Aortic dissection: Identification of entry site with CT virtual intravascular endoscopy
1 Department of Medical Imaging, Shandong Medical
College, Jinan, Shandong, People�s Republic of China
2 Discipline of Medical Imaging, Department of
Imaging and Applied Physics, Curtin University of Technology, Perth, Western
3 Department of Radiology, Qilu Hospital, Shandong University, Shandong, People�s Republic of China
Aortic dissection is a common vascular disease which has
high morbidity and mortality if it presents with acute onset. Early diagnosis,
characterisation of the type of dissection and identification of intimal tear
(entry site) is important for patient management. CT angiography, especially
with widely-used multislice CT imaging technique, is the method of choice for
diagnosis of aortic dissection. This article presents the additional value of
3D visualisation, using virtual intravascular endoscopy, for assessment of
aortic dissection when compared to conventional 2D views. The article focuses
particularly on identifying the entry site of dissection with the aid of
virtual intravascular endoscopy in three selected cases. It is expected that
the intraluminal findings will assist radiologists to accurately diagnose and
treat patients with suspected aortic dissection. � 2010 Biomedical Imaging
and Intervention Journal. All rights reserved.
Keywords: Aortic dissection, CT, 3D visualisation, virtual
Aortic dissection is a life-threatening condition which
occurs nearly three times as frequently as the rupture of abdominal aortic
aneurysm . It is critical to make a prompt diagnosis of aortic dissection,
as this can decrease mortality and increase the survival rate. Currently,
multislice CT (MSCT) imaging is the method of choice for diagnosis of aortic
dissection due to its high spatial and temporal resolution with nearly 100%
sensitivity and specificity [2, 3]. CT has been reported to be more sensitive
than invasive angiography for diagnosing aortic dissection .
Differentiation of true and false lumen, and
identification of the entry site of aortic dissection is important in planning
percutaneous treatment with endovascular grafts or surgical repair of aortic
dissection [5, 6]. In most of the cases, axial CT imaging is able to identify
the intimal flap which separates the true lumen from the false lumen. However,
this may not be possible in all cases due to variable appearances corresponding
to different types of aortic dissection [2, 7, 8]. Moreover, the
three-dimensional (3D) aortic arch is difficult to assess on an axial 2D plane.
3D virtual intravascular endoscopy (VIE) allows for acquisition of unique
intraluminal views of the blood vessel and it has been previously reported to
be valuable for assessment of aortic aneurysms and endovascular stent grafts
[9-11]. VIE has been shown to offer better understanding of the abnormalities
of aortic arch after endovascular repair ; however, its application in
pre-operative aortic dissection has not been studied before, to the best of the
authors� knowledge. The purpose of this paper is to illustrate VIE findings in
patients diagnosed with aortic dissection. This paper presents different types
of aortic dissection in three sample cases with the aim of exploring the
potential value of VIE for identification of the entry site of aortic
Multislice CT scanning protocols
The spiral CT angiography (CTA) was performed for all
three patients using a dual-source CT scanner (Siemens, Definition, Forchheim,
Germany), and scanning protocol was as follows: beam collimation 64x 0.6 mm
with slice thickness between 0.5-1.0 mm, and pitch 1.0 with reconstruction
interval of 50% overlap. 100 ml non-ionic contrast medium (Visipaque) was
injected intravenously at a flow rate of 4-6 ml/s via antecubital vein with a high-pressure
injector, followed by 40-60 ml of normal saline chasing. To account for
individual cardiac output, bolus triggering technique was used with the region
of interest placed at the proximal descending thoracic aorta and triggering threshold
was set at 120HU to initiate the scan. CT angiography was completed with a
single breath-hold technique and the length of scan required for acquisition of
data was determined by the total scan length in each patient.
Generation of virtual intravascular endoscopy images
Multislice CT volume data were converted from original
DICOM (Digital Imaging and Communication in Medicine) images using Analyze 7.0
(www.Analyzedirect.com). Similar to previous studies, a CT number thresholding
technique was used to produce virtual intravascular endoscopic (VIE) views of
the aorta and its branches, as well as abnormal changes [9, 10]. This was
determined by selecting an appropriate threshold value through measuring the CT
attenuation at the aorta (either ascending or descending aorta). An upper
threshold of 200-300 HU was applied to remove the contrast-enhanced blood from
the aorta while keeping the artery wall intact without luminal disruption.
