Dual-energy x-ray absorptiometry (DEXA) has sustained a niche for measuring bone mineral density since its approval by the Food and Drug Administration (FDA) for clinical use in 1988. The Bone Mass Measurements act in 1998 solidified its validity in light of other diagnostic modalities such as chemical analysis, direct dissection and ashing, quantitative ultrasonography, and later on against CT/MRI images. DEXA is comparatively inexpensive with notably shorter scan times and radiation exposure compared to other imaging options, and there is a long-standing consensus regarding guidelines for interpreting DEXA images.
A C-arm with the x-ray source below the supine patient emits photons at two distinct energy levels specific for soft tissue and cortical bone. A collimator is situated between the patient and x-ray source to minimize scatter. The attenuations from these low and high-energy photon emissions are detected above the patient and are combined to create a planar image to assess bone mass per unit volume (g/cm), for example, bone mineral density (BMD). A T-score is a number of standard deviations between the patient’s mean BMD and the mean of the population compared with reference populations matched in gender and race. The Z-score is the number of standard deviations above or below the mean of age-matched controls.
To flatten the lordosis of the lumbar spine, the patient lays supine with their hips and knees flexed on a supportive cushion. A PA film should display the spine as straight as possible as well as display the visible superior margin of bilateral iliac crests and the central portion of the T12/L5 vertebral body. Bone mineral density measurements are obtained using the L1 through L4 vertebral bodies.
The long axis of the femoral diaphysis is aligned with the scanner as the patient lies supine and a positioning device which internally rotates the femur to elongate the femoral neck on the PA image. If the femur is effectively internally rotated the less trochanter should be barely, if at all, visible. Bone mineral density measurements are obtained using the femoral neck, greater trochanter, Ward’s area, intertrochanteric region, and total hip.
The patient’s non-dominant arm is placed on the table with the forearm pronated, and the image should demonstrate the distal cortex of the radius/ulna and the diaphysis of each aligned with the long axis of the image. Bone mineral density measurements are obtained using the mid to distal radius and ulna.
The patient is placed supine on the table with arms pronated and feet in dorsiflexion. Bone mineral density measurements are obtained using the upper/lower extremities and the head.
Choosing Site to Scan
Two sites are routinely evaluated with DEXA: the lumbar spine and hip.
All women 65 years and older and men 70 years and older for asymptomatic screening.
Women younger than 65 years old at risk for osteoporosis:
Women younger than 65 years old or men younger than 70 years old with the following risk factors:
Individuals at any age with bone mass osteopenia or fragility fractures on imaging studies
Individuals 50 years and older who develop wrist, hip, spine, or proximal humerus fracture with minimal or no trauma, excluding pathologic fractures.
People of any age who develop one or more insufficiency fractures.
Individuals receiving (or expected to receive) glucocorticoid therapy equivalent to > or = to 5mg of prednisone or equivalent per day for > or = 3 months.
Individuals beginning or receiving long-term therapy with medications known to affect BMD adversely:
Individuals with an endocrine disorder known to affect BMD adversely
Hypogonadal men 18 years and older and men with surgically or chemotherapeutically induced castration
Individuals with medical conditions that could alter BMD:
Individuals considering pharmacologic therapy for osteoporosis.
Individuals monitored for:
Children or adolescents with medical conditions associated with abnormal BMD including but not limited to:
DEXA may be indicated in the diagnosis, staging, and follow-up of individuals with conditions that result in pathologically increased BMD, such as osteopetrosis or prolonged exposure to fluoride.
DEXA may be indicated as a tool to measure regional and whole body fat and lean mass (patients with malabsorption, cancer, or eating disorders).
There are no absolute contraindications to performing DEXA.
Possibly of limited value or require modification of the technique or rescheduling of the examination in some situations, including:
A C-arm with x-ray source allowing for variable photon energy levels, collimator, detector, and associated computer software.
Radiologic/nuclear medicine technologist under the supervision of a licensed physician.
Examination Day Discussion
A licensed radiologist interprets the scans and a T-score is determined to evaluate the standard deviation in the mean from the reference population and patient's average bone mineral density. The World Health Organization (WHO) defines T-scores as:
No complications considered due to procedure. Radiation dose is comparable to standard background radiation.
