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3D Virtual Intravascular Endoscopy of Aortic Disease
Chapter 17
3D Virtual Intravascular Endoscopy of Aortic Disease Zhonghua Sun Curtin University, Perth, WA, Australia
Chapter Outline Introduction181 Virtual Intravascular Endoscopy 181 Virtual Intravascular Endoscopy of Aortic Aneurysm 183 Virtual Intravascular Endoscopy of Aortic Dissection 185
Virtual Intravascular Endoscopy of Endovascular Stent Grafts 187 Summary191 References191
INTRODUCTION Cardiovascular disease is the leading cause of morbidity and mortality in advanced countries. Aortic disease refers to the large vessel disease, mainly including aortic dissection and aortic aneurysm. These diseases represent life-threatening conditions; thus early diagnosis is essential to improve patient treatment and reduce disease-related complications and mortality. Invasive angiography is the gold standard technique for diagnosing cardiovascular disease; however, it is an invasive procedure with associated morbidity and mortality [1]. Further, a short hospital stay is usually required for invasive angiography, which causes discomfort for the patients. Therefore, less invasive imaging techniques for diagnosis of cardiovascular disease are desirable. Presently, computed tomography (CT) is a fast-evolving technique reflected in the rapid development and wide availability of multislice CT (MSCT) scanners [2–7]. CT angiography (CTA) with MSCT systems has become the preferred imaging modality in the diagnostic assessment of cardiovascular disease with high sensitivity and specificity [7]. CTA allows for excellent visualization of cardiovascular disease with a combination of 2D and 3D reconstructions. These visualizations are sufficient to enable clinical diagnosis in most situations. However, they are limited to providing only external views of the cardiovascular system, without showing intraluminal changes of the arterial wall. This limitation is overcome with another visualization tool, virtual intravascular endoscopy (VIE). VIE presents a unique application of virtual endoscopy by providing intraluminal views of the hollow organs and structures. Virtual endoscopy is commonly used in the diagnosis of colonic polyps, with virtual colonoscopy being widely used as a screening tool for polyp detection [8–10]. VIE has been shown to be a valuable tool for visualizing vascular diseases, such as aortic aneurysm and endovascular stent grafts [11–14]. This chapter provides an overview of VIE applications in aortic disease, mainly focusing on aortic aneurysm, aortic dissection, and endovascular stent graft repair of the aortic aneurysm and dissection.
VIRTUAL INTRAVASCULAR ENDOSCOPY Generation of VIE images is different from virtual endoscopic views of other structures such as virtual colonoscopy or virtual bronchoscopy as air in the trachea and colon creates natural background contrast, which makes it easier to produce intraluminal views. Fig. 17.1A is an example of virtual colonoscopy looking at the transverse colon with fecal content resulting in false lumen stenosis, and this is confirmed by multiplanar orthogonal views (Fig. 17.1B). The minimum threshold is selected (−300 HU [Hounsfield unit]) to generate intraluminal views. New Approaches to Aortic Diseases from Valve to Abdominal Bifurcation. http://dx.doi.org/10.1016/B978-0-12-809979-7.00017-1 Copyright © 2018 Elsevier Inc. All rights reserved.
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FIGURE 17.1 Virtual colonoscopy. (A) Virtual colonoscopy of transverse colon with minimum computed tomography attenuation of −300 Hounsfield unit (HU). (B) Multiplanar orthogonal views confirm the narrowed lumen is due to presence of feces in the transverse colon.
Similar to virtual colonoscopy, a minimum threshold of −300 HU is applied to generate virtual bronchoscopic images as shown in Fig. 17.2A with a view positioned at the main bronchus looking at left and right main bronchi. Fig. 17.2B and C is the virtual bronchoscopic image of left and right bronchi, with Fig. 17.2D showing the effect of stent graft in the descending aorta on the visualization of left main bronchus. Due to the use of contrast enhancement in CTA of cardiovascular disease, the generation of VIE depends on the selection of appropriate threshold, which removes the contrast-enhanced blood from the arterial system so that intraluminal views can be obtained. This is achieved with the use of CT number thresholding technique, which has been well described previously [11,12]. In brief, appropriate CT threshold is determined by measuring the CT attenuation at different anatomical locations. For example, for VIE visualization of an abdominal aorta, three locations namely, abdominal aorta at the level of celiac axis or renal artery, aortic aneurysm, and common iliac arteries (Fig. 17.3) are chosen to measure CT attenuation, which is used to produce VIE images at corresponding levels, as shown in Fig. 17.4. The maximum threshold is changed from the 220 HU, which is considered appropriate for VIE visualization of the abdominal aorta to 160 HU with a view
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FIGURE 17.2 Virtual bronchoscopy. (A and B) Virtual bronchoscopy of main bronchi and right main bronchus with clear intraluminal views of right lobar bronchi. (C) virtual bronchoscopy of left main bronchus with irregular lumen change due to artifacts caused by the stent grafts placed in the descending aorta as shown in the orthogonal view (D).
