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Surgery and VAT

Surgery changes visceral adipose tissue in patients with obesity

Study sought to quantify fat fraction (FF) and T1 relaxation time by MRI after RYGB and compared the resulting values with the pre-operative ones

Bariatric surgery is not only related to a significant reduction in common parameters of adipose tissue distribution, BMI and total visceral fat volume, but also significant changes in T1 relaxation time and fat fraction of visceral adipose tissue (VAT), according to researchers from Ludwig-Maximilians-University, Munich and Leipzig University Hospital, Leipzig, Germany. The paper, ‘MRI assessment of changes in adipose tissue parameters after bariatric surgery’, was published in PlosONE.

The investigators acknowledge that little is known about potential therapy-related changes in the adipose tissue of obese patients and therefore, they established a study to quantify fat fraction (FF) and T1 relaxation time by magnetic resonance imaging (MRI) after Roux-en-Y gastric bypass surgery (RYGB) and compared the resulting values with the preoperative ones. They hypothesised that such quantitative MRI measures “may potentially serve as independent biomarkers for longitudinal and cross-sectional measurements in obese patients.”

In total, 23 patients (16 females and 7 males, mean age 44.3 years, mean BMI45.4, both at M0) underwent standard laparoscopic RYGB with a small pouch (20cm), an alimentary limb of 150cm and a biliopancreatic limb of 50cm. Anthropometrics were recorded for both MRI time points. All patients were examined in a 1.5-T MRI (Philips Achieva XR) at time points before (M0) and one month (M1) after surgery.

Fat volumetry involved axial in-phase/opposed-phase gradient echo images with 10mm thickness and 0.5mm gap that were acquired between diaphragm and femoral heads (typically 50 slices) in breath-hold technique (expiration).

Total abdominal VAT volumes (VVAT) were then evaluated by a semiautomatic fat segmentation tool. Automatic contouring used k-means clustering to detect the outer body (SAT) boundary and an iterative active-contours (snakes) algorithm in combination with region growing to define an inner SAT as well as a VAT boundary. Quantification of fat volumes involved a histogram-based analysis of the area under the fat-related peak structures (at higher signal intensities).

Figure 1 shows an example of a resulting T1 map with SAT and VAT regions of interest (ROI) used for quantification. VAT ROIs were generally placed within the mesentery at some distance away from signal heterogenities, sparing intestines, peritoneum and vessels.

Figure 1: Screenshot of custom-made software tool for the quantification of T1 relaxation times in selected regions of interest. Display shows presurgical axial T1 map of an obese, 31-year-old female patient with a BMI of 41.1 kg/m2. T1 values were determined in VAT (174 pixels) and SAT ROI (237 pixels) by fitting mean signal intensities (SI) in arbitrary units (a.u.) of single-shot fast spin-echo sequence with inversion recovery preparation at different inversion times TI to model function (Source: PlosONE)

Outcomes

The results revealed that a the BMI reduction of nearly –7% was significant (p<0.001) and the most prominent relative effect of nearly –11% was observed for total VAT volume, again significant (p<0.001). T1 coefficients of variation (CV) over all subjects and both time points were 1.8±0.3% (range 1.1–2.4%) for SAT and 2.2±0.8% (range 1.3–4.8%) for VAT.

At the ROI level, T1 relaxation times were significantly longer in VAT (+4.4%, p<0.001) but not in SAT (–0.9%, p = 0.141). The early relative changes in fat fraction were significant for VAT (–2.6%, p<0.01) but not for SAT (–0.2%, p = 0.564).

“In conclusion, there were detectable MRI changes in T1 relaxation time and fat fraction of VAT one month after bariatric surgery. Besides the widely used morphological measures derived from fat volumes, these potential MRI markers for the tissue response in obese patients deserve further attention,” the authors write.

To access this paper, please click here

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