Editorial for “Short‐Term Variability of Proton Density Fat Fraction in Pancreas and Liver Assessed by Multi‐Echo Chemical‐Shift Encoding‐Based MRI at 3 T”

Patients with nonalcoholic fatty liver disease or other conditions with metabolic involvement (eg, heart failure with preserved ejection fraction in patients with type 2 diabetes mellitus [T2DM]) are increasingly undergoing phenotyping using magnetic resonance imaging (MRI). The reason for this is 3-fold: diagnostic evaluation, prognostic evaluation, and inclusion and monitoring in clinical trials. Imaging-based quantitative MRI methods (eg, T1-, T * 2and proton density fat fraction [PDFF] mapping, magnetic resonance elastography) and magnetic resonance spectroscopy methods have been proposed for such abdominal studies. Technical influencing factors of quantitative methods have been established and well-characterized; however, the effect of diurnal and medium-term variability of metabolites on these quantitative methods remains unclear, even though they may influence measurement interpretation, especially in longitudinal studies. For instance, a previous study showed that extreme changes in liver glycogen concentration affect liver T1 measurements. 1 Given the natural diurnal variation of liver glycogen concentration, these T1 variations will likely be lower and within the single-session reproducibility range reported for liver T1 3 in healthy volunteers and most patients. However, this effect could be exacerbated by impaired glucose metabolism in patients who have T2DM. The same study also showed that variability in body hydration status has a significant effect on liver T1 that should be controlled for. When it comes to the variation of intrahepatic lipids, Colgan et al found no significant diurnal variation of liver PDFF when considering both healthy volunteers and patients with hepatic steatosis. In this issue of JMRI, Machann et al report on diurnal and medium-term pancreatic and liver fat variability in healthy individuals. As members of any such study group would have low amounts of liver and pancreatic fat, the authors first proved the accuracy of the chemical shift encoded gradient echo sequence in the low-fat fraction regime. This was followed by two substudies: one that examined the diurnal variability of pancreatic and liver fat and another evaluating the effects of a high caloric diet over 5 days. The 12 individuals enrolled in the first substudy showed no significant change in pancreatic fat on intrasession reproducibility scans, nor after a high-fat meal. A similar finding was reported for liver fat, in line with the results of Colgan et al. The 15 male volunteers participating in the second substudy experienced a minimal increase in their pancreatic PDFF after the high caloric diet, however, without reaching statistical significance. The observed increase in liver PDFF was significant (from 1.7% to 2.7%). In comparison, no changes from baseline to pancreatic or hepatic fat were observed after 5 days in a control group of 11 males. While these findings are a welcome addition to the growing body of evidence on factors that may influence the reliability of quantitative MRI biomarkers in longitudinal studies, there is a need for similar studies conducted in patient groups, with particular focus on patients with T2DM.