Tuesday, November 2, 2010
What is a Bone Density Test?
A BMD is also sometimes referred to as a DEXA (or DXA) scan, which stands for dual energy x-ray absorptiometry, the method by which bone density is measured by most bone density machines today. Basically, it is a specialized x-ray machine that solely measures bone density. It does not take x-ray images; rather it interprets the x-rays passing through your bones into numbers that a radiologist can interpret and make a report on your overall bone health and fracture risk.
Let’s take a closer look at the whole BMD testing process.
On the right a typical BMD machine (images supplied by GE Healthcare):
It is a flat table with a padded mattress, so it is not hard like a conventional x-ray table. The arm over the table is the sensor and it moves during the scan. The actual x-ray tube is beneath the table.
How do I prepare for my BMD test?
There is no advance preparation needed. You may wish to not take any calcium pills too close to your appointment time since they may not be fully dissolved. I have actually seen instances where they appear in the scan because they are still in the stomach. Other than that, you can carry out your normal daily routine.
Do I need to undress for a BMD test?
This will depend on the particular institution’s protocol. Some prefer to have all patients change into a patient gown since some materials will show on BMD scans, like heavy elastic waistbands, denim jeans, and of course any metal in the scan field (remove that belly-button piercing, please!). Often, it is easier to just change into a patient gown to prevent any repeat scans due to one of the above items showing up. Other BMD clinics may allow the patient to wear light clothing with no zippers, buttons, etc. Women may be asked to remove their bra due to the metals clips and/or under wiring in the cups.
What else can I expect?
Once the technologist is ready for you, he/she may go over a brief questionnaire. Some clinics may have you fill this out on your own while awaiting the test.
The purpose of the questionnaire is to gather any pertinent information as to your general health, medications you may be taken (bring a list!), family history of osteoporosis and so forth. This information assists the technologist as well as the radiologist in making a diagnosis and offering suggestions as to how you can improve your bone density.
One other thing to do before the actual test begins is to measure your height and weight. This information is primarily for setting up the scan modes of the BMD machine. As I tell my patients: ‘the scanner needs to know what size of patient it has to scan’. A tall patient will require a few more scan lines than a shorter patient, and a thinner patient requires less x-ray energy for their scan compared to a larger patient.
What is scanned in a BMD test?
Typically, the lower spine (Lumbar region) and the upper left femur (thighbone and hip joint area) is scanned. This is the standard for most, if not all BMD clinics. Why these two areas only? Primarily because these two areas are the best indicators of your overall bone health and are the worst areas to fracture. A vertebral fracture in the lower back or a hip fracture are very debilitating, to say the least.
If you have left hip replacement, a scan of the right hip will be done instead. If you have bilateral hip replacements, then a scan of the spine and the forearm will be done instead. A scan of the forearm will also be done if you have severe scoliosis (curvature of the spine in an ‘S’ shape) or if you have had extensive spinal surgery with metal rods and screws in the scanning area.
Next, you will be positioned on the table on your back, and unlike the patient in the picture above, you will (should) get a pillow. For the scan of the lower back, your legs will be elevated on a large block sponge (approximately 90°). This will help to reduce the natural curvature of the spine by getting the back as flat as possible against the table. This allows for more accurate measurements.Depending on patient size, the scan of the lumbar region can be anywhere from 30 seconds to over a minute for a large patient.
For the scan of the hip and femur, the block sponge will be removed and the feet placed on a triangular positioning device that helps to align the legs in a slight ‘toe-in’ position. This can be uncomfortable since it is not a natural position, but the scan is quick, typically 30-40 seconds or so.
After both scans are completed, you are free to go and the scan results will be examined and reported on by a radiologist at some point. Your referring doctor will get a printed copy of the radiologist’s report. The time this takes will vary from institution to institution, so be sure to ask the technologist performing your test as to when you can expect your doctor to have the results.
So that is what a BMD test is all about! There’s really nothing to be afraid of since there are no needles, drills, or any other invasive devices. If you can lay on your back for a few minutes, you can do this test!