We read with great interest the meta-analysis by Wang et al.,1 who evaluated the diagnostic accuracy of magnetic resonance elastography (MRE) and diffusion-weighted imaging (DWI) for the staging of hepatic fibrosis. The authors concluded that MRE is more reliable for staging hepatic fibrosis. This is a significant contribution to our knowledge, as recent advances in MRI techniques have made the use of these methods common in clinical practice. In our opinion, attention must also be focused on several technical parameters of MRI methods before physicians can safely interpret the results.
The standard MRI scanners currently use a 1.5-Tesla magnetic field. This is also the type of scanner that was used in all the studies included in the meta-analysis by Wang et al. Theoretically, new-generation 3-Tesla scanners could enhance the ability for hepatic fibrosis staging. Especially for DWI, another vital parameter is the apparent diffusion coefficient (ADC) and its b value. ADC reflects diffusion in a quantitative way and b value illustrates the sensitivity of a DWI sequence. The higher the b value, the more sensitive the sequence is to diffusion effects.2 Conflicting results have been published on the optimal b value for hepatic fibrosis staging.3 This was also remarkably reflected in the great variation of b values used in the studies of the meta-analysis.
We recently presented our preliminary results on hepatic fibrosis staging in a small cohort of patients with nonalcoholic fatty liver disease (NAFLD) using a 3-Tesla MRI scanner.4 DWI was performed in the axial plane with tridirectional diffusion gradients using three b values: 0, 500, and 1000 s/mm2. Fibrosis stage was poorly associated with ADC at a b value of 500 s/mm2 (r = −0.30, P = 0.27). However, it was significantly associated with ADC at a b value of 1,000 s/mm2 (r = −0.57, P = 0.01). For this b value the area under receiver-operating characteristic curve was 0.93 for fibrosis stage ≥3 and the optimal ADC cutoff value was 1.16 × 10−3 s/mm2 (positive predictive value: 100%, negative predictive value: 90%).
To our knowledge there are no published data on liver fibrosis staging with 3-Tesla MRI scanners in patients with NAFLD. The broader availability of this technology might enhance the performance of DWI for fibrosis staging. Given that DWI does not need additional equipment, as opposed to MRE, it might be an attractive option for liver fibrosis staging once all technical parameters like the b value are elucidated.