DEXA imaging serves a sentinel role in the evaluation of osteoporosis as the International Society of Clinical Densitometry, the United States Preventative Services Task Force, and the National Osteoporosis Foundation recommend all women over the age of 65 have their bone mineral density evaluated. It is considered the gold standard for diagnosing osteoporosis and predicting fracture risk with algorithms like the Fracture Risk Assessment tool. Although DEXA imaging has excellent reported accuracy and precision, consideration should be made if comparing results across different instruments from different manufacturers unless cross-calibration has been assured. Evaluation of primary and secondary osteoporosis can not by elucidated with DEXA imaging. This was exemplified by Tannenbaum and colleagues when 55 out of 173 women with the diagnosis of primary osteoporosis were found to have a secondary cause with hypercalciuria, malabsorption, hyperparathyroidism, and vitamin-D deficiency. DEXA imaging is the best clinical tool for assessing bone mineral density in the evaluation of osteoporosis and its validity is evident given its ubiquity among international guidelines.
It is important for the healthcare team to work together to ensure the appropriate DEXA test is ordered and that the test is done correctly. According to ACR Appropriateness Criteria Osteoporosis and Bone Mineral Density, there are specific cases in which Quantitative CT (QCT) is considered superior to DEXA. These include 1. Extremes in body height (i.e. very large and very small patients) 2. Patients with extensive degenerative disease of the spine 3. Severely obese patients (body mass index > 35 kg/meters squared). 4. A clinical scenario that requires increased sensitivity to small changes in trabecular bone density (parathyroid hormone and glucocorticoid treatment monitoring). 
Pitfalls in DEXA are common and errors can be categorized as patient positioning, data analysis, artifacts and/or demographics. Structural changes, such as osteophytes, calcifications, or fractures are more common in the lumbar spine than proximal femur  and potentially determine an artefactual increment of BMD.  Patient positioning may have the consequences of missing important anatomical regions, or excessive internal or external rotation of the proximal femur may cause non-negligible changes in BMD values.  Artefacts (e.g parts of bras, surgical clips, navel rings, vascular prosthesis) may alter the final BMD, resulting in overestimation if the metal is included in the region of interest or underestimation if outside the ROI.  Errors in demographics are important because the t-score is correlated to reference populations in gender and race. Some of the pitfalls can be avoided; however, some may just have to be observed and considered in patient care by the health professions team.
|||ACR Appropriateness Criteria<sup>®</sup> Osteoporosis and Bone Mineral Density., Ward RJ,Roberts CC,Bencardino JT,Arnold E,Baccei SJ,Cassidy RC,Chang EY,Fox MG,Greenspan BS,Gyftopoulos S,Hochman MG,Mintz DN,Newman JS,Reitman C,Rosenberg ZS,Shah NA,Small KM,Weissman BN,, Journal of the American College of Radiology : JACR, 2017 May [PubMed PMID: 28473075]|
|||Garg MK,Kharb S, Dual energy X-ray absorptiometry: Pitfalls in measurement and interpretation of bone mineral density. Indian journal of endocrinology and metabolism. 2013 Mar [PubMed PMID: 23776890]|
|||Preidler KW,White LS,Tashkin J,McDaniel CO,Brossmann J,Andresen R,Sartoris D, Dual-energy X-ray absorptiometric densitometry in osteoarthritis of the hip. Influence of secondary bone remodeling of the femoral neck. Acta radiologica (Stockholm, Sweden : 1987). 1997 Jul [PubMed PMID: 9240674]|
|||Lekamwasam S,Lenora RS, Effect of leg rotation on hip bone mineral density measurements. Journal of clinical densitometry : the official journal of the International Society for Clinical Densitometry. 2003 Winter [PubMed PMID: 14716045]|
|||Watts NB, Fundamentals and pitfalls of bone densitometry using dual-energy X-ray absorptiometry (DXA). Osteoporosis international : a journal established as result of cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA. 2004 Nov [PubMed PMID: 15322740]|