moving inside the aneurysm and to 135 HU with the viewing positioned at the common iliac arteries (Fig. 17.4A–C). This is due to the variation of contrast enhancement at different levels of abdominal aorta, in particular, inconsistent enhancement pattern observed in the aneurysm due to the turbulent blood flow resulting from the enlarged aortic lumen. Inappropriate selection of threshold leads to distortion of the aortic structures or interference with visualization due to presence of artifacts. Fig. 17.5 shows a series of VIE images generated with different CT threshold values ranging from maximum 230 HU to 290 HU with 10 HU increase at each step. With an increasing of threshold, the aortic ostia including celiac axis and superior mesenteric artery (SMA) starts to appear irregular, with floating artifacts present when the maximum threshold reaches 290 HU. Thus, selection of appropriate threshold plays an important role in determining the final VIE image appearances.
VIRTUAL INTRAVASCULAR ENDOSCOPY OF AORTIC ANEURYSM Aortic aneurysm is a common vascular disease, and it most commonly involves abdominal aorta, especially below the renal arteries, which is usually referred to as infrarenal abdominal aortic aneurysm (AAA). Because most aortic aneurysms are asymptomatic, it is difficult to estimate their prevalence. The incidence of aortic aneurysm depends on the age group studied, and its prevalence increases in elderly populations [15]. Once an aneurysm is detected by routine physical and imaging examinations, the assessment of the risk of rupture is crucial as it is directly related to the patient’s outcome. Aneurysm diameter is the major determinant for the risk of rupture, and this is usually performed with CTA, which serves as the method of choice in current clinical practice [15–18]. CT has been shown to have high sensitivity for the detection of AAA, thus serving as a reliable modality for the screening purpose compared to ultrasound screening [19].
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FIGURE 17.3 Measurement of appropriate computed tomography (CT) threshold for generation of virtual intravascular endoscopy (VIE) images. CT attenuation is measured at three locations including abdominal aorta, aortic aneurysm, and common iliac arteries to determine the appropriate threshold for producing VIE views corresponding to each anatomic location (A–C).
FIGURE 17.4 Virtual intravascular endoscopy (VIE) of aortic aneurysm. VIE of abdominal aorta with view of superior mesenteric artery with application of maximum threshold of 200–220 Hounsfield unit (HU) (A). (B) VIE view inside the aortic aneurysm with irregular wall due to turbulent flow within the aneurysm with maximum threshold of 160 HU applied to remove the contrast-enhanced blood. (C) VIE view towards the common iliac arteries with maximum threshold of 130–135 HU applied.
Axial CTA images are most commonly used to measure the maximal aneurysm diameter, detecting both aortic lumen and thrombus (Fig. 17.3). In addition, 2D and 3D reconstructions such as multiplanar reformation (MPR), maximum-intensity projection (MIP), and volume rendering visualizations are also commonly used to enhance the role of CTA in aortic aneurysm assessment. Fig. 17.6A is an example of 3D surface shaded display of an infrarenal AAA, whereas Fig. 17.6B–D shows MPR and MIP images of the same case. Compared to these 2D/3D visualizations, VIE offers different views of the aortic aneurysm as well as its relationship to the aortic ostia. Fig. 17.7 shows VIE views of aortic branches, moving toward the aneurysm with a view positioned at the SMA and looking at the common iliac arteries. VIE also enables the detection of ostial calcification in the aortic branches, which assists in the clinical assessment of degree of artery stenosis and morphological changes following the placement of endovascular stent grafts [20,21]. Fig. 17.8
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FIGURE 17.5 Effect of threshold selection on virtual intravascular endoscopy (VIE) appearance of the intravascular anatomy. (A) Maximum computed tomography (CT) threshold of 230 Hounsfield unit (HU) is chosen as an appropriate threshold for VIE visualization of aortic ostia and arterial wall. (B–G) With an increase of threshold at each step of 10 HU, intravascular views of the anatomical structures start to appear irregular with artifacts present when the threshold is increased to 290 HU. SMA, superior mesenteric artery.
FIGURE 17.6 Two-dimensional and three-dimensional reconstructed computed tomography angiography. (A) Surface shaded display of an infrarenal aortic aneurysm extending to the common iliac arteries. (B) Maximum-intensity projection shows the infrarenal aneurysm, but with clearly view of calcification in the aneurysm and arterial wall. (C and D) Coronary and sagittal reformatted images show the aneurysm in relation to the renal arteries.
shows VIE visualization of ostial calcification in SMA and left renal artery with intraluminal appearances. The aortic ostia, in particular, renal ostia with preexistent ostial calcification may be affected following stent graft treatment, although stent wires were not shown to have a significant impact on the ostial morphology [20].
