Water-Fat MRI Detects Increased Brown Adipose Tissue Volume In Anticipation of Hibernation in Ground Squirrels

Amanda D MacCannell¹, Kevin J Sinclair², Lanette J Friesen-Waldner², Charles A McKenzie², and James F Staples¹

¹Biology, Western University, London, ON, Canada, ²Medical Biophysics, Western University, London, ON, Canada

Synopsis

During winter, brown adipose tissue (BAT) is the primary source of heat production in hibernating animals. White adipose tissue volumes increase and BAT-specific genes are upregulated in autumn even when temperatures are warm, but the rhythm of changes in BAT volume is unknown. Water-fat MRI was used to measure total BAT volume in hibernating squirrels two months after arousing from hibernation in spring and again at 18 days following the first MRI scan. BAT volumes increased significantly in this 20 day time period.

Target Audience

This abstract is targeted to those interested in MRI of brown adipose tissue.

Introduction

During the winter hibernating mammals cycle between periods with very low body temperature (~5°C) that last for several days, and brief arousal periods with normothermic body temperature near 37°C. During these spontaneous arousals brown adipose tissue (BAT) is the primary source of heat production. In non-hibernating mammals proliferation of BAT requires extensive acclimation to cold environmental temperatures. In contrast, expression of brown fat-specific genes in hibernators follows an endogenous rhythm, increasing in autumn even when animals are held at constant, warm temperatures¹. We predicted that, similar to previously observed increases in white adipose tissue (WAT)², the total volume of BAT also increases as the hibernation season approaches, in the absence of acclimation to cold temperatures.

Purpose

To detect changes in BAT and WAT volume in squirrels following hibernation using water-fat MRI.

Methods

Three 13-lined ground squirrels (Ictidomys tridecemlineatus; 2 female, 1 male) were scanned using a 3T MRI scanner (Discovery MR750, GE Healthcare, Waukesha, WI, USA) and a 32 channel cardiac coil under a protocol approved by the institution’s Animal Use Subcommittee. Animals were anaesthetized using isoflurane and 100% oxygen and scanned at 2 months after arousing from hibernation, and again at 18 days after the first scan. T₁- and T₂-weighted images were acquired (TR/TE/flip angle/number of averages = 4.3ms/2.0ms/15°/4 and 2000ms/162ms/90°/2, respectively, with voxel dimensions = 0.9mm isotropic for both acquisitions). IDEAL water-fat images were also collected with TR/∆TE/flip angle = 7.96ms/0.856ms/1° and voxel dimensions = 0.9mm isotropic. The T₂-weighted images were used to manually segment total squirrel volume. BAT is known to have a lower fat fraction than WAT³ so fat fraction images generated from the water-fat images were used for semi-automatic segmentation of WAT and BAT. All segmentation was performed by (AM) using the “2D growing region” tool of Osirix (Pixmeo, Geneva, Switzerland). Segmented adipose tissue volumes were multiplied by the mean fat signal fraction of this region to correct for non-fat or partial-fat voxels. A paired sample Student’s t-test was used to test the null hypothesis that the BAT volume was unchanged between the baseline and 18 day time points.

Results and Discussion

Figure 1 shows a representative T₁-weighted image and a fat fraction image with the BAT locations indicated. Figure 2 demonstrates that a significant increase in BAT volume was seen over the scanning period in all three animals (P=0.02). The mean BAT volume increased by 10.6 +/- 2.4 mL during the 18 days between imaging sessions. Figure 3 shows an example of the BAT segmentation in a single squirrel at the two time points.

Previous studies have shown that the mass-specific BAT capacity for thermogenesis increases in anticipation of hibernation¹. Our results show, for the first time, that quantity of BAT increases as well.

Conclusions

This study shows, for the first time, that BAT volume increases in hibernating squirrels as the hibernation season approaches, in the absence of acclimation to cold temperatures.

Acknowledgements

The authors acknowledge support from NSERC.

References

1. Hindle AG and Martin SL. Intrinsic circannual regulation of brown adipose tissue form and function in tune with hibernation. Am J Physiol Endocrinol Metab. 2014 306(3):E284

2. Sheriff MJ, et al. Metabolic rate and prehibernation fattening in free-living arctic ground squirrels. Physiol Biochem Zool 2013 86: 515

3. Hu HH, et al. Identification of brown adipose tissue in mice with fat-water IDEAL-MRI. J Magn Reson Imaging 2010;31:1195–1202.

Figures

Figure 1. Representative images of a squirrel at the initial time point. A) T1 weighted image through the location of the abdominal BAT deposits (arrows). B) Fat fraction map of the same slice.

Figure 2. Bar chart showing the BAT volumes of the squirrels at baseline and 18 days later. All three squirrels had large increases in BAT volume in the 18 day interval between imaging.

Figure 3. Representative fat fraction maps of a squirrel at the initial time point. (A) and 18 days later (B). The location of the BAT deposits is indicated by the red ROIs.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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