26 Imaging Bone
Standard techniques for quantitative imaging of bone are dual energy X-ray absorptiometry (DXA) and quantitative computed tomography (QCT), these methods provide information on bone mineral density. Nuclear medicine techniques do not simply provide density information of bone but characterize bone metabolism; among these, positron emission tomography (PET) is a more novel technique. These technologies are currently used for both research and clinical applications.
26.1 Bone Densitometry
Bone densitometry can be used (1) to assess fracture risk based on the absolute or relative density of bone, (2) to provide recommendations on potential therapy, and (3) to monitor therapy. The most frequently measured sites include the proximal femur, the lumbar spine, and the distal radius, all sites that are also at risk for fragility fractures. Bone mineral density (BMD) is the single most important determinant of fracture, accounting for approximately 70% of bone strength. The lower the peak bone density at a young age, the higher the risk of fracture in later life. In addition to these indications, methods of measuring BMD are also relevant to the study of skeletal development and to diagnose osteopenia and osteoporosis. Most BMD measurement techniques are accurate, reproducible, and sensitive to small changes with time and to differences in patient groups with high and low fracture risk; they are also inexpensive and involve minimal exposure to ionizing radiation.
26.1.1 Dual Energy X-Ray Absorptiometry
DXA measurements of BMD have been universally adopted as a standard to define osteoporosis and osteopenia. DXA uses two X-ray beams with differing kVp (30 to 50 keV and > 70 keV), which enables subtraction of the soft tissue component. DXA measures “areal” BMD (g/cm2) typically of the lumbar spine (L1–L4), proximal femur (femoral neck and total), and distal radius (Fig. 26.1). The accuracy of DXA is between 3% and 8%, with a precision better than 1% (coefficient of variation in percent) at the anterior-posterior spine and the total femur, and 1 to 2% at the femoral neck. Also radiation dose is low (1 to 6 micro Sievert (Sv) for BMD and up to 50 microSv if performed with vertebral fracture assessment). 1 In addition to areal density values in g/cm2, DXA provides T-scores and Z-scores. Z-scores are standard deviations (SD) compared to an age-matched reference population, whereas T-scores are SD compared to a young adult, healthy reference population, matched for gender and ethnicity. In 1994, the World Health Organization (WHO)2 established T-scores at the proximal femur, the lumbar spine, and the distal radius to classify and define BMD measurements. According to the WHO, normal, osteopenic, and osteoporotic BMD are differentiated.
Normal: BMD above (≥) – 1 SD of the young adult reference mean (peak bone mass).
Osteopenia: BMD between (<) –1 and (>) –2.5 SD below that of the young adult reference mean.
Osteoporosis: BMD more than (≤) –2.5 SD below the young adult reference mean.
The WHO definition is not applicable to other bone densitometry techniques (QCT, quantitative ultrasound) or other anatomical sites (e.g., calcaneus). Whole-body DXA with regional analysis gives information not only on total and regional BMD but also on body composition (lean muscle mass and fat mass).
DXA has some limitations: (1) it measures density/area (in g/cm2) of integral (cortical and trabecular) bone and not the volumetric density (in mg/cm3) as is provided by QCT. That means areal BMD is dependent on bone size and will thus overestimate fracture risk in short individuals with small bones, who will have lower areal BMD than normal-sized individuals. (2) Spine and hip DXA are also sensitive to artifacts caused by degenerative changes, and individuals with significant degenerative disease will have falsely increased areal BMD, which will indicate a lower fracture risk than is actually present. Also all structures overlying the spine such as aortic calcifications, or morphological abnormalities of the vertebrae such as fractures (false elevation of BMD) or laminectomy (false reduction) will affect DXA BMD measurements.
26.1.2 Quantitative Computed Tomography
QCT provides a true volumetric density in mg/cm3, rather than the “areal” density (mg/cm2) of DXA. Using a calibration phantom, density values, measured in Hounsfield units, are transformed into BMD measured in mg hydroxyapatite/cm3. Typically, the L1–L3 vertebral bodies are measured (Fig. 26.2). In addition to the true volumetric measurements provided by QCT, the technique has several other important advantages over DXA. QCT can provide separate measures of cortical and trabecular BMD. Trabecular BMD is more sensitive to monitoring changes with disease and therapy, as trabecular bone is more metabolically active than cortical bone.3 Cross-sectional studies have shown that QCT BMD of the spine allows better discrimination of individuals with and without vertebral fractures. 4, 5 QCT is also better suited to examining obese patients as DXA makes assumptions about body composition and so has limitations in measuring BMD in patients with a body mass index over 25 kg/m2.
Limitations of QCT are a higher radiation dose (0.06 to 2.9 mSv depending on whether lumbar spine or hip are scanned and whether single-slice or volumetric techniques are used)1 and that the WHO T-score of −2.5 defining osteoporosis is not applicable to QCT. Currently, volumetric QCT techniques are preferred over single-slice techniques, 6, 7, 8, 9 and in clinical practice absolute measurements of BMD have been defined to characterize fracture risk: below 50 mg/cm3 = severe increase in fracture risk, 50 to 80 mg/cm3 = moderate increase in fracture risk, and 80 to 110 mg/cm3 = mild increase in fracture risk. According to the American College of Radiology Guidelines for QCT, a BMD range of 80 to 120 mg/cm3 is defined as osteopenic and values below 80 mg/cm3 as osteoporotic (ACR Practice Guideline for the Performance of Quantitative Computed Tomography, 2008).
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