Principles and Applications of Dual X-ray Absorptiometry Flashcards
What is dual x-ray absorptiometry (DXA) used for?
- To identify a deficiency in bone mineral (i.e. osteoporosis). This is there is a higher break down in bone in comparison to formation of new bones. This results in porous bones, decreasing bone density and increasing the risk of fracture.
- Identification is important due to the increased likelihood of fracturing a bone after a previous fracture and the increased morbidity associated with a fracture. Therefore, identification can improve a patient’s quality of life.
- Money will be saved in the long run as the need for bone protection is reduced.
What methods can be used to assess bone density?
- Dual x-ray absorptiometry (DXA).
- Ultrasound.
- CT (high dose).
How does dual x-ray absorptiometry (DXA) work?
- A narrow x-ray beam with two distinct x-ray energies (of restricted energy range) is sent through the bone. The bone density can be be determined by the amount of the beam that is attenuated by the bone.
- The two different energy x-rays allow for distinguishing from the attenuation effects of soft tissue compared with this of bone.
- Typically the lumbar spine and left hip are imaged - these two distinct sites with large area of bone provide reproducibility.
How are two different energies produced for each pixel in the Lunar and Hologic DXA systems?
- Lunar system uses a rare earth filter which produces a K-edge spectrum which absorbs photons in the mid energy range. Different energy portions of the spectrum are counted simultaneously by energy discrimination at each pixel.
- In the Hologic system, the voltage is switched every half cycle (~ 10 ms) to change the energy. The signals are then recorded consecutively at each pixel.
What is the equation relating incident radiation intensity, exit radiation intensity, attenuation coefficient of bone, that of soft tissue, bone mineral density and tissue density for DXA? What does this equation suggest?
- I = I_0 exp-(mu_b M_B + mu_s M_s) where I and I_0 are the exit and incident x-ray intensities, mu_b is the attenuation coefficient of bone and M_B is the bone density.
- There are too many unknowns to determine bone mineral density using one x-ray beam and this equation. Therefore, two x-ray beams of different energy (this making the mass attenuation coefficients different) are required. Can utilise nearby tissue to measure the intensity of the beam where no bone is apparent to produce a ratio for which the above equation can be solved for M_B.
How does the DXA process take place practically?
- Patient scanned in rectilinear fashion resulting in a tissue density map.
- Software employs edge detection to determine where bone begins and ends within the scanned area.
- Bone density is then calculated as the average bone density across the area considered bone by the edge detection algorithm.
- The above is typically an iterative process whereby the the soft tissue ratio is recalculated until its value no longer changes. This is used to determine areas of bone and soft tissue.
- The operator can define ROIs and determine the average bone density (or bone mineral density) within these regions.
What are the issues with pulse counting techniques employed in the Lunar DXA system? How are these avoided? How are energy cross over effects minimised?
- Pulse counting techniques (which allow for energy discrimination) causes detector pile up. This can be minimised by selecting different beam currents depending on patient size.
- Energy cross-over effects are minimised by subtracting a portion of the high level signal from the lower.
What types of detectors are typically employed in DXA scanners?
- Direct digital detectors (i.e. solid state crystals).
Why is it important to have accuracy and precision in DXA?
A healthy adult can lose 0.5% BMD at the hip or spine with osteoporotic bone losing more. Pharmaceuticals can increase BMD by several percentage points at these sites. Therefore, accuracy and precision is required to diagnose osteoporosis and to monitor treatment.
How are DXA results interpreted?
- Operator reviews and alters any ROIs to ensure correct position (matching with previous patient data, if available). Any artefacts are also removed and any errors are checked.
- Measured BMD is plotted against age and compared with young adult T-score and age match population reference (Z-score). This allows for the level of osteoporosis to be determined.
What ensures reproducibility in DXA results?
- Correct, consistent positioning of patients (requires operator training).
- Automatic positioning of ROIs if possible, rather than manual selection.
- Variations in both of the above can affect BMD results.
What is T-score in DXA? What is Z-score?
- T-score: The number of standard deviations BMD measurements are away from a young adult.
- Z-score: The number of standard deviations BMD measurements are away from an age match.
Why is bone mineral density useful in assessing osteoporosis?
It is independent of other risk factors for fragility fracture. It can, therefore, be fed into the fracture risk assessment tool separately from other risk factors to determine a 10 year probability of fracture.
What might full body DXA imaging be used for?
- Full body DXA imaging can be used to determine body composition.
- This can be used to determine paediatric growth.
- It can also be used for sports medicine assessments.
