Table 2.

Assessment of BMD without a calibration phantom measured simultaneously with the subject

Technique	Refs	Description	Pro and cons
Asynchronous calibration	[42–48]	Calibration phantom is not measured simultaneously with the subject but separately for example once a week or month. The CT value to BMD calibration procedure is the same as for simultaneous calibration.	+ X-ray field inhomogeneity effects can be avoided because the phantom can be scanned in the same location as the spine, for example, whereas in simultaneous calibration the phantom is positioned below the body - Scanner instabilities affecting the BMD calibration cannot be corrected if they occur between the patient and phantom scan - Workflow not implemented in a clinical routine yet
Internal calibration	[31, 49–57]	Use of air and body tissues such as muscle, blood, or subcutaneous adipose tissue for BMD calibration. Different approaches to calculate the linear calibration equation from CT values to BMD exist: 1. Calculation of ‘equivalent’ BMD values for the selected internal materials from a reference dataset using simultaneous calibration [49–54]. 2. Multiple linear regression between BMD values obtained from internal and from simultaneous calibration [31, 55, 56]. 3. BMD calibration based on the linear correlation between measured CT values of internal materials and their known density and absorption coefficients [57].	+ Scanner instability-related X-ray field inhomogeneity effects are smaller for internal calibration materials than for a calibration phantom positioned below the subject, as it is farther away from the bone for which BMD should be measured + Less affected by patient size than simultaneous calibration - Internal calibration is scanner and tube voltage specific - Methods 1 and 2 require a reference dataset with scans obtained on top of calibration phantom to derive the calibration equation and are not adequate for historic scans if such a dataset does not exist - Workflow not implemented in clinical routine yet
Direct use of HU values	[58–64]	No calibration equation used. Instead of BMD, CT values in HU units are used directly.	+ Most simplistic approach - CT scanner calibration to water does not guarantee identical HU values for hydroxyapatite - Thus, any HU threshold used instead of a BMD threshold will depend on the X-ray spectrum, i.e., on scanner, tube voltage, and potentially table height - All scanner instabilities directly affect CT values, which is not the case for simultaneous or internal calibration