Obesity is often linked to hyperglycemia and insulin resistance, and this low grade inflammatory condition is tightly associated to type 2 diabetes(1). As the main insulin-secreting organ in the body, the pancreas is an important target for disease assessment in obesity(2). In obesity, pancreas is the siege of fatty infiltration and fibrosis(3), all of which are risk factors for ductal adenocarcinoma(4). The associated hyperinsulinemia is accompanied with structural remodeling in the form of beta cell islets hyperplasia(5)(6). In the present study we sought to evaluate, using high field MRI, the effect of obesity on fibrosis, inflammation, fat content and general microstructural architecture in pancreas explants from healthy vs. diet-induced obese Wistar rats.

All animal experiments were performed in accordance with local institutional ethics committee approval. Male Wistar rats where assigned to high fat diet (obese group n=8) or normal diet (control group n=6) for six month and sacrificed. The pancreas tissue was then dissected from the surrounding intraperitoneal adipose tissue and maintained in physiological serum. Pancreas explants were immobilized on a custom-made MRE actuating cradle.

MRE: MRE was performed using a spin echo sequence with motion encoding gradients played sequentially along the three dimensions. To accommodate the softness of pancreatic tissue(7), relatively low frequencies of 400Hz, 600Hz and 800Hz were selected, and each was assessed at 4 mechanical offsets per cycle. Acquisition parameters were TR/TE of 1000ms/18ms, allowing 1, 2 or 6 periods of motion encoding, and 300µm isotropic resolution. Viscoelastic parameters (storage modulus, G' and loss modulus, G'' at all three frequencies, and their frequency dependence, γ, expressed as the exponent of a fit to a power law) were obtained by algebraic inversion of the wave propagation equation, applied to the shear components of the displacements recorded on unwrapped phase images.

T2* and fat fraction: acquisition consisted of a gradient echo sequence (TE=1.65ms+n·0.92ms, n=0···15) with 250µm in-plane resolution, 1mm slice thickness, 300kHz bandwidth, 950ms TR, 15° flip angle and 4 signal averages. T2* and fat fraction were extracted by fitting the magnitude image to a model accounting for T2* decay and signal interferences from water and the 7 most abundant lipid peaks.

Diffusion: Diffusion-weighted imaging was done with a spin echo sequence with 21 segments EPI readout, 4 b-values (0, 150, 300, 500 and 700 s/mm²), 195µm in-plane resolution, 400µm slice thickness, 2000ms/20ms TR/TE, and 3 signal averages).

Histology: Reference histological results were obtained with hematoxylin-eosin-saffron staining (for determination of presence or absence of adipocytes and numeration of endo-exo fibrous complexes). Perls staining was used to determine the percent surface area occupied by iron depots.

At the sacrifice, Rats from obese group weighted 863±141g and rats from control group weighted 586±42g.

MRE: Storage modulus was positively correlated to the number of fibrous complexes (r=0.63, p=0.02 and r=0.66, p=0.014, at 400Hz and 600Hz respectively), and were significantly higher in the obese group than in controls (400Hz: 0.87±0.07kPa vs. 0.57±0.06kPa, p<0.01; 600Hz: 1.63±0.12kPa vs. 1.16±0.07kPa, p<0.01). Storage modulus at 800Hz, and loss modulus at any frequencies were not significantly different between groups and were not significantly correlated to the number of fibrous complexes. The slopes of the frequency response for storage and loss moduli (γ_G' and γ_G'', respectively) were lower in the pancreas from obese group than from controls, but this difference was only significant for the slope of the storage modulus (1.69±0.14 vs. 1.25±0.05 for the control and obese groups, respectively, p<0.01). Furthermore, the slopes were negatively correlated to the number of fibrous complexes (r=-0.25 and r=-0.58 for γ_G' and γ_G''), and this correlation was significant in the case of γ_G'' (p<0.05).

T2* and fat fraction: The pancreas from the obese group had higher fat content (0.36±0.19 %) and shorter T2* (8.16±0.58 ms) than those from the control group (PDFF: 0.0024±0.0016 %, p<0.05; T2*: 12.44±0.74, p<0.001).

Diffusion: The diffusion coefficients ADC (0.75±0.03 and 0.81±0.07) and D (0.73±0.03 and 0.78±0.06) for obeses and controls, respectively, were not significantly different between groups. Both for ADC and D, the average values in the obese group were lower than in the controls.

Multiparametric assessment of pancreas explants from obese rats revealed a higher elasticity, a lower frequency response, an enhanced fat content, an enhanced R2* and a slightly lower diffusion coefficient in the pancreas of obese rats relative to controls. These findings were in agreement and reasonably well explained with quantitative and qualitative histologic markers of iron deposition, fibrotic and inflammatory signs. Multiparametric MRI is proposed as an important tool in the evaluation of obesity mediated disorders of the pancreas.