Chemoembolization is a widely used palliative treatment modality for inoperable hepatocellular carcinoma (HCC) (1). To perform chemoembolization safely and effectively, it is essential to achieve selective catheterization of the tumor-feeding branch based on a thorough knowledge of the hepatic artery and extrahepatic collateral arteries. However, multiple angiographic runs are needed to identify small tumor-feeding branches because multiple hepatic arteries overlap each other. In addition, there are several hypervascular non-tumorous stainings mimicking HCCs and fine non-hepatic arteries arising from the hepatic artery, which may cause complications if inadvertent or intentional embolization is undertaken.

C-arm cone-beam computed tomography (CBCT) technology using a flat-panel detector (FPD) is a useful tool for obtaining cross-sectional and three-dimensional (3D) images during interventional procedures. This technique has been popularly used in various interventional procedures such as adrenal vein sampling, transjugular intrahepatic portosystemic shunts, transthoracic needle biopsy, and chemoembolization for HCC (2, 3, 4). In intra-arterial treatment for HCC, C-arm CBCT can provide additional crucial information, including visualization of small tumors and their feeding-arteries, non-hepatic arteries, and possibly of extrahepatic collateral arteries. Recent potent intra-arterial treatment modalities such as drug-eluting beads and Y-90 radioactive beads may cause serious complications when non-hepatic arteries are treated (5). Thus, C-arm CBCT has become an inevitable tool for safe and favorable clinical outcomes. The purpose of this article is to review the technology of C-arm CBCT and its role in chemoembolization for HCC.

Digital technology launched FPD instead of the image intensifier system. The FPD provides markedly improved contrast and spatial resolution compared to image intensifier system. Whereas conventional multidetector CT (MDCT) adopts a fan-beam geometry and is equipped with multiple one-dimensional detectors, CBCT obtains information using cone-beam geometry and a two-dimensional (2D) FPD, which make it possible to generate a whole volumetric data set in a single gantry rotation. Thus, recent angiographic machines have CBCT mounted on a C-arm.

Various terms have described these new volumetric imaging systems in the literature, including C-arm CT, cone-beam CT, angiographic CT, volume CT, and flat-panel CT. In this article, the term C-arm CBCT will be used to refer to C-arm-mounted CBCT using a digital FPD.

In MDCT, detector row width ranges from 0.5 mm to 0.6 mm for the current 64-slice scanners and the patient is scanned in a helical fashion. An isotropic voxel size of 0.5 × 0.5 × 0.5 mm³ can be obtained with current state-of-the-art MDCT. For C-arm CBCT systems, current detector arrays are 40 × 30 cm², allowing 25 × 25 × 18 cm³ volumetric datasets to be generated. Isotropic voxel sizes of under 0.2 × 0.2 × 0.2 mm³ can be theoretically achievable with current C-arm CBCT systems. This higher spatial resolution of C-arm CBCT can make it possible to observe very thin subsegmental hepatic arteries which may not be discernible on CT hepatic arteriography using MDCT (Fig. 1).

The contrast resolution of C-arm CT in delineating soft-tissue structures is notably lower than that of MDCT owing to the increased beam scatter generated by the C-arm angiographic system compared with the conventional MDCT scanner (6). Whereas MDCT has approximately 3 Hounsfield units (HU) contrast resolution, CBCT allows a contrast resolution of 10 HU.

Three C-arm CBCT systems are commercially available in Korea: DynaCT (Siemens Medical Solutions, Forchheim, Germany), XperCT (Phillips Medical Systems, Eindhoven, the Netherlands), and Innova CT (GE Healthcare, Waukesha, WI, USA). Each of these systems has its own imaging protocol, in addition to different rotation times, numbers of projections acquired, image quality, image reconstruction time, and dedicated post-processing programs. In our hospital, DynaCT is available for chemoembolization, and the parameters of the C-arm CBCT scan are 0.5° increment, 512 × 512 matrix in projections, total angle of 211° at approximately 26° per second, a total of 419 projections, and an 8-second scan time. Recent C-arm CBCT machines only require a 5-second scan time.

