Emerging technology
C-arm Cone-beam CT: General Principles and Technical Considerations for Use in Interventional Radiology

https://doi.org/10.1016/j.jvir.2009.04.026Get rights and content

Digital flat-panel detector cone-beam computed tomography (CBCT) has recently been adapted for use with C-arm systems. This configuration provides projection radiography, fluoroscopy, digital subtraction angiography, and volumetric computed tomography (CT) capabilities in a single patient setup, within the interventional suite. Such capabilities allow the interventionalist to perform intraprocedural volumetric imaging without the need for patient transportation. Proper use of this new technology requires an understanding of both its capabilities and limitations. This article provides an overview of C-arm CBCT with particular attention to trade-offs between C-arm CBCT systems and conventional multi-detector CT.

Introduction

THE interventional radiology suite has historically used two-dimensional radiographic imaging techniques such as digital subtraction angiography (DSA) and standard fluoroscopy to visualize, manipulate, and intervene on three-dimensional (3D) structures. Advances in angiographic interventions, including vascular stent and stent graft placement, thrombolysis, transcatheter embolization, and targeted intravascular oncologic procedures, have increased the need for accurate 3D characterization of vessels and adjacent structures. Nonangiographic procedures such as percutaneous drain and stent placement and radiofrequency ablation frequently involve complex anatomical relationships, which are difficult to characterize fluoroscopically. In most, if not all of these cases, correlation with cross-sectional imaging is necessary. This often requires generation of pre- and post-intervention computed tomographic (CT) image sets. For procedures requiring real-time 3D imaging for which ultrasound localization is not feasible, intraprocedure CT guidance becomes necessary.

Cross-sectional imaging is a frequently used tool in most interventional radiology departments with CT-guided biopsies and percutaneous drain placements commonly performed procedures. However, these procedures are usually performed outside the interventional suite, thereby limiting access to commonly used interventional equipment while at the same time significantly impacting the diagnostic CT workflow. In busy radiology departments, this can decrease patient throughput where procedural complications can lead to significant scheduling conflicts. In certain types of interventions, there are situations in which both cross-sectional imaging and real time fluoroscopy are required.

Efforts to develop a robust system for generating 3D data sets suitable for use in interventional and surgical suites led to the development of several novel technologies. Made possible by advances in post-processing algorithms, computed rotational 3D DSA became the first 3D in-suite interventional technique, allowing 3D renderings of digitally subtracted contrast-enhanced vessels. With this technology, multiple DSA images at various projection angles are generated by rotating a conventional angiography unit around the patient. Three-dimensional image sets are generated using a cone-beam backprojection reconstruction algorithm (1, 2, 3). Following shortly thereafter, 3D digital angiography was developed, allowing 3D visualization of high-contrast structures including osseous structures and contrast-enhanced vessels. In the case of vascular imaging, 3D digital angiography uses unsubtracted rotational images. Three-dimensional digital angiography has the potential advantage over 3D DSA of no misregistration artifacts and lower patient dose (4). However, detectability of low contrast structures is still limited. In an attempt to overcome the limitation of poor low-contrast visibility, the angio-CT system was developed, which fuses a conventional angiographic system with a fan-beam CT scanner. The patient remains stationary, and a CT scanner on rails is rolled into position as needed. However, this system was expensive and required a large physical space (5).

In the late 90's, experimental systems were developed using cone-beam computed tomography (CBCT). Cone-beam CT research had been ongoing for over a decade in areas such as nuclear medicine and industrial testing before significant interest in diagnostic CT applications developed. Cone-beam CT enables generation of an entire volumetric data set in a single gantry rotation by using a two-dimensional detector system rather than a one-dimensional detector or series of one-dimensional detectors as used in conventional CT (6).

Cone-beam CT mounted on a Carm was originally performed using an image intensifier system over 20 years ago (7). However, image intensifier systems and charged couple devices suffer from limited spatial resolution. This led to the development of flat-panel detectors, which provide significantly increased contrast and spatial resolution compared to image intensifier detectors (8). The increased spatial resolution of flat-panel detectors has been demonstrated experimentally using a flat-panel detector system with significantly fewer pixels (970 x 768) than are available with today's systems (9). Furthermore, CBCT is reported to result in decreased radiation and intravenous contrast doses compared to angio-CT in head and neck applications (10).

