AJR 2004; 183:109-112
© American Roentgen Ray Society
Intimomedial Rupture: A New CT Finding to Distinguish True from False Lumen in Aortic Dissection
Vibhu Kapoor1,
James V. Ferris and
Carl R. Fuhrman
1 All authors: Department of Radiology, University of Pittsburgh Medical Center,
200 Lothrop St., Pittsburgh, PA 15213.
Received October 30, 2003;
accepted after revision January 14, 2004.
Address correspondence to V. Kapoor.
Abstract
OBJECTIVE. The purpose of our study is to describe a new imaging
finding observed on contrast-enhanced MDCT of discontinuity in the
intimomedial flap seen with aortic dissection that may identify the level of
intimomedial entrance tear and distinguish true from false lumen.
CONCLUSION. Direct visualization of an intimomedial rupture on MDCT
is a valuable cross-sectional imaging sign indicating direction of the
intimomedial entrance tear from true to false lumen in cases of aortic
dissection. The presence of this finding enables reliable distinction of the
true lumen, where the tear is broad-based and erupts into the false lumen,
which receives the evaginating tear. Improved scanning techniques through MDCT
may have enabled recognition of this relatively infrequent, yet highly
specific, finding.
Introduction
Aortic dissection can be a catastrophic event characterized by splitting of
the aortic wall by high-pressure arterial blood entering the media through an
intimomedial entrance tear. Unless diagnosed and treated quickly, ascending
and transverse aortic dissections are frequently fatal, and patients may die
before receiving medical treatment
[1,
2]. Cross-sectional imaging
with MDCT is currently the imaging technique of choice for diagnosis and
evaluation of aortic dissection and has a sensitivity and specificity of
nearly 100% [1,
3]. CT has been shown to be
more sensitive than catheter aortography for diagnosing aortic dissection
[4] and comparable to MRI and
transesophageal echocardiography
[5].
Aortic dissection results in the formation of true and false lumens
separated from each other by a flap of intimal lining and inner layers of
media. Outer layers of the aortic media and adventitia form the outer wall of
the false lumen. Differentiation between true and false lumens is important in
the planning of percutaneous treatment with endovascular grafts or surgical
repair of aortic dissections
[6,
7]. It is critical to identify
the lumen of origin in major branch vessels such as coronary, carotid, renal,
and mesenteric arteries before treatment because viscera supplied by the false
lumen are at risk when the false lumen is spontaneously or surgically
occluded. LePage et al. [8] and
Williams et al. [9] described
CT criteria to distinguish true from false lumens. The most reliable imaging
criterion is the identification of continuity of the true lumen of an aortic
dissection with the lumen of an uninvolved portion of the aorta proximal or
distal to the dissection. However, this may not be possible in all cases
[8], and secondary signs to
distinguish false from true lumens have been helpful
[8,
9]. We describe another
specific, but unreported, direct finding for distinguishing true from false
lumen in aortic dissection: direct visualization of the intimomedial entrance
tear from true to false lumen with MDCT.
Materials and Methods
Between February 2002 and March 2003, we retrospectively reviewed the
records of 148 patients from our radiology database when selecting patients
with CT findings of aortic dissection as a part of an ongoing educational
lecture and exhibit. In the process of review, we detected discontinuity
within an evaginating intraluminal aortic flap and communication between the
true and false lumens at the presumed level of intimomedial tear from the true
to the false lumen. We compared our findings to the accepted noninvasive
standard of assigning true and false lumens, which designates the true lumen
as that which becomes continuous with the lumen of an uninvolved segment of
aorta proximal or distal to the dissection. All CT scans were obtained on a
LightSpeed QX/i MDCT scanner (General Electric Medical Systems) using a
dedicated aortic protocol that involved unenhanced scanning followed by
enhanced arterial scanning from the aortic arch to the bifurcation. Time delay
for the enhanced scanning was achieved using automatic triggering with
contrast-monitoring cursor placed on distal descending thoracic aorta.
Contrast scanning was initiated when attenuation in aorta reached 100 H.
Between 125 and 150 mL of iodinated contrast material (Optiray 350 [ioversol],
Mallinckrodt) was delivered at a rate of 2.5–3 mL/sec during enhanced
scanning. The detector configuration during image acquisition was 4 x
1.25 mm interspaced (high-speed mode) helices at a table speed of 7.5 mm per
tube rotation (0.8 sec). The pitch was 6 (7.5 / 1.25 mm) in the high-speed
mode. The scanning could be completed in approximately 30–40 sec in a
single breath-hold. The helical data were retrospectively reconstructed at a
2.5-mm thickness with no overlap; if postprocessing, such as shaded surface
display, maximum intensity projection, or multiplanar reformatting, was
anticipated then the images were also reconstructed at a 1.25-mm thickness
without overlap. Cardiac gating is not routinely performed for CT of the
aorta.
