Osteoporosis is most common in elderly people. However, children can also be affected. The disease may also occur as a result of medication, specifically cortisol, an anti-inflammatory drug that leads to the direct inhibition of bone-forming cells (osteoblasts) and to a gradual loss of bone substance, when taken over long time. The main consequence of osteoporosis is an increased risk of bone fractures. Typical fragility fractures occur in the vertebral column and the upper thigh, and might result in life-long impairments.

Under the leadership of Prof. Dr. Dr. Fritz Schick, physicians and researchers of the Section of Experimental Radiology in the Department of Diagnostic and Interventional Radiology at the University Hospital of Tübingen (UKT) are developing new MRI methods that enable the precise representation of the typical alterations associated with osteoporosis. These insights into the filigree structures of the bones give osteoporosis diagnostics a completely new quality. This type of osteoporosis diagnostics does not bear risks for the patients.

Fracture risk does not only depend on bone density

Currently, the diagnosis of osteoporosis is based on the measurement of bone mineral density (BMD). However, the X-ray methods used for this purpose have two decisive disadvantages. “The patients are exposed to radiation, something that should be avoided in children, in particular,” said Jürgen Machann, a physicist at Tübingen University Hospital and in charge of the further development of MRI. “The microarchitecture of trabecular bone in the long bones and vertebrae is often disrupted and the weaker spicules break,” explained the scientist.

Machann uses famous buildings to illustrate the complex interaction of bone density and microarchitecture: “The Eiffel Tower in Paris is an excellent example. It consists of little material but has excellent stability. It is worth noting that stability does not primarily depend on the mass itself but on its distribution.” And that is almost the same in the human skeleton. “It is possible that a bone with very low bone density but excellent microarchitecture is more stable than a bone with high bone density but bad microarchitecture,” explains Machann.

In order to improve the assessment of osteoporosis-associated fracture risks, care has to be taken to assess bone density as well as the distribution and structure of trabecular bone. MR imaging provides promising results. “MRI enables us to accurately determine the proportion of trabecular bone structures and represent the results in colour,” said Machann.

This progress was made possible through huge technical progress in MRI. Higher field strengths enable the representation of increasingly detailed structures. “Five years ago, the devices had a field strength of 1.5 Tesla (T), modern devices have field strengths of three T, and seven T will soon be available,” said Machann summarising the rapid development in MRI technology. “This enables us to record images with a spatial resolution of 300 micrometers (µm). This enables us to look at the trabecular bone structures.” That is why the method is also referred to as “virtual bone biopsy”.

This high-resolution method now enables the in vivo analysis of the three-dimensional trabecular bone structure. This type of examination was previously only possible with tissue specimens that were surgically removed from the hip area. “Virtual bone biopsy enables us to take a close look into the tissue without having to remove a piece of the bone,” said Machann explaining a main advantage of the method.

However, before the new MRI methods become part of clinical everyday work, some technical problems have to be solved. For example, currently, only skeletal regions that do not contain red bone marrow (forms all of the blood cells with the exception of the lymphocytes) can be investigated. The iron-containing blood constituents would lead to false measurements. The lumbar portion of the spine, which is particularly prone to osteoporotic fractures, can therefore not be analysed effectively with the new method. Machann is however confident that the problem can be resolved, albeit with numerous complex experiments.

Costs are currently a problem that cannot be solved. MR imaging is currently far more expensive than all other investigations. Jürgen Machann therefore believes that broad osteoporosis screenings using MRI will not be possible due to the tremendous costs involved. However, there are many clinical scenarios in which the use of MRI is more than justified. These applications include the clinical monitoring of osteoporosis in children since MRI has the decisive advantage in that it does not involve X-rays.

sb - 26th June 2008
© BIOPRO Baden-Württemberg GmbH

Further information:
University Hospital Tübingen
Section of Experimental Radiology
Department of Diagnostic and Interventional Radiology
Jürgen Machann
Hoppe-Seyler-Str. 3
72076 Tübingen
Tel.: +49 (0)7071 29-87752
Fax: +49 (0)7071 29-5392
E-mail: juergen.machann@med.uni-tuebingen.de