The Evaluation of Aortic Dissections with Intravascular Ultrasonography

VASCULAR DISEASE MANAGEMENT 2011;8(4):E93–E97

Abstract

Intravascular ultrasound is an important intravascular imaging technology that may be used in the operating room or endovascular suite as an adjunct to other imaging modalities in the treatment of vascular disease. It is of critical importance in the endovascular treatment of aortic dissection where it facilitates true lumen access, identification of the primary fenestration, accurate measurement of the normal proximal aorta for suitable endograft choice and real-time assessment of the changes in flow in the true and false lumens.

Introduction

Aortic dissection is the most common aortic emergency, with 9,000 new cases in the U.S. each year, yet it remains associated with high overall morbidity and mortality rates.^1,2 Acute proximal aortic dissection is a surgical emergency because of high early mortality, and emergency open aortic repair remains the gold standard for treatment of this pathology. Despite years of experience and the evolution of perioperative techniques, early mortality rates of between 20 and 30% are consistently reported. For proximal dissection, medical management alone in the cohort of patients unfit for operative intervention is associated with an in-hospital mortality rate of 50% and poor long-term outcomes.³

Acute aortic dissection affecting the descending aorta is associated with less early mortality compared with proximal dissection. Early intervention in this cohort is reserved for those with a “complicated” dissection as evidenced by malperfusion, hemodynamic instability or persistent pain. Patients with uncomplicated or asymptomatic distal dissection have a 30-day mortality rate of 10%. Conversely, those who present with or develop limb ischemia, renal failure, visceral ischemia, or contained rupture often require urgent aortic or branch vessel intervention with a resultant 30-day mortality rate of 33%.^4,5

Advances in endovascular technologies in the last 20 years have led to a change in the treatment of aortic aneurysm disease. This paradigm shift has also been mirrored in the treatment of distal aortic dissections — both in the acute setting and in patients who develop aneurysmal degeneration or other branch vessel compromise of the aorta after the acute event. Increasingly, the use of endovascular technology in the setting of distal aortic dissection is becoming a mainstay of treatment, and devices specific to this pathology are being developed. One of these, the Cook Medical Zenith^® Dissection Endovascular System (Cook Medical, Inc., Bloomington, Indiana), features a bare-metal stent distal to the covered stent, which provides support for the delaminated segment of the aorta and has large openings, allowing access to the visceral branches should secondary intervention be necessary. It is postulated that the additional support provided to the delaminated aorta will permit aortic remodeling and prevent aneurysmal degeneration in the future. It is not yet FDA-approved in the United States, but preliminary results from its use in Europe are available. Although a small number of case reports^6–8 have been published describing endovascular methods utilized to treat proximal aortic dissections as well, no extensive series, comparative studies or even intermediate outcomes exist to date.

Increasing use of endografts for various aortic pathologies have demanded parallel improvements in aortic imaging to allow accurate planning, measuring and implantation of endografts. For endovascular treatment of aneurysms, computed tomographic (CT) angiography with centerline of flow (CLF) analysis has become the standard method to determine endovascular candidacy based on assessment of lesion morphology, vessel diameter, presence of thrombus or calcification and the presence of nearby branch vessels. Imaging requirements necessary for the treatment of aortic dissection with an endograft differ in some respects from that of aneurysmal disease. Akin to open surgical objectives, successful treatment depends on identification and sealing of the proximal fenestration, limitation of distal false lumen flow and the prevention of retrograde propagation of the dissection. Utilization of electrocardiographic (ECG)-gated CT scans reconstructed at either 55 or 70% of the R-R interval (based on the type of CT scanner) to minimize motion artifact in the ascending aorta have allowed identification of the primary fenestration in only 43–82% of cases, therefore other supplementary imaging modalities are necessary to permit successful endovascular treatment.^9,10 In this supplemental role, IVUS is a valuable tool for the endovascular surgeon.

