Diffusion-weighted imaging of the liver: comparison of image quality between monopolar and bipolar acquisition schemes at 3T.

PURPOSE: To compare image quality of monopolar and bipolar diffusion-weighted imaging (DWI) sequences of the liver at 3T.
METHODS: 32 healthy volunteers (mean 27 ± 8 years; 27 M/5F) and 11 patients (mean age 58 ± 14 years; 8 M/3F) underwent liver MRI using a 3T system incorporating 2-channel parallel transmission for B1-shimming and reduced B1-inhomogeneity. Scans included free-breathing DWI sequences (b-value 0, 400, 800 s/mm(2)) acquired using both monopolar and bipolar techniques. Estimated signal-to-noise ratio (eSNR), apparent diffusion coefficient (ADC), and measures of subjective image quality on b-800 images, scored on a 1-5 scale by two independent radiologists, were compared between sequences.
RESULTS: Monopolar sequence demonstrated significantly higher eSNR (volunteers: 12.7 ± 4.0 vs. 11.3 ± 3.5, patients: 11.4 ± 4.0 vs. 10.2 ± 3.3; p ≤ 0.013) compared with the bipolar sequence. Monopolar sequence also achieved significantly higher scores for reader 1 in volunteers and patients in terms of clarity of right lobe edge, clarity of intra-hepatic vessels, conspicuity of the left lobe, and overall diagnostic quality (p ≤ 0.031), as well as significantly higher scores for reader 2 in volunteers in terms of clarity of intra-hepatic vessels, conspicuity of the left lobe, and overall diagnostic quality (p ≤ 0.035). Respiratory motion artifact was not significantly different between sequences in patients or volunteers for either reader (p ≥ 0.191). Hepatic ADC was significantly lower using monopolar technique only in volunteers (1.28 ± 0.12 vs. 1.43 ± 0.15, p < 0.001).
CONCLUSION: In comparison with past studies performed at 1.5T, when using a modern 3T system, we observed improved image quality of liver DWI using a monopolar, rather than a bipolar, acquisition scheme, largely attributed to higher eSNR.