 The study will be conducted in 2 parts: first, 12 healthy adults will undergo SPECT and 3-D ultrasound in random order aim 1; one week after the first ultrasound examination, a second Ultrasound examination will be conducted in all participants aim 2 In the second part, 24 adolescents 12 in each gender, 12 aged 13-15, 12 aged 16-18 will undergo a single determination of fasting and postprandial gastric volumes during fasting and post-prandially. The methods and validation of the SPECT study have been previously published 2. Three D ultrasonography will be conducted using state of the art equipment following methods published in the literature. 3 