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EDUCATIONAL SERIES ON THORACIC AORTA (EDITED BY R. FATTORI)

The Importance of Imaging Assessment BeforeEndovascular Repair

H. Rousseau a,*, V. Chabbert a, M.A. Maracher a, O. El Aassar a, J. Auriol a,P. Massabuau b, R. Moreno a

a Service de Radiologie, CHU Rangueil TSA 50032, 31059 Toulouse Cedex 9, Franceb Service de Cardiologie A, CHU Rangueil TSA 50032, 31059 Toulouse Cedex 9, France

Submitted 22 June 2009; accepted 22 June 2009

KEYWORDSComputer tomography;MR angiography;Angiography;Stent graft;Descending aorta;Aneurysm anddissection

* Corresponding author. Tel.: þ33 524 92.

E-mail address: rousseau.h@chu-to

1078-5884/$36 ª 2009 European Sociedoi:10.1016/j.ejvs.2009.06.017

Please cite this article in press as: REndovasc Surg (2009), doi:10.1016/j.

Abstract Indications for and experience with placement of endovascular stent grafts in thethoracic aorta are still evolving. Recent advances in imaging technologies have drasticallyboosted the role of pre-procedural imaging. The accepted diagnostic gold standard, digitalsubtraction angiography, is now being challenged by the state-of-the-art computed tomog-raphy angiography (CTA), magnetic resonance angiography (MRA) and trans-oesophageal echo-cardiography (TEE). Among these, technological advancements of multidetector computedtomography (MDCT) have propelled it to being the default modality used, optimising thebalance between spatial and temporal resolutions and invasiveness. MDCT angiography allowsthe comprehensive evaluation of thoracic lesions in terms of morphological features andextent, presence of thrombus, relationship with adjacent structures and branches as well assigns of impending or acute rupture, and is routinely used in these settings.

In this article, we review the current state-of-the-art radiological imaging for thoracic en-dovascular aneurysm repair (TEVAR), especially focusing on the role of MDCT angiography.After analysing the technical aspects for optimised imaging protocols for thoracic aorticdiseases, we discuss pre-procedural determinants of candidacy, and how to formulate inter-ventional plans based on cross-sectional imaging.ª 2009 European Society for Vascular Surgery. Published by Elsevier Ltd. All rights reserved.

Since the first thoracic endovascular aortic repair (TEVAR),using homemade devices, published by the StanfordUniversity in July 1992, significant improvements were

61 32 28 81; fax: þ33 5 61 32

ulouse.fr (H. Rousseau).

ty for Vascular Surgery. Publishe

ousseau H et al., The Importancejvs.2009.06.017

made in the following generations, which are widely man-ufactured commercially. However, delivery systems remainlarge (22F to 24F), relatively rigid and difficult to deploysmoothly and accurately, owing to extensive frictionalresistance.1 Furthermore, hybrid procedures, whichcombine surgical and endovascular techniques, haveexpanded the potential applications of stent-grafting to

d by Elsevier Ltd. All rights reserved.

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more complex situations, such as stent-graft placementanchored to an elephant trunk or TEVAR of the arch. Forthese reasons, imaging is crucial for patient enrollment,device selection in correlation with the anatomy of thelesion, as well as formulation of a plan for the intervention.

Recent advances in imaging technologies, in partinspired by advancements in stent-graft technology, havedrastically changed the character and role of pre-proce-dural imaging. The accepted diagnostic gold standard,digital subtraction angiography, is now being challenged bythe state-of-the-art computed tomography angiography(CTA), magnetic resonance angiography (MRA) and trans-oesophageal echocardiography (TEE). These techniquesprovide information not only on the aortic lumen but alsoon the wall and surrounding mediastinal structures. Ofthese, technological advancements of multidetectorcomputed tomography (MDCT) have propelled it to beingthe default modality used, optimising the balance betweenspatial and temporal resolution and invasiveness. In thisarticle, we review the current state-of-the-art radiologicalimaging for TEVAR, especially focusing on the role of MDCTangiography.

Imaging Techniques

Chapter 1

CT angiographyComputed tomography (CT) currently is the most widelyused modality in the evaluation of the thoracic aorta due toits high diagnostic accuracy for detection of aorticpathology. MDCT can simultaneously acquire up to sub-millimetric sections with gantry rotation time of approxi-mately 0.5e0.33 s. Respiratory motion artefacts are nolonger a problem, since high-resolution imaging of theentire aorta can be obtained in a single breath hold.Imaging of all phases of contrast enhancement has alsobecome possible using a single-contrast agent bolus.Thinner section allows isotropic voxels, essential to obtainhigh-resolution three-dimensional (3-D) reconstructions inany selected plane. Despite ionising radiation hazards andthe nephrotoxicity of contrast agents, the technique iswidely available, fast, cost-effective and efficient.