The following are typical imaging process using C-arm CBCT for chemoembolization in our institution. C-arm CBCT is obtained at the proper hepatic artery or common hepatic artery by using a 5-Fr catheter (RH, Cook, Bloomington, IN, USA) or 2.6-Fr microcatheter (Asahi Intecc, Aichi, Japan). When the tip of the 5-Fr catheter can be advanced into the common hepatic artery, the C-arm CBCT is obtained by using 5-Fr catheter. When the tip of 5-Fr catheter cannot be advanced into the common hepatic artery, the C-arm CBCT is obtained by using 2.6-Fr microcatheter placed in the proper hepatic artery. In patients with hepatic artery variation (i.e., replaced right hepatic artery from the superior mesenteric artery or replaced left hepatic artery from the left gastric artery), an additional C-arm CBCT is obtained by using 2.0-2.6-Fr microcatheter placed in the replaced right or left hepatic artery. The injection rate of the contrast media is 3-4 mL/sec at the proper or common hepatic artery and 1.5-2.5 mL/sec at the right or left hepatic artery according to the vessel size and catheter size used. The preferred scan delay is 4 seconds, which means that scanning is initiated 4 seconds after the start of contrast material injection. Iodinated contrast media (Pamiray 300; Dongkook, Seoul, Korea) is used without dilution. C-arm CBCT images are reviewed on a dedicated workstation by multiplanar reformation images, maximum-intensity projection images, and volume-rendering images.

Most of the parameters of the C-arm CBCT are fixed according to the machines used, but two parameters can be adjusted by operators; scan delay and contrast media dilution. In most literature, scan delay ranges from 3 seconds to 8 seconds. A 4 second scan delay was recommended by Tognolini for better hepatic arterial visualization (7), and a 6 second scan delay was used for better visualization of small tumors (8). To optimize the image quality of C-arm CBCT, scan delay should be tailored to individual patients, considering the blood flow velocity through the hepatic artery and the position of the tip of catheter. A 6 second or more scan delay may be contaminated by portal vein enhancement, while a 3 second or less scan delay may not be enough to obtain sufficient enhancement of tumors.

Most interventional radiologists prefer half dilution of contrast media in C-arm CBCT imaging. Half-diluted contrast media (150-175 mg/mL as iodide) may reduce beam-hardening artifacts. However, according to our clinical experience, thin subsegmental hepatic arteries may be obscure when diluted contrast media is used, so undiluted contrast media (300 mg/mL as iodide) is preferred in our institute. The optimal concentration of iodine in contrast media for chemoembolization should be further studied. If contrast media diluted to 100 mg iodine/mL is used, longer scan delay (8 seconds) is needed to acquire adequate tumor-to-liver contrast (9).

The recent dual-phase CBCT prototype, which is vendor specific, allows two sequential scans, which can obtain CBCT images of both arterial and venous phases by using only one contrast material injection. The venous phase of the C-arm CBCT is useful to identify corona enhancement (10). Corona enhancement indicates venous drainage through hypervascular HCC nodules and can discriminate HCC from arterioportal shunts. Dual phase C-arm CBCT depicted corona enhancement in 88.7% of small tumors and diagnostic accuracy of small HCC nodules can be improved (10). With dual phase C-arm CBCT, tumor enhancement changes after chemoembolization using drug-eluting beads can be evaluated and is has been reported to serve as a useful prognostic indicator of short-term HCC response (11).

Routine use of C-arm CBCT can increase stochastic risk (i.e., radiation-induced cancer) because of the additional radiation exposure. However, Kothary et al. (38) demonstrated that C-arm CBCT can replace some angiographic runs, thereby resulting in a negligible increase in dose-area product which is considered an adequate surrogate for stochastic risks. In addition, C-arm CBCT decreases skin dose exposure because it is concentrated to the right upper abdomen, distributing the skin dose of radiation over a range of 210°, resulting in decreased deterministic risk (i.e., radiation-induced skin injury).

C-arm CBCT has small field of view, so the liver was entirely covered by C-arm CBCT in only 29% of cases (39). The field of view in CBCT scanning should be individually adjusted to include target tumors and the region of interest. In patients with large livers containing bilobar disease, two or more acquisitions of the CBCT is mandatory.

Cone-beam computed tomography images have motion artifacts caused by inadequate breath-holding and cardiac motion. In one report, 3% of patients could not cooperate with breathing instructions and were unable to undergo CBCT scanning (39). CBCT images are severely deteriorated by respiratory motion artifacts or cardiac motion in 5-10% of patients (7, 39). Cardiac motion can deteriorate CBCT image quality in the left lateral segments, especially in S2. Even in patients with good breath-holding, 25% of S2 hepatic arteries suffer from motion artifact caused by cardiac motion (39).

C-arm CBCT can provide additional useful information about HCCs and their feeding arteries and can be used as navigational tools with 3D roadmapping. The combination of current and future capabilities of C-arm CBCT may improve the efficacy and safety of chemoembolization and may widen the indication of chemoembolization. C-arm CBCT may improve the prognosis of HCC patients when interventional radiologists adopt this next generation of imaging technology to their daily clinical practice.