Overview of Dual-Energy X-Ray Absorptiometry
Dual-energy x-ray absorptiometry, or DEXA, is a type of medical test used to measure bone mineral density (BMD), which tells us how strong our bones are. The FDA approved DEXA for clinical use in 1988. Other tests like chemical analysis, dissection and ashing, ultrasound, and CT or MRI scans can also check our bones, but DEXA has some advantages. It’s relatively not too expensive, doesn’t take much time, and exposes the patient to less radiation than some other tests.
During the test, the patient lies down under a special x-ray machine called a C-arm. This machine sends out two types of x-ray energy that are good at looking at soft tissue and hard bone. A device called a collimator sits between the patient and the x-ray source to reduce scatter. The x-ray energy is detected above the patient and used to create an image that shows bone mass per area. This gives doctors the bone mineral density (BMD) reading.
The BMD result is often given as a T-score and a Z-score. The T-score compares your BMD to the average BMD of a young adult of your same gender and race. The Z-score compares your BMD to the average BMD of a person your own age, gender, and race. So if you have a positive score, you’re above average; if you have a negative score, you’re below average.
There are other ways of checking bone health too. For instance, CT scans can be used to get volumetric BMD measurements. This measures bone density in terms of weight per volume, and it can be done on scans that were taken for other reasons. This means doctors could potentially find cases of osteoporosis (a disease where the bones become weak) in people who didn’t know they had it.
Anatomy and Physiology of Dual-Energy X-Ray Absorptiometry
The lumbar spine, which is the lower part of your back, is examined using a procedure where you lie on your back with your hips and knees raised on a soft, supportive cushion. This way, your spine is made as straight as possible for the scan. The scan focuses on the central section of the 12th thoracic (middle back) and 5th lumbar (lower back) vertebrae. Bone mineral density (BMD), which is the amount of bone mineral in bone tissue, is measured using the first to fourth lumbar vertebrae.
Your hip is examined with you lying on your back while a device helps to rotate the thigh bone inward to lengthen the femoral neck (the part that sticks to the side of the hip joint) on the image. If set properly, a small bony portion called the lesser trochanter should be barely noticeable. BMD measurements are focused on the femoral neck, greater trochanter (bigger bony part of the thigh), Ward’s area, intertrochanteric region (part between the two trochanters), and the entire hip.
For the forearm examination, your non-dominant arm is placed on the table, with the forearm rotated so that the palm faces down. Images should show the lower end of the radius and ulna (the two bones in the forearm) and the main body of each bone lining up with the long direction of the image. BMD is measured using the middle to the lower end of the radius and ulna.
For a full body scan, you’ll lie flat on your back on the table with your arms and feet positioned a certain way. BMD measurements are taken from the upper or lower limbs and also the head.
The lumbar spine and hip are commonly chosen sites for DEXA scans. DEXA scans use low doses of X-ray to measure BMD and assess the risk of fractures and osteoporosis.
Why do People Need Dual-Energy X-Ray Absorptiometry
All women aged 65 or above and all men aged 70 or above should be checked for osteoporosis even if they do not have any symptoms. Osteoporosis is a condition that weakens bones and makes them fragile and more likely to break.
Women under the age of 65 who are at risk for osteoporosis might include those with low estrogen levels, a family history of osteoporosis, those who weigh less than 127 pounds or have a Body Mass Index (BMI) less than 18.5 kg/m², or those who’ve missed their periods for a year or more before they turned 42.
Additionally, men under 70 and women under 65 who either smoke cigarettes, have become shorter in height, or have a rounded upper back (thoracic kyphosis) should also be screened for osteoporosis.
Osteoporosis tests should also be considered for individuals of any age who have either a lower bone mass (osteopenia) or fragility fractures visible on X-rays, people over 50 who have fractures in the wrist, hip, or upper arm with minor or no accident, any person who has one or more fractures due to bone weakness, or those who have been taking or planning to take steroid medications like prednisone in doses of 5mg or more every day for three months or longer.
Tests for osteoporosis may also be needed for men who’ve low levels of sex hormones, or those men who lost their testicular function due to surgery or chemotherapy. People who are thinking about taking medication for osteoporosis should also be screened.