VIRTUAL INTRAVASCULAR ENDOSCOPY OF AORTIC DISSECTION Aortic dissection is formed by splitting the aortic wall into compartments, namely true lumen and false lumen. Aortic dissection is a life-threatening disease as it is associated with severe complications including aortic rupture and visceral organ ischemia [22–24]. Aortic dissection can be classified into Stanford type A dissection, which involves the ascending aorta and aortic arch, and Stanford type B dissection with dissection commencing distal to the left subclavian artery without
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FIGURE 17.7 Virtual intravascular endoscopy (VIE) of aortic ostia. (A) VIE visualization of aortic ostia, from proximal view of celiac axis and SMA (superior mesenteric artery) to distal view of renal ostia. (B) Close view of SMA toward the aneurysm. (C) VIE view of common iliac artery ostia.
FIGURE 17.8 Virtual intravascular endoscopy (VIE) of ostial calcification. (A) Two-dimensional (2D) axial computed tomography (CT) images show calcification in the SMA (superior mesenteric artery). (B) VIE reveals circular calcified appearance in the SMA ostium. (C) 2D axial CT images show calcification in the left renal artery ostium. (D) VIE displays renal ostial stenosis due to calcified plaque at the ostium.
involving the ascending aorta. Type A dissection requires surgical treatment as it is accompanied by high mortality, whereas type B dissection is the most common type with treatment involving different strategies depending on the presentation of complications [25–29]. Preoperative assessment of aortic dissection, in particular, type B dissection plays an important role in determining the extent of dissection in relation to the arterial branches. Currently, multislice CTA is the preferred method for the diagnosis of aortic dissection with a high diagnostic value [30–32]. Similar to aortic aneurysm assessment, axial CTA images supplemented by 2D and 3D reconstructions are able to identify the intimal flap, which separates the true lumen from the false lumen and determines the type and extent of dissection (Fig. 17.9). However, this may not be possible in all cases due to variable appearances associated with different types of aortic dissection. VIE serves as a supplementary tool in overcoming the limitations of conventional CTA images. VIE offers excellent views of looking inside the aorta and its branches to assist assessment of 3D relationship between the dissection and arteries. Despite narrowed true lumen in most of the cases, VIE is still able to produce intraluminal views of true and false lumens, in addition to identification of intimal flap or entry site [30]. Fig. 17.10 shows VIE visualization of
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FIGURE 17.9 Multiplanar reformation of aortic dissection. A series of sagittal reformatted image show the type B aortic dissection with dissection commencing distal to the subclavian artery, resulting in narrowed true lumen.
FIGURE 17.10 Virtual intravascular endoscopy (VIE) of aortic branches in type B aortic dissection. Three main arterial branches, left subclavian artery, left common carotid artery, and innominate artery ostia are clearly shown on VIE without being involved by the dissection.
the three main arteries arising from the aortic arch in a patient with type B dissection, whereas Fig. 17.11 is an example of VIE images with a viewing position placed at the true and false lumens, as well as the visualization of intimal flap.
VIRTUAL INTRAVASCULAR ENDOSCOPY OF ENDOVASCULAR STENT GRAFTS Endovascular aortic stent grafting (EVAR) is a minimally invasive technique, which is increasingly used in the treatment of aortic aneurysm and aortic dissection. It is associated with lower perioperative and postoperative mortality compared to open surgery [33–37]. Unlike open surgical repair, the success of EVAR highly depends on medical imaging assessment. Further, medical imaging techniques play an important role in the follow-up of EVAR in terms of monitoring the aneurysm changes, patency of true and false lumens in aortic dissection, and detecting complications associated with the procedure [15–17]. Currently, CTA is the preferred imaging modality in both preoperative assessment and postoperative follow-up of EVAR. 2D axial CTA images are enhanced by postprocessing methods to produce a 3D representation of aortic stent grafts in relation to the anatomical structures (Fig. 17.12). In contrast to the conventional visualizations, which offer external views of the stent grafts, VIE is a unique visualization tool providing intraluminal views of stent grafts relative to aortic branches [11–13], as shown in Fig. 17.13 showing the intravascular stents following EVAR of aortic dissection.
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FIGURE 17.11 Virtual intravascular endoscopy (VIE) of true lumen, false lumen and intimal flap in type B aortic dissection. (A) VIE shows a narrowed lumen with viewing position placed inside the true lumen. (B) VIE view of false lumen, with part of the true lumen displayed. Intimal flap is also shown in the image. (C) VIE shows the separation of true lumen from false lumen by a fibrous intimal flap.
FIGURE 17.12 Endovascular stent graft repair of type B aortic dissection. (A) Sagittal reformatted images show a type B aortic dissection. (B) The same patient as A is successfully treated with endovascular stent graft.
