Weight loss is a common phenomenon in ALS. During the course of the disease, difficulty in swallowing and mastication can be responsible for a decrease in caloric intake and thus for weight loss. However, significant weight loss can also be observed in patients with no feeding difficulties. About half of ALS patients have an increase in their resting energy consumption, but the origin of this "hypermetabolism" remains unknown. "Brown" fat is specialized in the production of heat. Unlike "white" fat that stores excess caloric intakes, brown fat consumes energy. In humans, brown fat has long been considered as absent in adults. However, recent imaging techniques have been able to detect brown fat deposits in some adult subjects. The aim of this study is thus to determine the role of brown fat on energy consumption in Amyotrophic Lateral Sclerosis.