Comparison of brown and white adipose tissues in infants and children with chemical-shift-encoded water-fat MRI

Abstract

Purpose: To compare fat-signal fractions (FFs) and T2* values between brown (BAT) and white (WAT) adipose tissue located within the supraclavicular fossa and subcutaneous depots, respectively.

Materials and methods: Twelve infants and 39 children were studied. Children were divided into lean and overweight/obese subgroups. Chemical-shift-encoded water-fat magnetic resonance imaging (MRI) was used to quantify FFs and T2* metrics in the supraclavicular and adjacent subcutaneous adipose tissue depots. Linear regression and t-tests were performed.

Results: Infants had lower supraclavicular FFs than children (P < 0.01) but T2* values were similar (P = 0.5). Lean children exhibited lower supraclavicular FFs and T2* values than overweight children (P < 0.01). In each individual infant and child, supraclavicular FFs were consistently lower than adjacent subcutaneous FFs. Supraclavicular T2* values were consistently lower than subcutaneous T2* values in children, but not in infants. FFs in both depots were positively correlated with age and weight in infants (P < 0.01). In children, they were correlated with weight and body mass index (BMI) (P < 0.01), but not age. Correlations between T2* and anthropometric variables existed in children (P < 0.01), but were absent in infants.

Conclusion: Cross-sectional comparisons suggest variations in FF and T2* values in the supraclavicular and subcutaneous depots of infants and children, which are potentially indicative of physiological differences in adipose tissue fat content, amount, and metabolic activity.

Keywords: T2* relaxation; brown adipose tissue; children; fat-signal fraction; water-fat MRI; white adipose tissue.

Figure 1

Anatomical grayscale images of water, fat, and opposed-phase images are used to manually create segmentation masks for the subcutaneous (arrowheads) and supraclavicular (brown arrows) adipose tissue depots. The binary masks are generated using commercial sliceOmatic® software using a combination of signal-intensity based thresholding, region growing, and morphological erosion steps. The generated masks are then automatically transferred by the software to the co-registered FF and T₂* maps, shown in color, to compute region-of-interest (ROI) values. Images are from a 66-days-old boy. The FF maps are illustrated on a 0% (purple) to 100% (red) scale. The T₂* maps are illustrated using a condensed scale from 0 to 26 msec for illustration clarity, such that any tissue with a computed T₂* value ≥ 26 msec is shown in red.

Figure 2

(A) FF map, (B) T₂* map, and (C) segmentation masks from a 15-years-old female with a healthy BMI of 22.2 kg/m², and (D-F) a 12-years-old female with an obese BMI of 31.4 kg/m². Note the relatively well-defined supraclavicular fossa depot and the detail with which the segmentation masks are generated by the sliceOmatic® software. Note the exclusion of non-adipose tissue structures. Same arrowhead and arrow symbols, as well as color scale notations, as Figure 1.

Figure 3

Representative FF (left column, A, C, E, G) and T₂* (right column, B, D, F, H) maps from (A-D) two infants and (E-H) two children. The infant illustrated in (A, B) is a 123-days-old girl with a weight percentile of 10%. The infant illustrated in (C, D) is a 100-days-old girl with a weight percentile of 25%. Bilateral supraclavicular fossa depots where BAT is typically found are highlighted (brown arrows). Note that these BAT-containing depots are visually distinguishable from the triglyceride-rich (red) subcutaneous white adipose tissue depot (arrowheads) and lean skeletal muscles and organs (purple) in the FF maps. Note that their appearance also differs between the two infants. Corresponding areas on the T₂* maps also show lower values in the supraclavicular fossa depot in comparison to the subcutaneous depot. FOV settings for (A, B) were: right/left 24 cm, anterior/posterior 14 cm, and superior/inferior 10 cm. For (C, D) they were: right/left 22 cm, anterior/posterior 14 cm, and superior/inferior 10 cm. The child illustrated in (E, F) is a lean 12-years-old male with a BMI of 17.2 kg/m² and a BMI percentile of 35.3%. The child illustrated in (G, H) is an obese 11-years-old male with a BMI of 32.7 kg/m² and a BMI percentile of 99.2%. The difference in both FF and T₂* appearances within the supraclavicular fossa BAT depot is markedly evident between the two children, with BAT in the lean subject characterized by lower values. FOV settings for (E, F) were: right/left 36 cm, anterior/posterior 16 cm, and superior/inferior 25 cm. For (G, H) they were: right/left 44 cm, anterior/posterior 26 cm, and superior/inferior 25 cm. Same arrowhead and arrow symbols, as well as color scale notations, as Figure 1.

Figure 4

Comparison of (A) FF and (B) T₂* distributions between all infants (n=12) and all children (n=39) from the study from measurements within the supraclavicular fossa depot (filled circles) and subcutaneous depot (open circles). The long horizontal bar in each distribution denotes the mean while the pair of short horizontal bars denotes standard deviation. Statistically significant comparisons are denoted by (*) for p<0.01. Statistically significant correlations were observed between FF values and infant age (C) and weight (D) in both depots. See also Table 3.

Figure 5

Comparison of (A) FF and (B) T₂* distributions between lean (n=17) and overweight/obese (n=22) children sub-groups, stratified at the 85^th BMI percentile. Same symbol notation as Figure 4. Statistically significant correlations were observed between (C) FF and (D) T₂* values versus BMI in both depots. See also Table 3.

Figure 6

Scatter plots of individual mean FF and T₂* values for (A) the infant cohort and (B) the children cohort. Filled circles represent measurements from the supraclavicular fossa depot. Open circles represent measurements from the subcutaneous depot. While there does not appear to be a data trend in the infants, it is clear that FF values in the supraclavicular fossa depot are lower than that of the subcutaneous depot. However, T₂* values are similar. The data trend in the children cohort is in contrast visually striking and suggests associations between FF and T₂* measurements in the two depots with body adiposity. For visual clarity, the two plots were not overlaid on top of each other. However, note that they share the same axes. In (A), the data point pair highlighted by arrows denote the infant illustrated in Figure 7.

Figure 7

FF maps from a 6-days-old girl, shown on the same 0-100% scale as in Figures 1-3. In (A) note the rather low (yellow-orange) FF appearance of the subcutaneous depot (~64.6%), suggestive of potential BAT presence. In (B), the triangular adipose tissue depot located at the nape is evident (arrow), with an FF appearance also supportive of BAT presence.