Optimising CT parameters

MDCT scanning requires an understanding of the basicprinciples for optimum results. Pitch and collimation aretwo important parameters of image acquisition. The slicethickness is dependent on the detector collimation. Thesmaller the collimation, the thinner is the available slicethickness. The quality of the 3-D reconstructions is directlyrelated to the thickness of the obtained axial slices. A CTarteriogram should be acquired with the thinnest availablecollimation (0.625e0.75 mm). Pre-contrast or delayedimages can be acquired with thicker collimations (1.5 mmor greater) to reduce irradiation, as they are not used inpost-processing. Currently, the standard tube voltage forCTA is 120 kV. The tube current should be approximately120 mAs, and automated dose modulation should be used. Atube voltage of 100 kV increases the contrast-to-noise ratio

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because of a more effective X-ray absorption by iodine atlower tube voltages, which improves image quality andreduces patient radiation exposure by 35% in comparisonwith 120 kV at a constant tube current.2 Consequently,lowering the voltage during CTA can reduce the volume ofrequired iodinated contrast medium.

Electrocardiographic gating

Although less-frequent with MDCT, the motion caused bytransmitted cardiac pulsation to the major arteries maycreate problems. These pulsation artefacts are particularlypronounced in the proximal ascending aorta and mayfrequently mimic an intimal flap resulting in a false-positivediagnosis of aortic dissection. Electrocardiogram (ECG)gating can avoid this problem; it is available in new CTscanners and may be applied prospectively or retrospec-tively. A decision should be made as to the need for ECGgating. As a general rule, if the heart, coronary arteries,aortic root or ascending aorta are to be evaluated, ECGleads should be placed for gating to minimise cardiacpulsation artefacts. Moreover, it has been shown thatdiameters of the thoracic aorta can change by up to 17.8%through the cardiac cycle.3 Considering that most cliniciansoversize endografts by only 10%, inadequate estimation ofthe fluctuations in aortic diameters may lead to seriouserrors of undersizing, hence contributing to the variousmeans of graft failure. For these reasons, the use of ECGgating has been proposed when endograft choice is to bemade so that the optimal sizing can be ascertained.

On the other hand, in emergency imaging for chest pain,some authors push the benefits of exploiting a single CTacquisition with the so-called triple rule-out technique tosimultaneously explore pulmonary arteries, coronaries aswell as thoracic aorta. ({Johnson, 2008 #155,{Urbania,2009 #156)}.4e6 We are reticent to embrace this approachas we believe that scans should be tailored for individualclinical questions, given that contrast timing and scanningparameters are much different if only one clinical questionneeds to be answered, that is, a pulmonary embolism CT ismuch different from a coronary CTA, where artefacts maylimit the study for certain structures while other structuresmay be better visualised. Moreover, in acute aorticsyndromes, we need to have a complete exploration of theaorta at an arterial phase, from the thoracic inlet down tothe bifurcation of the femoral arteries for completeassessment, which is not achievable with ECG gating. Thecoverage suffices to both detect disease extension and assessthe delivery route for endovascular devices that might be thepreferred therapeutic modality to treat the abnormality.

Contrast-enhancement methods

CT angiography requires intravenous injection of iodinatedcontrast agents to display the vessel lumen. The use ofadditional iodinated contrast media in TEVAR candidatescan be a problematic clinical issue, especially in patientspresenting with multiple co-morbidities where the risk ofcontrast-induced nephropathy is greatly accentuated.Faster scanners have resulted in shorter scan duration andcontrast bolus duration, allowing the use of less contrastagent. This advantage can be used to inject the same

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Figure 1 The thoracic aorta is divided into 5 segments (Z0 toZ4). The Z4 zone is further subdivided into 9 sub-segments (T4to T12) carrying the name of the corresponding vertebralprojection. This classification is important to define the preciselanding zone of a stent graft.

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amount of contrast at a higher flow rate to achievea greater luminal enhancement. Right ante-cubital veinsare the preferred injection sites because dense contrast inthe left brachio-cephalic vein may cause artefacts, whichcan limit the evaluation of the aortic arch and the proximalsegments of the great vessels. The type of contrast is alsoimportant, with most centres now using low osmolar andhigh iodine concentrations (350e370 mg ml�1). The patientshould be advised to take a medium-sized breath duringimage acquisition, as too much of a breath hold or a bearingdown may cause a Valsalva effect which can alter contrastdelivery to the vessel in question.