Fig. 17.14 shows a 3D volume rendering and MIP images of aortic stent grafts and renal stents in a patient of AAA after EVAR. Fig. 17.15 displays different intraluminal appearances of renal stents in patients treated with EVAR. The clinical application of VIE in the follow-up of aortic aneurysm and dissection after EVAR is to demonstrate the renal stent in relation to renal ostia because the main concern of EVAR treatment is to develop renal dysfunction due to interference of blood flow by stent wires across the renal arteries [38–43]. Although the long-term outcomes of EVAR in aortic aneurysm and dissection remain to be well understood, the ability of VIE to demonstrate the endovascular stent
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FIGURE 17.13 Virtual intravascular endoscopy (VIE) visualization of three-dimensional (3D) relationship between aortic stent graft and aortic branches in aortic dissection. (A) Two-dimensional (2D) axial images show the stent graft implanted distal to the left subclavian artery. (B and C) VIE demonstrates endovascular aortic stent graft distal to the left subclavian and common carotid arteries. Arrows refer to the indentation in the arterial lumen caused by endovascular stents.
FIGURE 17.14 Two-dimensional (2D) and three-dimensional (3D) visualizations of aortic stent graft in a patient with abdominal aortic aneurysm. (A) 3D volume rendering shows the aortic stent graft with renal stents in both renal arteries with blood vessels and stent graft coded with red color, while bones with green color. (B) Maximum-intensity projection shows the stent wires including the left renal stent inside the abdominal aorta.
appearances will serve as a complementary diagnostic tool to assist the detection of potential detrimental effects on renal function or other aortic branches. Unlike virtual colonoscopy, which is widely recommended as a screening tool for colonic polyps in clinical practice [44,45], applications of VIE in aortic disease are mainly limited to research purpose in most studies reported in the literature. However, VIE does offer additional value when compared to conventional CT scans [46–48]. Louis et al. analyzed the diagnostic performance of VIE in 103 patients undergoing thoracic aortic procedures consisting of EVAR of aortic dissection and aortic aneurysm. In comparison with conventional MPR CT images, VIE provided additional information in more than 76% of cases, which is reflected by showing improved localization of abnormalities in relation to supra-aortic arterial branches, measurement of inadequate stent graft apposition with respect to the aortic wall, and analysis of kinking of stent grafts [48]. Fig. 17.16 shows the clinical value of VIE in the follow-up of patients with aortic dissection treated by stent graft insertion. This indicates the potential value of VIE to facilitate the accurate analysis of abnormalities associated with stent graft treatment, although further studies correlating VIE findings with patient follow-up of treatment outcomes are warranted.
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FIGURE 17.15 Virtual intravascular endoscopy (VIE) visualization of intravascular renal stent appearance. (A) Two-dimensional (2D) axial images show patency of left renal stent. (B) Corresponding VIE shows circular intravascular appearance of the left renal stent. (C) 2D axial images indicate right renal stent with slight angulation. (D) Corresponding VIE reveals irregular with some kind of distortion of the right renal stent.
FIGURE 17.16 Follow-up virtual intravascular endoscopy (VIE) visualization of stent graft repair of aortic dissection. (A) Follow-up computed tomography (CT) scan of a patient treated by stent graft insertion after reimplantation of supra-aortic trunks (SATs) into the ascending aorta: coronal cut (top right) shows retrograde dissection of the ascending aorta. VIE precisely locates the intimal tear just on the suture line of the bypass to the SAT (top left image) and shows inadequate apposition of the proximal rim of the stent graft. (B) Follow-up CT scan of a patient treated by a thoracic endovascular aortic repair for a traumatic aortic transection: sagittal cut (bottom left) and axial cut (top right) shows an inadequate apposition of the proximal rim of stent-graft, VIE shows in a single view the of stent graft components involved and allows accurate measurements. Reprint with permission from Louis N, Bruguiere E, Kobeiter H, Desgranger P, Allarire E, Kirsch M, Becquemin JP. Virtual angioscopy and 3D navigation: a new technique for analysis of the aortic arch after vascular surgery. Eur J Vasc Endovasc Surg 2010;40:340–7.
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SUMMARY Due to the complex anatomy of the cardiovascular system, in particular the aortic arch, which is associated with curvature, angulation, and variation of origin of aortic branches, it is difficult to accurately assess these anatomic structures with conventional CT images. VIE, an additional 3D visualization tool with navigation mode, is able to represent the aortic anatomy in three dimensions [48–50]. VIE has been shown to be superior to conventional CT images in the analysis of endovascular stent grafts with respect to the aortic aneurysm and aortic dissection, therefore, playing an important role in the follow-up of stent graft repair of these aortic diseases. Future research is needed to determine how VIE findings will impact patient care and guide interventions in patients with a potential risk of developing complications following endovascular stent grafting.
