Bone mineral density (BMD) is defined as the amount of mineral (calcium hydroxyapatite) per unit of bone.

Radiographic features

BMD can be measured by various methods: with DEXA the most prevalent

• gamma rays: replaced by radiographic methods
  • single-energy photon absorptiometry (SPA) was superseded by the introduction of single x-ray absorptiometry; both methods measure density at peripheral sites in the skeleton (e.g. distal radius, calcaneus), but the disadvantage is that it cannot measure trabecular and cortical bone separately
  • dual-energy photon absorptiometry (DPA) also has been replaced by dual-energy x-ray absorptiometry (DEXA)
• x-ray
  • single-energy x-ray absorptiometry
  • dual-energy x-ray absorptiometry (DEXA)
  • quantitative computed tomography (QCT)
  • radiogrammetry
• ultrasound
  • calcaneal ultrasonography: determines bone mineral density and bone strength
  • speed sound (SOS): measures velocity in the bone
  • broadband ultrasound attenuation (BUA): measures the frequency in the bone
• CT
  • advantageous for opportunistic screening but research not advanced for implementation into clinical practice ⁴
  • HU measurements vary on anatomic site including by individual vertebra ⁴
  • scanner variability is also a significant factor with HU threshold for diagnosis of osteoporosis varying between 99 and 136 HU ⁴
• MRI
  • spectroscopy
  • quantitative magnetic resonance imaging: depicts changes in the trabecular structures at several sites of the peripheral skeleton