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ABOUT
BONE DENSITOMETRY
Introduction
Different Techniques
Dexa or Ultrasound?
BONE
DENSITOMETRY EQUIPMENT
Types of Equipment
New or Preowned?
Operator Requirements
BONE
DENSITOMETRY PRACTICE
Why offer Bone Densitometry?
Indications for Bone Densitometry
Patient Reimbursements
Favorable Legislation
ABOUT
OSTEOPOROSIS
Basic Facts
Not Just a Woman's Disease
Prevention & Treatment
Links
INTEGRITY'S
B D EQUIPMENT PROGRAM
Why Bone Densitometry is Important
Now
How IMS Can Help
TERMS OF USE
© 2004, Integrity Medical Systems, Inc. |
Introduction
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.
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| Dual energy x-ray absorptiometry
scan of the proximal femur in a 37-year old woman with
femoral-neck osteopenia (T-score = -1.6). |
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| 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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