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3D Virtual Intravascular Endoscopy of Aortic Disease Zhonghua Sun Curtin University, Perth, WA, Australia
Chapter Outline Introduction181 Virtual Intravascular Endoscopy 181 Virtual Intravascular Endoscopy of Aortic Aneurysm 183 Virtual Intravascular Endoscopy of Aortic Dissection 185
Virtual Intravascular Endoscopy of Endovascular Stent Grafts 187 Summary191 References191
INTRODUCTION Cardiovascular disease is the leading cause of morbidity and mortality in advanced countries. Aortic disease refers to the large vessel disease, mainly including aortic dissection and aortic aneurysm. These diseases represent life-threatening conditions; thus early diagnosis is essential to improve patient treatment and reduce disease-related complications and mortality. Invasive angiography is the gold standard technique for diagnosing cardiovascular disease; however, it is an invasive procedure with associated morbidity and mortality [1]. Further, a short hospital stay is usually required for invasive angiography, which causes discomfort for the patients. Therefore, less invasive imaging techniques for diagnosis of cardiovascular disease are desirable. Presently, computed tomography (CT) is a fast-evolving technique reflected in the rapid development and wide availability of multislice CT (MSCT) scanners [2–7]. CT angiography (CTA) with MSCT systems has become the preferred imaging modality in the diagnostic assessment of cardiovascular disease with high sensitivity and specificity [7]. CTA allows for excellent visualization of cardiovascular disease with a combination of 2D and 3D reconstructions. These visualizations are sufficient to enable clinical diagnosis in most situations. However, they are limited to providing only external views of the cardiovascular system, without showing intraluminal changes of the arterial wall. This limitation is overcome with another visualization tool, virtual intravascular endoscopy (VIE). VIE presents a unique application of virtual endoscopy by providing intraluminal views of the hollow organs and structures. Virtual endoscopy is commonly used in the diagnosis of colonic polyps, with virtual colonoscopy being widely used as a screening tool for polyp detection [8–10]. VIE has been shown to be a valuable tool for visualizing vascular diseases, such as aortic aneurysm and endovascular stent grafts [11–14]. This chapter provides an overview of VIE applications in aortic disease, mainly focusing on aortic aneurysm, aortic dissection, and endovascular stent graft repair of the aortic aneurysm and dissection.
VIRTUAL INTRAVASCULAR ENDOSCOPY Generation of VIE images is different from virtual endoscopic views of other structures such as virtual colonoscopy or virtual bronchoscopy as air in the trachea and colon creates natural background contrast, which makes it easier to produce intraluminal views. Fig. 17.1A is an example of virtual colonoscopy looking at the transverse colon with fecal content resulting in false lumen stenosis, and this is confirmed by multiplanar orthogonal views (Fig. 17.1B). The minimum threshold is selected (−300 HU [Hounsfield unit]) to generate intraluminal views. New Approaches to Aortic Diseases from Valve to Abdominal Bifurcation. http://dx.doi.org/10.1016/B978-0-12-809979-7.00017-1 Copyright © 2018 Elsevier Inc. All rights reserved.
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FIGURE 17.1 Virtual colonoscopy. (A) Virtual colonoscopy of transverse colon with minimum computed tomography attenuation of −300 Hounsfield unit (HU). (B) Multiplanar orthogonal views confirm the narrowed lumen is due to presence of feces in the transverse colon.
Similar to virtual colonoscopy, a minimum threshold of −300 HU is applied to generate virtual bronchoscopic images as shown in Fig. 17.2A with a view positioned at the main bronchus looking at left and right main bronchi. Fig. 17.2B and C is the virtual bronchoscopic image of left and right bronchi, with Fig. 17.2D showing the effect of stent graft in the descending aorta on the visualization of left main bronchus. Due to the use of contrast enhancement in CTA of cardiovascular disease, the generation of VIE depends on the selection of appropriate threshold, which removes the contrast-enhanced blood from the arterial system so that intraluminal views can be obtained. This is achieved with the use of CT number thresholding technique, which has been well described previously [11,12]. In brief, appropriate CT threshold is determined by measuring the CT attenuation at different anatomical locations. For example, for VIE visualization of an abdominal aorta, three locations namely, abdominal aorta at the level of celiac axis or renal artery, aortic aneurysm, and common iliac arteries (Fig. 17.3) are chosen to measure CT attenuation, which is used to produce VIE images at corresponding levels, as shown in Fig. 17.4. The maximum threshold is changed from the 220 HU, which is considered appropriate for VIE visualization of the abdominal aorta to 160 HU with a view
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FIGURE 17.2 Virtual bronchoscopy. (A and B) Virtual bronchoscopy of main bronchi and right main bronchus with clear intraluminal views of right lobar bronchi. (C) virtual bronchoscopy of left main bronchus with irregular lumen change due to artifacts caused by the stent grafts placed in the descending aorta as shown in the orthogonal view (D).