A number of terms have emerged in the literature to describe these new volumetric imaging technologies, including C-arm CT, cone-beam CT, cone-beam volume CT, volume CT, angiographic CT, and flat-panel CT. In this article, the term C-arm CBCT will be used to refer to C-arm-mounted cone-beam CT units employing a digital flat-panel detector. Because all commercially available C-arm CBCT units employ digital flat-panel detectors, the flat-panel detector term will not be explicitly stated. A number of the issues discussed in this article refer to properties of CBCT in general, whether mounted on a traditional CT gantry or on a C-arm. In these cases, the more generic term CBCT will be used, again with the understanding that the detectors are digital flat-panel detectors.

C-arm CBCT allows volumetric data acquisition in a single rotation of the source and detector. A photograph of a commercially available unit is shown in Figure 1. This setup is ideally suited for imaging in the interventional suite for several reasons. The system is compact enough to allow mounting on a moving C-arm, thereby allowing the patient to remain stationary during the examination. In a single orbit about the patient, a complete volumetric dataset covering a large anatomic region of interest is generated from which submillimeter isotropic reconstructions can be created. The high efficiency two-dimensional detector allows excellent low-contrast detectability, a capability not present on image intensifier detector-based 3D-angiographic systems.

The applications of in-suite 3D imaging are many. For the neurointerventionalist, utility has been demonstrated in intra- and extracranial arteriography, particularly for aneurysm characterization (1, 2, 3). Recent investigations suggest that current generation C-arm CBCT systems should be able to reliably discriminate the 40 Hounsfield Units necessary to detect intracranial hemorrhage (11). For characterization of intra- and extracranial stent placement in which the low profile, highly flexible stents lead to minimal radiopacity, C-arm CBCT demonstrated residual stent narrowing and calcified plaque that was not visualized by either projection radiography or DSA (12). C-arm CBCT has also demonstrated utility in the repair of endoleak following endovascular repair of abdominal aortic aneurysms (13). Limited studies using C-arm CBCT for transjugular intrahepatic portosystemic shunt placement and transcatheter arterial embolization are encouraging (14, 15).

Outside interventional radiology, clinical studies have been performed and are ongoing to investigate the use of CBCT for brachytherapy and external beam radiotherapy, as well as for surgical planning in orthopedic, thoracic, abdominal, head and neck, and neurosurgical procedures (16, 17, 18). Preclinical investigations suggest that with continued refinement of CBCT imaging, dedicated systems for use in routine diagnostic CT may someday become feasible (19, 20).

This article provides an introduction to C-arm CBCT systems utilizing high spatial resolution flat-panel detectors, which in a single compact unit provide the interventionalist with the ability to generate projection radiographic, fluoroscopic, DSA, and CT data sets on a stationary patient. This article focuses on comparisons with standard diagnostic multidetector CT, as successful implementation of C-arm CBCT systems is predicated on the ability to provide diagnostic cross-sectional images, which while perhaps not equivalent in quality to diagnostic multidetector CT images, provide adequate image quality to answer the relevant clinical question while offering the advantage of in-suite 3D imaging.

Section snippets

Cone-beam versus Fan-beam Geometries

The central difference between conventional multidetector CT (fan-beam) and CBCT is that CBCT acquires information using a high-resolution two-dimensional detector instead of multiple one-dimensional (1D) detector elements. In standard multidetector CT, a series of detector element rows is used. Illustrations demonstrating cone-beam and fan-beam geometries are shown in Figure 2, Figure 3.

In multi-detector spiral CT, the patient is scanned in a helical fashion with gantry speeds on the order of

Technical Limitations and Challenges

The increased scatter generated by CBCT systems compared to conventional multidetector CT accounts for the most significant differences in image quality between the two systems, resulting in image artifacts, decreased contrast-to-noise (CNR), and inaccuracies in CT number calculations (19). The majority of the following discussion deals with the consequences of scatter on image quality and strategies to mitigate these effects. Differences in dynamic range and temporal resolution between C-arm

Conclusion

In this article, the main cause of image quality degradation with C-arm CBCT, increased scatter radiation, was discussed along with several strategies for scatter reduction and compensation. Many of the scatter reduction strategies discussed are outside the control of the operating physician. Imaging volume, however, is within the operating physician's control and is the largest single factor determining the amount of scattered radiation. Furthermore, for a given set of imaging parameters,

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    M.J.W. received an honorarium for speaking and grant support from Siemens Medical Solutions, Malvern, Pennsylvania.

    This article first appeared in J Vasc Interv Radiol 2008; 19:814–820.

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