Techniques for measuring bone density
- Dual energy x-ray absorptiometry (DXA) scanning is the internationally accepted gold standard means for measuring bone density.
- Bone density should be measured in the spine and/or proximal femur. Spinal measurements are more precise and sensitive in detecting change, but can be falsely elevated by sclerosis associated with degenerative change, osteophytes, or vertebral fractures.
- Central osteoporosis is determined by a T-score of ≤-2.5 in the spine or in either the femoral neck or total femur.
- Wrist scanning is advised in assessing primary hyperparathyroidism and is a useful second area of assessment when axial scanning is affected by artifact and hip scanning not possible due to previous THR or ORIF.
- Guidelines for providing a densitometry service including quality control, interpretation and reporting standards, have been developed and workshops for reporting physicians and technicians have been provided in conjunction with Osteoporosis New Zealand. Details are available via e-mail.
- Quantitative CT (QCT) is an axial x-ray technique allowing selective assessment of cortical and trabecular bone, generally confined to evaluation of the spine only. The disadvantages include a higher radiation dose (10 times DXA) and lesser precision and reproducibility. Additionally, spinal scanning does not predict hip fracture risk as accurately as does DXA of the hip. WHO T-scores do not apply to QCT of the spine. Using QCT to guide therapy decisions is not at present supported by evidence.
- Ultrasound of the heel is sometimes used as a screening technique. It has the advantage of being inexpensive, portable and non-radiation based. However, the technique also suffers from lesser precision and is therefore not suitable for monitoring osteoporosis. It is not clear that it is sufficiently predictive of the DXA result to justify a routine place in osteoporosis assessment. Results indicating a low bone density by heel ultrasound should be confirmed by DXA scan before therapy is instituted.