moving inside the aneurysm and to 135 HU with the viewing positioned at the common iliac arteries (Fig. 17.4A–C). This is due to the variation of contrast enhancement at different levels of abdominal aorta, in particular, inconsistent enhancement pattern observed in the aneurysm due to the turbulent blood flow resulting from the enlarged aortic lumen. Inappropriate selection of threshold leads to distortion of the aortic structures or interference with visualization due to presence of artifacts. Fig. 17.5 shows a series of VIE images generated with different CT threshold values ranging from maximum 230 HU to 290 HU with 10 HU increase at each step. With an increasing of threshold, the aortic ostia including celiac axis and superior mesenteric artery (SMA) starts to appear irregular, with floating artifacts present when the maximum threshold reaches 290 HU. Thus, selection of appropriate threshold plays an important role in determining the final VIE image appearances.
VIRTUAL INTRAVASCULAR ENDOSCOPY OF AORTIC ANEURYSM Aortic aneurysm is a common vascular disease, and it most commonly involves abdominal aorta, especially below the renal arteries, which is usually referred to as infrarenal abdominal aortic aneurysm (AAA). Because most aortic aneurysms are asymptomatic, it is difficult to estimate their prevalence. The incidence of aortic aneurysm depends on the age group studied, and its prevalence increases in elderly populations [15]. Once an aneurysm is detected by routine physical and imaging examinations, the assessment of the risk of rupture is crucial as it is directly related to the patient’s outcome. Aneurysm diameter is the major determinant for the risk of rupture, and this is usually performed with CTA, which serves as the method of choice in current clinical practice [15–18]. CT has been shown to have high sensitivity for the detection of AAA, thus serving as a reliable modality for the screening purpose compared to ultrasound screening [19].
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FIGURE 17.3 Measurement of appropriate computed tomography (CT) threshold for generation of virtual intravascular endoscopy (VIE) images. CT attenuation is measured at three locations including abdominal aorta, aortic aneurysm, and common iliac arteries to determine the appropriate threshold for producing VIE views corresponding to each anatomic location (A–C).
FIGURE 17.4 Virtual intravascular endoscopy (VIE) of aortic aneurysm. VIE of abdominal aorta with view of superior mesenteric artery with application of maximum threshold of 200–220 Hounsfield unit (HU) (A). (B) VIE view inside the aortic aneurysm with irregular wall due to turbulent flow within the aneurysm with maximum threshold of 160 HU applied to remove the contrast-enhanced blood. (C) VIE view towards the common iliac arteries with maximum threshold of 130–135 HU applied.
Axial CTA images are most commonly used to measure the maximal aneurysm diameter, detecting both aortic lumen and thrombus (Fig. 17.3). In addition, 2D and 3D reconstructions such as multiplanar reformation (MPR), maximum-intensity projection (MIP), and volume rendering visualizations are also commonly used to enhance the role of CTA in aortic aneurysm assessment. Fig. 17.6A is an example of 3D surface shaded display of an infrarenal AAA, whereas Fig. 17.6B–D shows MPR and MIP images of the same case. Compared to these 2D/3D visualizations, VIE offers different views of the aortic aneurysm as well as its relationship to the aortic ostia. Fig. 17.7 shows VIE views of aortic branches, moving toward the aneurysm with a view positioned at the SMA and looking at the common iliac arteries. VIE also enables the detection of ostial calcification in the aortic branches, which assists in the clinical assessment of degree of artery stenosis and morphological changes following the placement of endovascular stent grafts [20,21]. Fig. 17.8
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FIGURE 17.5 Effect of threshold selection on virtual intravascular endoscopy (VIE) appearance of the intravascular anatomy. (A) Maximum computed tomography (CT) threshold of 230 Hounsfield unit (HU) is chosen as an appropriate threshold for VIE visualization of aortic ostia and arterial wall. (B–G) With an increase of threshold at each step of 10 HU, intravascular views of the anatomical structures start to appear irregular with artifacts present when the threshold is increased to 290 HU. SMA, superior mesenteric artery.
FIGURE 17.6 Two-dimensional and three-dimensional reconstructed computed tomography angiography. (A) Surface shaded display of an infrarenal aortic aneurysm extending to the common iliac arteries. (B) Maximum-intensity projection shows the infrarenal aneurysm, but with clearly view of calcification in the aneurysm and arterial wall. (C and D) Coronary and sagittal reformatted images show the aneurysm in relation to the renal arteries.
shows VIE visualization of ostial calcification in SMA and left renal artery with intraluminal appearances. The aortic ostia, in particular, renal ostia with preexistent ostial calcification may be affected following stent graft treatment, although stent wires were not shown to have a significant impact on the ostial morphology [20].