Optimum intra-luminal enhancement during peak aorticopacification is of utmost importance for a good CTA study.Automated bolus tracking is used more often than the test-bolus technique, because it is easier to use, more efficientand reduces the total contrast medium dose. Anotherimportant consideration with regard to synchronisation ofthe contrast bolus and the scan is the table feed. We canrecommend using a fixed table feed of 40e48 mm s�1,combined with bolus tracking to perform aortic CTA.

Post-processing methods

Once CTA acquisition is complete, the raw data arereconstructed at the thinnest slice thickness available forthat acquisition, so that small structures can be adequatelyvisualised and optimum 3-D reconstructions can beobtained from these axial source images. Although axialsections are still the mainstay of interpretation, 2-D and3-D reformatting techniques such as maximum intensityprojection (MIP), multiplanar reformation (MPR), curvedplanar reformation (curved MPR) and volume rendering (VR)may facilitate interpretation and improve communicationwith referring physicians (Figs. 1 and 2).

Study protocol

As a rule, oral contrast agent is not given before CTA. Aninitial non-enhanced scan of the whole thoracic aorta isobtained (collimation: 1.5e0.6 mm; slice thickness: 5 mm;and reconstruction interval: 5 mm). This non-enhancedscan is important for proper planning of the contrast-enhanced scan and also useful in the evaluation of certainentities, such as intramural haematoma (Fig. 3). ECG-gatedcontrast-enhanced scanning is useful only for the ascendingaorta. For the other cases, a CT without ECG gating couldbe sufficient, allowing a complete examination of thethoracic and abdominal aorta in one step (collimation:0.75 mm; slice thickness: 0.75 mm; reconstruction interval:0.4 mm) (Fig. 4). In case of dissection, a delayed scan isroutinely obtained at 60-s delay with acquisition parame-ters similar to the non-enhanced scan to visualise theparenchyma and the late opacification of false lumen.

MR angiography

Magnetic resonance (MR) is an important alternative tech-nology for pre-procedural imaging of TEVAR patients whenMDCT angiography is contraindicated. MR of the thoracicaorta usually requires a combination of several available MR

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imaging methods, each of which has certain advantages andcontributes to the diagnostic versatility of the technique.

Dynamic Contrast-Enhanced MR angiography (CE-MRA) isthe most widely used method because of its speed androbustness, providing projection images of the aorta similarto conventional invasive angiography (Fig. 5). CE-MRA usesthe T1 shortening effects of gadolinium-based contrastagent, so that the blood appears bright regardless of flowpatterns or velocity. Contrast amount at a rate of 40 ml/2.5 ml, and saline amount 20 ml/2.5 ml are used. As for CT,the synchronisation of image acquisition and arrival of thebolus of contrast agent in the region of interest is crucial toobtain high image quality. As for CT scanner, severalmethods are used for determining the correct contrastinjection timing, especially in patients with low cardiacoutput. A timing-bolus scan, automatic detection ofcontrast bolus passage, and, more frequently, MR imaging

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Figure 2 CT scan and 3-D reconstruction with multi-planar reformatting showing an aortic aneurysm with an adequate landingzone. A virtual line in the center of the lumen allows accurate measurement of the aortic diameters perpendicular to this line.Providing specific precise measurements of the extent of aortic pathological conditions, including diameter and length, that areessential for endovascular stent design and placement. Factors to be considered include involvement of arch and mesentericvessels, aortic diameters at the landing zone and the distance separating these structures from the pathologic condition. Thevascular access possibilities must be checked as well.

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fluoroscopy, are used to ensure correct timing.7 In case ofdissection, as for CT, a routine use of a second scan also ishelpful to evaluate late-enhancing vascular structures, andparenchymal perfusion. Both mask and contrast-enhancedimages are obtained with breath-holding and the maskimage is subtracted from the contrast-enhanced sequencebefore MIP reconstructions.Most commonly, post-processinginvolves the use of an MIP algorithm to create a projectionimage, but axial reformatting is also very helpful.

Figure 3 Unenhanced axial CT image (A) shows intramural hematohyperdense corona. Note also the displacement of small calcificatioatherosclerotic plaques. After contrast media injection, a focal pe

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The usage of gadolinium chelate contrast agents inpatients with chronic renal insufficiency was long consid-ered to be safe. However, there have been recent reportsthat gadolinium-based contrast agents increase the risk fordeveloping a serious medical condition called nephrogenicsystemic fibrosis (NSF) in patients with severe renal insuf-ficiency. NSF may result in a fatal or severe form ofsystemic fibrosis affecting the skin, muscle and internalorgans.8 In patients with severe renal insufficiency

ma in the arch and the descending aorta with a circumferentialns of the aortic wall inside the lumen, a major sign to identifynetrating atherosclerotic ulcer was seen.

