Estimation of bone status using X-rays: current techniques

Assessment of low-energy fractures
Spinal radiography is the most widely used imaging method for identification of vertebral fractures. Vertebral fractures on radiographs are not always reported and remain under-diagnosed radiologically with false negative rates up to 45% [2]. The assessment of vertebral fractures is possible using visual, morphometric and semiquantitative methods [3–6]. The method for identification of vertebral fractures using computational techniques has also been applied to spine images acquired by dual-energy X-ray absorptiometry (DXA). Vertebral fracture assessment (VFA) developed by DXA manufacturers provides information on the vertebral body heights and their ratios and the patient’s fracture status is given. A recent study evaluated the utility of VFA to detect vertebral fractures [7]. Although the sensitivity of VFA was found to be less than that of radiography, in certain circumstances results support the use of VFA for the detection of prevalent vertebral fracture.

Dual-energy X-ray absorptiometry (DXA)
DXA technology has evolved from pencil beam to fan beam, allowing short acquisition time and improved image quality. In clinical practice, ‘areal’ bone mineral density (BMDa; g/cm2) assessment of lumbar spine (L1–L4), proximal femur (femoral neck and total hip) and forearm (distal) is made by central DXA. Interpretation of BMDa measurements is based on the World Health Organisation (WHO) recommendations. Osteoporosis can be diagnosed if the value of BMDa is 2.5 or more standard deviations (SD) below the mean value of a young reference population (T score at or below −2.5). Central DXA can also provide whole-body imaging for total and regional BMDa, body composition (lean muscle and fat mass) and VFA. Total body imaging is useful for the assessment of the growing skeleton and forearm imaging can be performed in patients with hyperparathyroidism [8]. DXA at peripheral sites can be performed using either general purpose body DXA or smaller dedicated peripheral DXA for measurements in peripheral skeletal sites. Using device-specific thresholds peripheral DXA may play a role in identifying those at risk of osteoporotic fracture, especially when there is limited or no access to central DXA.

Quantitative computed tomography (QCT) using body CT
Using QCT, bone mineral density (BMD; mg/cm3) measurements can be obtained in central and peripheral skeletal sites. Examinations are performed using an application-specific software package and a dedicated bone-equivalent calibration phantom imaged simultaneously with the patient to convert the CT numbers into bone-equivalent values (mg/cm3; g/l). QCT requires a lateral scout image of the lumbar spine. A typical single-slice 2D QCT protocol consists of a 10-mm section in the mid plane of each of three or four adjacent vertebrae (T12, L1, L2 and L3) acquired with 80-kVp tube potential and 125-mAs tube load. As this 2D technique has a limited precision, 3D volumetric QCT protocols have been developed based on multi-detector CT (MDCT) imaging. Using MDCT, 3D volume sets are acquired and from these BMD values and bone geometry can be measured [9, 10]. In spine multi-detector QCT (MDQCT) two or three vertebrae are usually imaged, L1–L2 or L1–L3, to reduce dose. Hip MDQCT is capable of analysing the main regions of the hip i.e. the femoral neck, the trochanter and the intertrochanteric region.

High-resolution CT imaging
MDCT is not capable of depicting individual trabeculae. However, important information can be obtained from structure analysis of high-resolution image data. A recent study compared MDCT-derived apparent structure measures with high-resolution (HR) peripheral QCT (pQCT)-derived structure measures as the ‘gold standard’ using intact human cadaver forearm specimens [11]. Most MDCT-derived microarchitectural parameters correlated highly significantly with HR pQCT measures. This study shows that MDCT is capable of quantifying characteristics of the trabecular bone network in the radius [11]. Image processing techniques such as fuzzy distance transformation have been used to provide information on trabecular distance measurements in vertebrae imaged by HR CT [12]. Findings confirmed that this technique can potentially be used as a tool for monitoring osteoporosis treatment. However, currently, these techniques are limited to research applications.

Peripheral QCT
pQCT permits in vivo assessment of bone morphology and BMD at appendicular bones such as the distal radius and tibia. pQCT can be used simply for BMD and bone geometry, or in HR to provide information on trabecular bone structure. A recently developed device (Scanco, Bruttisellen, Switzerland) has an isotropic voxel size in the order of 80 μm, which allows direct or indirect evaluation of cortical and trabecular bone architecture. Specifically, assessment of parameters such as trabecular number, cortical thickness, trabecular thickness and porosity, and trabecular separation is possible with this technique. Recent studies have focused on HR pQCT imaging of bone microstructure in both adults and adolescents [13, 14].