Fig 1.
Illustration of sampling methods for intrapancreatic quantification.
A: Representative MRI (3-point Dixon) slice of the pancreas was selected (upper panels). An anatomical scan was also acquired in parallel to the Dixon scan for localization purpose and for differentiation pancreas parenchymal tissue from main vessels and other adjacent abdomen tissues (lower panels). Regions of interest were carefully positioned away from pancreas borders to avoid contamination from visceral fat and away from main vessels. Conventional ROI: Polygon tool of ImageJ was used to select single ROI on the head, body and tail of pancreas away from visceral fat and main vessels. MR-opsy: Three ROIs (100mm2 each) were placed uniformly to represent different parts of the pancreas using ImageJ Oval tool away from visceral fat and main vessels. B: Magnified region of the pancreas to illustrate the size of biopsy selection (100mm2) relative to the size of an individual pixel (1.93mm2 = 1 pixel).The software reshape the oval selection (b, right) to take the nearest pixel shape (b, left).
Fig 2.
An illustration of adipocytes distribution within single MRI voxel of parenchymal tissues of pancreas.
Histological section of background normal pancreatic tissue of a 48-year female undergoing pancreatectomy for a neuroendocrine tumour. The average size of single adipocyte is approximately 100μm, but adipocytes can occur in clusters. Based on average adipocyte size of 100μm, the maximum number of adipocytes likely to be present in one voxel is ~400x50 = 20000 adipocytes. The upper threshold of 20% assumes that the maximum number to be 4000 adipocytes within a single voxel of pancreas.
Fig 3.
Example of ductal system architecture within the pancreas.
A: different MRI axial acquisitions of the pancreas in T2DM subject (a1: T2-SPAIR, a2: BTFE, a3: 3-point Dixon). Pancreas of T2-SPAIR (a1) sequence was segmented and volume rendered in Drishti as described before [28], volume rendered image was colour tagged then opacity level was manipulated to show the distribution of pancreatic ductal system in white colour (Drishti version 2.6.3).
Fig 4.
Colour map of pancreatic fat distribution in type 2 diabetes.
The colour map shows the wide range of fat distribution within the sampling area. This underlies the rationale for thresholding to exclude non-parenchymal tissues. The colour bar on the right shows fat level from 0% (dark blue) to 25% (red). Threshold levels were set to exclude areas of fat content less than 1% (possible blood vessels or main duct) or above 20% (visceral fat contamination). Parenchymal fat was considered to range between 1–20%. a1-a6 represent areas of varied fat content within the single MR-opsy selection (a1: < 1%, a2: 1–5%, a3:6–10%, a4:11–15%, a5:16–20%, a6: > 20%).
Fig 5.
Reproducibility of fat quantification methods.
The inter-observer variation for each method is shown for low level (3%) of pancreatic fat (A), and high level (6%) of pancreatic fat (B). Data for both methods are shown with and without 1–20% thresholding. * p<0.05 Conventional vs MR-opsy before thresholding. ‡ p<0.05 Conventional vs MR-opsy after thresholding. † p<0.05 Conventional without thresholding vs with thresholding.
Fig 6.
Example of areas selected by two observers using conventional and MR-opsy methods.
ROIs of participants with the high level of pancreatic fat (6%) were shown using conventional (a,b), and MR-opsy (c,d) methods for observers 1 and 2. Two regions with potential contribution to wrong estimation of pancreatic fat content were selected: Region 1 represents a region of focal fat which appears bright on the Dixon scan (a,c), and dark on the anatomical BTFE scan (b,d). Region 2 represents blood vessel and appears dark on the Dixon scan (a,c), and bright on the anatomical scan (b,d). It is clear that observer 1 included both areas within the selection using conventional method whereas biopsy method avoided those regions by the same observer.
Table 1.
Counterbalance study: Pancreas fat change in responders and non-responders before and after weight loss.
Table 2.
Bariatric surgery study: Pancreas fat change in type 2 diabetes (T2DM) and normal glucose tolerance (NGT) participants before and after weight loss.
Table 3.
Fat% in different regions of the pancreas before and after intervention studies.
Table 4.
Summary of up-to-date studies employed MR for fat quantification in the pancreas.