Live 3D Guidance in Endovascular Procedures
A showcase of patient studies illustrating the benefits of advanced live 3D guidance tools in a busy day-to-day clinical practice.
Today's endovascular interventionists must be skilled with the latest device and imaging technology to best treat their patients. An accurate morphological assessment of blood vessels and their relationship with interventional devices is essential to the management of vascular lesions, and digital subtraction angiography (DSA) has had a central role in the deployment of interventional procedures. High resolution, the ability to selectively evaluate individual vessels, and the access to direct physiological information make DSA a cornerstone technology in endovascular procedures. The addition of three-dimensional rotational angiography (3D RA) and flat panel detector technology have further contributed to greater diagnostic accuracy, faster procedures, and improved outcomes.1-4 In particular, 3D reconstructions from rotational angiograms have overcome limitations of two-dimensional (2D) imaging such as false vascular foreshortening due to projection angle and vessel obscuration due to overlying vessel or bone. These 3D reconstructions are now routinely used to reveal complex vascular relationships. Major developments in hardware and software of x-ray angiographic systems have significantly increased the usability of this technique, and 3D reconstructions are now performed and available for analysis and manipulation in real-time. The availability of 3D reconstructions also carries along a wealth of 3D tools that can improve current procedural workflows starting from disease assessment to treatment evaluation through navigation and treatment planning.
The use of 3D RA and associated tools in endovascular procedures is, however, not yet common practice. Although evidence of the clinical benefit and reduction in radiation exposure to patients and staff following the use of 3D RA has been extensively reported for vascular lesions of the neck and brain,5-10 there is not enough information in the literature on how the use of 3D tools can improve the (peripheral) vascular procedural workflow with potentially no changes (or reduction) to radiation and contrast administration. This article provides a showcase of minimally invasive endovascular applications in which the availability of 3D tools was distinctively beneficial to achieve effective disease management in a busy day-to-day clinical practice.
MATERIALS AND METHODS
The interventional radiology department at Paoli Hospital performs approximately 5,400 procedures per year, covering the full spectrum of vascular and nonvascular interventions. In order to provide a suitable range of endovascular procedures, 10 patients were selected retrospectively from a data pool of more than 200 subjects treated between 2006 and 2007 with clinically indicated rotational angiography. The clinical indication was made by the interventional radiologist on a per-case basis based on his judgment as to whether the 3D tools would help in problem solving or clarification of anatomy, offer any potential time savings, possibly reduce contrast or x-ray dose to the patient, or uncover a suspected hidden lesion. All rotational examinations were performed on a Philips Allura Xper FD20 system (Philips Medical Systems, Best, The Netherlands) using an automated "two-button" 3D acquisition process. Contrast material was injected during acquisition through a catheter placed into the vascular territory of interest. A total of 120 frames were obtained during a 240¡ rotation for a total scan time of 4 seconds. The frames were automatically sent to a dedicated workstation where 3D reconstructions with a field of view of 25 X 19 X 25 cm3 and a matrix size of 1283 were generated in real time. These real-time 3D images were displayed in the procedure room, alongside or even embedded within the fluoroscopic images. All 3D analyses were performed live by the physician who carried out the examination.
The 3D reconstructions were used to visualize the vascular anatomy from multiple angles, select the optimal working projection, and simultaneously synchronize the position of the C-arm. This is now possible because modern workstations are fully integrated with the acquisition system and directly coupled with the
C-arm geometry. The synchronization could also be exploited to achieve live 3D navigation through the real-time superimposition of live fluoroscopy images on a surface-rendered 3D reconstruction (Dynamic 3D Roadmap, Philips Medical Systems). Changes to the live fluoroscopy images after adjustment of x-ray/detector distance, position, and/or magnification are transferred to the 3D reconstruction so that the matching is maintained throughout the procedure. When available, preinterventional computed tomography (CT) and magnetic resonance (MR) scans could also be transferred to the workstation, matched with 2D live fluoroscopy, and used for 3D navigation.
Three-dimensional reconstructions were further processed using quantitative analysis tools. Vessel size and stenosis grading were measured in 3D using advanced vascular analysis and virtual stenting functionalities, which provide an accurate delineation of local vascular diameter and length and a simulation of stent placement that facilitates the selection of commercially available stents. In the case of aneurysms, a built-in, computer-assisted aneurysm analysis tool was used to automatically detect significant vascular dilatations and obtain quantitative information such as aneurysmal volume and neck size.
