Bone densitometry, often referred to as a Dual-Energy X-ray Absorptiometry (DEXA or DXA) scan, is a medical imaging technique that utilizes a minimal dose of ionizing radiation. This procedure generates detailed images of the body’s interior, typically concentrating on the lower spine (lumbar region) and hips, to precisely measure bone loss. It’s a crucial tool in diagnosing osteoporosis and evaluating an individual’s susceptibility to osteoporotic fractures. DEXA scans are recognized for being straightforward, rapid, and non-invasive, establishing themselves as the gold standard for osteoporosis diagnosis.
Preparing for a DEXA scan is generally uncomplicated. Patients are advised to inform their doctor and the technician if they suspect pregnancy or have recently undergone a barium exam, CT scan with contrast, or radioisotope scan. It’s recommended to leave jewelry at home and wear comfortable, loose-fitting attire. In some instances, a gown may be provided. Importantly, calcium supplements should be avoided for at least 24 hours prior to the examination.
What is a Bone Density Scan?
A bone density scan, technically known as dual-energy x-ray absorptiometry (DXA) or bone densitometry, is an advanced x-ray technology designed to quantify bone loss. DXA is the current, widely accepted benchmark for assessing bone mineral density (BMD).
X-ray examinations are invaluable diagnostic tools in medicine. They employ a small amount of ionizing radiation to create images of internal body structures, aiding in the diagnosis and management of various medical conditions. X-rays represent the oldest and most frequently utilized form of medical imaging.
DXA scans are most commonly performed on the lower spine and hips. In specific cases, such as with children and some adults, a full-body scan may be conducted. Peripheral DXA (pDXA) devices, which use x-ray or ultrasound technology, are sometimes employed to screen for reduced bone mass, primarily in the forearm. Furthermore, specialized software integrated with CT scans can also diagnose or monitor low bone mass (Quantitative Computed Tomography or QCT). While accurate, QCT is less frequently used compared to DXA scanning.
Common Applications of Bone Density Scans
DXA scans are primarily used to diagnose osteoporosis. Osteoporosis is a condition frequently observed in post-menopausal women but can also affect men and, less commonly, children. It is characterized by a progressive decrease in bone density and alterations in bone structure, leading to increased bone fragility and fracture risk.
Beyond diagnosis, DXA scans are instrumental in monitoring the effectiveness of treatments for osteoporosis and other conditions contributing to bone loss.
The DXA test is also valuable for assessing an individual’s fracture risk. This risk is influenced by factors such as age, body weight, previous fracture history, family history of osteoporotic fractures, and lifestyle choices like smoking and excessive alcohol consumption. These elements are crucial when determining the necessity of therapeutic intervention.
Bone density testing is highly recommended in the following scenarios:
- Post-menopausal women not undergoing estrogen therapy.
- Individuals with a personal or maternal history of hip fracture or smoking.
- Post-menopausal women who are notably tall (over 5 feet 7 inches) or thin (weighing less than 125 pounds).
- Men with medical conditions linked to bone loss, such as rheumatoid arthritis, chronic kidney or liver disease.
- Individuals using medications known to induce bone loss, including corticosteroids like Prednisone, certain anti-seizure medications such as Dilantin, specific barbiturates, or high-dose thyroid replacement drugs.
- Those with type 1 diabetes, liver disease, kidney disease, or a family history of osteoporosis.
- Individuals exhibiting high bone turnover, indicated by excessive collagen in urine samples.
- Individuals with thyroid conditions, such as hyperthyroidism.
- Individuals with parathyroid conditions, such as hyperparathyroidism.
- Individuals who have experienced fractures from minor trauma.
- Individuals with x-ray evidence of vertebral fractures or other signs of osteoporosis.
Vertebral Fracture Assessment (VFA), a low-dose x-ray examination of the spine performed using the DXA machine, is often recommended for older patients, particularly if they:
- Have experienced a height loss of more than an inch.
- Report unexplained back pain.
- Have DXA scan results that are borderline.
- Show vertebral deformity or fracture indications on DXA spine images.
Preparing for Your Bone Density Scan
On the day of your DXA exam, you can maintain your regular dietary habits. However, it’s important to avoid taking calcium supplements for at least 24 hours prior to your appointment.
Wear loose-fitting, comfortable clothing, and avoid garments with metal components like zippers, belts, or buttons. Remove any metal objects such as keys or wallets from the area to be scanned.
You may be required to remove some clothing and change into a gown for the procedure. Remember to take off jewelry, removable dental appliances, eyeglasses, and any metallic items that could interfere with the x-ray images.
Inform your doctor if you’ve recently had a barium examination or received an injection of contrast material for a CT or radioisotope scan. These procedures may necessitate a waiting period of 10 to 14 days before undergoing a DXA test.
It is crucial for women to inform their physician and technologist if they are pregnant. To protect the fetus from radiation exposure, many tests are avoided during pregnancy. If an x-ray is deemed necessary, precautions will be taken to minimize radiation exposure to the baby.
Alt text: A modern DEXA machine used for bone density scanning, featuring a patient table and overhead arm.
Understanding the Equipment
DXA equipment comes in two main types: central devices and peripheral devices.
Central DXA devices, primarily used to measure bone density in the hip and spine, are typically found in hospitals and medical offices. These devices consist of a large, flat table with an overhead “arm.”
Peripheral DXA (pDXA) devices are designed to measure bone density in the wrist, heel, or finger. These are often available in pharmacies and mobile health units. Smaller and more portable than central DXA devices (weighing around 60 pounds), pDXA devices may have a box-like structure where the foot or forearm is placed for imaging. Other portable technologies, such as specialized ultrasound machines, are also sometimes used for screening, although central DXA remains the standard technique for bone density measurement.
