AJR 2001; 177:719-720
© American Roentgen Ray Society
Paradoxical Attenuation Values in Acute Aortic Dissection
Max F. Ryan and
Paul Hamilton
Sunnybrook and Women's College Health Sciences Centre Toronto,
Ontario, Canada M4N 3M5
Acute Stanford type A thoracic aortic dissections, which account for up to
70% of all dissections, are well described in the literature. Enhanced CT is
useful in both diagnosis and follow-up of this aortic disease
[1]. Typically, the literature
describes characteristic locations of the intimal flap as well as early
contrast enhancement of the true lumen followed by opacification of the false
lumen. With rapid helical CT techniques, the attenuation values within the
true and false lumens of aortic dissection may be the opposite of what is
described in the literature as being found in the "classic" case.
These previous descriptions were based on the use of conventional CT
techniques in which multiple images were usually acquired at three different
levels with three different injections
[2]. Attenuation values should
not be the only considerations when deciding which is the false and true
lumen.
We recently reviewed a case of a previously healthy 79-year-old man who
presented with a typical history of dissection—a sharp tearing
intractable posterior chest pain that radiated to the lower back. The patient
had signs of congestive heart failure but not pericardial tamponade.
Helical CT scans obtained using IV contrast material confirmed an acute
Stanford type A aortic dissection; spontaneous longitudinal separation of
aortic intima and adventia by circulating blood had opened an access to the
media of the aortic wall and had split it in two (Figs.
1A,1B,1C).
Both lumina were patent and separated by an intimal flap that extended the
entire length of the aorta and into the common iliac arteries where there was
occlusion of the false lumen on the right side. Retrograde dis-section to the
aortic valve was noted associated with pericardial fluid.
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Fig. 1A. —79-year-old man with acute aortic dissection (Stanford type
A). CT scan obtained at level of aortic arch shows higher contrast density in
true lumen (T, 374 H) and lower attenuation in false lumen (F, 318 H).
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Fig. 1B. —79-year-old man with acute aortic dissection (Stanford type
A). CT scan obtained at level of main pulmonary artery shows higher contrast
density within false lumina (F) of both ascending (345 H) and proximal
descending thoracic aorta (350 H) compared with true lumina (T, 306 and 322
H).
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Fig. 1C. —79-year-old man with acute aortic dissection (Stanford type
A). Coronal oblique maximum-intensity-projection CT reconstruction of aortic
arch shows density differences between false (F) and true (T) lumina.
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In this patient, however, we observed a feature that, to our knowledge, has
not been previously described: Paradoxical attenuation values in the false
lumen were found, which led to some confusion in interpretation. The
attenuation values in the false lumen were relatively hyperdense compared with
the true lumen near the aortic root (Fig.
1B), which is the opposite of what is classically described.
Moreover, the attenuation values in the false lumen were lower in the aortic
arch (Fig. 1A) but were higher
in the descending thoracic and abdominal aorta relative to the true lumen
(Fig. 1C).
The explanation for this finding is related to the technique (timing of
image acquisition after IV contrast enhancement) and flow dynamics within the
separate lumina. For example, at the aortic arch level (at which we acquire
images before acquiring those of the ascending aorta), the faster flow of the
contrast media resulted in greater opacification of the true lumen relative to
the false lumen. However, by the time images of the proximal ascending aorta
were later acquired, washout from the faster flowing true lumen had occurred,
leading to greater attenuation values in the slower flowing false lumen.
Moreover, attenuation values in the slower flowing false lumen of the
descending aorta were of significantly higher attenuation than those in the
true lumen.
Although the reported in-hospital mortality rate is nearly 35% for acute
aortic dissection [3], the
immediate surgical graft reinforcement of the aortic wall fortunately
prevented rupture and progressive aortic valve insufficiency, and our patient
is alive and well 1 month after the surgery.
References
-
Small JH, Dixon AK, Csulden RA, Flower CDR, Housden BA. Fast
computed tomography for aortic dissection. Br J Radiol
1996;69:900
-905[Abstract/Free Full Text]
-
Suchato C, Pekanan P, Singjaroen T, Sereerat P. Indication of
dissecting aortic aneurysm on non-contrast computed tomography. J
Comput Assist Tomogr
1980:1;115
-116
-
Sebastia C, Pallisa E, Quiroga S, Alvarez-Castells A, Dominguez R,
Evangelista A. Aortic Dissection; Diagnosis and Follow-up with Helical
Computed Tomography. RadioGraphics
1999:1;45
-60
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