A bone mineral density (BMD) test, also known as bone densitometry (DEXA scan), enables the clinician to obtain a diagnosis of osteoporosis or osteopenia, often before fractures occur, by measuring BMD. No radiopharmaceuticals are used in this procedure, but special imaging techniques are used. X-ray absorptiometry for measuring BMD includes these special modalities:

1. Dual-energy absorptiometry (DEXA or DXA) to measure spine, hip, and forearm density

2. Peripheral dual-energy absorptiometry to measure forearm density

3. Single-energy x-ray absorptiometry to measure the heel and forearm density

4. X-ray absorptiometry to measure the density of the phalanges

DEXA is the most common and preferred method of measuring BMD because of its precision and low radiation exposure. With the use of laser x-ray imaging and specific computer software, DEXA can assess fracture risk with relative ease and patient comfort. Fracture risk is measured in standard deviations (SDs) by comparing the patient’s bone mass to that of healthy 25- to 35-year-olds. Test scores are printed out and reported with a T-score and a Z-score. The T-score is the number of SDs for the patient compared with normal young adults with mean peak bone mass. Fracture risk increases about 1.5–2.5 times for every SD. According to the World Health Organization, T-scores of less than 2.5 SDs may confirm a diagnosis of osteoporosis; scores of 2.5–1.0 SDs are associated with osteopenia; and scores of 1.0 SD or greater are considered normal. The Z-score is defined as the number of SDs for the patient compared with normal persons in the same age category. The T-score is the score most commonly reported and is the preferred reference point for diagnosing osteoporosis.

1. Position the patient so the area being imaged is immobile.

2. Place a foam block under both knees during the spine imaging. Use a leg brace immobilizer during the femur imaging and use an arm brace when imaging the forearm.

3. DEXA images of the spine and hip take approximately 20 minutes to complete. An additional 15 minutes is needed to image the forearm.

4. See Chapter 1 guidelines for safe, effective, informed intratest care.

Abnormal imaging may be associated with the following:

1. Estrogen deficiency in postmenopausal women

2. Vertebral abnormalities

3. Patients with x-ray osteopenia

4. Hyperparathyroidism

5. Patients receiving long-term corticosteroid therapy

Pretest Patient Care

1. Explain the purpose and procedure for measuring bone density of spine, hip, forearm, heel, and phalanges. No radiopharmaceuticals are administered.

2. Encourage patients to wear loose, comfortable, cotton garments that are free of metal or plastic zippers or buttons.

3. Refer to standard NMI pretest precautions.

4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

False readings may occur with the following:

1. NMI within the previous 72 hours (longer for gallium or indium imaging) may cause residual emission that can be misinterpreted.

2. Barium studies within the previous 7–10 days may interfere with the spine imaging.

3. Prosthetic devices or metallic objects surgically implanted in areas of interest may interfere with the image.

Normal

Absence of osteoporosis or osteopenia

T-score: <1.0 SD below normal (>−1.0)

Osteopenia: 1.0–2.5 SD below normal (−1.0 to −2.5)

Osteoporosis: >2.5 SD below normal (<−2.5)