Orthogonal views were used to assist VIE identification of the true, false
lumen as well as intimal flap and entry site of aortic dissection (Figure 1).
Case report and VIE findings
A 64-year-old patient with a history of chest pain for 2
days involving his back, chest and abdomen underwent CTA for exclusion of
cardiovascular disease. Stanford type B aortic dissection was diagnosed based
on 2D axial images as the entry site was located in the convexity of the aortic
arch distal to the left subclavian artery (Figure 2A). VIE clearly
demonstrated a circular entry site of the dissection inside the medical wall of
the ascending aorta (Figure 2B). The true and false lumens were separated by an
intimal flap, which is clearly shown on both axial and VIE images (Figures 2B
and C). Extending from the aortic arch to the descending aorta, another small
re-entry point was identified by VIE in the aorta (Figure 2D).
In contrast to 2D CT views, VIE accurately identified the
entry site and clearly demonstrated the relationship of aortic dissection to
the surrounding anatomic structures.
A CTA scan was performed on a 39-year-old patient with 1
day of chest pain involving her interscapular region and anterior chest. Stanford
type A dissection was diagnosed on 2D views as aortic dissection arises from
the ascending aorta extending to the descending and abdominal aortic branches
(Figures 3A and B). The ultrasound examination detected an abdominal aortic aneurysm.
VIE showed the entry site of dissection which is located in the ascending
aorta, adjacent to the left ventricle (Figure 3 C). In addition, the intimal
flap which separates the true from false lumens is clearly visualised on VIE
image, as shown in Figure 3D.
This case demonstrates the superiority of intraluminal
appearance provided by VIE as it is able to precisely recognise the location of
the aortic dissection entry site which is close to the left ventricle. It is difficult
to directly locate the entry site or intimal tear of aortic dissection based on
conventional 2D images, especially for type A dissection.
A 65-year-old man presenting with a history of hypertension
and two days of interscapular, chest and abdomen pain underwent a CTA scan. Chest
radiograph showed mediastinal widening, and this suggested aortic artery
disease. Stanford type A dissection was diagnosed since the entry site was located
in the middle section of the ascending aorta based on CT images (Figure 4A),
and this was confirmed by VIE showing a very narrowed true lumen and a long
entry site (Figures 4B and C). The false lumen was apparently larger than the
true lumen which was compressed to a greater extent, leading to a protrusion
sign inside the true lumen, which was clearly visualised on VIE views. The
curved intimal flap formed a ridge towards true lumen and the intimal tear was
located posterior to the intimal flap (Figure 4C).
The communication between true and false lumen was
demonstrated on 2D axial CT images; however, the accurate location of intimal
tear could not be adequately assessed on 2D views. In contrast, VIE possesses
the advantage of being able to confirm the entry site and intimal tear, in
terms of both location and extent, as shown in Figures 4 B and C.
Aortic dissection can be characterised as the splitting of
the aortic wall by high-pressure arterial blood entering the media through an
intimomedial entrance tear. CT imaging has been the method of choice for
diagnosis and characterisation of aortic dissection, due to the high diagnostic
accuracy achieved [2, 3]. However, in some cases, conventional CT views fail to
detect the entry site of aortic dissection due to variable appearances
presented by aortic dissection or the fact that the location of intimal tear
makes it difficult for 2D views to follow [7, 13]. The ability of VIE to provide
intraluminal views offers an advantage over conventional views, and this was
confirmed in a recent report by Louis et al. . This case report
corroborates their findings about the diagnostic value of VIE through
identification of the intimal tear, thus it is believed that VIE could be used
as a complementary tool to 2D CT imaging for accurate evaluation of aortic
Detection of the entry site in aortic dissection is
important for several physiologic and therapeutic reasons [6, 14]. It is
imperative to determine whether major aortic arteries originate from true or
false lumens before placement of endovascular grafts or stents because any
branch arteries being supplied by the false lumen may be occluded with an
intervention unless surgically bypassed . VIE not only allows for
generation of static intraluminal views of the aorta and its branches, but also
provides virtual fly-through and navigation of the entire aorta lumen, despite
the presence of a very-narrowed true lumen (video clips, online viewing only).
