Diffusion-weighted imaging of the liver in patients with chronic liver disease: comparison of monopolar and bipolar diffusion gradients for image quality and lesion detection.

OBJECTIVE. The objective of our study was to compare diffusion-weighted imaging (DWI) sequences using a bipolar versus a monopolar single-shot echo-planar imaging (EPI) gradient design for image quality and for lesion detection and characterization in patients with liver disease. MATERIALS AND METHODS. In this retrospective study, 77 patients with chronic liver disease who underwent MRI including bipolar and monopolar DWI at 1.5 T were assessed. Two independent observers reviewed the DWI studies for image quality and the detection and characterization of liver lesions. The reference standard for diagnosis was established by consensus review of two different observers using imaging characteristics on conventional MRI sequences, lesion stability over time, pathologic correlation, or a combination of these findings. The estimated signal-to-noise ratio (SNR) of liver parenchyma and apparent diffusion coefficients (ADCs) of the liver and lesions were calculated for both sequences. ROC analysis was conducted to evaluate the performance of ADC for the diagnosis of hepatocellular carcinoma (HCC). RESULTS. Eighty-five lesions, 50 HCCs and 35 benign lesions, were detected with the reference standard in 38 patients. There was equivalent image quality for the bipolar and monopolar sequences (p = 0.24-0.42). The HCC detection rate for observers 1 and 2 was slightly better with bipolar DWI (50.0% and 52.0%, respectively) compared with monopolar DWI (44.0% and 46.0%); however, this difference did not reach statistical significance. The estimated SNR was higher with the monopolar sequence than with the bipolar sequence (p = 0.001). The AUC for the ROC curve was 0.691 for bipolar DWI and 0.649 for monopolar DWI when ADC was used for the characterization of HCC, which is not a statistically significant difference (p = 0.59). CONCLUSION. The higher estimated SNR yielded by the monopolar DWI sequence did not translate into better HCC detection compared with the bipolar DWI sequence. ADC has a limited role for HCC characterization in patients with liver disease.