Certain medications and endocrine disorders are known to negatively impact Bone Mineral Density (BMD), which is a measure of the amount of minerals, like calcium, in your bones. Medications that may affect BMD include epilepsy drugs (anticonvulsants), long-term use of heparin, and certain cancer treatments. Disorders such as overactive parathyroid gland (hyperparathyroidism), overactive thyroid gland (hyperthyroidism), and Cushing syndrome can also lower BMD.
Bone health in patients is monitored to track the effectiveness of medication for osteoporosis and to follow up on health conditions that can affect bone density.
Children or teenagers with certain medical conditions may also need monitoring for bone health. These may include young individuals who need to take corticosteroids for more than three months or need radiation or chemotherapy for cancer, those with endocrine disorders like hyperthyroidism, or conditions that make bones very fragile or dense.
People with certain medical conditions that could lower bone density should also be monitored. These conditions include kidney failure, arthritis, eating disorders, organ transplants, conditions that cause long-term immobility, past gastric bypass surgery, or conditions that cause secondary osteoporosis.
Additionally, DEXA scans (a type of X-ray that measures bone density) can help diagnose, stage, and follow up on conditions that cause abnormally high bone mass, such as fluoride exposure or osteopetrosis (disorder that causes bones to harden).
DEXA scans can also be used to measure body fat and muscle mass in people with conditions that affect nutrient absorption, such as in patients with cancer, eating disorders, or malabsorption.
When a Person Should Avoid Dual-Energy X-Ray Absorptiometry
There are no absolute reasons why a DEXA, a type of bone density test, cannot be performed. However, there are situations that might require the test to be modified, rescheduled, or might affect the quality of the results. These include:
-If you have recently had a type of X-ray that uses a special dye or a procedure that uses radioactive substances.
-Pregnancy
-Having a curved spine (scoliosis), which might affect measurements taken from the spine.
-Severe changes in your bones due to aging or a previously fractured bone in the area that will be measured.
-If there are implants, hardware, devices, or other foreign materials where the scan will be done.
-If you have difficulties getting into the right position or staying still for the measurements.
-A very high or very low Body Mass Index (BMI) might make it harder to get accurate measurements. In these cases, a different type of scan (Quantitative computed tomography or QCT) might be a better option.
In general, anything that prevents you from being positioned properly for the scan, which is necessary to get accurate bone density values, might affect the quality of the DEXA test.
Equipment used for Dual-Energy X-Ray Absorptiometry
A DEXA scan is basically a procedure done using a piece of equipment called a C-arm. This C-arm has a feature that can release x-rays of different energy levels. Other features of the C-arm include a device to shape the x-ray beams called a collimator, a sensor or detector which captures the x-rays, and a special computer software that simplifies the process.
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Preparing for Dual-Energy X-Ray Absorptiometry
Before your scan, there are some important things to know and steps to take:
* You will need to lie on your back for up to 10 minutes during the scan. Make sure you’re comfortable doing so.
* If you weigh more than 300 pounds, a different type of bone density testing may be needed because some scanning machines have weight restrictions.
* If you’ve had a recent medical scan using contrast materials, like barium or gadolinium, you will need to wait 2 weeks before undergoing this scan.
* If you are a woman who has not gone through menopause, the doctor will need to make sure you are not pregnant before performing the scan. A pregnancy test might be needed.
* To prepare for your scan, avoid taking calcium tablets within 24 hours beforehand.
* Wear comfortable, loose-fitting clothes for your scan, and avoid wearing anything that contains metallic components like zippers.
* If you’ve had a DEXA (a type of bone density scan) before, please bring your previous results with you.
If your scan is for a body composition study, it’s best to have your scan in the morning after not eating for 12 hours (an overnight fast) to ensure consistent results.
On the day of your scan:
* Make sure that you’ve followed all the necessary steps mentioned above.
* The doctor will need to confirm your menopause status, and ensure that you are not pregnant.
* Wear hospital scrubs or a gown, with only your underwear and socks (if needed) underneath. A thin sheet might be added for warmth.
* Any objects that could show up on the scan (like jewelry) should be removed from the area being scanned.
How is Dual-Energy X-Ray Absorptiometry performed
When it comes to diagnosing a bone-related condition like osteoporosis and assessing whether you are at risk of a bone fracture, one crucial test is the Bone Mineral Density (BMD) measure. This method helps to identify how strong your bones are. However, another important tool is the Fracture Risk Assessment Tool, or FRAX. It was created by the World Health Organization in 2008 and aims to assess whether you might experience a fracture within the next decade.
