Three-Dimensional Rotational Angiography

A new imaging technology takes a detailed look at peripheral angioplasty and stenting.

By JOS C. van den BERG, MD, PhD
 

The combined approach of balloon angioplasty and stenting is a well-accepted treatment plan for peripheral stenotic and occlusive arterial disease.1 Over the past decade, the essentials of percutaneous transluminal angioplasty and stenting procedures have not changed fundamentally; however, the materials used and methods of diagnostic work-up are continually evolving.

As endovascular treatments for peripheral arterial disease have been introduced, opinions regarding the radiological work-up needed to determine patient eligibility for minimally invasive treatment are changing. Proper patient selection and device sizing are mandatory to prevent treatment failure and ensure long-term success; immediate, short- and long-term results depend substantially on proper sizing of the materials used. Some of the modalities employed for preinterventional interpretation of anatomic morphology and measurement of the targeted vessels are helical CT, CT angiography, MR angiography, calibrated angiography, or intravascular ultrasound.

This article describes the use of three-dimensional rotational angiography (3D-RA) in the assessment of patients undergoing repair for stenotic and occlusive arterial disease.

TECHNICAL CONSIDERATIONS
3D-RA (V5000, Philips Medical Systems, Best, The Netherlands) is a technique based on conventional rotational angiography. Images can either be acquired at a frame rate of 12.5 frames per second, a rotation speed of 30° per second and a total arc of movement of 180° (roll movement), or at a frame rate of 25 frames per second through 240° of rotation at 55° per second. Injection protocols are adjusted according to the area of interest (common carotid artery, 4 mL/sec; aortoiliac bifurcation, 8 mL/sec). The acquisition requires 8 seconds for the roll movement and 6 seconds for the propeller movement (including 1 second for starting and 1 second for ending the run), and yields 100 images per run (Figures 1 and 2).

The images are transferred online to a workstation and a predefined default volume is automatically reconstructed within 75 seconds (Figure 2). The obtained three-dimensional volume can be rotated and viewed in any direction. Cut planes can be made at any position in the volume and measurements are taken. The interventionalist uses a reconstructive zoom technique to make extra reconstructions of the area of interest, a step that saves time, minimizes the radiation dose to the patient, and reduces the volume of contrast medium (Figure 1A and 2A through C).

Software Capabilities
The use of automated vessel analysis software allows determination of vessel geometrical properties on the user-defined vessel segments, such as the length, diameter, and area of the vessel segment of interest. The analysis is divided into successive steps: definition of the central vessel axis and any branching points along the vessel trajectory and analysis of the cross-sections positioned perpendicular to the defined vessel axis. The interventionalist performs both steps virtually and interactively.

The software provides the endoscopic view used for evaluation of the vessel interior and accurate depiction of the hyperdense plaque locations. Any point of the analyzed vessel segment along the traced trajectory can be viewed in a cross-sectional perspective, which provides the maximum and minimum vessel diameter measured at that particular position (Figure 2C). Finally, a plot of vessel diameter distribution between the start and the end point of the vessel analysis is drawn (Figure 2C). The interventionalist measures the stenotic size and segment, indicating the information in a histogram. Interactive dragging of the analysis borders allows enhanced definition of stenting location and the desired stent length.

Recent software developments also allow virtual angioscopy as well as visualization of calcifications. Assessment and visualization of the extent and location of the vessel’s hyperdense plaque is based on a new acquisition protocol, especially designed for this particular purpose. The protocol requires creating a calcification run and a contrast run. The first run is based on lowered kV values that decrease the radiation penetration rate and enhance visualization of the soft tissue. As the vessel calcifications increase attenuation in comparison with the surrounding soft tissue, the depiction is greatly enhanced. The second, standard contrast run is taken immediately after completion of the first run. After both the runs are acquired and reconstructed, the interventionalist performs the reconstruction superimposition.

In order to allow a perfect alignment of the reconstructions, a motion compensation technique is employed. The compensation diminishes consequences of possible patient movements introduced in the interim between the acquisition of the runs and counteracts for vessel pulsation. The final reconstruction delineates the calcified masses and allows the assessment of the relation between the residual vessel lumen and the hyperdense plaque that normally protrudes into the lumen (Figure 1E and F). The same method of superimposition of the two runs, taken with different acquisition settings, provides an improved depiction of stent location and its relation to the calcified plaque and vessel wall (Figure 1E and F).

ROTATIONAL ANGIOGRAPHY
The use of three-dimensional reconstructions is well known in MR imaging and helical (multislice) CT. 3D-RA is a relatively new technique that has been applied successfully in neuroradiological interventions, in which it is a helpful tool for assessing intracranial aneurysms and arteriovenous malformations.2,3 One of the most critical issues in dealing with vascular diseases of the brain such as aneurysms or arteriovenous malformations is the accurate delineation of the pertinent anatomy. Rotational angiography (without three-dimensional reconstruction) has been demonstrated as a reliable technique for the multidirectional depiction of the internal carotid artery. In general, this technique shows a more severe maximum internal carotid artery stenosis than does conventional two- or three-directional digital subtraction angiography.4,5 Rotational angiography has been used successfully in stent-graft placement for iliac aneurysms.6,7 This article focuses on its use in peripheral angioplasty and stenting.