For procedures requiring intraoperative visualization of soft tissues, CT-like 3D reconstructions were acquired using the XperCT (Philips Medical Systems) technique. XperCT is an additional rotational acquisition mode performed over a scan range of 240° resulting in 310 frames and a total scan duration of 10 seconds. When required, XperCT and 3D RA were matched to visualize interventional devices and soft tissues along with contrast-filled vessels and assess treatment completion.
Three-dimensional imaging is useful in the assessment and grading of vascular stenoses. Not only can it provide a view of the anatomy from angles not achievable with 2D imaging, but 3D imaging also helps in the detection of hidden stenoses not visible in 2D projections and other diagnostic images (Figure 1).
The benefits of 3D information also extend to a better estimation of the required stent characteristics. For instance, when performing iliac stenting, most interventional radiologists are trained to acquire an anteroposterior (AP) aortoiliac arteriogram and two additional oblique angiographic projections. Due to the high tortuosity of the external iliac arteries, the use of conventional 2D images and "eyeballing" may lead to underestimation of the length of the vessel and ultimately to incorrect stent sizing. By using 3D RA and quantitative tools, the radiologist can accurately evaluate the extension and grading of the stenosis and select the right stent from the start, thus avoiding the need of overlapping stents with the associated extra costs and risks of recurrent stenosis (Figure 2).
Three-dimensional imaging provides detailed anatomical information that assists decision making for emergency and trauma patients. Three-dimensional information can be used to elucidate the relationship between foreign bodies and vascular structures and support/exclude the need for complex surgical procedures (Figure 3). In this case, 3D RA was superior to high-resolution multiplanar CT visualization, film radiography, and conventional 2D venography in providing quick, optimal anatomical viewing and facilitating timely and confident decisions.
Accurate imaging information on volume, location, neck size as well as the relationship with parent vessel and side branches is essential in the diagnostic assessment and treatment planning of aneurysms. Often, this type of information cannot be obtained—neither from 2D DSA due to complex angles required to view the aneurysm neck nor from CT angiography (CTA) and magnetic resonance angiography (MRA)—because of inadequate spatial resolution, especially for small branch vessels. CTA and MRA typically provide a spatial resolution greater than 0.4 to 0.5 mm, whereas a resolution of 0.1 to 0.2 mm can be expected from 3D RA.
Renal arteries are common locations of vascular aneurysms, and appropriate anatomical information is critical. For example, incorrect deployment of endovascular coils due to underestimation of neck size may lead to nontarget embolization with possible kidney infarction. Three-dimensional RA offers high spatial resolution and can achieve a detailed 360¡ view of the vessels of interest. Computer-assisted aneurysm analysis tools can also be used to automatically define the aneurysm sac, calculate its dimensions, and analyze its relationship with the surrounding vessels, facilitating both diagnostics and interventional planning (Figure 4). In this case, a video file of the 3D RA reconstruction was supplied to transplant surgeons prior to surgery to help them plan the best method of vascular repair during bench surgery (autotransplantation). This resulted in minimizing the ischemia time of the explanted native kidney during repair, because much of the repair planning could be done prior to surgery.
Inferior Vena Cava Filter Removal
Retrievable inferior vena cava (IVC) filters are increasingly being used to prevent pulmonary embolism. Several weeks or months after implantation, patients are typically brought back for filter removal. Most retrievable IVC filters incorporate a "retrieval hook" that is used to capture the filter, and these filters are designed in a manner to avoid tilting and ensure correct positioning of the hook in the middle of the IVC. When properly placed, the filter is typically removed within minutes with a snare and sheath.
Often, the removal process requires several snare attempts and lengthy fluoroscopic exposures. Even symmetric filters, while appearing untilted in standard 2D venograms, may hide a significant degree of tilt, which is often revealed with unusual projection angles.11 Our approach is therefore to routinely perform a rapid, 4-second 3D RA at the start of the procedure to achieve a fast and confident removal, even in complex cases. A single acquisition provides all the necessary information to quickly and accurately visualize the filter and the IVC, plan the appropriate removal approach, and select the optimal working projection, often becoming a significant time saver (Figure 5).