How DXA Procedure Works
The DXA machine operates by emitting a narrow, imperceptible beam of low-dose x-rays at two distinct energy levels through the bones being examined. One energy peak is predominantly absorbed by soft tissue, while the other is absorbed by bone. By differentiating between these absorption rates, the machine calculates a patient’s bone mineral density by subtracting the soft tissue component from the total absorption.
DXA machines are equipped with specialized software that processes and displays bone density measurements on a computer screen, providing immediate results for analysis.
Step-by-Step Procedure
Typically performed on an outpatient basis, the central DXA examination assesses bone density in the hip and spine. The patient lies down on a cushioned table, with an x-ray generator positioned underneath and an imaging detector located above.
For spine assessment, the patient’s legs are supported on a padded box to ensure the pelvis and lower (lumbar) spine are flattened. Hip assessment involves placing the patient’s foot in a brace that rotates the hip inward. In both scenarios, the detector arm moves slowly across the area of interest, generating images on a computer monitor.
During the x-ray emission, it’s crucial to remain still and possibly hold your breath for a few seconds to prevent image blurring. The technologist will step behind a protective barrier or into an adjacent room to operate the x-ray machine.
Peripheral DXA tests are simpler. The finger, hand, forearm, or foot is placed into a compact device that yields a bone density reading within minutes.
Vertebral Fracture Assessment (VFA), an additional procedure, is increasingly common. VFA is a low-dose x-ray examination of the spine, conducted on the DXA machine, to screen for vertebral fractures and adds only a few minutes to the overall DXA procedure time.
A complete DXA bone density test usually takes between 10 to 30 minutes, varying based on the equipment used and the body parts examined.
Patients are often asked to complete a questionnaire to help doctors identify pre-existing medical conditions or medications that might influence fracture risk. The World Health Organization has developed an online tool that combines DXA results with basic questionnaire data to predict the 10-year risk of hip fracture or other major osteoporotic fractures in post-menopausal women.
What to Expect During and After the Scan
Bone density tests are generally quick and painless.
Regular evaluations, typically every two years, are recommended to detect significant changes in bone mineral density, whether increases or decreases. Some patients, particularly those on high-dose steroid medication, may require more frequent follow-ups, such as every six months.
Results Interpretation and Communication
A radiologist, a physician specializing in interpreting radiology examinations, will analyze the images from your DXA scan. The radiologist will then send a detailed report to your primary care or referring physician, who will discuss the findings with you.
Besides radiologists, other specialists like rheumatologists and endocrinologists may also interpret DXA scans. Clinicians consider your DXA scan results alongside clinical risk factors, such as rheumatoid arthritis, chronic renal and liver disease, respiratory disease, and inflammatory bowel disease, for a comprehensive assessment.
Your test results will be presented as two scores:
T-score: This compares your bone density to that of a healthy young adult of the same gender at peak bone mass. A T-score of -1.0 or higher is considered normal. A score between -1.1 and -2.4 indicates osteopenia (low bone mass), and a score of -2.5 or lower signifies osteoporosis. The T-score is crucial in estimating fracture risk and determining the need for treatment.
Z-score: This score compares your bone density to others of your age, size, and gender. An unusually high or low Z-score may prompt further medical investigation.
Minor variations between scans can occur due to differences in positioning and are typically not clinically significant.
Benefits and Risks of Bone Density Scans
Benefits
- DXA bone densitometry is a straightforward, rapid, and noninvasive procedure.
- It requires no anesthesia.
- The radiation exposure is minimal, less than one-tenth of a standard chest x-ray and less than the daily natural background radiation.
- DXA is the most standardized and reliable method for diagnosing osteoporosis and accurately predicting fracture risk.
- It is instrumental in deciding if treatment is necessary and in monitoring treatment effectiveness.
- DXA equipment is widely available, making testing easily accessible for both patients and physicians.
- No radiation remains in the body after the x-ray exam.
- X-rays, in diagnostic ranges, typically have no side effects.
Risks
- There is a minimal risk of cancer from excessive radiation exposure. However, the low radiation dose in medical imaging is generally outweighed by the diagnostic benefits.
- Pregnant women should always inform their doctor and x-ray technologist due to potential risks to the fetus.
- Radiation dose varies with each procedure.
- Complications from DXA procedures are rare.
Minimizing Radiation Exposure
Healthcare professionals are committed to using the lowest possible radiation dose while ensuring high-quality images for diagnosis. Radiology protection organizations continuously refine safety standards and techniques.
Modern x-ray systems minimize scatter radiation through controlled x-ray beams and dose management, ensuring that areas outside the imaging target receive minimal radiation exposure.
Alt text: A radiology technologist positions a patient for a DEXA scan, highlighting the non-invasive nature of the procedure.
Limitations of Bone Density Scans
- DXA scans assess fracture risk but cannot predict fractures definitively; they guide treatment decisions based on risk assessment.
- DXA effectiveness is limited in individuals with spinal deformities or prior spinal surgery. Conditions like vertebral compression fractures or osteoarthritis can affect test accuracy, potentially making CT scans more suitable in these cases.
- Central DXA devices are more sensitive and standardized than peripheral DXA devices but are also more costly.
- Peripheral DXA tests, such as heel or wrist scans, can indicate fracture risk in the spine or hip but are less effective for monitoring treatment response. If pDXA suggests medication is needed, a baseline central DXA scan is recommended.
- For follow-up, DXA exams should ideally be conducted at the same facility and using the same machine to ensure comparability. Measurements from different DXA equipment cannot be directly compared.
This page was last reviewed on March 11, 2024.
