dexa

• A bone mineral density (BMD) test measures calcium and other minerals in bone.
• Bones containing more minerals are denser, so they tend to be stronger and less likely to break.
• Bones can become less dense as we age or if we develop certain medical conditions.
• Bone density tests are noninvasive tests that measure bone mineral density (BMD) such as calcium and other minerals in your bones.
• Bone density tests measure the strength and thickness, or mass, of your bones.
• Overlapping images are matched and reconstructed, resulting in a more accurate estimation of the depth of the bone and reducing the magnification effect.
• DXA is the primary test used to monitor therapy approved by the FDA.
• DXA is a noninvasive method with short scan times that measures critical 'central' or 'axial' skeletal sites where early bone loss and fractures occur: hip and spine.
• The path of the X-ray beam in successive dual-energy X-ray absorptiometry (DXA) systems includes a pencil beam, a fan beam, and a narrow fan beam.
• DXA offers high precision and provides high resolution imaging at low cost.
• DXA equipment has a stable calibration of equipment.
• As we age, bones naturally become thinner.
• Osteopenia occurs when bones are thinner than normal.
• Osteoporosis generally affects those over the age of 65, especially women.
• Patients with osteoporosis can more easily develop bone fractures of the hips or wrists.
• Dual Energy X-ray Absorptiometry (DXA/DEXA scan) is a gold standard of noninvasive methods in the role of diagnosis osteoporosis, predicting fracture risk, monitoring therapy.
• DXA/DEXA scan uses two x-ray beams of different energy levels.
• Soft tissue absorption is subtracted out and a bone mineral density (BMD) can be calculated.
• BMD results for individual are related to databases containing the values of young, healthy adults at peak bone mass (20 – 29 years old), matched in terms of gender and ethnicity and applied in form of a “T-score”.
• T-score marks how far a patient’s BMD is deviated from the mean of a sex-matched young adult population (ages 20 - 35) of individuals who have reached their peak bone mass.
• When no bone is present, the ratio of the attenuation of the two photon energies is linearly related to the proportion of fat in the soft tissue.
• Pencil-beam x-ray scans in a rectilinear fashion, taking relatively long scan times (5-10 min per site, 10-20 min for total body), while fan-beam x-ray measures the whole body with a single sweep of the X-ray arm, resulting in faster scan times (1-2 min per site, 5-10 min for total body).
• After the attenuation of the X-ray beam has been analyzed in regions with soft tissue and bone, as well as regions with soft tissue only, fat mass, lean tissue mass, and bone mineral mass can be discriminated.
• Disadvantages of fan-beam densitometers include a higher radiation dose and inherent magnification of scanned structures as the distance from the X-ray source decreases, significantly affecting bone mineral content and bone area measurements.
• Each pass of the narrow fan beam across the body overlaps the previous one.
• Reduction of xray beam attenuation consists of absorption and scatter, with higher energy leading to higher scatter and lower absorption, resulting in more penetration.
• Narrow fan-beam densitometers offer a compromise between older technologies, scanning in a rectilinear fashion with a fan beam that is wider than the original pencil beam but still narrower than the first fan beams, reducing magnification effects.
• DXA determines the amount of fat and fat-free soft tissue in the body by measuring the ratio of attenuation of the two photon energies at anatomical sites that do not contain any bone, typically the pure soft tissue that is adjacent to bone.
• The scanning system of DXA involves a transition from x-ray pencil-beam to x-ray fan-beam, with pencil-beam x-ray using a highly collimated pencil beam of X-rays with a single detector and fan-beam x-ray using a fan-beam X-ray source and a set of detectors.
• Attenuation of the X-ray beam decreases as photon energy increases.
• The Z-score is recommended for use when reporting in pre-menopausal females and males of ages less than 50.
• There are two types of DXA equipment: central device (cDXA) and peripheral device (pDXA).
• As photons traverse the subject's tissues, physical interactions take place that reduce beam intensity.
• Central devices are used to measure bone density in the hip and spine.
• Z-score compares your bone density to the average bone density of people your own age and gender.
• The attenuation depends on the energy of the photons and the density and thickness of the human tissues through which they pass.
• DXA is an X ray imaging technique primarily used to derive the mass of one material in the presence of another through knowledge of their unique X ray attenuation at different energies.
• Z-score is conceptually similar to a T-score, except the reference population is matched by the patient's age.
• DXA defines the composition of the body as three materials having specific X ray attenuation properties: bone mineral, lipid (triglycerides, phospholipid membranes, etc.) and lipid free soft tissue.
• Low density material (i.e., soft tissues) allows more photons to pass through, thus attenuating the X-ray beam less than high-density material such as bone.