Pancreatic disease in obesity: observations on fat content, diffusion, T2* relaxometric and mechanical properties in the rat ex vivo
Philippe Garteiser1, Sabrina Doblas1, Jean-Baptiste Cavin1, André Bado1, Vinciane Rebours1,2, Maude Le Gall1, Anne Couvelard1,3, and Bernard E Van Beers1,4

1Center For Research on Inflammation, Inserm U1149, Paris, France, 2Pancreatology Unit, AP-HP, Beaujon Hospital, Clichy, France, 3Pathology department, AP-HP, Bichat Hospital, Paris, France, 4Radiology department, AP-HP, Beaujon Hospital, Clichy, France

Synopsis

Multiparametric assessment of pancreas in the obese rat was used to evaluate alterations linked to obesity-mediated inflammation. Mechanical properties and T2* values are significantly affected by disease, and reflect accurately the histological features of the obese pancreas.

Purpose

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.

Methods:

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.

Results

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 (γG' and γG'', 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 γG' and γG''), and this correlation was significant in the case of γG'' (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.

Conclusion

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.

Acknowledgements

No acknowledgement found.

References

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2. Macauley M, Percival K, Thelwall PE, Hollingsworth KG, Taylor R. Altered volume, morphology and composition of the pancreas in type 2 diabetes. PloS One. 2015;10:e0126825.

3. Rebours V, Gaujoux S, d’Assignies G, et al. Obesity and Fatty Pancreatic Infiltration Are Risk Factors for Pancreatic Precancerous Lesions (PanIN). Clin Cancer Res Off J Am Assoc Cancer Res. 2015;21:3522–3528.

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Figures

Fig. 1: Feasibility of MR elastography on pancreas explants. A, B, C: images of the curl wave field at the three tested frequencies. D: total amplitude map. E: magnitude image depicting the localization of the region of interest (green). Regions of interest was positioned on the 800Hz images and in close proximity to the actuation mechanism.

Fig. 2: Monofrequency results. A, B: storage and loss modulus at all frequencies in the control vs. obese groups (2.5-97.5 percentiles). C: Correlation between the storage modulus (blue: 400Hz, green: 600Hz) and the number of fibrotic complexes. Significant differences between groups were found for the storage modulus at 400 and 600Hz. Significant correlations were obtained between the mechanical properties and the histologically determined fibrosis

Fig. 3: Multifrequency MR elastography results. A: slope of the frequency response for storage and loss modulus in the obese and control experimental groups. B: correlation between the mechanical slope (blue: storage modulus slope, γG'; green: loss modulus slope, γG'') and the number of fibrotic complexes determined at histology. The slopes are lower in the obese group vs. controls (significantly in the case of γG'), and are well and significantly correlated to the number of fibrotic complexes.

Fig. 4: PDFF and relaxometry results. A, B, C: fat fraction, T2* and R2*, respectively. D, E: correlation between T2* (green traces) and R2* (blue traces) values, and histologically determined iron levels in terms of number of positive spots (D), or surface area of positive zones (E). Obese animals have significantly more fat and a shorter T2* than control animals. Relaxometric properties are well and significantly correlated to iron deposition.

Fig. 5: Apparent diffusion coefficient results for an example obese (A, B) and control (C, D) explant. Magnitude images (A, C) and apparent diffusion coefficient maps (B, D) are depicted. ADC was not found to be significantly altered by any of the tested histology markers.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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