VIRTUAL INTRAVASCULAR ENDOSCOPY OF AORTIC DISSECTION Aortic dissection is formed by splitting the aortic wall into compartments, namely true lumen and false lumen. Aortic dissection is a life-threatening disease as it is associated with severe complications including aortic rupture and visceral organ ischemia [22–24]. Aortic dissection can be classified into Stanford type A dissection, which involves the ascending aorta and aortic arch, and Stanford type B dissection with dissection commencing distal to the left subclavian artery without
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FIGURE 17.7 Virtual intravascular endoscopy (VIE) of aortic ostia. (A) VIE visualization of aortic ostia, from proximal view of celiac axis and SMA (superior mesenteric artery) to distal view of renal ostia. (B) Close view of SMA toward the aneurysm. (C) VIE view of common iliac artery ostia.
FIGURE 17.8 Virtual intravascular endoscopy (VIE) of ostial calcification. (A) Two-dimensional (2D) axial computed tomography (CT) images show calcification in the SMA (superior mesenteric artery). (B) VIE reveals circular calcified appearance in the SMA ostium. (C) 2D axial CT images show calcification in the left renal artery ostium. (D) VIE displays renal ostial stenosis due to calcified plaque at the ostium.
involving the ascending aorta. Type A dissection requires surgical treatment as it is accompanied by high mortality, whereas type B dissection is the most common type with treatment involving different strategies depending on the presentation of complications [25–29]. Preoperative assessment of aortic dissection, in particular, type B dissection plays an important role in determining the extent of dissection in relation to the arterial branches. Currently, multislice CTA is the preferred method for the diagnosis of aortic dissection with a high diagnostic value [30–32]. Similar to aortic aneurysm assessment, axial CTA images supplemented by 2D and 3D reconstructions are able to identify the intimal flap, which separates the true lumen from the false lumen and determines the type and extent of dissection (Fig. 17.9). However, this may not be possible in all cases due to variable appearances associated with different types of aortic dissection. VIE serves as a supplementary tool in overcoming the limitations of conventional CTA images. VIE offers excellent views of looking inside the aorta and its branches to assist assessment of 3D relationship between the dissection and arteries. Despite narrowed true lumen in most of the cases, VIE is still able to produce intraluminal views of true and false lumens, in addition to identification of intimal flap or entry site [30]. Fig. 17.10 shows VIE visualization of
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FIGURE 17.9 Multiplanar reformation of aortic dissection. A series of sagittal reformatted image show the type B aortic dissection with dissection commencing distal to the subclavian artery, resulting in narrowed true lumen.
FIGURE 17.10 Virtual intravascular endoscopy (VIE) of aortic branches in type B aortic dissection. Three main arterial branches, left subclavian artery, left common carotid artery, and innominate artery ostia are clearly shown on VIE without being involved by the dissection.
the three main arteries arising from the aortic arch in a patient with type B dissection, whereas Fig. 17.11 is an example of VIE images with a viewing position placed at the true and false lumens, as well as the visualization of intimal flap.
VIRTUAL INTRAVASCULAR ENDOSCOPY OF ENDOVASCULAR STENT GRAFTS Endovascular aortic stent grafting (EVAR) is a minimally invasive technique, which is increasingly used in the treatment of aortic aneurysm and aortic dissection. It is associated with lower perioperative and postoperative mortality compared to open surgery [33–37]. Unlike open surgical repair, the success of EVAR highly depends on medical imaging assessment. Further, medical imaging techniques play an important role in the follow-up of EVAR in terms of monitoring the aneurysm changes, patency of true and false lumens in aortic dissection, and detecting complications associated with the procedure [15–17]. Currently, CTA is the preferred imaging modality in both preoperative assessment and postoperative follow-up of EVAR. 2D axial CTA images are enhanced by postprocessing methods to produce a 3D representation of aortic stent grafts in relation to the anatomical structures (Fig. 17.12). In contrast to the conventional visualizations, which offer external views of the stent grafts, VIE is a unique visualization tool providing intraluminal views of stent grafts relative to aortic branches [11–13], as shown in Fig. 17.13 showing the intravascular stents following EVAR of aortic dissection.
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FIGURE 17.11 Virtual intravascular endoscopy (VIE) of true lumen, false lumen and intimal flap in type B aortic dissection. (A) VIE shows a narrowed lumen with viewing position placed inside the true lumen. (B) VIE view of false lumen, with part of the true lumen displayed. Intimal flap is also shown in the image. (C) VIE shows the separation of true lumen from false lumen by a fibrous intimal flap.
FIGURE 17.12 Endovascular stent graft repair of type B aortic dissection. (A) Sagittal reformatted images show a type B aortic dissection. (B) The same patient as A is successfully treated with endovascular stent graft.
