Dual-Energy X-Ray Absorptiometry

Anatomy and Physiology

Lumbar Spine

To flatten the lordosis of the lumbar spine, the patient lies supine with their hips and knees flexed on a supportive cushion. A PA film should display the spine as straight as possible, with the visible superior margin of bilateral iliac crests and the central portion of the T12/L5 vertebral body. BMD 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 that internally rotates the femur to elongate the femoral neck on the PA image. If the femur is effectively internally rotated, the lesser trochanter should be barely, if at all, visible. BMD measurements are obtained using the femoral neck, greater trochanter, Ward's area, intertrochanteric region, and total hip.

Forearm

The patient's nondominant arm is placed on the table with the forearm pronated. The image should demonstrate the distal cortex of the radius and ulna and the diaphysis of each aligned with the long axis of the image. BMD 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. BMD measurements are taken from the upper or lower extremities and the head.

Choosing Sites for Scanning

Two sites are routinely evaluated with DEXA—the lumbar spine and hip.[3]

Indications

All women 65 or older and men 70 or older should be screened for asymptomatic osteoporosis.[4]

Women younger than 65 at risk for osteoporosis include the following groups:

• Those with estrogen deficiency
• Those with a history of maternal hip fracture before 50
• Those with low body mass (<127 pounds or body mass index (BMI) <18.5 kg/m²)
• Those with a history of amenorrhea for 1 year or more before 42

Women younger than 65 or men younger than 70 with the following risk factors should also be screened.

• Current cigarette smoker
• Loss of height
• Thoracic kyphosis

The following groups should also be screened for osteoporosis:

• Individuals at any age with bone mass osteopenia or fragility fractures on imaging studies.
• Individuals 50 or older who develop wrist, hip, spine, or proximal humerus fractures with minimal or no trauma.
• Individuals of any age who develop 1 or more insufficiency fractures.
• Individuals receiving (or expected to receive) glucocorticoid therapy equivalent to 5 mg or more of prednisone daily for 3 or more months.
• Individuals considering pharmacological therapy for osteoporosis.
• Hypogonadal men 18 or older and men with surgically or chemotherapeutically induced castration.

Medications known to adversely affect BMD include:

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

Endocrine disorders known to adversely affect BMD include:

• Hyperparathyroidism
• Hyperthyroidism
• Cushing syndrome

Patients may be monitored for the following reasons:

• To assess the effectiveness of osteoporosis drug therapy
• Follow-up for medical conditions associated with abnormal BMD

Children or adolescents with medical conditions associated with abnormal BMD, including but not limited to the following categories:

• Individuals receiving (or expected to receive) glucocorticoid therapy for more than 3 months
• Individuals receiving radiation or chemotherapy for malignancies
• Individuals with endocrine disorders known to adversely affect BMD, such as hyperparathyroidism, hyperthyroidism, growth hormone deficiency, or Cushing syndrome
• Individuals with bone dysplasias known to have excessive fracture risk, such as osteogenesis imperfecta and osteopetrosis, or high bone density

Individuals with medical conditions that could decrease BMD include the following conditions:

• Chronic renal failure
• Rheumatoid arthritis and other inflammatory arthritides
• Eating disorders, including anorexia nervosa and bulimia
• Organ transplantation
• Prolonged immobilization
• History of a gastric bypass for obesity (noting that the accuracy of DEXA in these patients might be affected by obesity)
• Conditions associated with secondary osteoporosis, such as gastrointestinal malabsorption, inflammatory bowel disease, malnutrition, osteomalacia, vitamin D deficiency, acromegaly, cirrhosis, HIV infection, chronic alcoholism, and multiple myeloma.

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, such as in patients with malabsorption, cancer, or eating disorders.

Contraindications

There are no absolute contraindications for performing DEXA. The possibility of inadequate studies or the need for modification of the technique or rescheduling of the DEXA test may be required in the following situations:

• Recently administered gastrointestinal contrast or radionuclides
• Pregnancy
• Scoliosis, which may affect spine measurements
• 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 the correct position or remain motionless for the measurement
• Extremes of high or low BMI may adversely affect the ability to obtain accurate and precise measurements. Quantitative computed tomography (QCT) may be a desirable alternative for these individuals.
• Any condition that precludes proper positioning of the patient to be able to obtain accurate BMD values

Equipment

A DEXA scan is typically performed using a C-arm with an x-ray source that allows for variable photon energy levels, a collimator, a detector, and associated computer software.

Personnel

DEXA imaging is typically performed by radiologic or nuclear medicine technologists under the supervision of a licensed physician.

Preparation

Pre-Scan Discussion

• Patients should be able to tolerate laying on the back for up to 10 minutes.
• Patients who weigh more than 300 pounds require alternative BMD testing. Different models have different weight restrictions, and the equipment limitations should be checked.
• Recent medical imaging with contrast, such as barium or gadolinium, precludes imaging for 2 weeks after contrast is 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 within 24 hours before the examination.
• Patients should wear comfortable, loose-fitting clothes, avoiding metal components such as zippers.
• If a 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

• Compliance with the above-listed recommendations should be ensured.
• The menopausal status should be re-checked, and negative pregnancy status should be confirmed.
• Subjects should be dressed in hospital gowns or scrubs, wearing only underpants and, if necessary, thin socks. A thin sheet may be placed over them for warmth.
• All radio-opaque objects should be removed from the scan area.

