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Aortic Dissection

CT Features that Distinguish True Lumen from False Lumen

¹ Department of Radiology, University of Michigan Health Center, Box 0030, 1500 E. Medical Center Dr., Ann Arbor, MI 48109-0030.
² Zuedema Program for Surgical Core Outcomes Research and Evaluation, University of Michigan Health Center, Ann Arbor, MI 48109.
³ Department of Surgery, Section of Cardiothoracic Surgery, University of Michigan Health Center, Ann Arbor, MI 48109.

Received November 15, 2000; accepted after revision January 16, 2001.

 
Address correspondence to L. E. Quint.

Abstract

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OBJECTIVE. The purpose of this study was to determine which CT findings are reliable indicators of the true or false lumen in an aortic dissection.

CONCLUSION. The beak sign and a larger cross-sectional area were the most useful indicators of the false lumen for both acute and chronic dissections. Features generally indicative of the true lumen included outer wall calcification and eccentric flap calcification. In cases showing one lumen wrapping around the other lumen in the aortic arch, the inner lumen was invariably the true lumen.

Introduction

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Acute aortic dissection is readily diagnosed using CT scanning, with the reported diagnostic accuracy ranging from 88% to 100% [1,2,3]. After making a diagnosis of aortic dissection, it is important for the radiologist to determine the luminal origins of branch vessels in patients who may undergo surgical repair or percutaneous treatment with endografts [4,5,6,7,8]. Although some authors have written about compression of the true lumen by the false lumen [6], few authors have directly addressed differentiation of the true and false lumens using imaging [9, 10]. In most CT examinations, the identity of the true lumen may be determined by its continuity with an undissected portion of aorta. However, in some patients, this continuity is difficult to appreciate. In other patients, it may be impossible to establish continuity because only a portion of the aorta has been scanned or because the entire aorta is dissected. In these circumstances, it may be useful to base the distinction between true and false lumens on other imaging findings. The purpose of this study was to determine whether certain CT findings are reliable indicators of the true or false lumen in aortic dissections.

Materials and Methods

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Thoracic surgery records were reviewed to identify all patients who were seen at our institution for acute or chronic aortic dissection between January 1993 and May 2000. Patients who underwent contrast-enhanced CT and had involvement of the descending thoracic aorta were included in this study. Patients were excluded if they had undergone previous surgery or endograft placement for the dissection or if complete (or near complete) thrombosis of the false lumen was seen at CT. Fifty-one patients were identified in this manner.

Imaging
All but six patients underwent helical CT imaging at our institution using a dedicated aortic protocol with a 2.5- to 5-mm-section collimation and a 1.3- to 5-mm-section spacing (the technique varied according to type of helical scanner used). Between 120 and 200 mL of iodinated contrast material was IV administered at a rate of 3-4 mL/sec. Patients were scanned from the aortic arch to the aortic bifurcation (superior to inferior). Three patients had contrast-enhanced CT examinations at our institution using a conventional (nonhelical) scanner, and three others had contrast-enhanced CT performed at outside institutions using various techniques.

Chronic dissections were defined as those persisting more than 2 weeks after the acute event. If more than one CT examination was available for a particular patient, the earliest acute scan and the latest chronic scan were used for the purposes of this study.

Hard copies of the axial images were retrospectively reviewed independently by two experienced radiologists who were unaware of the acute versus chronic nature of each patient's disease. When there was disagreement between the two CT reviewers' interpretations of an individual feature, a consensus was reached via joint reevaluation of the images. Each scan was evaluated for the presence of the following imaging features on one or more images in each lumen of the descending aorta: the beak sign, outer wall calcification, intraluminal thrombus, eccentric flap calcification, and cobwebs. The beak sign was defined as the presence of an acute angle between the dissection flap and the outer wall; the space formed by the acute angle could be filled with high-attenuation material (contrast-enhanced blood) or low-attenuation material (hematoma). The lumen in which this feature was predominant was identified, after taking into account all the images in the examination. Eccentric flap calcification in one lumen was called when the side of the dissection flap facing that lumen contained calcification, whereas the side of the flap facing the other lumen was of soft-tissue attenuation without apparent calcification. Cobwebs were defined as thin, linear radiolucent filling defects in the lumen that were attached to the wall at one end; the other end could be attached to the flap or the wall or have no attachment.

