骨密度測量方法的最新進展

Recent Advances in Bone Density Measurement

骨質疏鬆最終的結果即是骨折，骨折主要決定因素在於骨骼礦物質密度（bone mineral density，簡稱骨密度）及骨骼強度的高低；因此，骨密度的測量被視為現階段主要評估量化骨鬆症診斷的依據及治療的指標。目前用於測量骨密度方法包括：單光子測量法（SPA）、雙光子測量法（DPA）、單能X－光測量法（SXA）、雙能X－光測量法（DXA）、定量超音波法（QUS）、定量電腦斷層掃描（QCT）、磁振造影（MRI）、中子激活法（NAA）、康普吞散射法（Compton Scattering）、放射測量法（Radiogrametry）等。各種骨質密度測量方法均有其優缺點與適應範圍，而以DXA為目前測量骨密度的最佳選擇。雖然直接測量腰椎與股骨雖能得到最好的診斷率與骨折預測。然而，骨骼強度與骨折危險除受骨密度影響外，也與骨骼組成及結構有關，這是單測骨密度方法所無法測量的。骨質疏鬆症所造成的不只是骨量流失，同時骨微結構脆弱、斷裂，這種結構變化與彈性鍵結可藉由超音波通過骨骼所造成的音波衰減及速度變化等反應出來。由於定量超音波不僅能測量骨景（bone quantity），也能反應骨質（bone quality），因此，比DXA所測量之骨密度多出獨立預測骨折之變數。此外，其優點尚包括：正確預測骨折危險率、系統穩定、無放射線、操作簡便、掃描時間短、價格便宜、可攜帶式等。其中，以測量跟骨要比測量脛骨更能預測股骨骨折。隨著美國FDA核可若干QUS機器之使用，預計未來幾年QUS之臨床使用將更為普遍。

Osteoporosis is a disease characterized by low bone mass and microarcbitectural deterioration of bone tissue leading to bone fragility and a consequent increase in fracture risk. Because a reduction in bone density often correlates to an increased risk of fracture, bone density is usually measured in an attempt to establish a diagnosis, to monitor treatment response, and to predict the risk of fracture, Recent advances in bone density measurement, such as Single-energy photon absorptiometry (SPA) Dual energy photon absorptiometry (DPA) Single-energy x-ray absorptiometry (SXA) Dual-energy x-ray absorptiometry (DXA), Quantitative ultrasound (QUS) Quantitative computed tomography (QCT), Quantitative magnetic resonance (QMR) and Neutron activation analysis (NAA) provide highly accurate and precise means of determining bone density. Among these, DXA is currently the best method of measuring bone density and the best available indicators of osteoporotic fracture risk. The precision is 1-1.5% and scanning time 1-5 minutes. The limitation of DXA is its relative high cost, low dose radiation and requirement of a patient referral to the hospital. Factors other than bone mass, e.g., spatial distribution of bone, structural properties, etc. also contribute to bone strength. Most of these factors cannot be determined by roentgenographical examinations, but can be assessed roughly by sonographic approach. QUS represents a rather new approach toward bone density measurement, and several devices are now commercially available. These usually measure bone density in the calcaneus. In addition to broadband ultrasound attenuation (BUA), the speed of sound (SOS) is bone can also be measured. It has been shown that SOS in bone is influenced by orientation of porous bone. QUS can thus provide information on bone elasticity (quality) in addition to bone density (quantity). Measurements in the same calcaneus by QUS and DXA have yield correlations of 0.5-0.8, suggesting an influence of other factors. Precision has varied between 2% and 4% and accuracy approximately 20%. QUS has the capability of providing a cheap, portable, radiationfree assessment of osteoporosis. Following recent regulatory approval of ultrasound devices by the FDA and the availability of reimbursement in the United States, QUS is becoming increasingly accepted and will be more widely used in the future.