Technique or Treatment

Interpretation of Results

BMD is the standard for measuring the diagnosis of osteoporosis and fracture risk assessment. The Fracture Risk Assessment Tool (FRAX) uses clinical risk factors, excluding the measure of BMD, to identify individuals at risk for osteoporosis or fractures. The FRAX was implemented in 2008 by the World Health Organization (WHO) and is used to calculate the chance of a fracture in a 10-year timeframe.

The clinical risk factors included in FRAX are age, sex, race, height, weight, BMI, a history of fracture, use of oral glucocorticoids, rheumatoid arthritis, other secondary causes of osteoporosis, smoking, and alcohol use of 3 or more drinks daily. The geographic location of a patient's home is also considered in the calculation.

In the United States, the National Osteoporosis Foundation recommends using FRAX to calculate fracture risk for patients with T-scores between −1.0 and −2.5 in the spine, femoral neck, or total hip region.

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 the patient's average BMD. The WHO defines T-scores as follows:

• Greater than or equal to −1.0: normal
• Between −1.0 and −2.5: osteopenia
• Less than or equal to −2.5: osteoporosis
• Less than or equal to −2.5 plus fragility fracture: severe osteoporosis

Complications

Complications are uncommon with this procedure. The radiation dose is comparable to standard background radiation.

Clinical Significance

DEXA imaging plays a crucial role in evaluating osteoporosis. The International Society of Clinical Densitometry, the United States Preventative Services Task Force, and the National Osteoporosis Foundation recommend that all women over 65 have a BMD evaluation. DEXA imaging is the gold standard for diagnosing osteoporosis and predicting fracture risk with algorithms such as the FRAX. Although DEXA imaging has excellent reported accuracy and precision, consideration should be made when comparing results across different instruments from different manufacturers unless cross-calibration has been assured. 

DEXA imaging is the best clinical tool for assessing BMD in the evaluation of osteoporosis, and its validity is evident, given its ubiquity among international guidelines.

Evaluation of primary and secondary osteoporosis cannot be elucidated with DEXA imaging. This limitation was exemplified in a study showing that 55 out of 173 women (32%) diagnosed with primary osteoporosis were found to have secondary causes, most often calcium disorders and hyperparathyroidism. The study suggested that additional testing for all women, including 24-hour urine calcium, serum calcium, serum parathyroid hormone, and serum thyroid-stimulating hormone for women on thyroid replacement therapy, is cost-effective in identifying secondary causes of osteoporosis.[5]

Enhancing Healthcare Team Outcomes

The collaboration of the interprofessional healthcare team is crucial to ensure the appropriate DEXA test is ordered and performed correctly. According to ACR Appropriateness Criteria Osteoporosis and Bone Mineral Density, there are specific cases in which QCT is considered superior to DEXA. These include:

• Extremes in body height, such as very large and very small patients
• Patients with extensive degenerative disease of the spine
• Severely obese patients (BMI >35 kg/m²)
• A clinical scenario that requires increased sensitivity to small changes in trabecular bone density (parathyroid hormone and glucocorticoid treatment monitoring) [3]

Pitfalls in DEXA are common, and errors can be categorized as patient positioning, data analysis, artifacts, or demographics. Structural changes, such as osteophytes, calcifications, or fractures, are more common in the lumbar spine compared to in the proximal femur and potentially cause an artefactual component of BMD.[6] 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.[7] Artifacts such as bra components, surgical clips, navel rings, and vascular prostheses may alter the final BMD, resulting in overestimation if the metal is included in the region of interest or underestimation if it is outside the region of interest.[8] Errors in assigning demographics are significant because the T-score is correlated to reference populations in gender and race. Some of the pitfalls can be avoided; however, some are unavoidable and must be considered by the healthcare team.

Nursing, Allied Health, and Interprofessional Team Interventions

The DEXA scan should be used as a decision-making tool for Allied Health Professionals, such as physical and occupational therapists.[9] The multi-component training consists of a combination of different exercises, such as aerobics, strengthening, progressive resistance, balancing, and dancing, aimed at increasing and preserving bone mass.[10]

Several types of exercise are recommended for patients with osteoporosis to counter bone mass reduction.[11] Combining multiple types of exercise can significantly affect BMD at three sites—the femoral neck, greater trochanter, and especially the spine.

A meta-analysis of postmenopausal women found that resistance training increased lumbar BMD, and adding low-moderate impact exercises such as jogging, walking, and stair climbing was even more effective in preserving BMD at both lumbar and femoral levels.[12][13] Another study involving older and middle-aged men showed that resistance training and load-bearing exercise improved bone mass density rather than just decreasing the loss. A suggested exercise frequency is 3 or more sessions between 30 and 60 minutes each for at least 10 months to show BMD improvement.[14]