pQCT – Advantages over other techniques
Advantages compared to DXA and QCT
In contrast to DXA pQCT measures actual density values in g/cm³ and not the area-projected mass in g/cm². When projecting bone mass to and area the information about the size of bone is lost. It is then no longer possible to separate between density and size. Therefore shorter subjects are often erroneously diagnosed as osteopenic or osteoporotic because of their smaller bone mass. Especially in children and adolescents it is not possible to differentiate between growth and an actual increase in bone density with projectional methods . As a consequence many children with chronic diseases that show growth retardation have been treated against osteoporosis. In contrast pQCT can determine size and density independently. Bone density is independent of bone size.
With pQCT not only density but also bone geometry can be measured. Bone strength depends on the spatial distribution of bone material. From the cross sectional geometry the pQCT software calculates the moment of inertia and section modulus allowing to predict the fracture load with high accuracy. With the analysis of cortical bone in the diaphysis osteoporosis can be distinguished from and osteomalacia. Cortical density in patients with osteoporosis is normal (1100–1200 mg/cm³) while in patients with Osteomalacia it is reduced (<1000 mg/cm³).
Most DXA scans are done at the spine. With increasing age the number and severity of degenerative changes at the spine increases resulting in falsely high values. As a consequence an increase of degenerative changes can be misinterpreted as a success of a therapy. Therefore it is suggested not to measure the spine if degenerative changes are visible in the x-ray.
Measurements with DXA or QCT are influenced by changes in the fat content of the marrow. While at peripheral sites from the third decade on mostly fatty marrow is found, the composition of the marrow can change considerably at the spine and the proximal femur. In addition the DXA results can be influenced by the composition of the soft tissue surrounding the bone. In follow up measurements a change in the fat content can change the bone results by up to 11%.
The pQCT measurement allows the comparison of muscle and bone parameter. Bone strength is adapted to the maximum muscle force. With pQCT the muscle cross sectional area can be determined. With a comparison of muscle and bone cross sectional area it can be recognized if bone is adapted to muscle and an osteopenia caused by a sarcopenia can be differentiated from a primary osteoporosis. In the former case the reduced bone strength is a result of a physiological adaptation process to the reduced muscle force. In this case bone is healthy and the muscle should be treated. In the latter case bone is not adapted to muscle and the bone must be treated.
Advantages of pQCT over axial QCT
Axial QCT shares many advantages with pQCT as the measurement of true density units in g/cm³ and the separate analysis of cortical and trabecular bone. But there are also some disadvantages:
- The radiation dose is considerably higher. (Effective Dose: 50 µSv for QCT, 1 µSv for pQCT)
- Precision is better for pQCT (1% in vivo , 2% for QCT)
- Osteophytes can also influence the results of axial QCT
- Higher costs
