Purpose: To compare a free breathing navigator triggered single shot echoplanar imaging (SS EPI) diffusion-weighted imaging (DWI) sequence with prospective acquisition correction (PACE) with a breathhold (BH) DWI sequence for liver imaging. Materials and Methods:Thirty-four patients were evaluated with PACE-DWI and BH DWI of the liver using bvalues of 0, 50, and 500 s/mm 2 . There were 29 focal liver lesions in 18 patients. Qualitative evaluation was performed on a 3-point scale (1-3) by two independent observers (maximum score 9). Quantitative evaluation included estimated SNR (signal to noise ratio), lesion-to-liver contrast ratio, liver and lesion apparent diffusion coefficients (ADCs), and coefficient of variation (CV) of ADC in liver parenchyma and focal liver lesions (estimate of noise contamination in ADC).Results: PACE-DWI showed significantly better image quality, higher SNR and lesion-to-liver contrast ratio when compared with BH DWI. ADCs of liver and focal lesions with both sequences were significantly correlated (r ϭ 0.838 for liver parenchyma, and 0.904 for lesions, P Ͻ 0.0001), but lower with the BH sequence (P Ͻ 0.02). There was higher noise contamination in ADC measurement obtained with BH DWI (with a significantly higher SD and CV of ADC). Conclusion:The use of a navigator echo to trigger SS EPI DWI improves image quality and liver to lesion contrast, and enables a more precise ADC quantification compared with BH DWI acquisition. THERE IS A GROWING interest in using liver diffusionweighted imaging (DWI) for lesion detection and characterization, prediction of treatment response and assessment of chronic liver disease, by means of apparent diffusion coefficient (ADC) measurement (1-20). The interest in DWI is motivated by the recently described risk of nephrogenic systemic fibrosis (21-24), which makes DWI an attractive alternate method to Gadolinium-enhanced sequences. Most prior studies have used breathhold (BH) single shot echoplanar imaging (SS EPI) to acquire diffusion-weighted images, using various sets of b-values. SS EPI sequences are very fast, however, suffer from limited image quality, mostly related to limited signal to noise ratio (SNR) especially at higher b-values, and limited spatial resolution, which constitute an obstacle for its widespread use in clinical practice. Techniques such as parallel imaging (25,26) and pulse triggering (27) have been proposed to improve SS EPI image quality and precision of ADC measurement.There are several recent reports on the use of respiratory triggered acquisitions for liver DWI (15,16,[28][29][30][31]. However, only two studies have specifically compared different diffusion acquisition schemes for liver imaging (28,30). The first study (28) included a population of patients with liver lesions, and compared image quality and ADC of free breathing versus BH DWI. The second study included healthy volunteers, and compared BH, free breathing and respiratory triggered DWI (without navigator echoes) (30). There are, however, no reports on the use of a...