Adipose tissue as well as other depots of fat (triglycerides) are increasingly being recognized as active contributors to the human function and metabolism. In addition to the fat concentration, also the fatty acid chemical composition (FAC) of the triglyceride molecules may play an important part in diseases such as obesity, insulin resistance, hepatic steatosis, osteoporosis, and cancer. MR spectroscopy and chemical-shift-encoded imaging (CSE-MRI) are established methods for non-invasive quantification of fat concentration in tissue. More recently, similar techniques have been developed for assessment also of the FAC in terms of the number of double bonds, the fraction of saturated, monounsaturated, and polyunsaturated fatty acids, or semi-quantitative unsaturation indices. The number of papers focusing on especially CSE-MRI-based techniques has steadily increased during the past few years, introducing a range of acquisition protocols and reconstruction algorithms. However, a number of potential sources of bias have also been identified. Furthermore, the measures used to characterize the FAC using both MRI and MRS differ, making comparisons between different techniques difficult. The aim of this paper is to review MRS-and MRI-based methods for in vivo quantification of the FAC. We describe the chemical composition of triglycerides and discuss various potential FAC measures. Furthermore, we review acquisition and reconstruction methodology and finally, some existing and potential applications are summarized. We conclude that both MRI and MRS provide feasible non-invasive alternatives to the gold standard gas chromatography for in vivo measurements of the FAC. Although both are associated with gas chromatography, future studies are warranted.

Purpose To compare MR‐based fatty acid composition (FAC) quantification methods against the gold standard technique, gas chromatography (GC), with comparison of a free and a constrained signal model. The FAC was measured in the healthy and edematous legs of lymphedema patients. Methods In vivo MRS and MRI data were acquired from 19 patients at 3 T. Biopsies were collected from subcutaneous adipose tissue of both thighs during liposuction. The saturated, monounsaturated, and polyunsaturated fatty acid fractions (fSFA, fMUFA and fPUFA, respectively) were estimated with the MR‐based methods using two signal models: free and constrained (number of methylene‐interrupted double bonds expressed in number of double bonds, based on GC data). Linear regression, Bland–Altman plots, and correlation coefficients were used to evaluate the MR methods against the GC of the biopsies. Paired t‐test was used to compare the FAC difference between edematous and healthy legs. Results The estimated parameters correlated well with the GC data (rSFA, rMUFA, and rPUFA = 0.82, 0.81 and 0.89, respectively) using the free model MRI‐based approach. In comparison, the MRS‐based method resulted in weaker correlations and larger biases compared with MRI. In both cases, correct estimation of fMUFA and fPUFA fractions were not possible using the constrained model. The difference in FAC of healthy and edematous legs were estimated to 0.008 (P = .01), −0.009 (P = .005), and 0.002 (P = .03) for fSFA, fMUFA, and fPUFA. Conclusion In this study, MRI‐based FAC quantification was highly correlated, although slightly biased, compared with GC, whereas the MRS‐based approach resulted in weaker correlations. Small but significant differences could be found between the healthy and edematous legs of lymphedema patients using GC analysis.

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Our findings do not indicate that pancreatic atrophy in Type 1 diabetes is associated with an increased pancreatic fat fraction, unlike some other diseases featuring reduced pancreatic volume. We speculate that our results may support the hypotheses that much of pancreatic atrophy in Type 1 diabetes occurs before the clinical onset of the disease and that exogenous insulin administration decelerates pancreatic atrophy after diabetes onset.

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Purpose Previous studies have shown that at a similar body mass index, Middle Eastern immigrants are more insulin resistant and at higher risk for type 2 diabetes (T2D) than native Europeans. Insulin resistance is strongly associated with disturbed fat metabolism and cardiovascular disease (CVD). However, fat metabolism is poorly investigated comparing Middle Eastern and European ethnicities. Methods This observational study included 26 Iraqi and 16 Swedish-born men without T2D or clinical risk factors for CVD. An oral fat tolerance test (OFTT) was performed, where plasma triglycerides (p-TG) were measured for 6 h. mRNA expression and adipocyte size were measured in subcutaneous adipose tissue biopsies collected prior to OFTT, and magnetic resonance imaging was conducted to assess body fat distribution. Results The median p-TG accumulation was higher and the clearance slower among Iraqis than Swedes. None of the groups reached their fasting p-TG (Iraqis 1.55 mmol/l; Swedes 0.95 mmol/l) after 6 h (Iraqis p-TG 3.10 mmol/l; Swedes p-TG 1.50 mmol/l). Adipocyte size, mRNA expression, and fat accumulation in the liver, muscle and abdomen were similar in both groups. Conclusion Postprandial p-TG levels rather than fat distribution may reflect early signs of disturbed fat metabolism in Iraqi immigrants without CVD risk factors.

Middle Eastern immigrants are at high-risk for insulin resistance. Fatty acid composition (FAC) plays an important role in the development of insulin resistance but has not been investigated in people of Middle Eastern ancestry. Here, the aim was to assess the FAC in visceral and subcutaneous adipose tissue (VAT and SAT) in healthy Iraqi- and Swedish-born men using a magnetic resonance imaging (MRI) method.This case-control study included 23 Iraqi- and 15 Swedish-born middle-aged men, without cardiometabolic disease. Using multi-echo MRI of the abdomen, the fractions of saturated, monounsaturated, and polyunsaturated fatty acids (fSFA, fMUFA, and fPUFA) were estimated in VAT and SAT. SAT was further analyzed in deep and superficial compartments (dSAT and sSAT). In all depots, fPUFA was significantly higher and fSFA significantly lower in Iraqi men, independently of age and BMI. In both Iraqi- and Swedish-born men, higher fPUFA and lower fMUFA were found in sSAT vs. dSAT. Among Iraqi men only, higher fPUFA and lower fMUFA were found in SAT vs. VAT.Iraqi-born men presented a more favorable abdominal FAC compared to Swedish-born men. This MRI method also revealed different FACs in different abdominal depots. Our results may reflect a beneficial FAC in Middle Eastern immigrants.

Chemical shift-encoded imaging (CSEI) is the most common magnetic resonance imaging fat–water separation method. However, when high spatial resolution fat fraction (FF) images are desired, CSEI might be challenging owing to the increased interecho spacing. Here, 3 T2-based methods have been assessed as alternative methods for obtaining high-resolution FF images. Images from the calf of 10 healthy volunteers were acquired; FF maps were then estimated using 3 T2-based methods (2- and 3-parameter nonlinear least squares fit and a Bayesian probability method) and CSEI for reference. In addition, simulations were conducted to characterize the performance of various methods. Here, all T2-based methods resulted in qualitatively improved high-resolution FF images compared with high-resolution CSEI. The 2-parameter fit showed best quantitative agreement to low-resolution CSEI, even at low FF. The estimated T2-values of fat and water, and the estimated muscle FF of the calf, agreed well with previously published data. In conclusion, T2-based methods can provide improved high-resolution FF images of the calf compared with the CSEI method.