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Figure 4 Axial CTA and 3D images of an 81-year-old man with a large aneurysm of the descending aorta revealing a containedrupture with a left hemothorax. Note marked dilation of the aortic root at the level of the sinus of Valsalva resulting in a ‘‘pear-shaped’’ aortic root.

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Figure 5 A 55-year-old male patient treated for coarctation with a post stenting pseudoaneurysm. 1/ CTA with 3D recon-structions. A large partially thrombosed pseudoaneurysm is seen with the bare stent placed at the level of the isthmus. 2/ T1-weighted axial image, and contrast-enhanced MRA of the aorta with sagittal oblique source and MIP images showing the falseaneurysm. The subtracted image, obtained by subtracting the unenhanced mask image from the contrast-enhanced source image,demonstrates better suppression of the background signal. Mild artefacts are seen related to the stent.

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(glomerular filtration rate <30 ml min�1 1.73 m�2), onlyunenhanced MR angiography should be performed, but thisdoes not allow an accurate evaluation of a potentialcandidate for a stent-graft implantation. In fact, risks ofcontrast injection must be individually evaluated inpatients with renal insufficiency in terms of risk/benefitratio, in particular if a stent graft is recommended.

Black-blood vascular imagingIn conventional spin echo MR imaging, blood usually is lowin signal intensity because of movement of spins betweena pair of (90� and 180�) slice-selective radiofrequencypulses. If blood flows out of the plane of the section in thetime interval between successive radiofrequency pulses,the result is absence of signal, called a signal flow void. Forbetter depiction of intra-luminal or mural abnormalitydedicated black-blood technique is preferred because itprovides better suppression of the signal from flowingblood.

Phase-contrast imagingPhase-contrast imaging is a unique MR imaging techniquethat measures blood flow and can be used in many clinicalapplications to evaluate physiologic properties of bloodflow. Two scans are acquired (flow-sensitive scan and

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a flow-compensated reference scan), which are automati-cally subtracted from each other. By knowing the cross-sectional areas of a vessel (measured from anatomicimages), blood flow volume and velocity can be calculatedquantitatively.

Echocardiography

TEE is a semi-invasive procedure that provides optimalimaging of the ascending and descending aorta and cansometimes visualise the upper part of the abdominal aorta.TEE can be performed quickly, at bedside and without usingnephrotoxic contrast agents, providing accurate measure-ments of different aortic segments, and is an importantdeterminant for diagnosis, management and follow-up. Theuse of multiplane probes allows reduction of the classicalblind zone, caused by the trachea at the junction ofascending aorta and proximal part of the arch. ColourDoppler enhances detection of the entry tear and canreveal the presence of additional multiple small commu-nications between the true and false lumens in case ofaortic dissection, especially those involving the descendingaorta. Intravenous infusion of an echographic contrast

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Figure 6 Trans-Esophageal echocardiography TEE can provide optimal imaging of the ascending and descending aorta. ColorDoppler enhances detection of the entry tear and can reveal the presence of additional multiple small communications betweentrue and false lumens in case of aortic dissection. Color Doppler flow through two intimal tears. A: towards false lumen (larger one)in systole. B: towards true lumen (smaller one) in diastole. Echographic contrast agent can enhance the performance of TEE todetect small re-entry tears of dissection and to demonstrate turbulence in the false lumen.

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agent can enhance the performance of TEE to detect smallre-entry tears of dissection and to demonstrate turbulencein the false lumen (Fig. 6). Screening of contraindicationsand preparation (local anaesthesia, intravenous access,material and structures for resuscitation) are especiallyimportant in cases of aortic pathologies.

Catheter Angiography

During the early years of TEVAR, diagnostic catheter angi-ography was routinely performed as a mandatory pre-procedural examination. However, catheter angiography isan invasive, expensive and time-consuming examination.Moreover, the projectional nature of fluoroscopic andcatheter angiographic imaging does not adequately portraythe 3-D configuration of the vessels, so that pre-proceduralplanning based on these images is subject to increasederror. Today, CT and MR angiography with 3-D reconstruc-tion abilities have replaced catheter angiography and hasbeen established as the gold standards for pre-proceduralevaluation (Fig. 7).

