DXA Study of Precision and Reliability

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:

1. The smallest change (least significant change, LSC) in bone density that is biologically significant
2. 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."