The goal of imaging in endovascular aortic interventions is to enable adequate visualization of the aorta and its branches. Fluoroscopy remains the gold standard against which other imaging technologies are compared. IVUS is an important intravascular imaging technology that may be used in the operating room as an adjunct to other imaging modalities. Due to its intraluminal perspective, it provides information that the other modalities cannot — and gives information about flow as well as anatomic landmarks (Figure 1). It allows real-time luminal and cross-sectional imaging of blood vessels and provides detailed information with regard to vessel anatomy, lesion morphology, intimal fenestrations, branch vessel origin and the presence or absence of thrombus. It has been used quite extensively in the aorta to allow accurate placement of endografts for aneurysmal disease.^11–13This review will examine the role that IVUS plays in the evaluation and treatment of aortic dissection.

IVUS: General Characteristics

Since its introduction in the 1980s, IVUS has been increasingly used in the interrogation of the coronary arteries, the aorta and its branch vessels.^14,15 It has become more user-friendly over time with improvements in the catheters used and the display monitors. It is a catheter-based technology and consists of a cylindrical ultrasonic transducer mounted on the distal end of the catheter that provides real-time images of blood vessels. Large-vessel catheters can be accommodated by 8 Fr sheaths, although coronary IVUS probes are much smaller. Mostly because of the predominance of use in the coronary bed, but also due to cost and limited ability to interpret images, IVUS has been underutilized by vascular surgeons in the past, but it is a very valuable adjunct when performing endovascular procedures.

A wide range of IVUS catheters are commercially available, and catheter choice depends on the size of the particular vessel being interrogated and the resolution required to perform the endovascular procedure. Current IVUS catheters function in high-resolution B mode and use frequencies ranging from 10–40 MHz depending on the size of the target vessel. For the evaluation of large-diameter thoracic aorta pathology, lower-frequency catheters (8–10 MHz) are used with a slightly higher frequency (20–30 MHz) used for iliac procedures. Lower-frequency catheters allow us to scan the entire circumference of the aorta and even though the resolution is not as good as with higher-frequency devices, the branches can still be identified, as they are of large caliber in this region. In the presence of a dilated aorta, however, even low-frequency catheters may not be adequate to visualize the entire circumference of the aorta, and branch vessel visualization may be dependent on the path of the catheter through the aneurysmal segment. High-frequency catheters are used to evaluate finer details in smaller vessels such as the superficial femoral artery (SFA). At our institution, we currently use either the Visions^® PV 8.2 Fr IVUS imaging catheter (Volcano Corp., San Diego, California) or the Sonicath Ultra^® Ultrasound Catheter (12 MHz) (Boston Scientific Corp., Natick, Massachusetts) for our aortic cases. Access may be achieved percutaneously or via an open femoral arterial approach. Other considerations include the sheath necessary to insert the catheter and the choice of wire over which to insert the catheter. The lower-frequency catheters require an 8–10 Fr sheath to facilitate insertion. Wire choice is usually a stiff 0.025 inch or 0.035 inch guidewire. This allows a more controlled insertion of the device through diseased tortuous iliac arteries. Catheters that track over centrally placed guidewires have been shown to provide superior imaging to those with an eccentrically placed guidewire and thus are used preferentially now.¹⁶

Acquisition of images is obviously operator-dependent, but the basic principles of ultrasound apply. Two-dimensional images are acquired by inserting the catheter over the wire into the area to be interrogated. Localization using fluoroscopy can also be helpful. Rotation of the beam around the axis of the catheter 360º then provides additional real-time views of the vessel. Interpretation of images requires familiarity with image artifact from both wires and catheters and also the knowledge that angulation will result in overestimation of the size of the vessel; the resultant images are not always in a plane perpendicular to the vessel. Arterial calcification and synthetic graft material can also interfere with the quality of the signal. Whereas Dacron does allow accurate visualization beyond the fabric, other synthetic materials such as polytetrafluoroethylene interfere with ultrasound images due to their absorptive and reflective qualities. Optimal aortic visualization is achieved when the catheter is directed perpendicular to the aortic surface. As a general rule, superior images tend to be acquired on withdrawal of the probe rather than during insertion — the so-called “pull-back technique.” This can be achieved by withdrawing the probe manually or using a motorized device.

