Several measures for bone health have been used in the past. Some have used T directly, and others have preferred the metacarpal index 2T/W, where W is the bone width. Finally Exton-Smith used T/L, where L is the bone length.
BoneXpert’s BHI does something in between, using BHI approximately proportional to T / (LW)^0.33. As explained at great length here, this BHI formula was designed to provide the optimal compensation for the highly variable size and proportion of children.
The exact definition is BHI = π T (1 − T/W) / (LW)^0.33

The figure above shows the BHI reference curve, i.e. the average BHI versus bone age, constructed from healthy Dutch children studied in 1997. BoneXpert computes BHI standard deviation scores (SDS) accordingly. Notice the rapid increase of BHI at puberty, illustrating the importance of assessing BHI as a function of bone age rather than age.

As a result of the large number of measurements underlying BHI, its precision (coefficient of variation, or CV) is 1.4% at age 10. This error is due mainly to variations in the way the hand is placed on the imaging detector. Since the standard deviation of BHI at fixed bone age is 7.5%, this CV corresponds to a precision of BHI of 0.19 SDs, or 0.19 units of BHI-SDS.

In growth hormone deficient children it was found that BHI-SDS increases on average 0.74 in the first year of treatment, which means that this change can be readily measured in the individual child _{(see DD Martin, C Heckmann, MB Ranke, G Binder, HH Thodberg, Differentiation of GH effects on metacarpal bone in children with GHD, ICCBH5, Cambridge, UK. June 23-26, 2009 Bone, Volume 45, Page S60)}

BoneXpert automatically rejects the BHI determination if the bones are reconstructed inconsistently, or if the image quality is insufficient, e.g. due to blur of lack of contrast.