Images were reviewed retrospectively by two experienced radiologists who
reached a consensus to the presence of the intimomedial rupture finding. This
finding was deemed present when a distinct defect could be identified in the
dissection flap with the free edges of the flap pointing toward the false or
true lumen. The lumen that was continuous distally or proximally with the
lumen of an uninvolved segment of aorta was considered to be the true lumen.
Pathologic correlation was available in one patient.
Results
Fifty-nine of 148 patients reviewed had true aortic dissection, and five of
these 59 patients had positive findings for an intimomedial rupture with
clinical evidence of acute dissection and onset of pain within 48 hr before
CT. The true lumen of their aortic dissection was established using criteria
described in all five patients and proven pathologically in one. Direct
evidence of intimomedial rupture was identified in the thoracic aorta, with
the free edges of the dissection flap uniformly pointing toward the false
lumen (Fig. 1) in all five
patients. The intimomedial tear was present in the ascending aorta in three
patients (Fig. 2A,
2B), at the level of the
transverse aorta (Fig. 3) in
one patient, and in the descending thoracic aorta (Fig.
4A,
4B,
4C,
4D) in the remaining patient.
The aortic dissection from the level of the intimomedial entrance tear
extended both proximally and distally in three patients (Fig.
4A,
4B,
4C,
4D) and only distally in two.
The finding of intimomedial rupture was seen on MRI in one patient who
underwent both CT and MRI (Fig.
4A,
4B,
4C,
4D). Because neither the CT
scans nor the MR images were cardiac gated, evaluation of the dynamic nature
of the dissection flaps was not possible.
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Fig. 1. —61-year-old man with chest pain and acute type A aortic
dissection. Axial enhanced CT scan of ascending aorta shows type A aortic
dissection with intimomedial tear (arrows) entering false lumen (F)
from true lumen (T). DA = descending thoracic aorta, PA = pulmonary
artery.
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Fig. 2A. —63-year-old woman with severe chest and back pain from acute
type A dissection resulting in death 2 days later. Axial enhanced CT scan at
level of ascending aorta shows type A dissection with discontinuous dissection
flap (arrows) pointing to false lumen (F). T = true lumen, S =
superior vena cava.
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Fig. 2B. —63-year-old woman with severe chest and back pain from acute
type A dissection resulting in death 2 days later. Photograph of gross
pathology specimen from corresponding segment of ascending aorta resected at
surgery shows intimomedial tear (arrows) erupting from true (T) into
false (F) lumen. Arrowheads mark adventitia.
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Fig. 3. —72-year-old man with acute type B dissection. Axial enhanced
CT scan at level of aortic arch shows type B dissection with intimomedial
dissection flap (arrows) and evagination of torn ends of flap into
false lumen (F). T = true lumen, asterisk = innominate artery, arrowhead =
left carotid artery.
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Fig. 4A. —47-year-old man with acute chest pain and type B aortic
dissection. Axial enhanced CT scan at level of ascending aorta shows
intimomedial entrance tear (arrows) with central true lumen (T)
erupting into false lumen (F). True lumen was continuous proximally with lumen
of uninvolved ascending aorta (AA), which has similar attenuation.
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Fig. 4B. —47-year-old man with acute chest pain and type B aortic
dissection. Axial enhanced CT scan at level of superior mesenteric artery
(SMA) shows distal extension of dissection flap to abdominal aorta and SMA.
True (arrow) and false (F) lumens within proximal SMA are patent and
show unequal contrast enhancement. Although true obstruction does not exist in
this patient, dissection flap has potential of causing mechanical SMA
obstruction (static obstruction). Right and left renal arteries (not shown)
arise from markedly narrowed true lumen and larger false lumen, respectively.
Decreased right renal perfusion is evident by decreased enhancement of right
kidney (RK) compared with left kidney (LK). P = pancreas.
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Fig. 4C. —47-year-old man with acute chest pain and type B aortic
dissection. Axial (C) and oblique sagittal (D) reformatted
gadolinium-enhanced T1-weighted first-pass MR angiograms (TR/TE, 4.9/1.2) of
thoracic aorta show discontinuity in dissection flap (arrows),
similar to that seen in A, with true lumen (T, C) opening into
false lumen (F) posterolaterally. Ascending aorta (AA), descending thoracic
aorta (DA, D), and pulmonary artery (PA, D) are also
visible.
