This ICRU report systematically addresses the quantities and units used in bone densitometry utilizing both x-ray and ultrasound techniques. For x-ray techniques the most important quantities are bone mineral density (BMD) and bone mineral mass (BMM), and for ultrasound techniques the speed of sound (SOS) or velocity of sound (VOS), and broadband ultrasonic attenuation (BUA). In the first section of the report these quantities are precisely defined in accordance with the SI system and ISO standards and ambiguities in their current use in the literature are addressed. For example, it is suggested that areal bone mineral density measured with dual energy x-ray absorptiometry (DXA) be indicated by a subscript “a” (BMDa) to differentiate it from the true volumetric quantity BMD measured by quantitative computed tomography (QCT). Another suggestion is the renaming of bone mineral content (BMC) to bone mineral mass (BMM).
The various densitometric techniques and their associated quantities are based on different physical principles. However, from a clinical perspective the physical nature of these densitometry quantities is of secondary interest to their usefulness in the diagnosis of osteoporosis, in the prediction of fractures, in making decisions about treatment and further diagnostic procedures and ultimately in monitoring disease progression or treatment related changes. Hence, in addition to a discussion of physical performance measures such as accuracy, trueness and precision, a large section of this Report discusses diagnostic performance measures to assess and compare the clinical usefulness of the physical quantities specifically in the study of osteoporosis although many of these performance measures are used in other medical fields as well.
In the last part of the Report the physical principles and error sources are described for the densitometric modalities utilizing x-ray and ultrasound based techniques. Radiographic absorptiometry, radiogrammetry, single and dual x-ray absorptiometry, and quantitative computed tomography are based on the well-understood physics of x-ray absorptiometry. In contrast the propagation and absorption of ultrasonic waves through bone and soft tissue is much more complex. The densitometric modality quantitative ultrasound (QUS) provides an umbrella of techniques that do not just yield one quantity such as the absorption coefficient as in the case of x-rays but potentially a multitude of variables that in principle should allow an advanced assessment of bone strength since the ultrasound propagation characteristics are determined by a combination of structural and material properties of the propagation medium. However, to date this potential advantage of ultrasound has not been fully exploited and the physical framework is not completely understood despite recent advances in theoretical models and numerical computations.
In summary this Report is dedicated to researchers as well as clinicians in the field of osteoporosis as a concise reference of the physical foundations and clinical performance of bone densitometry. It provides a clear and unambiguous definition of the quantities and units used in this field.