When CT is used, a circular region of interest is drawn around a portion of one of five lumbar vertebrae. These are annotated L1, L2, L3, L4, and L5 with the L1 vertebra being the preferred level and the L3 and L4 levels as secondary choices. L2 and L5 are typically not used. Amongst other information, the region of interest’s primary objective is to quantify the density of tissue in Hounsfield Units (HU) within the circle. When measuring bone, this measurement directly relates to the bone density. Independent studies have validated using this method to differentiate normal from abnormal bone. For example, the following HU values can be used as cutoffs to determine the threshold of the lower limit normal bone density:
135 HU at L1: Using this threshold, 90% of all patients that have a higher HU number will have a bone mineral density within the expected range for age.
99 HU at L1: Using this threshold would correspond to -2SD below the mean which would mean any patient with a higher HU would have a BMD within the expected range for age.
92 HU at L1: Using this threshold would correspond to -2SD below the mean which would mean any patient with a higher HU would have a BMD within the expected range for age.
145 HU at L3 and L4: Using this threshold, any patient with a higher HU would indicate BMD within the expected range for age.
There is no reliable data to determine fracture risk based on standard XR or CT imaging. Fracture risk is determined by calculation of a FRAX score. This calculation takes into consideration: A) femoral neck BMD; B) age; C) gender; D) height; E) weight; F) previous fracture history; G) parent hip fracture history; H) current smoking history; I) glucocoritcoid use; J) rheumatoid arthritis history; K) secondary osteoporosis; and, L) alcohol use. These data cannot be entirely obtained by interpretation of XR or CT, but rather require both a DXA examination and a detailed clinical history. Any calculation of risk without these criteria is speculative.