Different Techniques
Dexa or Ultrasound?

Types of Equipment
New or Preowned?
Operator Requirements

Why offer Bone Densitometry?
Indications for Bone Densitometry
Patient Reimbursements
Favorable Legislation

Basic Facts
Not Just a Woman's Disease
Prevention & Treatment

Why Bone Densitometry is Important Now
How IMS Can Help

BUY A Bone Densitometer
SELL Your Bone Densitometer


© 2013, Integrity Medical Systems, Inc.


Bone densitometry is a noninvasive technology that is used to measure bone mass. Bone mass, simply put, is the weight of the skeleton, overall or in specific regions. Bone mineral density, or BMD, reveals a risk factor for fractures. BMD is usually expressed as the amount of mineralized tissue in the area scanned (g/cm2); with some technologies it is expressed as the amount per volume of bone (g/cm3). A bone density more than 2.5 standard deviations below the young adult reference population indicates osteoporosis, a disease occurring most often (but not exclusively) among women after the menopause in which the bones become very porous, break easily, and heal slowly.

In the United States, osteoporosis affects over 25 million people, and is associated with more than 1.3 million fractures annually. The prevention, detection and treatment of osteoporosis will become an increasingly important medical concern for the next decade, as the American population ages in record numbers. Bone measurement tests are simple, painless and cost-effective, usually costing less than $200. Increased reimbursement for screening would have a direct impact on the the billions of dollars a year spent on direct medical costs for osteoporosis and related fractures.

Clinical Indications for Bone Densitometry
Currently, bone densitometry tests are the only accurate measurements of bone mass and fracture risk. Bone mass cannot be deduced accurately from any other clinical data, and risk factors for osteoporosis based on medical history or physical examination are not good predictors of bone mass.

Bone mass measurements also influence the choice of therapy. Bone mass measurements can influence clinical decisions in four key indications: to decide about hormone replacement therapy in estrogen deficient women, to diagnose spinal osteoporosis, to adjust therapy of patients on long-term glucocorticoids, and to decide about surgery in patients with primary hyperparathyroidism.

Bone mass measurements should lead to fracture reduction. Bone loss is the only risk factor for fracture that can be manipulated in mid-life to achieve fracture reduction later. Bone loss can be reduced by treatment, but it is difficult, if not impossible, to restore the biomechanical competence of the skeleton once bone has been lost because bone architecture has been disrupted. Thus, bone loss must be prevented, which needs to be done sooner rather than later in life.

Bone Mass Measurement Equipment

The most commonly used tool to diagnose low bone density is the DEXA (Dual Energy X- ray Absorptiometry) densitometer, a specialized x-ray device that precisely quantifies bone at the spine, femur and other skeletal sites. DEXA scans are non-invasive and comfortable for the patient, with very low radiation. The patient lies on the scanner bed while a small beam of radiation passes through the region of interest. Patient bone density is measured automatically. The entire examination for the spine and the femur requires about 10 minutes.

Dual-energy x-ray absorptiometry (DEXA)

DEXA, the gold standard in densitometry, uses one of two methods to create a dual-energy spectrum from an x-ray source. One method involves alternating pulses of low and high kV that are applied to the x-ray tube. The low- and high-energy spectra are then measured separately. The other method applies a constant potential to the x-ray source while using a K-edge filter to separate the energy spectrum into two narrow energy bands. An energy-discriminating detector with a dual-channel analyzer counts the resultant photons. The use of two energies allows bone mineral to be assessed independently of soft-tissue inhomogeneities. Bone mineral content (BMC) and bone mineral density (BMD) are calculated in g/cm and g/cm2, respectively.

DEXA scanners use either a pencil beam coupled to a single detector (first generation) or a fan beam coupled to a linear array of detectors (second generation). The pencil-beam scanner performs a two-dimensional raster scan, while the fan-beam scanner performs a single sweep across the patient.

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Spine or hip?

Bone density measurements can be done at the hip (proximal femur), total body, spine, radius, and calcaneus. Recent research has focused on comparisons of the hip and spine measurements to maximize the practitioner's abililtiy to get the most accurate BMD measurement for each patient.

Spine BMD may be preferable to assess changes early in menopause and after bilateral ovariectomy. With most antiresporptive therapies there is a greater change at the spine than at the hip. The spine does have a higher percentage of trabecular bone that is more metabolically active. However, spine measurements are frequently inaccurate because of osteoarthritis, aortic calcifications or other skeletal problems such as surgery or compression fractures. This is especially true in elderly patients.