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Pre-interventional Planning

Chapter 2

Pre-procedural imaging combines three main tasks: todetermine the patient’s eligibility for TEVAR, to choose theappropriate stent-graft devices and to simulate a plan forthe intervention. For peri-procedural planning, variousanatomical factors need to be assessed, including thelocation and morphology of the neck in relation to themajor branches in order to determine the stent-graftlanding zones, the diameters of the neck to decide theappropriate size of the stent-graft device and the conditionof femoral and iliac arteries to choose vascular accesspathways (Fig. 8).

To ensure an adequate stent-graft placement, thefollowing conditions must be evaluated:

Vascular Access

Many major adverse events during TEVAR are related todifficulties with access arteries. To ensure that large-profile

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Figure 7 Angiography with a calibrated catheter, of a post-traumatic pseudoaneurysm located in the descending aorta.The length of the proximal landing zone along the lessercurvature of the aorta is only half that along the greatercurvature in the strongly angulated distal arch. The expectedlength of the device should be predicted in consideration of itsasymmetric redundancy in the proximal landing zone. Inaddition, a proximal landing zone in the strongly angulateddistal arch could be problematic with poor device appositionand risk of type I endoleak.

Figure 8 Prestent-graft measurement: Identification anddesignation of appropriate proximal and distal landing zones iscrucial for TEVAR planning. A central line (red line) must bedrawn to obtain strict perpendicular measurements to theaortic axis (double arrows). Proximally, the distance from theorigin of the left subclavian artery or the left common carotidartery to the beginning of the aneurysm defines the landingzone. The distance from distal end to the coeliac artery mustbe also measured. The length of aortic segment to be treatedcan be estimated using 3D reconstructions (blue line),measuring the length of the lumen from the beginning of theproximal landing zone to the end of the distal landing zone.The angulations of the aorta (yellow lines) and the plannedposition of the stent graft after deployment must be wellestimated. The degrees of accordion and bow redundancy maybe difficult to predict and are frequently underestimated. Forthe choice of an aortic stent-graft diameter, we use themaximum diameter from adventitia to adventitia. If the cross-sectional shape of the lumen at the landing zones is ellipticalor even crescentic rather than circular, mathematical model-ling is done (maximumþminimum diameter/2) to obtaincorrect device sizing.

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devices can safely navigate the selected vascular route tothe thoracic aorta, preoperative imaging must include high-resolution characterisation of the femoral arteries, iliacarteries and infra-renal aorta. Diameters, tortuosity,degree of atherosclerosis and calcifications of potentialaccess vessels all need to be evaluated. The presence ofprevious stenting of the iliac arteries or small aorto-iliacbypass could be another contraindication for a femoralapproach as they are inextensible, and therefore may notallow the introduction of stiff devices. In such situations,alternative routes such as the abdominal aorta, thoracicaorta or supra-aortic branches could be used.9 Recently, inselected patients, entirely percutaneous stent-graftprocedures have been performed with the puncture sitesclosed by commercially available suture-mediated access-closure devices. In comparison to routine surgical expo-sures, this approach has the advantages of a shortened timeto recovery and possible reduced risk of access sitecomplications, namely infection, lymphocoele, seroma andpostoperative scars. The major limitations are clearly smallfemoral arteries and calcification that must be checkedfirst before shifting to entirely percutaneous procedures.

Length and morphology of landing zones

Identification and designation of appropriate proximal anddistal landing zones is crucial to the clinical success ofTEVAR.

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The length of the proximal landing zone is usuallymeasured along the greater curvature of the aorta, but theactual length along the lesser curvature may be signifi-cantly shorter when the distal arch is involved (Fig. 7).Proximally, the distance from the origin of the leftsubclavian artery (LSCA) or the left common carotid artery(LCCA) to the beginning of the aneurysm defines the landingzone. When the neck is shorter than 15 mm, the stent graftcan be deployed to intentionally cover the LSCA. It remainscontroversial whether re-vascularisation of a covered LSCAis necessary to reduce the risk of stroke, paraplegia, ver-tebro-basilar insufficiency and/or left upper extremityclaudication.10 Nevertheless, if intentional exclusion of theostium of the LSA is unavoidable, complete imaging of the

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vertebral arteries and collaterals must be performed beforeimplantation to avoid major neurological complications.

The length of the distal neck defined as the distancefrom the lower end of the lesion to the coeliac axis is a less-frequent limiting issue. Coverage of the coeliac axis canalso be performed intentionally, and is usually well toler-ated as long as there is no occlusive disease of the superiormesenteric artery.11 Again, pre-stent-graft placementimaging of the mesenteric vasculature to identify thepathways between the coeliac trunk and mesenteric arterythrough the gastro-duodenal artery is mandatory.