In addition, the following features of each lumen were subjectively evaluated at one quarter, one half, and three quarters of the distance along the dissected length of the descending thoracic aorta and abdominal aorta: relative cross-sectional area and the presence and direction of flap curvature. If the central portion of the flap was curved toward one lumen, at a particular level, then the flap curvature was coded as present for that lumen. If the flap was flat, then no curvature was coded. These features were evaluated at three different points along the length of the dissected portion of the aorta because of the possibility that results might differ depending on location.

For patients with involvement of the transverse portion of the aortic arch and proximal descending aorta (depicted as a long-axis view on a transverse CT image), one lumen sometimes appeared to wrap partially or completely around the other lumen, occasionally even giving the appearance of three separate lumens. In these cases, the identity of the inner lumen was noted.

To determine which lumen was true and which was false, we followed the two lumens in the descending aorta proximally and distally on the CT scans. The lumen in the dissected portion of aorta that was continuous with the lumen of an undissected portion of aorta was deemed to be the true lumen. If a lumen ended in a blind sac, it was deemed to be the false lumen. These standards were used to define the lumens in all patients except four, in whom continuity could not be determined at CT. In these four patients (all with an acute type A dissection), the boundary between the lumens was blurred in the aortic root. In two of the four patients, the determination was made via comparison with a later postoperative scan. On the postoperative study, the lumen within the remaining native dissected portion of aorta that was continuous with the aortic interposition graft was deemed to represent the true lumen. In one of the four patients, one lumen completely surrounded the other lumen; the inner lumen was deemed to represent the true lumen, and this was confirmed via comparison with surgical notes. Luminal identification in the fourth patient was also made via comparison with detailed surgical notes. Differentiation between the true and false lumens was readily made at open aortotomy because of the distinctive appearance of the intima in the true lumen.

Approval of the institutional review board was obtained; informed consent from patients was waived.

Statistics
Frequencies of CT findings were compared between true and false lumens using the McNemar test. This test was used because of the paired nature of the data and the correlation between the result from the true lumen and the false lumen for a given subject. Frequencies of CT findings were compared between acute and chronic dissections using Fisher's exact test. Eight patients had scans of both acute and chronic dissections; for statistical analysis, these patients were excluded from the acute group, because it was the larger of the two groups. The level of significance for each test was a p value of less than 0.05.

Results

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Fifty-one patients were included in the study. There were 21 type A dissections and 30 type B dissections. Eight patients had CT examinations during both the acute and chronic phases of disease (one type A and seven type B), leading to a total of 59 CT examinations (35 of acute dissections and 24 of chronic dissections). For patients with chronic dissection, the mean age of the dissection was 23 months (range, 2-84 months).

CT findings for the true and false lumens in acute and chronic cases are delineated in Table 1. The beak sign and cobwebs were seen only in the false lumen (p < 0.05 for the beak sign, p > 0.05 for cobwebs) (Figs. 1 and 2). However, whereas the beak sign was present on all scans, cobwebs were present in only 9% of the scans of acute dissections and 17% of those of chronic dissections. Outer wall calcification (Figs. 3 and 4) was never present in the false lumen on scans of acute dissections (p < 0.05), although it was occasionally present on scans of chronic dissections (17% of the cases) (p < 0.05). Intraluminal thrombus was more common in the false lumen (46% of the acute cases, 83% of the chronic cases) compared with the true lumen (6% of the acute cases, 4% of the chronic cases) (p < 0.05) (Fig. 3). Except for one case, eccentric flap calcification was only seen on the true lumen side of the flap (p < 0.05) (Fig. 5).

View larger version (111K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —41-year-old man with acute aortic dissection. CT scan obtained at one-quarter distance along length of dissected portion of aorta shows descending aortic dissection flap (arrows) that is curved toward false lumen (F). Beak sign (arrowheads) is present in false lumen. Note that false lumen area is larger than true lumen area.

View larger version (103K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —51-year-old woman with chronic aortic dissection. CT scan obtained at one-half distance along length of dissected portion of aorta shows flat dissection flap. False lumen beaks are filled with lowattenuation thrombus (arrowheads). Faintly visualized cobweb (arrows) is present in false lumen (F).

