Current Applications and Selected Technical Details of Dual-Energy X-Ray Absorptiometry

Abstract

The application of dual-energy X-ray absorptiometry (DXA) examinations in the assessment of bone mineral density (BMD) in the lumbar spine, hip, and forearm is the basic diagnostic method for recognition of osteoporosis. The constant development of DXA technique is due to the aging of societies and the increasing importance of osteoporosis as a public health problem. In order to assess the degree of bone demineralization in patients with hyperparathyroidism, forearm DXA examination is recommended. The vertebral fracture assessment (VFA) of the thoracic and lumbar spine, performed by a highly-skilled technician, is an interesting alternative to the X-ray examination. The DXA total body examination can be useful in the evaluation of fat redistribution among patients after bariatric surgery, in patients infected with HIV and receiving antiretroviral therapy, and in patients with metabolic diseases and suspected to have sarcopenia. The assessment of visceral adipose tissue (VAT) and detection of abdominal aortic calcifications may be useful in the prediction of cardiovascular events. The positive effect of anti-resorptive therapy may affect some parameters of DXA hip structure analysis (HSA). Long-term anti-resorptive therapy, especially with the use of bisphosphonates, may result in changes in the DXA image, which may herald atypical femur fractures (AFF). Reduction of the periprosthetic BMD in the DXA measurements can be used to estimate the likelihood of loosening the prosthesis and periprosthetic fractures. The present review aims to present current applications and selected technical details of DXA.

Figure 1

DXA examination of the lumbar spine. At the bottom of the picture is visible the borderline L5/S1, at the top the last pair of ribs connected to the Th12 vertebra. The presented image of the lumbar spine is not suitable for diagnostic objectives, but it is used to identify the ROI in the lumbar spine (vertebral bodies from L1 to L4). The diagnostic result is the T-score parameter estimated for the entire examined ROI area (circled in red). If, due to degenerative changes or artifacts, all 4 lumbar vertebrae cannot be used for analysis, 3 vertebrae should be used. The result of examination is reliable if at least 2 vertebrae are suitable for analysis.

Figure 2

DXA examination of the hip. The diagnostic result is the T-score parameter estimated for the femoral neck (the upper value in the red circle) and for the total proximal femur (the lower value in the red circle). The ROI of the femoral neck (marked as green frame) should be perpendicular to the femoral neck, should not include the greater trochanter and the ischial bone, but should include soft tissues on both sides of the femoral neck. The midline (marked as green line) should cover the long axis of the femoral neck. In the blue circle is marked the FRAX for major osteoporotic fracture (upper value) and FRAX for hip fracture (lower value).

Figure 3

The DXA examination of the right forearm. The diagnostic result is the T-score parameter estimated for the 1/3 ROI of nondominant forearm (blue frame). The reference line (marked in purple) should be positioned at the distal end of the styloid process of the ulna, the radius plate should be located outside the ROI UD (green frame), and a vertical line crossing the middle of the 1/3, MID, and UD ROI (marked in red) should cut the airspace between the radius and ulna.

Figure 4

Result of the VFA examination in the thoracic and lumbar spine. The table shows individual vertebral heights in millimeters (marked in the black circle – the posterior, middle and anterior height, respectively, in order from the left) and the degree of deformation (%) of vertebral bodies (marked in the green circle – the wedge, biconcave and crush deformation, respectively, in order from the left). The red arrow indicates the position of the sacrum and the blue arrow the position of the L5 vertebra. Clear visualization of the sacrum and vertebra L5 is necessary for identification of examined vertebrae.

Figure 5

Severe wedge fracture of the Th8 vertebra (the degree of deformation (%) marked in red circle) and mild crush fracture of the L4 vertebra (the degree of deformation (%) marked in blue circle), based on the VFA examination.

Figure 6

Result of the body composition assessment - skeletal system. ROIs are defined by lines 1–7. The line number 1 separating the head should pass directly below the chin. The line number 2 separating the spine from the chest should pass as close to the spine as possible. The line number 3 separating the upper limbs from the body should pass through the shoulder joints as close to the body as possible. A perpendicular line inside the area defined by line number 2 should separate the thoracic spine from the lumbar spine. The line number 4 separating the pelvis from the trunk should pass over the upper edge of the iliac crests. The line number 5 separating the pelvis from the lower limbs should cross the femoral necks. Line number 6 should cut the airspace between the upper and lower limbs and line number 7 the airspace between the lower limbs.

Figure 7

Result of the body composition assessment – soft tissues. The table on the left shows (parameters from the left, marked in the black circle, respectively): the mass of adipose tissue, the sum of the mass of other soft tissues and bone tissue, the total mass, and the percentage of fat tissue in individual ROIs, including the android and gynoid region (A and G in red circle, respectively). In the tables on the right are presented, among others: percentage of total body fat mass (blue circle), parameters of visceral adipose tissue (green circle), and parameters useful in the assessment of loss of muscle mass (purple circle).