Osteoporosis and obesity are highly prevalent and disabling chronic diseases.
Osteoporosis is a disease of low bone mass, which predisposes people to bone fragility
and increased risk of bone fractures. Obesity is a disease characterized by excess body
fat levels, which predisposes people to diabetes, heart disease, stroke, hypertension,
cancer, sleep apnea, osteoarthritis, and gallbladder disease.
An essential component to research on the prevention and treatment of osteoporosis and
obesity is a valid method for monitoring body fat levels and bone mineral density. Dual
energy X-ray absorptiometry (DXA) is a method for analyzing bone mineral density and body
composition, particularly bone mineral content, fat mass and lean mass in children and
adults. DXA scanners use an X-ray tube to produce radiation, which is then filtered into
low- and high-energy beams. These beams of radiation are emitted from beneath a table
that the supine human body is lying on. The arm, which passes above the person, detects
the attenuation of the radiation for each pixel the body occupies. Based on known
attenuation levels of different human tissues, the imaging software provides information
on the composition of each pixel of the body. These pixels are then summed to provide
information on: total body bone mineral content, total bone mineral density, fat mass,
and lean mass. The DXA can also assess regional body composition (e.g. trunk fat), which
is of importance in evaluating health effects of body fatness patterns. Using standard
protocols, bone mineral content and density can be assessed in areas of the body
indicating high risk of fractures (e.g. hip and lumbar spine) or areas that are likely to
be responsive to dietary or physical activity manipulations (e.g. lumbar spine and
radius).
Peripheral quantitative computed tomography (pQCT) is a 3-dimensional imaging technique
that goes beyond the 2-dimensional imaging of DXA to assess both true volumetric bone
density and bone geometry, the two key components of bone strength. Additionally, it can
divide bone into its component parts, i.e. separately assessing bone density and bone
geometry of cortical and trabecular bone. As such, the measurements obtained from pQCT
provides a more complete picture of what may be occurring within bone tissue that
contributes to either bone gain or bone loss, depending on the population and question of
interest. pQCT assesses parameters of bone strength at the radius (forearm) and tibia
(lower leg).
A vital component of any clinical and research program is precision testing, which
assesses the reproducibility of DXA and pQCT measurements within an individual technician
and/or between multiple technicians. This study was designed to address our facility's
needs and ethical requirement to complete precision testing. This precision testing is a
necessary component of verifying the feasibility and validity of the method, as well as
cross-calibrating DXA scanners in our multi-site studies. We are completing this testing
with the two DXA scanners, the GE Lunar iDXA and the Hologic Horizon W DXA, as well as
with the Stratec XCT 3000 pQCT.
A vital component of any clinical and research program is precision testing, which
assesses the reproducibility of DXA measurements within an individual technician and/or
between multiple technicians. As alluded to above, the validity of bone and soft tissue
measurements by DXA and pQCT is related to the skill of the technician in: 1) properly
positioning the person before scanning and 2) properly analyzing the scan images
afterward. Both of these components are subjective, requiring experience and feedback to
improve technique.
From an ethical standpoint, a level of technician competency is important in ensuring
that both research volunteers and the technician are not exposed to radiation without the
benefit of acceptable scan results. From a clinical and research standpoint,
understanding the inherent variability in testing is an important component in planning
and executing research studies having bone or body fat outcomes. As described the
Conference of Radiation Control Program Directors, Inc. (CRCPD), and as established by
the International Society of Bone Densitometry (ISCD), measurement of precision is a key
component for assessing:
The smallest change (least significant change, LSC) in bone density that is
biologically significant
The time interval between measurements necessary to detect changes.
Precision testing requires that multiple scans be completed on individuals representative
of the primary target population of interest, sometimes within a fairly short period of
time. This exposure of additional radiation to a select group of individuals has created
controversy about whether precision testing is necessary and ethical. The CRCPD and ISCD
has fully supported precision testing, and recommended that it be a routine practice in
all DXA sites. The CRCPD states that, "Some states without understanding the need for
precision testing have prohibited the measurement despite low radiation doses and limited
numbers of repeat densitometry determinations. Their major concern, of course, is the
apparent unnecessary radiation to a few, select patients. To address the benefit versus
risk issue, those exposed to the additional, small amount of radiation (equal to
approximately an additional 6-12 hours of background radiation) are providing a benefit
to themselves and all others by validating the results of bone mineral density (BMD)
exams for that facility. Without precision testing, the BMD study is of no value
resulting in thousands of patients being exposed to unnecessary radiation."