IVUS and Aortic Dissection

IVUS has proved useful as an adjunct during implantation of endografts in both the abdominal and thoracic aorta for the treatment of aneurysmal disease.^11–13 It allows measurement of iliac and aortic diameters for sizing endografts, identification of branch vessels thus permitting accurate fixation of endografts, as well as confirmation of cannulation of the contralateral limb. Post deployment of the endograft IVUS can be used to assess patency of the branch vessels and it is very useful for assessing how well the device has conformed in the aneurysm neck. In-folding of the graft or poor apposition to the aortic wall can be identified, allowing supplementary ballooning and thus avoiding secondary procedures. IVUS can also be used to assess for post-deployment endoleaks, although several groups have shown that both conventional angiography and transesophageal echocardiography (TEE) are more useful at identifying endoleaks.^17–19 Utilization of IVUS for these cases should result in a reduction in the amount of fluoroscopy time and contrast load. While not routinely used in the majority of centers, utilizing IVUS in patients with known renal impairment allows limitation of contrast dose, thus decreasing the risk of post-operative renal failure.

First described over 20 years ago in the evaluation of aortic dissection, IVUS now has an even greater role to play in its management.²⁰ The treatment of aortic dissection, both acute and chronic, remains one of the most controversial topics in cardiovascular surgery. With improvements in medical management, more patients are surviving the acute phase of the illness and thus chronic dissection with aneurysmal growth and recurrent dissection has become a more common phenomenon, mandating a need for less invasive treatment strategies with acceptable perioperative morbidity. Endovascular treatment for complicated acute distal aortic dissection is now recognized as a good alternative to open surgery.⁴ The treatment of chronic distal aortic dissection is more controversial, however, recent results from our own institution suggest favorable outcomes with endovascular treatment.²¹ IVUS plays several critical roles in facilitating the endovascular treatment of aortic dissection.

Accessing the true lumen in an aortic dissection is one of the key steps when approaching treatment of aortic dissection and confirmation of placement of the catheter and wire in the true lumen was very challenging prior to the use of IVUS because angiographic images can be difficult to interpret. Indeed, conventional angiography alone has been shown to be inaccurate at confirming the presence of the guidewire within the true lumen.¹⁷ IVUS allows real-time imaging to first ensure wire access in the true lumen, but also to ensure that one stays within the true lumen. We now routinely use an IVUS probe prior to stent-graft placement in the setting of dissection to confirm placement of the graft in the true lumen (Figure 2). The primary goal of treatment involves identification of the primary fenestration and excluding it with the endograft, thus obviating false lumen flow. With IVUS the primary fenestration can be accurately identified and its proximity to the arch branches assessed. While IVUS, TEE and conventional angiography appear equivalent at identifying the primary entry site, IVUS and TEE have been shown to be superior when assessing for subsequent entry sites.¹⁷ This is of significant importance, as it may necessitate implantation of additional devices to successfully treat the dissection. Identification of other critical branches such as the visceral branches can then be performed before measurements are taken and an appropriate endograft chosen (Figure 3). Once the endograft is deployed, systolic flow patterns in the true lumen would be expected to accompany stasis of flow and thrombosis in the false lumen and movement of the septal flap back towards the false lumen, indicating adequate stent length coverage. IVUS allows for the assessment of these changes in flow patterns within the false and true lumens. Correct apposition of the stent to the aortic wall can then be assessed, as well as ensuring patency of the branch vessels.