Fig. 17.14 shows a 3D volume rendering and MIP images of aortic stent grafts and renal stents in a patient of AAA after EVAR. Fig. 17.15 displays different intraluminal appearances of renal stents in patients treated with EVAR. The clinical application of VIE in the follow-up of aortic aneurysm and dissection after EVAR is to demonstrate the renal stent in relation to renal ostia because the main concern of EVAR treatment is to develop renal dysfunction due to interference of blood flow by stent wires across the renal arteries [38–43]. Although the long-term outcomes of EVAR in aortic aneurysm and dissection remain to be well understood, the ability of VIE to demonstrate the endovascular stent
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FIGURE 17.13 Virtual intravascular endoscopy (VIE) visualization of three-dimensional (3D) relationship between aortic stent graft and aortic branches in aortic dissection. (A) Two-dimensional (2D) axial images show the stent graft implanted distal to the left subclavian artery. (B and C) VIE demonstrates endovascular aortic stent graft distal to the left subclavian and common carotid arteries. Arrows refer to the indentation in the arterial lumen caused by endovascular stents.
FIGURE 17.14 Two-dimensional (2D) and three-dimensional (3D) visualizations of aortic stent graft in a patient with abdominal aortic aneurysm. (A) 3D volume rendering shows the aortic stent graft with renal stents in both renal arteries with blood vessels and stent graft coded with red color, while bones with green color. (B) Maximum-intensity projection shows the stent wires including the left renal stent inside the abdominal aorta.
appearances will serve as a complementary diagnostic tool to assist the detection of potential detrimental effects on renal function or other aortic branches. Unlike virtual colonoscopy, which is widely recommended as a screening tool for colonic polyps in clinical practice [44,45], applications of VIE in aortic disease are mainly limited to research purpose in most studies reported in the literature. However, VIE does offer additional value when compared to conventional CT scans [46–48]. Louis et al. analyzed the diagnostic performance of VIE in 103 patients undergoing thoracic aortic procedures consisting of EVAR of aortic dissection and aortic aneurysm. In comparison with conventional MPR CT images, VIE provided additional information in more than 76% of cases, which is reflected by showing improved localization of abnormalities in relation to supra-aortic arterial branches, measurement of inadequate stent graft apposition with respect to the aortic wall, and analysis of kinking of stent grafts [48]. Fig. 17.16 shows the clinical value of VIE in the follow-up of patients with aortic dissection treated by stent graft insertion. This indicates the potential value of VIE to facilitate the accurate analysis of abnormalities associated with stent graft treatment, although further studies correlating VIE findings with patient follow-up of treatment outcomes are warranted.
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FIGURE 17.15 Virtual intravascular endoscopy (VIE) visualization of intravascular renal stent appearance. (A) Two-dimensional (2D) axial images show patency of left renal stent. (B) Corresponding VIE shows circular intravascular appearance of the left renal stent. (C) 2D axial images indicate right renal stent with slight angulation. (D) Corresponding VIE reveals irregular with some kind of distortion of the right renal stent.
FIGURE 17.16 Follow-up virtual intravascular endoscopy (VIE) visualization of stent graft repair of aortic dissection. (A) Follow-up computed tomography (CT) scan of a patient treated by stent graft insertion after reimplantation of supra-aortic trunks (SATs) into the ascending aorta: coronal cut (top right) shows retrograde dissection of the ascending aorta. VIE precisely locates the intimal tear just on the suture line of the bypass to the SAT (top left image) and shows inadequate apposition of the proximal rim of the stent graft. (B) Follow-up CT scan of a patient treated by a thoracic endovascular aortic repair for a traumatic aortic transection: sagittal cut (bottom left) and axial cut (top right) shows an inadequate apposition of the proximal rim of stent-graft, VIE shows in a single view the of stent graft components involved and allows accurate measurements. Reprint with permission from Louis N, Bruguiere E, Kobeiter H, Desgranger P, Allarire E, Kirsch M, Becquemin JP. Virtual angioscopy and 3D navigation: a new technique for analysis of the aortic arch after vascular surgery. Eur J Vasc Endovasc Surg 2010;40:340–7.
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SUMMARY Due to the complex anatomy of the cardiovascular system, in particular the aortic arch, which is associated with curvature, angulation, and variation of origin of aortic branches, it is difficult to accurately assess these anatomic structures with conventional CT images. VIE, an additional 3D visualization tool with navigation mode, is able to represent the aortic anatomy in three dimensions [48–50]. VIE has been shown to be superior to conventional CT images in the analysis of endovascular stent grafts with respect to the aortic aneurysm and aortic dissection, therefore, playing an important role in the follow-up of stent graft repair of these aortic diseases. Future research is needed to determine how VIE findings will impact patient care and guide interventions in patients with a potential risk of developing complications following endovascular stent grafting.
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