The extension of the disease to the visceral arteriescould also be resolved by the use of fenestrated orbranched stent grafts.12 The size, patency and angulationsof each visceral artery must be taken in consideration toselect the stent graft accordingly.

The radiculo-medullary artery of Adamkiewicz maysometimes be preoperatively identified by CT or catheterangiography. Ischemia involving this artery is the mostcommonly accepted mechanism of postoperative para-plegia. Coverage of an inter-costal artery supplying theartery of Adamkiewicz should be avoided if possible,requiring additional tailoring of the landing zone. Mean-while, the risk of this complication seems to be directlyrelated to the number of excluded collaterals. In otherwords, paraplegia is observed in less than 4% of cases andthe associated risk factors are the combination of abdom-inal and thoracic treatment, exclusion of the LSCA, longstent grafts and the situation at the level of the diaphragm(Greenberg, 2000 #1317).13,14

Diameter of the stent graft

Stent-graft diameter is oversized by 10e15% to providegood apposition to the aortic wall. The reference diameteris the external diameter of the aorta at the systolic phase ifgated MDCT is used.3 Furthermore, the cross-sectionalshape of the lumen at the landing zones may not becircular, but elliptical or even crescentic, requiring addi-tional mathematical modelling (maximumþminimumdiameter/2) to obtain correct device sizing. Since themaximum diameter of commercially available stent graftsis 46 mm, a maximum aortic landing zone diameter of 40e42 mm must be selected. Diameters of proximal and distallanding zones may differ widely enough to requirea tapered or flared device or combination of devices.

Device length

Most device manufacturers only have standardised lengthsavailable, although some will also accommodate customlengths. The length of aortic segment to be treated can beestimated using 3-D reconstructions, measuring the lengthof the lumen from the beginning of the proximal landingzone to the end of the distal landing zone (Fig. 8). Thedegrees of accordion and bow redundancy may be difficultto predict and are frequently underestimated. Beingprepared with longer devices and extension componentsmust be always warranted at the time of treatment.

Arteriosclerotic aneurysms of the descending thoracicaorta typically occur in elderly patients and often manifest

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in a diffusely diseased and tortuous aorta, relativelynormal-looking segments of which may dilate over time.Meanwhile, the aneurysms tend to elongate, producingvector forces that may eventually cause the stent graft tomigrate. Therefore, it is advisable that the landing zones ofthe stent graft be at least 2 cm long. Moreover, whenmultiple stent grafts are deployed, generous overlapping(>5 cm) should be considered to avoid late disconnection.

The angulations of the aorta and the predictable posi-tion of the stent graft after deployment must be wellestimated. Sharp angulations and/or high tortuosities mayentail shifting the landing zone proximally or distally.Different designs of devices have different capabilities ofconforming to angulations and tortuosities, and newergenerations of devices are being designed with this featurein mind.

Specificity according to clinical applications

Chapter 3

Aortic dissectionRecently introduced interventional techniques, such asaortic fenestration and stent graft implantation, haveopened new therapeutic options, which can be performedbefore or after surgery.15 Multidetector contrast-enhancedCT is the best imaging tool in the emergency setting toevaluate a clinically suspected dissection. The extent ofthe dissection, true and false lumen sizes, false lumenpatency, proximal and distal tear sites as well as branchvessel involvement must be evaluated by CT, which shouldextend from the neck base to the femoral arteries. Peri-aortic fluid, pericardial effusion and pleural effusion areimportant additional signs.

In order to obliterate the primary tear, an adequate sealzone is required. We recommend placing the device, asoften as possible, in disease-free landing zones. In otherwords, in the setting of a classic entry tear located justdistal to the LSCA origin, the segment between the LCCAand LSCA should be used as a safer landing zone. Never-theless, in cases of aortic dissection, the distal true lumenis usually tapered and compressed by the false lumen; asa result, the distal end of the device is deployed in a lumenmuch narrower than that of the proximal landing zone. Inacute dissection, expansion of the true lumen is not hard toachieve if the false lumen is adequately excluded by thestent graft, and the true lumen will expand gradually to thenominal diameter of the stent graft. Inversely, in chronicdissection, the intimal flap is fibrotic and cannot expand.This reduction of the true lumen diameter may posea challenge when using cylindrical stent grafts, witha possible risk of collapse or incomplete expansion of thegraft. In these situations, a tapered stent-graft device maybe a better option.

