The Importance of Bone/Tissue Area Selection in the Clinical Practice of Dual Energy X-ray Absorptiometry

To demonstrate the bone/tissue area selection in dual energy X-ray absorptiometry (DXA) practice significantly affect the bone mineral density (BMD) quantification. Worldwide DXA is the most widely applied method for BMD assessment. It is applied for purposes of diagnostic classification and response to therapy. To best serve patients, it requires understanding of the underlying principles of the method, attention to detail in performing the exam and the ability to recognize numerical values that are inappropriate. DXA uses two energies of X-ray to estimate BMD in the body. The material decomposition is achieved because bone and tissues have different attenuations in different X-ray energies. Due to the existence of fat, a fat correction procedure must be performed to accurately quantify BMD. In clinical practice, the accurate selection of bone and tissue areas can significantly affect BMD quantification. Manufacture's software automatically assigns these areas to an image and its user manual claims that it usually requires no adjustment. We present 2 cases of unexpected large change in BMD resulting from inappropriate tissue reference area. The cases demonstrate the importance of tissue/bone selection in DXA. Radiologists identified cases based on unexpected large changes in BMD. A Medical Physicist worked with the technologists to review these cases, particularly on bone/tissue areas selection. If inappropriate bone/tissue areas were identified, a correction would be made. The BMD changes over 1 year period before and after the correction were calculated. One AP Spine case was identified by a Radiologist due to 10% change in BMD over 1 year period without any history of medical therapy or diseases that accelerated changes in BMD. The bone/tissue areas were reviewed and found the tissue area contained bone. After removing bone from the tissue area, BMD was re- calculated and patient's diagnosis changed from osteoporosis to osteopenia (Figure 1). Another AP Spine case was identified due to 50% change in BMD in 1 year. The review process found no tissue area was selected. This led to incorrect fat correction, which underestimated BMD. After adding the tissue area, BMD increased. Patient's diagnosis changed from osteoporosis to osteopenia (Figure 2). When an unexpectedly large change in BMD occur between two DXA exams for the same patient, a review process needs to be performed to seek potential technical problems. In this presentation we showed that inappropriate tissue/bone areas is one such technical problem but it must be considered by the physician since the tissue areas are usually shown on the images. Additionally, DXA technologists shall visually check the tissue/bone area selections after each exam, and manually adjust these areas when it is necessary.