small pseudoaneurysm. **(a–c)** Axial CT angiogram **(a)** in a 42-year-old man involved in a motor vehicle collision shows a small intimal flap (arrow). **(b)** Axial CT angiogram obtained 3 weeks later shows the small intimal flap (arrow) to be stable. However, the patient reported persistent chest pain, which prompted TEVAR stent-graft. **(c)** Axial CT angiogram shows the TEVAR stent-graft (arrowheads). The patient had complete resolution of symptoms after surgery. **(d, e)** Sagittal CT angiogram **(d)** and volume-rendered reconstruction **(e)** in a 37-year-old man show a small (<1 cm) pseudoaneurysm (arrow) that was associated with a small mediastinal hematoma (not shown) but was otherwise asymptomatic. The pseudoaneurysm underwent semielective TEVAR repair (based on SVS guidelines) on day 8. **(f, g)** Sagittal CT angiogram **(f)** and volume-rendered reconstruction **(g)** depict the TEVAR stent-graft (arrowheads) that was placed without intraoperative or postsurgical complications.

TEVAR has almost completely replaced the open surgical approach in the last several decades, given its far superior mortality and morbidity profile (eg, low stroke and paraplegia rates) and high technical success (16,64). The complication rate from TEVAR in overall BTAI management is reported at 3%–18% (16,64). While urgent (performed in less than 24 hours) TEVAR is required for patients in an unstable condition with aortic lesions manifesting with external contour abnormalities (pseudoaneurysm regardless of the size, SVS grade III injury and complete aortic transection, or SVS grade IV injury), patients with grade I or II injuries may be chosen for delayed (performed after 24 hours) or semielective (performed after 15 days) repair, depending on the severity of concomitant injuries.

Pre-TEVAR CT angiography is used for appropriate sizing of the endograft. In acutely injured patients, stent-graft oversizing is limited to approximately 10% and certainly no greater than 20%. In a patient with acute trauma, if there is any concern for decreased intravascular volume contributing to undersizing of the true aortic diameter, then intravascular US can be used to help gauge diameter. However, undersizing is generally not a problem given that trauma remains a condition affecting primarily patients in younger age ranges with smaller native aortic diameters (65–67). A significant proportion of patients with MAI require coverage of the left subclavian artery ostium in order to stent the injury, with reported rates as high as 40%. However, this technique is usually well tolerated in this young patient demographic, owing to preserved collateral sufficiency (9,24,68). Periprocedural anticoagulation therapy with the associated risk of bleeding in patients with polytrauma is also proven to be low when using low-dose heparin as an anticoagulative agent (69).

Small Aortic Pseudoaneurysm and Large Intimal Flap

Currently, the SVS does not distinguish between small and large pseudoaneurysm and recommends endovascular repair regardless of size (32). A growing body of literature suggests that small aortic pseudoaneurysms may be successfully nonoperatively managed on the basis of reported stability (26–28). Another controversial lesion is a large intimal flap without external aortic wall abnormality, and there are current debates about whether it belongs to a mild nonoperative type of injury or represents a moderate degree of aortic injury and should be repaired (18,25). However, most institutions, including our own, tend to perform delayed or semielective TEVAR of a pseudoaneurysm or large intimal flap on the basis of current SVS recommendations. This approach may change in the future as more data are collected and results from the Aortic Trauma Foundation national registry are finalized. As of January 2020, the registry has accrued 537 cases from multiple centers across North America, with the main goal of helping standardize patient selection for the most appropriate treatment (70).

Conclusion

MAI represents a mild form of BTAI with a favorable prognosis and is traditionally defined as a small intimomedial abnormality with no external contour irregularity, most commonly occurring at the aortic isthmus. The imaging spectrum of manifestations of MAI may expand in the future as more long-term data are collected regarding the course of and survival rates for several other traumatic aortic lesions. High-resolution multidetector CT with multiplanar reconstructions and awareness of imaging findings are essential for the diagnosis. Asymptomatic resolution of this imaging abnormality with conservative management is expected in the majority of patients. The consensus on the imaging follow-up timeline and the need for intervention in persistent cases is currently not defined.

Acknowledgment

Illustration support by Tom Dolan, MS, Department of Medical Illustrations, University of Kentucky.

Presented as an education exhibit at the 2019 RSNA Annual Meeting.

For this journal-based SA-CME activity, the authors, editor, and reviewers have disclosed no relevant relationships.

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Article History

Received: Apr 5 2020
Revision requested: May 8 2020
Revision received: July 3 2020
Accepted: July 9 2020
Published online: Oct 02 2020
Published in print: Nov 2020

Vol. 40, No. 7

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Abbreviations

Abbreviations:
BTAI	blunt traumatic aortic injury
ECG	electrocardiographic
MAI	minimal aortic injury
SAI	significant aortic injury
SVS	Society for Vascular Surgery
TEVAR	thoracic endovascular aortic repair

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