While the majority of patients would undergo a CT angiogram in the diagnostic workup for aortic dissection, it is not always entirely accurate for sizing an endograft due to the tortuosity of the thoracic aorta. IVUS can be very useful for accurately sizing the normal proximal aorta to allow correct choice of endograft and identifying length of fixation available distal to branch vessels. Initial reports indicated that IVUS tended to undersize the aorta when compared with digital subtraction angiography and CT angiography.^13,22 While this difference has been substantiated in more recent reports, the difference is deemed to be so small that it would not be clinically relevant when choosing endografts.²³ IVUS is particularly helpful when sizing an endograft for completion of an elephant trunk procedure for chronic dissection. In this situation, very often the Dacron graft is folded in on itself or has dilated over time, so accurately sizing an endograft based on a CT angiogram or on the size of the original graft implanted can be difficult. Instead, IVUS can be used first to confirm cannulation of the synthetic graft and then to measure the diameter of the graft so an appropriately sized endograft can be chosen.

IVUS is also a superior technique to CT angiography for assessing true lumen patency and compression in aortic dissection. CT provides images at a single time point, which is not particularly useful when assessing something such as a dynamic dissection flap, which is constantly changing. IVUS provides real-time images of the flap and can evaluate the degree of true lumen flow and compression during the different phases of the cardiac cycle and thus assess the impact of the dissection on critical visceral and renal arteries. It also provides information about whether these come off the true or false lumen.

Current conventional IVUS technology utilizes a circular probe, as discussed above, but this provides limited tissue penetration. It also possesses no Doppler or color Doppler capabilities. As other imaging modalities have advanced over the last number of years, IVUS technology is advancing too. There now exists a phased-array IVUS probe with Doppler and Doppler capabilities called AcuNav (Siemens, Mountain View, California). It has tissue penetration capabilities of up to 15 cm and is mounted on the tip of a steerable 10 Fr catheter that is 90 cm in length. It has been shown to be safe and effective when used in combination with fluoroscopy in EVAR procedures.²⁴It does have some limitations including poor levels of detection of endoleaks and also difficulties obtaining accurate aortic diameters, as the probe has to be rotated 180º and the two measurements added together. As the technology continues to advance, it will undoubtedly play an important role in the treatment of patients via endovascular means with renal impairment, as contrast can be excluded.

Penetrating Aortic Ulcer and Intramural Hematoma

A greater awareness of aortic pathologies has led to an increase in cross-sectional imaging for patients who present to the emergency department with chest pain. This has led to a greater number of diagnoses of abnormalities such as penetrating aortic ulcers and intramural hematomas of the aorta. IVUS is very useful in this context for identifying the exact area of intimal disruption.²⁵The actual hematoma may extend for a distance on either side of the disruption, and IVUS can accurately measure this to allow effective treatment of the problem without covering unnecessarily large areas of normal aorta and increasing the risk of spinal cord ischemia.

Conclusion

Limiting radiation exposure is important for both patients and operating teams, as cumulative radiation doses for either cohort are detrimental in the long term. As vascular surgeons we must continue to look for alternative imaging strategies to limit fluoroscopy times and to improve aortic visualization. IVUS has become an invaluable resource for the endovascular treatment of aortic diseases. It is particularly useful in the treatment of aortic dissection and has expanded our ability to perform endovascular surgery for this pathology. Utilizing IVUS in these endovascular procedures traditionally performed using only conventional angiography and fluoroscopy has allowed us to decrease both fluoroscopy time and contrast load. Further advances in IVUS technology with vector phase-array probes should continue to reduce the amount of contrast and radiation exposure for our patients treated using endovascular techniques.

Disclosure: Dr. Mastracci reports that she is a paid consultant to Cook Medical, Inc.

Address for correspondence: Dr. Tara M. Mastracci, Department of Vascular Surgery, Cleveland Clinic, 9500 Euclid Avenue, Desk H-32, Cleveland, OH 44195. E-mail: mastrat@ccf.org

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