The possibility of visualizing the rotational acquisition as a "CT-like" multiplanar reconstruction further facilitates the analysis of the hook positioning with respect to the IVC lumen, thus making the removal process simple and fast (Figure 6).
Endovascular treatment of arteriovenous malformations (AVMs) can be quite complex. Although diagnostic multiplanar 3D CTA or MRA images are valuable for AVM detection, they do not offer adequate spatial resolution to precisely analyze the "spaghetti-like" arterial supply and determine the optimal angle of approach for each feeding vessel. Three-dimensional RA offers more detailed anatomical information that enables the radiologist to untangle the tangled web of vessels surrounding the AVM, choose the angles to selectively show the feeders, and define treatment approach and chronology.
A comprehensive treatment plan based on 3D RA is achieved with a single run and helps to minimize the number of 2D DSA and fluoroscopic images required during treatment. The minimization of iodinated material is particularly important for patients affected by renal AVMs because renal function may be impaired due to preparenchymal shunting of blood, with resultant bypassing of renal filtration. Additionally, further kidney injury may result from imprecise or nontarget embolization. Thus, optimal viewing angles are critical. The use of 3D imaging is particularly beneficial in these patients and helps us to achieve a favorable balance between contrast minimization, optimal viewing, and a prompt recovery of kidney function (Figure 7).
Accurate 3D imaging plays a significant role for malformations affecting the uterine arteries. Uterine AVMs are particularly complex and tortuous, and, even if the proximal uterine artery is correctly embolized, parasitization and collateral vessels such as the ovarian artery may rapidly take over and continue feeding the malformation. Should the malformation be inadequately embolized, uterine expansion during pregnancy may lead to vessel rupture and to life-threatening hemorrhage for the mother. The availability of 3D RA reconstructions is therefore essential to quickly identify the nidus, unravel the intricate vascular configuration of the feeding vessels, select the optimal working projections to access all feeding vessels, and achieve a complete embolization (Figure 8).
Uterine Fibroid Embolization
Uterine fibroid embolization (UFE) is an important application making use of 3D imaging.12 UFE is performed by introducing small embolic particles into uterine arteries feeding the fibroid, resulting in fibroid shrinkage. The imaging technique of choice for this procedure is conventional angiography. Most interventional radiologists perform abdominopelvic aortography with additional, randomly chosen oblique selective internal iliac angiograms. The angle selected is quite variable from operator to operator and is performed in hopes of localizing the origin of the uterine artery and to produce a road map to guide superselective catheterization of this vessel. While occasionally this randomly selected angle may be adequate, very often additional angiographic road maps in other angles are performed to successfully show the origin of the uterine artery. This increases procedural radiation dose, contrast administration, and procedure time. Our approach is instead to acquire one 3D RA at the beginning of the procedure using a distal aortic contrast injection to produce a 360¡ view of the feeding arteries. A single 3D RA 4 second acquisition with near-instant reconstruction is then used to select the optimal projection to show the origins of the uterine arteries. A 3D dynamic road map can then be used to swiftly access the feeding arteries with no need for any additional contrast or DSA runs prior to the embolization (Figure 9).
The combination of live 3D tools is particularly valuable in transarterial chemoembolization (TACE) procedures. Starting from a 3D RA scan, accurate anatomical information is available to identify the vessels feeding the tumor and plan the treatment approach. The 3D RA volume can also be registered and fused with diagnostic CTA or MRA to obtain valuable multimodal visualizations of feeding vessels and tumors. During navigation, the 3D RA volume can then be matched with live 2D fluoroscopy to achieve a Dynamic 3D Roadmap and a smooth catheter placement into the feeding vessels. Finally, soft tissue information obtained with the XperCT protocol can be used to confirm treatment completion by comparison with pretreatment CTA or MRA. The availability of CT-like imaging in the interventional suite enables the interventionist to promptly tackle possible tumor residuals and improves the clinical workflow eliminating the need of moving the patient to a CT unit (Figure 10).
