Quantitative Measurement of Iodine Concentration in the Liver Using Abdominal C-Arm Computed Tomography 1 Katsuyuki Taguchi, PhD, Yoshinori Funama, PhD, Mengxi Zhang, BS, Elliot K. Fishman, MD Jean-François H. Geschwind, MD Rationale and Objectives. C-arm computed tomography is an option on a C-arm angiographic system capable of acquiring projections while rotating the C-arm around the patient and reconstructing cross-sectional images with improved contrast resolu- tion of 5 to 10 Hounsfield units. Typical abdominal C-arm computed tomographic (CCT) images, however, exhibit artifacts with spatially varying and drifting pixel values. Considering liver tumor oncologic procedures, the aim of this study was to evaluate the accuracy of liver iodine concentration (IC) estimated from CCT images under such challenging conditions. Materials and Methods. The proposed method estimates the IC in a region of interest (ROI) using pixel values of CCT im- ages measured at the ROI and a nonenhanced background. Two approaches to measure the background value were tested: one approach, L-BG, measured a corresponding local background value near each ROI, and the other, G-BG, used one global back- ground value for the entire object. The accuracy of estimations using CCT and computed tomographic scanners was evaluated; an elliptical cylinder water phantom with iodine solution inserts and seven patient data sets with transcatheter arterial chemoembolization were used. Results. With the least “truncation” (the edge of the object being outside the field of view) of 27 mm, the IC was accurately estimated with CCT images (n = 9; root-mean-square error [RMSE], 1.60 –1.63 mg/mL; normalized RMSE, 11.8%; r 2 = 0.97; P  .001), with the true concentration ranging from 2.32 to 31.82 mg/mL. With truncations of up to 100 mm (n = 88), the estimation by L-BG remained accurate independent of the amount of truncation (RMSE, 1.58 mg/mL; normalized RMSE, 12.5%; r 2 = 0.06; P = .02), whereas the estimation by G-BG reduced the accuracy (RMSE, 4.61 mg/mL; normalized RMSE, 34.3%; r 2 = 0.10; P = .003). Clinical data ( n = 37) showed that the estimation from CCT images using the L-BG method agreed well with that from computed tomographic images (RMSE, 2.86 mg/mL; normalized RMSE, 38.7%; r 2 = 0.76; P  .001). Conclusion. The liver IC can be accurately estimated with abdominal CCT images. Key Words. C-arm computed tomography; iodine concentration; quantitative measurement; abdominal imaging; live tu- mor; drug distribution. © AUR, 2009 Many image-guided interventional procedures remain sub- jective, even with the introduction of digital flat-panel detectors. Many interventional procedures that use x-ray images involve iodine, for example, nonionized iodine contrast agent material (1,2), iodine-mixed lipiodol oil for transcatheter arterial chemoembolization (TACE) (3,4), and embolic particles in an iodized solution (5). Measur- ing the concentration of iodine, which is likely associated with drug concentration or blood perfusion, can lead to objective, evidence-driven procedures. Acad Radiol 2009; 16:200 –208 1 From the Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, 601 North Caroline Street, JHOC 4263, Baltimore, MD 21287. Received December 4, 2007; accepted August 5, 2008. This study was supported in part by a research grant from Philips Medical Systems (Best, The Netherlands). Address correspondence to: K.T. e-mail: ktaguchi@jhmi.edu © AUR, 2009 doi:10.1016/j.acra.2008.08.002 200