With newer techniques the hip is about as reproducible as the spine, so the hip is now considered the best overall site to measure. The total hip has the best ability to predict hip fractures and can predict spine fractures as well as the spine density. Because both bisphosphonates and estrogens will increase bone density at the hip as well as the spine, this site can also be used to follow most patients.

Some investigators have argued that both spine and hip should be measured to achieve greater sensitivity in prediction of fractures. A large study which prospectively measured fracture incidence documented that this is not true. To increase sensitivity (to detect more patients who might get a hip fracture) one could measure just the hip and choose the higher threshold.

There are some clinical situations in which the spine and the hip would biologically be different, and in these infrequent cases measurements of both the spine and the hip are justified. The most common is hyperparathyroidism, both primary and secondary, in which the bone density at the hip is more affected than at the spine. This is because PTH tends to increase bone loss from cortical bone more than trabecular bone. In fact, PTH may increase trabecular bone mass while decreasing cortical bone mass. Fluoride also preferentially increases trabecular bone mass. Corticosteroids, on the other hand, may have a greater effect on the trabecular bone, but spine fractures may falsely increase the spinal bone density.
Dual energy x-ray absorptiometry scan of the proximal femur in a 37-year old woman with femoral-neck osteopenia (T-score = -1.6).
Dual energy x-ray absorptiometry scan of the lumbar spine in a 37-year old woman with lumbar spine osteopenia (T = -1.8)

While many centers measure both the spine and the hip, research shows that dual hip and spine measurements are in most cases unnecessary.

DEXA or Ultrasound?

DEXA is a diagnostic test used to assess bone density in the spine, hip, or wrist using radiation exposure about one tenth of that of a standard chest x- ray. This is the method used to determine efficacy in the recent large clinical trials, and to characterize fracture risk in large epidemiological studies. Older methods such as single photon absorptiometry do not predict hip fractures as well as DEXA. The largest companies manufacturing these densitometers are Lunar, Hologic, and Norland.

Newer diagnostic techniques such as ultrasound appear to offer a more widely-available, and possibly cost- effective method of screening bone mass; however careful comparison of DEXA and ultrasound is required.

The question of ultrasound vs. DEXA requires deeper scrutiny for the practitioner seeking to prevent and treat osteoporosis. Although some have said that ultrasound measures the "quality" of the bone, more careful studies suggest that it mainly measures the bone mass, revealing structure more than actual density. Ultrasound measurements are used to assess bone density at the calcaneus or patella. It is not possible to measure sites of osteoporotic fracture such as the hip or spine using ultrasound densitometry. Ultrasound measurements correlate only modestly with other assessments of bone density in the same patient. Most experts would agree that adding an ultrasound measurement to DEXA does not improve the prediction of fractures.

Ultrasound reports calculate a T-score (the number of standard deviations above or below the mean for young normal adults) based on an impedance and ultrasound attenuation. Again, since most patients who have a low density have also lost bone structure, a low T-score with the ultrasound units is generally accurate. However, there are many patients with low bone densities who don't have fractures because they still have good structure; and there are other patients who still have fractures even if they have good density because they have lost the structure. At best, the ultrasound complements, but does not compete with, the DEXA.

Further research needs to be done to determine the percentage of false negatives produced with ultrasound systems which may tell patients they have a good T-score when in fact they have low bone mineral density. This is a work in progress and again is another screening tool being endorsed and being pushed mainly by the drug companies who are frustrated, especially in rural America where space limitations and capital equipment costs are limiting the number of DEXA tables available. Most practitioners would agree that some type of testing for low bone mineral density is better than no testing whatsoever. There are still serious concerns about giving patients a false sense of security and telling them they are normal within normal ranges with the ultrasound studies, when in fact, a hip scan may show that they have osteopenia or osteoporosis. Again, the issue is accessibility.

Less common methods are QCT, RA and SXA. QCT, or Quantitative computed tomography of the spine, reflects three-dimensional bone mineral density. It is usually used to assess the lumbar spine, but has been adapted for other skeletal areas. QCT must be done following strict protocols in laboratories that do these tests frequently; in community settings the reproducibility is poor. The QCT measurements decrease more rapidly with aging, so the "T scores" in older individuals will be much lower than DEXA measurement. RA, or radiographic absorptiometry, is a diagnostic test used to assess bone density at a peripheral site, usually the hand. Such techniques are referred to as aluminum equivalence, photodensitometry, and radiographic densitometry. SXA, or Single x-ray absorptiometry, is a diagnostic test used to assess bone density. Limited to peripheral sites, it cannot measure bone density in the hip or spine, nor can it discriminate between cortical and cancellous bone.

"A forearm scan using QCT technology"

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