The length of the stent graft is another issue to discuss.Whereas false-lumen thrombosis is consistently seen at thelevel of the implanted stent graft, thrombosis distal to theimplant, particularly in the distal descending aorta, is lesscommon. This may be related to the to-and-fro movementof the intimal flap along uncovered segments of the aortawith retrograde flow through secondary distal tears. This

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has implications for the selection of device length. In fact,most investigators use longer stent grafts than required tosimply cover the primary tear. The additional distalcoverage provides a more normal anatomic configuration byplacing the distal margin of the device within the mid-thoracic segment, where the aorta is usually less curved.Further distal extension may also expedite the rate of false-lumen thrombosis. In light of this, some investigators haveadvocated the placement of bare stents into the distal

Figure 9 Drawing of the different mechanisms of visceral ischeorifice. �Ischemia does not develop in a since there is a large natuoccluded. B. Dynamic dissection. In this pattern, the aortic branccompromised owing to true lumen compression by the false lumecould be observed. If no re-entry within the branch is observed, iscComplete rupture could be observed also. Ischemia does not develoischemia could develop because the intimal flap is retracted thus ocbe proposed. TL Z True lumen; FL Z False lumen.

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thoracic aorta to provide structural stability without riskingocclusion of additional inter-costal arteries.16

Ischaemic complications are encountered in 30e50% ofthe patients and represent one of the major indications forsurgical intervention.17 The most commonly affectedarteries are the iliac vessels; however, the most seriouscomplications are encountered with renal and/or mesen-teric ischemia. Occlusion of aortic side branches can becaused by two different mechanisms, which in fact may

mia. A. Static dissection. The aortic branch is involved at itsral fenestration. �Ischemia could develop in b as the branch ish itself is not involved by dissection. However, blood supply isn. C. During dynamic dissection intimal stretching of a branchhemia could develop due to compression of the true lumen. D.p if the intima of the collateral is retracted in the aorta. E. In ecluding the branch. In such a case, stenting of the branch could

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Figure 10 A 60-year-old male patient with a history of hypertension and family history of aortic dissection presenting with sharpepigastric pain radiating to the chest. CTA revealed type B dissection. Axial image at the isthmus level shows intimal flap and tearsite. Axial image at the level of the right pulmonary artery clearly shows the intimal flap separating the false lumen from the truelumen. However, blood supply is compromised at the distal end of the descending aorta, owing to true lumen compression by thefalse lumen. Axial image at a more cranial level reveals involvement of major arch arteries. The intimal flap was seen involving theostia of the coeliac trunk and the superior mesenteric artery (Static dissection). Dynamic compression of the right renal artery andintimal flap retraction of the left renal artery can also be noted.

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occur simultaneously as well (Figs. 9 and 10). The firstmechanism is static vascular obstruction, where thedissection involves the affected side branch. Additionaloutflow obstruction may be caused by thrombosis of thefalse lumen at the level of the side branch, thus com-pressing the true lumen. Fortunately, in some cases theaortic side branches are perfused through a re-entry tear,thereby re-perfusing the true lumen with subsequentoutflow. The second mechanism, dynamic obstruction issecondary to compression of one of the two aortic chan-nels, (usually the true lumen) without extension of theintimal flap in the occluded side branches. A prolapse of theintimal layer into the ostium of the non-dissected sidebranch can accompany compression of the true lumen.(Fig. 11).

Alternatively to stent-graft insertion, fenestration andbare stent insertion could be proposed separately orcombined.

Fenestration continues to be a valuable option in casesin which stent-graft treatment is not feasible owing toanatomic or organisational constraints.18 Fenestration may

Please cite this article in press as: Rousseau H et al., The ImportancEndovasc Surg (2009), doi:10.1016/j.ejvs.2009.06.017

also serve as a valuable adjunctive treatment when stent-graft repair inadequately addresses the indications forintervention. Several approaches to dissection flap fenes-tration have been described.19 The use of uncovered stentsin the aorta and side branches may improve flow in aorticdissection. The most common indications include (1) inad-equate relief of dynamic obstruction after surgery, stent-graft treatment or fenestration, and (2) static obstructionof abdominal aortic branch vessels. Similarly, uncoveredaortic stents may be a useful adjunct to dissectionfenestration.

Nevertheless, before the fenestration or stenting,a complete exploration of the mechanism of the dissectionby MDCT is crucial.

