Intrahepatic Tumor and Vessel Identification in Intravenous Contrast Enhanced Liver kV Cone Beam CT

Abstract

Without intravenous (IV) contrast, most liver tumors are not visible on kV cone beam CT (CBCT). Visualization of hepatic vessels and tumor could improve accuracy of image guided liver cancer stereotactic body radiation therapy (SBRT). The aim of this study was to measure the improvement in vessel and tumor enhancement on contrast CBCT compared to non-contrast CBCT. On an IRB approved study, IV contrast was given on up to 2 occasions per patient concomitantly with CBCT acquired in the treatment position immediately following SBRT (volume 2 mL/kg, range 120–170, injection rate 3–5 mL/sec). CBCT imaging (120 kVp, 100 mAs, 200–360 degree gantry rotation, 163–200 frames) was initiated at 0–30 seconds following injection and acquired over 60–120 seconds. Exhale breath hold (BH) gated CBCT when feasible; otherwise images were obtained during free breathing (FB). A non-contrast CBCT was acquired immediately prior to the contrast CBCT. Assessment of change in visualization of tumor and hepatic vasculature between contrast and non-contrast CBCT was made by measuring changes in CT number in regions of interest (ROI) within visible vessels or tumor and adjacent liver. The changes in mean CT number in tumor and vessel ROIs were compared to that in ROI in adjacent liver, for non-contrast and IV CBCTs. The changes in contrast to noise ratio (CNR) were also measured. Eight pairs of contrast and non-contrast CBCT scans were obtained in 5 patients (5 exhale BH, 3 FB). Tumors and intrahepatic vasculature were not visible on the non-contrast CBCT scans. In 5 scans (3 patients, 2 with BH), hepatic vasculature was visible on contrast CBCT as regions of relative increased contrast. In 5 scans (4 patients, 2 with BH), hepatic tumor was visible on contrast CBCT as regions of hypodensity relative to the liver. A significant difference in vessel contrast was observed between non-contrast and contrast CBCTs. The mean absolute difference in CT number between vessel and liver for contrast CBCT and non-contrast CBCT was 32.2 and 1.6 respectively (p < 0.05), with no significant difference in CNR (1.03, 0.16; p = 0.25). Differences in tumor contrast were also observed (Figure 1, CNR 2.8) but were not significant for all patients. Figure 1 illustrates a contrast CBCT (a) showing a 1 cc red ROI on tumour and a 1 cc green ROI on adjacent liver. (b) The tumour shows a decrease in linear attenuation coefficient compared to liver. The addition of IV contrast to CBCT enhances the visibility of hepatic vessels and tumor, particularly in breath hold scans. Optimization of IV contrast timing and methods to reduce artifacts are being investigated to improve tumor visualization.