Single institution experience. Take it with "a pinch of salt".
(Mt Sinai Medical Center, New York, NY - Dr. Randall Griepp is the senior author)
Ratio (Aortic ratio) = Observed diameter/Predicted diameter at SoV
Predicted diameter of SoV is determined from regression formula ("reportedly" derived from Roman MJ et al. AJC 1989:64:507-12). It "appears" that the regression formulas dervice the expected mean value - not clarified in the referenced article (Leget et al. Heart 1996;75:389-95).
Regression formula for "Predicted diameter of SoV" in cm. are as follows:
Child (< 18 yrs): 1.02 + (0.98 x BSA in m2)
Adult (18-40 yrs): 0.97 + (1.12 x BSA in m2)
Adult (> 40 yrs): 1.92 + (0.74 x BSA in m2)
Ann Thoracic Surg 1999;67:1834-9.