Traumatic aortic injuries

Because of its non-invasiveness and availability in mostemergency departments, CT has become the main imagingmodality in the evaluation of traumatic aortic injuries. Itallows rapid assessment of the entire thorax, abdomen as

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Figure 11 CTA showing an abdominal extension of a type Bdissection. Blood supply of the left kidney is compromisedbecause the intimal flap is retracted (arrow) thus occluding thebranch. In such a case, stenting of the branch could beproposed.

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well as the brain, while patients are being monitored.Intramural haematoma, intimal tear, pseudoaneurysm andextravasation of the contrast material from the aorta aretypical CT findings of acute traumatic aortic injuries(Fig. 12). The aortic isthmus is the most commonly injuredaortic site.20 In this case, the reference diameter is thesegment of the arch between the LCCA and LSCA. In spite ofattractive aspects of stent-grafting, some technical diffi-culties have to be overcome such as small and spasticarteries, short proximal neck and important curvature ofthe distal aortic arch. However, because aortic traumatism

Figure 12 CTA of a 21-year-old male patient who sustained injuclassic features of a posttraumatic pseudoaneurysm located in the dto a ‘‘pseudo coarctation syndrome’’. Note mediastinal hematoma

Please cite this article in press as: Rousseau H et al., The ImportancEndovasc Surg (2009), doi:10.1016/j.ejvs.2009.06.017

occurs more often in young patients with immature(<20 mm) aorta, the main limitation is the commondilemma of selection of the correct stent-graft dimension.In the last few years, because of the lack of commerciallyavailable endovascular devices with small diameters,a device of a minimum 26 mm was used. Theoretically, bydoing so, we are exposing the patient to aortic rupture,a complication that has not yet been described in theliterature. On the other hand, very recently, endovasculardevices with small diameters (20 mm) are becomingcommercially available, raising the question of a lateaorta/stent graft diameter mismatch. Theoretically, as weare treating an immature aorta, we can postulate that theaortic diameter will increase by at least 20e30% with time.Consequently, if a small stent graft is used today, after 10e20 years, the patient will be exposed to an undersized stentgraft in comparison to his then normal aortic diameter, witha subsequent risk of migration, endoleak or pseudo-coarctation.

Future of imaging

Chapter 4Computational fluid dynamics (CFD) obtained with anentirely non-invasive 4-D MRI protocol provides time-varying geometry and flow rates21,22 (Fig. 13). Froma practical viewpoint, we start by performing a routinecontrast-enhanced MR angiography using a 3-D slabcovering the vessels of interest using an injection of gado-liniumediethylenetriamine penta acetic acid (Gd-DTPA).Secondly, 3-D velocity phase-contrast imaging is performedwith the same orientation of contrast-enhanced acquisi-tion. An initial computational grid is obtained by the dis-cretisation of segmented contrast-enhanced MRangiography.23 The flow simulations are performed usingthe finite volume (FV) method to solve the full NaviereStokes equations that govern the flow.

On the whole, the importance of the implementation ofquantitative evaluation strategies to assess flow rates, wall

ry from a motorcycle accident. Axial images elegantly displayistal aortic arch. Compression of the distal aortic lumen leadingand left pleural effusions.

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Figure 13 Aortic coarctation causing dilation of the ascending aorta with pre- and post-stenotic dilatation. On this figures,obtained with conventional MRA (A), Q Flow measurements and CFD obtained by the finite elements (B), we can see the differentparameters analysed: WSS (C), Pressure measurement (D), Flow (E) and turbulences (in red) (F). Coarctation is responsible for lowWSS in the arch, ascending and descending aorta and branch arteries. At the level of the stenosis, increased WSS, pressure andturbulences can be observed.

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shear stress and vorticity is therefore warranted in order todevelop and apply objective diagnostic criteria for thedescription of vascular pathologies. Furthermore, the post-processing virtual 4-D modelling represent theoretically animportant purpose for future investigations on the aorticdissection, in order to develop an imaging-based methodfor planning the interventional strategies that are actuallybeing performed on an empirical basis. Finally, computing,in conjunction with suitable experimental data, can beimportant by helping understand the complex relationshipbetween biomechanics and device failure and by aiding indesign of better devices while shortening design cycle time.

Conclusions

TEVAR has become widely accepted as an important optionfor treatment of thoracic aortic diseases. Recent advancesin cross-sectional imaging, particularly MDCT angiography,have helped to establish a new gold standard for pre-procedural assessment of potential TEVAR candidates.Optimisation of imaging protocols, as well as a thoroughunderstanding of the indications, candidacy criteria,procedures and potential complications are essential toderive maximal benefits from these advances in modernimaging technology.

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