Rotational angiography employs a continuous rotation around the region of interest during continuous intra-arterial contrast infusion. This area of importance is placed in the isocenter in both the frontal and lateral planes. In this way, many contrast-enhanced cinefluoroscopic images are obtained (we can take a total of 100 images with our system). To obtain three-dimensional reconstructions from the rotational angiographic images, they must be precisely matched to each other. This calibration and quality assessment is done using phantoms during routine maintenance. Measurements performed using well-defined test phantoms demonstrated that in all orientations of the phantom, the measured size deviated less than 2% from the actual size.8 Because of precise calibration, the interventionalist can perform accurate length and diameter measurements without using (costly) calibrated catheters.7,9 Additionally, viewing the volume from different angles allows the physician to determine the optimal projection (angulation and skew) of the x-ray tube needed to facilitate the endovascular intervention (Figure 2D).

SIZING ISSUES
Sizing is always important when considering endovascular treatment of occlusive peripheral arterial disease. The unwanted complications of undersizing are insufficient dilatation or incomplete deployment of the stent—occurrences that have been identified as possible causes for restenosis10 or stent migration. Underdeployment of a stent cannot always be discerned on conventional angiographic studies alone.6,10-14 Oversizing the stent may have deleterious effects on the vessel wall, leading to pseudoaneurysm formation.

Although simple techniques (using the movement of the guidewire in a digitally subtracted image) have been proposed as reliable measurement tools, usually more sophisticated methods are used. These approaches include intravascular ultrasound, calibrated angiography, conventional or spiral CT, CT angiography, and MR imaging.15 The optimal measurement tool is still a matter of debate. Modern CT workstations allow physicians to measure diameter, length, and volume, create three-dimensional reconstructions, and central lumen line length measurements. One disadvantage of this method is that these workstation reconstructions are very time-consuming and all measurements must be made manually.

PERSONAL EXPERIENCE
Our initial experience shows that 3D-RA is a fast and reliable adjunct modality that provides for performance measurements online. The measurement error of less than 2% is acceptably low. Radiation exposure of the patient is less than that of a normal angiographic run because a cinefluoroscopic mode of image acquisition is used (data on file, Philips Medical Systems).

Although the amount of contrast for one rotational angiographic run exceeds the amount for a single, classic angiographic injection, the total amount of contrast is not significantly increased because one run is sufficient. For example, in our institution, standard projections of the aortoiliac region include three views (AP, LAO, and RAO), using a total contrast volume of 45 mL (three times 15 mL at a flow rate of 15 mL/sec). Using 3D-RA to visualize this region requires a total contrast volume of 48 mL (at an injection rate of 8 mL/sec).

The amount of time needed to perform the default three-dimensional reconstruction and the reconstructive zoom is relatively short (<5 min), and should not be considered a loss of time, as the necessity to perform various runs is lacking with the 3D-RA technique (unlike conventional angiography). A disadvantage of the 3D-RA technique is the inability to visualize the thrombus; the same is true for conventional angiography. 3D-RA can, however, demonstrate calcification using either the source images showing indirect signs of the presence of thrombus (discrepancy between angiographic lumen and location of calcification), or the calcified plaque software. Furthermore, the cinefluoroscopic angiographic images allow the physician to obtain information on flow characteristics.

BENEFITS OF 3D-RA
Similar to conventional digital subtraction angiography, and unlike CT and MRI, image artifacts caused by metallic stents or coils do not hamper 3D-RA. In fact, 3D-RA is capable of demonstrating stent anatomy as well as stent adaptation to the vessel wall (Figure 1 and 2E). We appreciate the technique’s ability to reveal stenosis of the inferior mesenteric artery or aberrant arteries (made possible by the availability of true lateral projections).7 3D-RA can also detect an inadvertent subintimal course of the guidewire and/or catheter and will reveal misalignment of intravascular stents. In addition, this technique allows the interventionalist to obtain orthogonal views in the plane of any vessel, such as the renal artery.16

CONCLUSION
In conclusion, 3D-RA can be used in peripheral arterial angioplasty and stenting, providing useful additional data on target vessel and lesion morphology without adding substantial procedure time. Furthermore, it helps in monitoring the procedure and in assessment of treatment results. n

Jos C. van den Berg, MD, PhD, is an interventional radiologist and is Chairman of the Department of Radiology, St. Antonius Hospital in Nieuwegein, The Netherlands. He holds no financial interest in any product mentioned herein. Dr. van den Berg may be reached at +31 306099111, pager 656; jc.berg@antonius.net.

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Endovascular Today is a publication dedicated to bringing you comprehensive coverage of all the latest technology, techniques, and developments in the endovascular field. Our Editorial Advisory Board is composed of the top endovascular specialists, including interventional cardiologists, interventional radiologists, vascular surgeons, neurologists, and vascular medicine practitioners, and our publication is read by an audience of more than 22,000 members of the endovascular community.