Quantitative and exact measurements of fat and muscle volume in the body are important for prevention and diagnosis of diseases related to obesity and muscles degeneration. Water-fat separated MRI, based on Dixon imaging techniques [1] enables high soft-tissue contrast and separation of fat and muscle compartments. Scanning neck-to-knee with sufficient resolution may today be accomplished in a mere six minutes. It is, however, challenging to maintain high throughput and data acquisition quality as the number of subjects increases, in particular in population-based studies.

The UK Biobank imaging study [2] will include 100,000 subjects who undergo radiological examinations, including neck-to-knee MRI. In order to handle these vast data quantities it is crucial to develop methods for rapid, accurate and robust acquisition and body composition analysis.

The aim of this study was to investigate the feasibility and success-rate of one recently described method for MR data-acquisition and body composition analysis in the UK Biobank imaging study.

In-vivo acquisition

The first 1,000 subjects in the UK Biobank imaging cohort were included in this study, demographics of the subjects was kept hidden and only imaging data was available. The North West Multicenter Research Ethics Committee (MREC), UK, approved the study and written informed consent was obtained from all subjects prior to study entry.

All subjects were scanned with a Siemens Aera 1.5 T scanner (Siemens, Erlangen, Germany) using the two-point Dixon Vibe protocol, covering neck-to-knees without localizer. The protocol covered a total of 1.1 m from the subjects clavicles, divided over six overlapping slabs of axial 3D spoiled gradient dual-echo images with Dixon reconstruction to water and fat images. Common parameters for all slabs were; TR=6.69 ms, TE=2.39/4.77 ms. The first slab, over the neck consisted of 64 slices, voxel size 2.23×2.23x3 mm³; slabs two to four were acquired during 17 sec expiration breath-holds with 44 slices, voxel size 2.23×2.23×4.5 mm³; slab five consisted of 72 slices, voxel size 2.23×2.23×3.5 mm³; slab six consisted of 64 slices, voxel size 2.23×2.23×4 mm³.

Quality Assurance

Five operators trained to assess radiological images performed quality assurance, to investigate prevalence of artifacts and asses analyzability, using an extensive questionnaire including; respiratory artifacts, metal contamination, water-fat swaps, outer field-of-view inhomogeneities, and scan coverage. Measurements were considered not analyzable if any part of the anatomical region was missing.

Body Composition Analysis

Body composition analysis was performed using AMRA Profiler (AMRA AB, Linköping, Sweden), for all approved datasets [3-5]. Briefly the analysis consisted of the following steps; (1) intensity-inhomogeneity correction as described in [4,6], (2) atlases with ground truth labels for fat and muscle compartments were registered to the acquired volumes as described in [3], (3) quantification of fat and muscle volumes were performed using a voting scheme, and (4) visual inspection by an operator, who then could interactively adjust the final segmentation, as described in [5].

Finally, volumes of visceral adipose tissue (VAT) and abdominal subcutaneous tissue (ASAT) were calculated by integrating the calibrated fat-image over the fat-labels. Thigh muscle volumes were calculated as fat-free muscle volume using the method described in [3].

Statistical Analysis

Descriptive statistics were calculated for all volume measurements and quality assurance metrics. Histograms were generated for VAT, ASAT, total trunk fat defined as VAT+ASAT and total thigh volumes. All statistical analyses were performed in SPSS 21 (SPSS Inc., Chicago, USA, 2012).

Of the 1,000 included subjects, 981 were completely analyzable, 986 were analyzable when only one thigh was included, and 997 were analyzable when omitting both thighs. Figure 1 shows a 1%-sample of the total imaged cohort.

Reasons for not being able to analyse dataset were in all cases; either due to missing slabs in the acquisition, or patient positioned so that large parts of the volume was outside of the field-of-view. Quality assurance are reported in Table 1, representative samples of each type of artifact are demonstrated in Figure 2.

The mean VAT volume was 3.71±2.26 L, the mean ASAT volume was 6.74±3.12 L, and the mean total thigh volume was 10.45±2.54 L. Complete descriptive statistics in Table 2 and histograms in Figure 3.

The high success-rate indicated that the general population within the UK Biobank imaging study complied with the Dixon imaging approach. Most types of artifacts were uncommon, except outer FOV inhomogeneities, which may have led to lower sensitivity for the ASAT measurement in some of the subjects. Water-fat swaps were uncommon and the Dixon Vibe reconstruction was sufficiently robust.

Over 98% of the subjects were analysed for fat and muscle compartments and over 99 % were analysed for fat compartments.