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3. —65-year-old woman with chronic aortic dissection. CT scan obtained at one-quarter distance along length of dissected portion of aorta shows flat dissection flap. Outer wall calcification (straight arrow) is present in true lumen (T). Thrombus (arrowheads) is present in false lumen. Curved arrow indicates thrombus within false lumen beak.

View larger version (163K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —76-year-old man with chronic aortic dissection. CT scan obtained at three-quarters distance along length of dissected portion of aorta shows flat dissection flap. Outer wall calcification (arrows) and thrombus (asterisk) are present in false lumen (F). T = true lumen.

View larger version (144K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5. —59-year-old man with chronic aortic dissection. CT scan obtained at one-quarter distance along length of dissected portion of aorta shows flat dissection flap. Eccentric flap calcification (arrow) is present along true lumen side of flap. Notice that false lumen (F) contains thrombus (arrowheads) and is larger than true lumen at this level.

At one quarter of the distance along the dissected length of the aorta, for both acute and chronic cases, the larger lumen was usually the false lumen (85% of the acute cases, 83% of the chronic cases) (p < 0.05) (Figs. 1 and 5). For acute dissections, the flap was most commonly curved toward the false lumen at this level (56% of the cases) (p < 0.05), although it was often flat (38% of the cases), and occasionally curved toward the true lumen (6% of the cases) (Figs. 1 and 6). For chronic dissections, the flap was usually flat (75% of the cases) and occasionally curved toward the false lumen (25% of the cases) (p < 0.05).

View larger version (147K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6. —65-year-old man with acute aortic dissection. CT scan obtained at one-quarter distance along length of dissected portion of aorta shows dissection flap that is curved toward true lumen. Anterior false lumen beak (arrowheads) is partially opacified and partially filled with thrombus. F = false lumen.

At one half of the distance along the dissected length of the aorta, for both acute and chronic cases, the larger lumen was almost always the false lumen (94% of the acute cases, 96% of the chronic cases) (p < 0.05). For acute dissections, the flap was almost equally likely to be flat (36% of the cases), curved toward the false lumen (33% of the cases), or curved toward the true lumen (30% of the cases) (p > 0.05) (Fig. 6). For chronic dissections, the flap was usually flat (67% of the cases) (Fig. 2) and occasionally curved toward the false lumen (29% of the cases) (p < 0.05); in only one case was it curved toward the true lumen.

At three quarters of the distance along the dissected length of the aorta, the larger lumen was always the false lumen in chronic cases and almost always the false lumen in acute cases (91% of the cases) (p < 0.05). For acute dissections, the flap was nearly equally likely to be flat (33% of the cases), curved toward the false lumen (39% of the cases), or curved toward the true lumen (27% of the cases) (p > 0.05). For chronic dissections, the flap was always flat at this level.

Statistically significant differences (p < 0.05) between acute and chronic dissections were as follows. Chronic dissections were more likely to show eccentric flap calcification of the true lumen, outer wall calcification of the false lumen, and intraluminal thrombus of the false lumen compared with acute dissections. Acute dissections were more likely to show a curved flap (as opposed to a flat flap) at one-quarter and three-quarters distance compared with chronic dissections.

Forty-five of 51 patients had involvement of the transverse portion of the aortic arch and proximal descending aorta. In 24 of the 45 patients, one lumen appeared to wrap around the other lumen. In all 24 patients showing this finding, the inner lumen was the true lumen (Fig. 7).

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7. —69-year-old woman with acute aortic dissection. CT scan obtained at level of transverse aortic arch shows that outer false lumen (F) wraps around inner true lumen (T). Dissection flap extends into innominate artery. Note cobweb in false lumen (arrow) and bilateral pleural effusions (P).

Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
It is important to determine the luminal origins of branch vessels before endograft or bare stent placement within the aorta or within the branch vessels, to plan optimal deployment and avoid end-organ ischemia. In most cases, the true lumen may be identified on CT by its continuity with an undissected portion of aorta. Sometimes, however, this rule is difficult to apply, particularly in cases with involvement of the aortic root and especially in those with circumferential dissection of the root. In addition, patients occasionally undergo only abdominal CT (e.g., in the setting of abdominal pain), and therefore the true lumen cannot be traced back to the aortic root. Our study showed several additional CT imaging features that were helpful in differentiating the true and false lumens. The best feature was the beak sign; it was only seen in the false lumen, and it was present in all cases, both acute and chronic. Moreover, it was generally easily identified on CT. The beak sign is the cross-sectional imaging manifestation of the wedge of hematoma that cleaves a space for the propagating false lumen and that is present microscopically in all dissections [10]. Cobwebs (ribbons of media that are incompletely sheared off by the dissection) were also present only in the false lumen; however, this feature was of limited usefulness because of its rarity. This finding is in agreement with previous studies that have reported cobwebs as a specific, but relatively insensitive, indicator of the false lumen [9, 10]. The lower prevalence of cobwebs in our study compared with that reported in a previous intravascular sonographic study [10] may reflect, in part, the greater spatial resolution of intravascular sonography relative to CT.

Outer wall calcification always indicated the true lumen on scans of acute dissections. In chronic dissections, however, this finding was somewhat unreliable because the false lumen lining may endothelialize and subsequently calcify in long-standing dissections [11, 12], leading to calcification in the outer wall of the false lumen. Although, theoretically, this phenomenon could also lead to calcification on the false luminal surface of the flap in chronic dissections, no such cases were identified in our study. Intraluminal thrombus was a fairly good marker for the false lumen, although in patients with an underlying aneurysm, thrombus may be present in the true lumen as well. False lumen thrombus was significantly more frequent in chronic dissections than acute dissections. Thrombus formation in acute dissection is due partly to the thrombogenic exposed media, which lines the false lumen, and stasis in low-flow segments of the false lumen. In cases of chronic dissection, the false lumen remains prone to thrombus formation because of stasis related to aneurysmal enlargement and because of atheromatous changes in the neointima, which can outstrip atheromatous degeneration in the native intima.

In an in vitro model of dissection with two patent lumens, one published study showed that the true lumen collapses and the false lumen distends even when the pressure gradient across the dissection flap is zero [13]. Moreover, for in vivo aortic dissections in humans, pressure in the false lumen frequently exceeds that in the true lumen [8] and superimposes an element of compression onto the underlying true lumen collapse. Thus, it is not surprising that in our study, larger lumen size was a good marker for the false lumen in both acute and chronic dissections. This finding was commonly present, and usually obvious, particularly at one half or three quarters along the distance of the dissected aorta.

The direction of flap curvature was generally not useful in distinguishing the true from false lumen. However, acute dissections usually had some curvature to the flap, whereas chronic dissections usually had a flat flap over much of its length. This appearance of the flap is probably caused by cellular and mechanical changes in the aorta wall. As the flap heals, fibrosis and neointima formation [14] lead to thickening and rigidity of the flap. In addition, we hypothesize that as the false lumen dilates, the lines along which the dissection flap joins the outer wall of the aorta are drawn apart, stretching the flap taut. Our impression is that this flattening of the dissection flap is particularly notable in larger false lumens.

In patients with involvement of the aortic arch, it is common for one lumen to appear to wrap partially or completely around the other lumen, sometimes even giving the appearance of three lumens. The different appearance in the transverse arch, compared with the appearance in the descending aorta, is because of the different angle of sectioning, relative to the long axis of the aorta. In such cases, the inner lumen is invariably the true lumen.

Accurate CT differentiation between the true and the false lumen has previously been relatively unimportant, because surgery has been the mainstay for therapy, and therapeutic decisions have relied predominantly on the presence or absence of involvement of the ascending aorta [15, 16]. However, at the current time, percutaneous treatment methods are maturing and have become more prevalent, partly fueled by advances in CT angiography. Reliable CT findings that differentiate the true and false lumen may become particularly important in assisting the cardiovascular surgeon and radiologist in planning endovascular treatment of dissection. We conclude that the beak sign and large cross-sectional area on contrast-enhanced CT examinations are the most useful indicators of the false lumen in classical acute and chronic aortic dissection. The pattern of mural calcification, presence of intraluminal thrombus or cobwebs, and wraparound feature in the transverse arch are less common and less reliable identifiers of the true and false lumens.

References

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