Accurate fatty acid composition estimation of adipose tissue in the abdomen based on bipolar multi‐echo MRI

Abstract

Purpose To develop a bipolar multi‐echo MRI method for the accurate estimation of the adipose tissue fatty acid composition (FAC) using clinically relevant protocols at clinical field strength. Methods The proposed technique jointly estimates confounding factors (field map, R2*, eddy‐current phase) and triglyceride saturation state parameters by fitting multi‐echo gradient echo acquisitions to a complex signal model. The noise propagation behavior was improved by applying a low‐rank enforcing denoising technique and by addressing eddy‐current‐induced phase discrepancies analytically. The impact of the total echo train duration on the FAC parameter map accuracy was analyzed in an oil phantom at 3T. Accuracy and reproducibility assessment was based on in vitro oil phantom measurements at two field strengths (3T and 1.5T) and with two different protocols. Repeatability was assessed in vivo in patients (n = 8) with suspected fatty liver disease using test–retest acquisitions in the abdominal subcutaneous, perirenal and mesenteric fat depots. Results Echo train readout durations of at least five times the conventional in‐phase time were required for accurate FAC estimation in areas of high fat content. In vitro, linear regression and Bland–Altman analyses demonstrated strong (r > 0.94) and significant (P ≪ 0.01) correlations between measured and reference FACs for all acquisitions, with smaller overall intercepts and biases at 3T compared to 1.5T. In vivo, reported mean absolute differences between test and retest were 1.54%, 3.31%, and 2.63% for the saturated, mono‐unsaturated, and poly‐unsaturated fat component, respectively. Conclusions Accurate and reproducible MRI‐based FAC quantification within a breath‐hold is possible at clinical field strengths.

Quantitative 1H MRI and MRS of fatty acid composition

“…62 In oil phantoms, a bias was found between the estimates at 1.5T and 3T, 20 and a lower reproducibility was demonstrated between field strengths than between 2D and 3D acquisition protocols with a lower accuracy found at 1.5T. 60 Apart from the lower spectral resolution, an increased impact of J-coupling is also expected at the lower field strength which may motivate the choice of 3T over 1.5T. 60 However, the increased frequency differences also reduces the suitable range of inter-echo times as described above, potentially making high resolution imaging more difficult at a higher field strength and possibly making the use of a bipolar readout necessary.…”

“…20,60,61 Although the stability of the method has been demonstrated to increase with the number of echoes, inaccuracies of the model have an increasing impact on estimation accuracy with increasing total read-out time. 60,61 At 3T, a maximum inter-echo time of 1.8 ms and a minimum total readout time of 15 ms (corresponding to nine echoes) has been suggested by Berglund et al for a joint separation of four free FAC parameters. 20 Note that the method by Berglund et al 20 separates an alternative set of four FAC parameters as opposed to the more commonly used ndb, nmidb, and cl.…”

“…However, the optimal acquisition parameters are likely similar. In another study using 3T, a maximum inter-echo time of 1.23 ms and a readout time of 14-18 ms (corresponding to 11-15 echoes) was suggested for a sequential reconstruction of ndb and nmidb with cl expressed in terms of ndb and with correction of bipolar phase errors by Schneider et al 60 Imaging-based FAC estimation has been compared between 1.5T and 3T, 20,60 and the feasibility also at 7T has been demonstrated, using a preclinical system. 62 In oil phantoms, a bias was found between the estimates at 1.5T and 3T, 20 and a lower reproducibility was demonstrated between field strengths than between 2D and 3D acquisition protocols with a lower accuracy found at 1.5T.…”

“…60 Apart from the lower spectral resolution, an increased impact of J-coupling is also expected at the lower field strength which may motivate the choice of 3T over 1.5T. 60 However, the increased frequency differences also reduces the suitable range of inter-echo times as described above, potentially making high resolution imaging more difficult at a higher field strength and possibly making the use of a bipolar readout necessary. The use of a bipolar readout introduces additional phase errors of the acquired images with a detrimental impact on fat/water separation if not corrected.…”

“…63,64 For FAC estimation, correction of these phase errors has been suggested as the first part of sequential reconstruction approaches. 59,60,62 The data acquisition time for imaging-based FAC assessment is typically a few minutes, but may also be limited to a single breath-hold for liver applications. 6,51,59…”