The patient studies in this article demonstrate the utility of intraprocedure 3D guidance and imaging during endovascular procedures. Three-dimensional RA acquisitions provide accurate anatomical information and advanced quantitative analysis tools, multimodality matching, and 3D navigation; soft tissue imaging can then be routinely used in synergy with 2D imaging to achieve improved clinical outcome and faster procedural workflow for a variety of applications. Our results suggest that 3D tools have clinical scope, particularly in optimal 3D viewing and navigation of tortuous and highly bifurcating vascular networks such as renal, hepatic, uterine, and external iliac arteries and veins. Three-dimensional runs also allow fast selection of favorable working projections during the embolization of aneurysms, AVMs, tumors, and uterine fibroids, and in a comprehensive treatment planning and evaluation with detailed visualization of devices and soft tissue in CT-like imaging (which is particularly useful in oncologic TACE procedures). In addition, the fact that a full 3D acquisition can now be obtained in a matter of seconds (approximately 4 seconds with the system and settings used in our institution) makes 3D RA readily available for all cases involving a number of initial unsuccessful attempts during navigation and/or embolization based on 2D imaging. Other potential endovascular applications not considered here but reported in the literature include anatomical visualization during aortic endograft procedures, evaluation and follow-up imaging of transplanted kidneys, and the evaluation of pancreas allografts.13-15
An optimal combination of 2D and 3D imaging may also lead to a reduction of radiation dose and contrast administration during endovascular procedures. Although quantitative studies should be pursued to evaluate procedure-based benefits of 3D RA, the use of a single 3D RA for 3D road mapping and for pretreatment selection of optimal working projections showed clear potentials for cases requiring multiple oblique 2D DSA runs and injections. It should be noted that, although 3D RA series involve a larger number of images, depending on the exposed anatomical region, a single 3D RA image requires a dose that is 10 to 80 times lower than a single 2D DSA image.9 This means that a 3D RA scan is equivalent to less than 4 seconds of a 2D DSA acquisition at 3 frames per second. A recent multicenter comparison has shown that the use of 3D RA for carotid artery treatment involves a cumulative radiation dose three times lower than for procedures based on 2D angiography only, mainly due to quicker selection of working projections and consequent reduction in fluoroscopy time and number of 2D DSA series.10
A correct combination of 2D and 3D information is also essential to support the interventionist in the deployment of new minimally invasive procedural approaches. For example, the recent combination of fluoroscopy and 3D imaging in an integrated tracking and navigation system has allowed interventional radiologists to achieve live 3D guidance in the interventional suite and to open new application areas for deep injections, biopsies, and drainages.16 Intraprocedure 3D information also allows for a smooth integration of CTA and MRA diagnostic data in the interventional workflow and is essential for the introduction of advanced modeling and simulation technologies.17 Further technology development should focus on the compensation of motion artifacts affecting the integration of live 2D and 3D imaging, with the potential of extending the usability and application range of 3D tools in endovascular procedures.
Vendors of angiography systems offer competing versions of the 3D interventional tools mentioned in this article, which go by names like Innova 3D and Innova CT (GE Healthcare, Chalfont St. Giles, United Kingdom); syngo InSpace 3D, DynaCT, iPilot, and iGuide (Siemens Healthcare, Erlangen, Germany); Allura 3D RA, XperCT, Dynamic 3D Roadmap, and XperGuide (Philips Medical Systems); and 3D Angio (Toshiba Medical Systems Corporation, Tokyo, Japan). These vendors' 3D packages do vary significantly in their speed of acquisition, image quality, feature set, reconstruction times, and ease of use. We encourage the interventionist to carefully compare the various vendors' 3D technologies when choosing an angiography system, because we expect the use of these 3D tools to significantly increase and be an integral part of future interventions.
The availability of 3D tools is indispensable in a busy day-to-day endovascular practice and promotes good clinical outcome and efficient procedural workflow. We encourage the use of 3D tools to achieve fast, accurate, and confident decisions with a potential reduction in contrast administration and radiation dose to patients and staff.
Atul Gupta, MD, is the Director of Interventional Radiology at Paoli Hospital, Main Line Health, in Paoli, Pennsylvania. He has disclosed that he is a member of the Medical Advisory Board for Philips Healthcare. Dr. Gupta may be reached at (610) 648-1255; email@example.com.
Alessandro G. Radaelli, PhD, is a clinical scientist (cardiovascular x-ray) employed by Philips Healthcare in Best, The Netherlands. Dr. Radaelli may be reached at +31 40 27 66685; firstname.lastname@example.org.