Monitoring Fatty Liver Disease with MRI Following Bariatric Surgery: A Prospective, Dual-Center Study

Abstract

Purpose To longitudinally monitor liver fat before and after bariatric surgery by using quantitative chemical shift-encoded (CSE) MRI and to compare with changes in body mass index (BMI), weight, and waist circumference (WC). Materials and Methods For this prospective study, which was approved by the internal review board, a total of 126 participants with obesity who were undergoing evaluation for bariatric surgery with preoperative very low calorie diet (VLCD) were recruited from June 27, 2010, through May 5, 2015. Written informed consent was obtained from all participants. Participants underwent CSE MRI measuring liver proton density fat fraction (PDFF) before VLCD (2-3 weeks before surgery), after VLCD (1-3 days before surgery), and 1, 3, and 6-10 months following surgery. Linear regression was used to estimate rates of change of PDFF (ΔPDFF) and body anthropometrics. Initial PDFF (PDFF₀), initial anthropometrics, and anthropometric rates of change were evaluated as predictors of ΔPDFF. Mixed-effects regression was used to estimate time to normalization of PDFF. Results Fifty participants (mean age, 51.0 years; age range, 27-70 years), including 43 women (mean age, 50.8 years; age range, 27-70 years) and seven men (mean age, 51.7 years; age range, 36-62 years), with mean PDFF₀ ± standard deviation of 18.1% ± 8.6 and mean BMI₀ of 44.9 kg/m² ± 6.5 completed the study. By 6-10 months following surgery, mean PDFF decreased to 4.9% ± 3.4 and mean BMI decreased to 34.5 kg/m² ± 5.4. Mean estimated time to PDFF normalization was 22.5 weeks ± 11.5. PDFF₀ was the only strong predictor for both ΔPDFF and time to PDFF normalization. No body anthropometric correlated with either outcome. Conclusion Average liver proton density fat fraction (PDFF) decreased to normal (< 5%) by 6-10 months following surgery, with mean time to normalization of approximately 5 months. Initial PDFF was a strong predictor of both rate of change of PDFF and time to normalization. Body anthropometrics did not predict either outcome. Online supplemental material is available for this article. © RSNA, 2018.

Figure 1:

Proton density fat fraction (PDFF, %) maps in a 55-year-old woman who underwent gastric bypass with preoperative very low calorie diet (VLCD) demonstrate progressive reduction in liver fat content. Regions of interest (circles) were obtained from each Couinaud segment of the liver, with care taken to avoid major vessels, and were averaged to obtain mean liver PDFF. Initial mean liver PDFF was 28.3% at the preoperative MRI (top left), which decreased to 18.9% following the preoperative VLCD period (top middle). Following gastric bypass, further decreases in PDFF were seen, measuring 9.0% at 1 month (top right), 5.8% at 3 months (bottom left), and finally normalizing to 4.1% at 6 months (bottom middle). A PDFF fat content scale is provided. As an internal reference, note that PDFF signal remains consistently low within the spleen (S), while the liver progressively darkens as fat content decreases. Also note the reduction in the quantity of subcutaneous adipose tissue. The woman’s body mass index at each time point was 43.7, 41.7, 38.7, 36.4, and 32.4 kg/m², respectively.

Figure 2:

Flow diagram of study cohort. VLCD = very low calorie diet, NAFLD = nonalcoholic fatty liver disease, BMI = body mass index, SD = standard deviation.

Figure 3:

Liver proton density fat fraction (PDFF), body mass index (BMI), weight, and waist circumference trajectories over time for each study participant. The thick trajectory on each plot represents sample averages at average visit times. Note the relatively rapid decrease in both liver PDFF and body anthropometrics over the first 6–8 weeks (time spanning initiation of very low calorie diet [VLCD] through the 1-month postoperative MRI: visits MRI₀, MRI_VLCD, MRI_1mo), compared with the remainder of the study period (time spanning the 1-month postoperative MRI through the 6-month postoperative MRI: visits MRI_1mo, MRI_3mo, MRI_6–10mo). Upper bounds for normal liver PDFF (5%) and normal BMI (25 kg/m²) are represented on the corresponding plots with red lines.

Figure 4:

Graphs show the rate of change in liver proton density fat fraction (PDFF) versus rates of change in other body anthropometrics: body mass index (BMI), weight, and waist circumference. The red circles and dashed line represent the short-term period (time spanning from initiation of very low calorie diet through the 1-month postoperative MRI). The black triangles and solid line represent the longer term period (time spanning from the 1-month postoperative MRI through the 6-month postoperative MRI). The relationships are stronger in the acute period. Changes in waist circumference were not significantly correlated with changes in PDFF.

Figure 5:

Estimation of time to proton density fat fraction (PDFF) normalization, with the upper limit of normal PDFF for liver defined as 5%. Time 0 corresponds to baseline visit. Mixed-effects linear regression modeling of natural log-transformed PDFF (y-axis) as a function of time (x-axis) demonstrates a mean time to normalization (the point where the natural log-transformed trajectory crosses the ln [5%] = 1.6% line) of 22.5 weeks. Trajectories have been extended (dashed lines) for 13 participants for whom the estimated crossing time occurs outside the observation window. Of note, in this sample it was estimated that 90% of study participants achieved normal PDFF of 5% by 42 weeks.