The purpose of this study was to compare the accuracy of lesion detection and diagnostic confidence between 18 F-FDG PET/CT, gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid (Gd-EOB-DTPA)-enhanced MRI, and retrospectively fused PET and MRI (PET/MRI). Methods: Thirty-seven patients (mean age 6 SD, 60.2 6 12 y) with suspected liver metastases underwent PET/CT and Gd-EOB-DTPA-enhanced MRI within 0-30 d (mean, 11.9 6 9 d). PET and Gd-EOB-DTPA-enhanced MR image data were retrospectively fused. Images were reviewed independently by 2 readers who identified and characterized liver lesions using PET/CT, Gd-EOB-DTPA-enhanced MRI, and PET/MRI. Each liver lesion was graded on a 5-point confidence scale ranging from definitely benign (grade of 1) to definitely malignant (grade of 5). The accuracy of each technique was determined by receiver-operating-characteristic analysis. Histopathology served as the standard of reference for all patients with malignant lesions. Results: A total of 85 liver lesions (55 liver metastases [65%] and 30 benign lesions [35%]) were present in 29 (78%) of the 37 patients. Twenty-four (65%) of the 37 patients had liver metastases. The detection rate of liver lesions was significantly lower for PET/CT than for Gd-EOB-DTPA-enhanced MRI (64% and 85%; P 5 0.002). Sensitivity in the detection and characterization of liver metastases for PET/CT, Gd-EOB-DTPA-enhanced MRI, PET/MRI in reader 1, and PET/MRI in reader 2 was 76%, 91%, 93%, and 93%, respectively; the respective specificity values were 90%, 100%, 87%, and 97%. The difference in sensitivity between PET/CT and PET/MRI was significant (P 5 0.023). The level of confidence regarding liver lesions larger than 1 cm in diameter was significantly higher in PET/MRI than in PET/CT (P 5 0.046). Accuracy values (area under the receiver-operating-characteristic curve) for PET/ CT, Gd-EOB-DTPA-enhanced MRI, PET/MRI in reader 1, and PET/MRI in reader 2 were 0.85, 0.94, 0.92, and 0.96, respectively. Conclusion: The sensitivity of Gd-EOB-DTPA-enhanced MRI and PET/MRI in the detection of liver metastases is higher than that of PET/CT. Diagnostic confidence was significantly better with PET/MRI than with PET/CT regarding lesions larger than 1 cm in diameter. Compared with Gd-EOB-DTPA-enhanced MRI, PET/MRI resulted in a nonsignificant increase in sensitivity and diagnostic confidence. Imagi ng of liver metastases is still challenging. In the last few years, dynamic contrast-enhanced CT has become the primary imaging modality for detection of liver metastases, with reported sensitivities of 73%285% (1,2). 18 F-FDG PET has been reported to be superior to CT for detecting liver metastases from colorectal cancer (3) and by some was found the most sensitive noninvasive imaging investigation for detecting liver metastases (4,5). 18 F-FDG PET/ CT has been performed with the objective of improving detectability and increasing diagnostic performance and has shown sensitivities of between 61% and 97% in the detection of liver metastases (6-10).Dynamic contrast-en...