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Fig. 4D. —47-year-old man with acute chest pain and type B aortic
dissection. Axial (C) and oblique sagittal (D) reformatted
gadolinium-enhanced T1-weighted first-pass MR angiograms (TR/TE, 4.9/1.2) of
thoracic aorta show discontinuity in dissection flap (arrows),
similar to that seen in A, with true lumen (T, C) opening into
false lumen (F) posterolaterally. Ascending aorta (AA), descending thoracic
aorta (DA, D), and pulmonary artery (PA, D) are also
visible.
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Discussion
The distinction between true and false lumens in aortic dissection is
important for several physiologic and therapeutic reasons
[7,
10]. Branch arteries supplied
only by the false lumen are rarely compromised before intervention. However,
branch arteries supplied by the true lumen—even in the absence of static
obstruction (dissection flap involving vessel origin)—may undergo
dynamic obstruction (dissection flap covers vessel origin without entering its
lumen causing obstruction during systole)
[7]. It is imperative to
determine whether major branch vessels originate from true or false lumens
before placement of endovascular grafts or stents
[7,
8,
10], because any branch
vessels being supplied by the false lumen may be occluded with an intervention
unless surgically bypassed. Detection of aortic dissection, distinction of
true from false lumens, and extension of the dissection to major branch
vessels are important when planning vascular access for percutaneous
procedures, particularly if iliac or subclavian arteries are involved, because
groin or axillary catheterization procedures may extend or even rupture
existing dissections within these vessels.
The most reliable direct imaging sign for distinguishing true and false
lumens is the ability to show direct continuity between the true lumen and the
lumen of the uninvolved aortic lumen distal or proximal to the dissected
aortic segment. This may not always be possible because the dissection extends
proximally into the aortic root, the origin of the intimomedial entrance tear
is at the convexity of the aortic arch where the true and false lumens may be
difficult to follow, and limited or incomplete imaging of the aorta in cases
in which dissection was not suspected
[8]. Numerous secondary or
indirect signs for distinguishing true from false lumens with imaging have
been described as diagnostic aids
[3,
6,
8,
9,
11,
12].
We report a helpful, direct sign in cases of aortic dissection:
visualization of the level of intimomedial tear from true to false lumens
using MDCT (Fig. 5) seen in
approximately 8% (5/59) of patients with aortic dissection. MDCT has
facilitated faster scanning times, improved thin-section reconstructions, and
decreased pulsation artifacts in aortic imaging. Although further studies
spanning other cross-sectional techniques and additional pathologic
confirmation will be helpful, use of this finding to identify true and false
lumens showed uniform congruity with the established method of delineating
continuity of the true lumen of an aortic dissection with the lumen of an
uninvolved portion of the aorta proximal or distal to the dissection in all
patients (Fig. 4A,
4B,
4C,
4D) and was pathologically
proven in one patient. Although we observed that the free edges of the
dissection flap uniformly pointed toward the false lumen in all five patients
(Figs. 1,
2A,
2B,
3,
4A,
4B,
4C,
4D), given the dynamic nature
of the dissection flap [7], it
is conceivable the free edges of the tear may point toward the true lumen
during a different phase of the cardiac cycle. Because no patients underwent
cardiac gating during CT or MRI, assessment of dynamic flap motion was not
possible. One patient who underwent both CT and MRI showed an intimomedial
rupture with the free edges of the dissection flap pointing toward the false
lumen with both techniques. However, we suspect that pulsation and other
artifacts will make identification of this finding on MRI more challenging and
less reliable than on CT. CT has the added utility of being more readily
accessible at most centers and is more compatible with the life-support and
monitoring devices that are required by these patients.
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Fig. 5. —Illustration shows utility of intimomedial rupture finding in
distinguishing true from false lumen in aortic dissection. Top left image
shows longitudinal section of aortic lumen; bottom left image is longitudinal
section of aortic wall in profile; and images on right are transverse
cross-sectional images above (A and B), at (C), and below (D) level of
intimomedial entrance tear. True lumen is depicted in red and false lumen, in
pink. At level of tear (C) direction of blood flow is from true to false lumen
with intimomedial flaps pointing toward false lumen.
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Aortic dissections can be acute, catastrophic conditions that require
prompt diagnosis to decrease morbidity and prolong or enable survival.
Differentiation of true from false lumens and depiction of luminal origins of
branch arteries are important before definitive endovascular grafting or
surgical repair. The finding of intimomedial rupture may directly identify the
level of intimomedial entrance tear and aid in the distinction of true from
false lumens in cases of aortic dissection.
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Abstract
Introduction