When using the FRAX, doctors take into account a variety of different factors about your lifestyle and health. Things like your age, gender, race, height, weight, and Body Mass Index (BMI) are all considered. The tool also looks at your history of fractures, whether you take oral steroid medicines, if you have rheumatoid arthritis, if you’ve been diagnosed with other conditions that might weaken your bones, and your habits related to smoking and drinking alcohol. As another factor, it also takes into account where you live.
In the United States, the National Osteoporosis Foundation suggests using FRAX to calculate the risk of fractures in people who have certain results from a bone scan – specifically, those with T-scores (a kind of rating that shows how close you are to the average bone density) between -1.0 and -2.5 in the spine, hip, or complete hip region.
The scans from a bone scan, or BMD test, are read by a specially trained doctor known as a radiologist. This doctor calculates your T-score, which is a way of comparing your bone density to that of a healthy 30-year-old of the same sex. The World Health Organization defines these scores as follows:
- A T-score of -1.0 or higher is considered normal.
- A T-score between -1.0 and -2.5 suggests your bones might be getting thinner (a condition known as osteopenia).
- A T-score of -2.5 or lower indicates osteoporosis, which means your bones are weak and at a higher risk of fractures.
- A T-score of -2.5 or lower, plus a history of easily getting fractures, points to severe osteoporosis.
With these tools and methods, your doctor can make the best decision in treating your bone health.
Possible Complications of Dual-Energy X-Ray Absorptiometry
After an appendix surgery, a common complication people might experience is a surgical site infection. This may include wound infections and abscesses formed inside the abdomen. Although these types of infections are not very common in routine appendix surgeries, they can occur in up to 10% of patients whose appendix has ruptured.
Infections related to the surgical wound can be superficial or deep. Superficial wound infections occur within a month after surgery and affect the skin and tissues underneath. Doctors diagnose this by checking if there’s pain, swelling, or redness around the surgical cut, or if there’s infectious fluid leaking from it.
Deep wound infections can occur up to 3 months after surgery and affect deeper tissues, such as muscles and fascia. Symptoms similar to those of superficial infections, such as pain around the surgical site and fever could suggest deep wound infections.
To decrease the risk of post-surgery wounds getting infected, doctors use preventative antibiotics, clean the wound properly during surgery, use special surgical tools, and cleanse the surgical site with medical liquids. There is not much difference in the likelihood of wound infection between closing the surgical cut immediately after the surgery and a bit later.
When compared to traditional surgery to remove the appendix, laparoscopic surgery – a less invasive method – carries a lower risk of skin site infection, but a higher risk of infection within the abdomen or pelvic region.
Sometimes, a pelvic abscess, or a collection of pus, might form after surgery. In such complications up to 9.4% of people with a difficult appendicitis situation might experience it. Another rare condition is stump appendicitis, where if a large amount of appendix tissue is left behind, it can cause repeated appendicitis.
The death rate associated with appendix surgery is quite low, making it a generally safe procedure. That said, the death rate can vary based on where the patient is located in the world, with developed countries having lower rates (0.09% to 0.24%) than in developing countries (1% to 4%).
What Else Should I Know About Dual-Energy X-Ray Absorptiometry?
DEXA imaging, or dual-energy x-ray absorptiometry, is a type of x-ray that measures bone strength. It’s especially important for understanding osteoporosis – a condition that weakens your bones – and is recommended for all women over 65. This method is recognised as the best way to diagnose osteoporosis and helps to predict the risk of bone fractures.
Note that different machines from different manufacturers might give different results, so it’s key that the machines have been adjusted to ensure the results match up (this is what “cross-calibration” means).
However, DEXA imaging isn’t without its limitations. While it’s excellent for understanding bone strength and the risk of osteoporosis, it can’t always identify the cause of osteoporosis. For example, a study found that out of 173 women diagnosed with primary osteoporosis, 55 (32%) actually had secondary causes – most frequently problems related to calcium and overactivity of the parathyroid glands (a condition called hyperparathyroidism).
This is why other tests such as 24-hour urine calcium test, serum calcium test, checking serum parathyroid hormone level (to see if the parathyroid glands are working properly), and checking thyroid hormone level (for women taking thyroid hormone replacement), may be beneficial. These additional tests can help identify other conditions that may be contributing to osteoporosis and can be cost-effective in the long run.