Therefore, accurate assessment of the involvement of the artery branches by
dissection, in addition to the identification of entry site, can be achieved
with VIE visualisation. Moreover, the extent of the entry site can be further
explored by VIE as shown in case 3.
Although VIE is a visualisation tool that enables
demonstration of intimal flap and entry site of aortic dissection, the image
quality is determined by the original CT data, which depends on the scanning
protocols selected for CTA. With the current CT scanners such as 64-slice or
dual-source CT, high spatial and temporal resolution allow for acquisition of
isotropic volume data which ensures VIE image quality. Generation of VIE images
is affected by the selection of appropriate CT threshold, since artifacts
arising from an inappropriate threshold could interfere with visualisation of
the aortic dissection with regard to intimal flap or intimal tear. Detecting
full-directional information about the dissection should be assisted by
cross-sectional and multiplanar reformatted CT images. Correlation with
orthogonal views is necessary to confirm the exact position of anatomic details
on VIE visualisation.
In conclusion, VIE provides unique intraluminal
information to demonstrate the location and extent of entry site of aortic
dissection. VIE could be used as a complementary tool to conventional 2D CT
views for accurate evaluation of aortic dissection.
Figure 1 Virtual intravascular endoscopy (VIE) demonstration of the intimal flap (arrows) in a dissection aneurysm. Corresponding orthogonal views (axial, coronal and sagittal) confirm the location of VIE position inside the ascending aorta. Three main artery branches arising from the aortic arch can be clearly seen on VIE image which are shown in the top left VIE image.
Figure 2 Stanford type B dissection in case 1 with the dissection arising from the descending aorta distal to the left subclavian artery (arrow in A). Intimal tear was identified on VIE image (B) when viewed from the false lumen. The false lumen is much larger than the true lumen (C). F indicates false lumen and T refers to true lumen.
Figure 3 Stanford type A dissection in case 2 with the dissection originating at the ascending aorta. The intimal flap is clearly displayed on 2D axial and coronal reformatted images (long arrows in 3A and B) with left coronary artery arising from the true lumen (arrowhead). VIE shows the intimal tear which is located at the ascending aorta (arrow in C) proximal to the left ventricle. VIE clearly identifies the intimal flap which separates the true lumen from the false lumen (D). IF indicates intimal flap. T and F refer to true lumen and false lumen, respectively. Short arrows in A indicate the cobweb sign in the false lumen.
Figure 4 Stanford type A dissection in case 3 with communication between the true lumen and false lumen (arrow in A). The true lumen was obviously narrowed due to compression by the false lumen (B), and a protrusion sign was observed on VIE images (C). The intimal tear was identified posterior to the protrusion from the false lumen (D). Thrombus is formed in the false lumen (F).
Movie 1 Virtual fly-through of the thoracic aorta looking at the intimal flap, which separates the true lumen from the false lumen. The three artery branches arising from the aortic arch are located in the true lumen.
Movie 2 Virtual fly-through inside the narrow true lumen. A protruding sign arising from the false lumen is noticed from the anterior artery wall.
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|Received 31 December 2009; received in revised form 24
February 2010, accepted 6 March 2010
Correspondence: Discipline of Medical Imaging, Department of Imaging and Applied Physics, Curtin University of Technology, GPO Box, U1987, Perth, Western Australia 6845, Australia. Tel.: +61-8-9266 7509; Fax: +61-8-9266 2377; E-mail: firstname.lastname@example.org (Zhonghua Sun).
Please cite as: Cao Y, Sun Z, Shang Y, Jiang B, Ma X,
Aortic dissection: Identification of entry site with CT virtual intravascular endoscopy, Biomed Imaging Interv J 2010; 6(3):e22