Dual Energy Xray Absorptiometry (DEXA)

Article Author:
Marissa Krugh
Article Editor:
Michelle Langaker
Updated:
9/6/2018 11:03:55 AM
PubMed Link:
Dual Energy Xray Absorptiometry (DEXA)

Introduction

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. 

Anatomy

Lumbar Spine

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.

Hip

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.

Forearm

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.

Whole Body

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.[1]

Indications

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:

  • Estrogen deficiency
  • History of maternal hip fracture before the age of 50
  • Low body mass (less than 127 pounds)
  • History of amenorrhea more than 1 years before the age of 42

Women younger than 65 years old or men younger than 70 years old with the following risk factors:

  • Current cigarette smoker
  • Loss of height
  • Thoracic Kyphosis

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:

  • Anticonvulsants
  • Androgen deprivation therapy
  • Aromatase inhibitor therapy
  • Chronic heparin

Individuals with an endocrine disorder known to affect BMD adversely

  • Hyperparathyroidism
  • Hyperthyroidism
  • Cushing’s syndrome

Hypogonadal men 18 years and older and men with surgically or chemotherapeutically induced castration

Individuals with medical conditions that could alter BMD:

  • Chronic renal failure
  • Rheumatoid arthritis and other inflammatory arthritides
  • Eating disorders, including anorexia nervosa and bulimia
  • Organ transplantation
  • Prolonged immobilization
  • Conditions associated with secondary osteoporosis, such as gastrointestinal malabsorption or malnutrition, sprue, osteomalacia, vitamin D deficiency, endometriosis, acromegaly, chronic alcoholism or established cirrhosis, and multiple myeloma
  • Individuals who have had a gastric bypass for obesity (The accuracy of DEXA in these patients might be affected by obesity)

Individuals considering pharmacologic therapy for osteoporosis.

Individuals monitored for:

  • Assess the effectiveness of osteoporosis drug therapy
  • Follow-up medical conditions associated with abnormal BMD.

Children or adolescents with medical conditions associated with abnormal BMD including but not limited to:

  • Individuals receiving (or expected to receive) glucocorticoid therapy for more than 3 months
  • Individuals receiving radiation or chemotherapy for malignancies
  • Individuals with an endocrine disorder known to adversely affect BMD (e.g., hyperparathyroidism, hyperthyroidism, growth hormone deficiency or Cushing’s syndrome)
  • Individuals with bone dysplasias known to have excessive fracture risk (osteogenesis imperfecta, osteopetrosis) or high bone density
  • Individuals with medical conditions that could change BMD, for example:
  1. Chronic renal failure
  2. Rheumatoid arthritis and other inflammatory arthritides
  3. Eating disorders, including anorexia nervosa and bulimia
  4. Organ transplantation
  5.  Prolonged immobilization
  6.  Conditions associated with secondary osteoporosis, such as gastrointestinal malabsorption, sprue, inflammatory bowel disease, malnutrition, osteomalacia, vitamin D deficiency, acromegaly, cirrhosis, HIV infection, prolonged exposure to fluorides

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).

Contraindications

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:

  • Recently administered gastrointestinal contrast or radionuclides
  • Pregnancy
  • Severe degenerative changes or fracture deformity in the measurement area
  • Implants, hardware, devices, or other foreign material in the measurement area
  • The patient’s inability to attain correct position and/or remain motionless for the measurement
  • Extremes of high or low body mass index (BMI) which may adversely affect the ability to obtain accurate and precise measurements. Quantitative computed tomography (QCT) may be a desirable alternative in these individuals.
  • Any condition that precludes proper positioning of the patient to be able to obtain accurate BMD values

Equipment

A C-arm with x-ray source allowing for variable photon energy levels, collimator, detector, and associated computer software.

Personnel

Radiologic/nuclear medicine technologist under the supervision of a licensed physician.

Preparation

Pre-Scan Discussion

  • Patients can tolerate laying on back for up to 10 minutes.
  • If the patient is greater than 300 pounds, they will require alternative BMD testing. Different models have different weight restrictions, and the system owner’s manual should be checked.
  • Recent medical imaging with contrast, such as barium or gadolinium, will preclude imaging 2 weeks after contrast was administered
  • Premenopausal patients should be asked whether there is any possibility that they might be pregnant. A pregnancy test may need to be administered before the examination.
  • Calcium tablets should not be taken in the 24 hours before the examination
  • Patients should wear comfortable, loose-fitting clothes with avoidance of metal components such as zippers.
  • If prior DEXA was done, the patient should be instructed to bring results.
  • For body composition studies, patients should be scanned in the morning after a 12-hour overnight fast for consistency.

Examination Day Discussion

  • Confirm they have complied with the recommendations listed above
  • The menopausal status should be re-checked and whether a pregnancy test or question relating to possible pregnancy has been administered
  • Subjects should be dressed in a hospital gown or scrubs, wearing only underpants and, if necessary, thin socks. A thin sheet may be placed over subjects for warmth.
  • All radio-opaque objects should be removed from the scan area.

Technique

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:

  • Greater than or equal to -1.0: normal
  • Less than -1.0 to greater than -2.5: osteopenia
  • Less than or equal to -2.5: osteoporosis
  • Less than or equal to -2.5 plus fragility fracture: severe osteoporosis

Complications

No complications considered due to procedure. Radiation dose is comparable to standard background radiation.

Clinical Significance

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.

Enhancing Healthcare Team Outcomes

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). [1]

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 [2] and potentially determine an artefactual increment of BMD. [3] 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.  [4]  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.  [5] 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.