There are no existing treatment guidelines for the management of inherited long-chain fatty acid oxidation disorders (LCFAOD). The majority of patients receive a fat-modified diet with medium-chain triglycerides (MCT). With increasing number of patients identified by newborn screening, there is a growing demand for sensitive and effective monitoring parameters, in order to access outcome and concordantly to develop evidence-based treatment strategies. There is a large body of evidence from the very long-chain acyl-CoA dehydrogenase (VLCAD) knockout mouse, that dietary modification with MCT causes abnormal fat distribution/accumulation as well as an abnormal fat composition1. Long-term MCT treatment did not prevent the development of cardiac dysfunction in this mouse model but aggravated the phenotype into a dilative cardiomyopathy2. The aim of this pilot study was to demonstrate that the complex multimodal MRI/MRS protocol from an animal study can successfully be transferred to humans (children and young adults) without using any sedation.Fifteen subjects of the age 8-19 (mean 13.1+/-3.5) years were included in this pilot study. Depending on their condition, subjects were assigned to one of the three groups ‘healthy controls’ (mean BMI = 18.1), ‘obese controls’ (mean BMI = 30.5) or ‘LCFAOD patients’ (mean BMI = 28.5). MRI measurements were carried out on a 3T Siemens TIM-Trio System. For fat-water Dixon imaging a breath-hold spoiled 2D gradient echo protocol covering the abdomen between the top of the femoral heads and the liver apex was used. Under free breathing and with prospective acquisition correction (PACE) based on navigator triggering, liver MR spectroscopy was performed using single voxel PRESS³. The measurement voxel (3 x 3 x 3 cm3) was positioned in the lateral part of the liver, avoiding contamination from larger blood vessels. Using this setup, non-water-suppressed as well as water-suppressed MRS data with 32 spectral averages were acquired using an echo time TE = 35 ms and a minimal TR = 1 s. To avoid unwanted patient motion during MR scans leading to corrupted data the children were given special attention before and during the examination. Fat and water images were reconstructed from the acquired multi-TE gradient echo data, using the graph cuts algorithm⁴ and intra-abdominal and subcutaneous fat volumes were distinguished using an active contour algorithm for image segmentation⁵. The liver spectra were fitted and quantified with LCModel, using a dedicated analysis protocol for lipid detection in the liver⁶. The lipid signal was modeled with peaks at [0.9, 1.3, 1.6, 2.1, 2.3, 2.8, 4.1, 4.3, 5.2, 5.3] ppm by LCModel. Cardiac data were analyzed using the Siemens build-in analysis software ARGUS determining ejection fraction, end-diastolic and end-systolic volume, stroke volume and wall thickness. Figure 1 gives an overview of the measurement locations within the abdomen.

The combined MRI/MRS protocol did successfully run in all subjects without sedation and valid data could be gained in all cases. In Figure 2 three exemplary parameters (one for each MR modality) is plotted. Comparing the control group with the two other groups, clear differences are visible in the peak ejection fraction and the subcutaneous and intra-abdominal fat volume fractions. The latter is obviously reflecting the difference in body mass index of the control group compared to the other two groups. More interesting are details of the liver MRS analysis such as mean chain length and saturation of lipid molecules, even though they did not reach statistical significance within this limited number of subjects. Myocardial systolic wall thickening parameters based on the left ventricle showed differences in maximum and minimum thickening which did not reach statistical significance.

The pilot study demonstrated that a complex multimodal MRI/MRS protocol including fat/water Dixon MRI , liver MRS and cardiac MRI can be applied without sedation in children and young adults to gain information about fat distribution and composition and key parameter of the cardiac system. Such an approach will help to run a larger multi-center study to develop new monitoring and treatment strategies for patients with LCFAODs by improving the understanding of the pathophysiology and long-term outcome of these disorders. However, even in this small pilot study, larger heterogeneity was observed in the patient group compared to the control groups.