Measurement of abdominal anatomic characteristics on CT images. RAT: the maximum sagittal distance from the top to the visceral side of rectus abdominis; SFT: the maximum sagittal distance from the top to the visceral side of the subcutaneous fat; AD: the distance between the bottom of umbilicus and the top of vertebra; AW: the maximum transverse distance of the abdominal cavity perpendicular to the measurement line of the AD.