Semester of Graduation

Summer 2020

Degree

Master of Science (MS)

Department

Physics and Astronomy

Document Type

Thesis

Abstract

Introduction: Obesity has become a major societal issue. Many researchers are looking for ways to combat this growing epidemic. Brown adipose tissue (BAT) might be a way to help individuals overcome the challenges associated with weight loss and maintenance of weight loss, but a better understanding of BAT and how to control and utilize it is needed. BAT differs from white adipose tissue (WAT) in that BAT is rich with mitochondria and therefore is metabolically active. BAT is a source of non-shivering thermogenesis and can be activated both by cold exposure and pharmacologically. Current methods of assessing BAT activity are invasive; a noninvasive method to visualize BAT is highly desirable.

X-ray interferometry may be applicable to BAT imaging. Interferometry yields three images from one acquisition: an absorption image, a dark-field (DF) image, and a phase contrast image. The absorption image represents attenuation by the material, equivalent to conventional x-ray imaging; the phase contrast image shows refraction at interfaces through which the beam travels. Small angle scatter caused by microscopic structures in the material cause the DF image; DF has potential interest for BAT visualization. This study evaluated DF imaging as a means to image BAT. The expectation was the large number of mitochondria in BAT will cause a large DF signal, and furthermore that BAT activated by cold exposure would have a different DF signal than BAT at normal conditions.

Materials and Methods: Mice were kept for one week at 8°C to activate BAT; control mice were kept at 22°C. Biochemical markers were used to verify BAT activation by the cold exposure regimen. DF images of cold-exposed and control mice were assessed visually and by region-of-interest analysis to determine if activated BAT could be distinguished from tissue in the same region in control mice. Absorption images provided the identification of an intrascapular region of interest for examination in the DF images. In vivo 99mTc-sestamibi SPECT was used as an independent means to assess BAT activation.

Results: Biochemical markers showed that the cold exposure regimen caused activation of intrascapular BAT as well as the beiging of inguinal WAT; only the intrascapular BAT region was investigated by DF imaging. A region between the scapula and posterior to the spine was apparent in both 8°C and 22°C mice; this region did not show substantial differences in DF signal between the two groups, however. Region of interest analysis of the SPECT images showed increased uptake in the intrascapular region for cold-exposed mice, but the increase was not substantial enough to allow direct visual observation.

Conclusion: Both absorption and DF imaging were capable of contrasting BAT depots from adjacent tissue in the intrascapular region. However, no significant difference in DF signal was seen for this intrascapular BAT between the cold-exposed group and the mice kept at 22°C. This indicated that BAT activation did not result in cellular changes, such as changes in cell size or number of mitochondria, that would alter the small-angle scattering signal.

Committee Chair

Matthews, Kenneth L II

DOI

10.31390/gradschool_theses.5178

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