Abstract
Introduction. More than 40 per cent of all women older than 50 years will
suffer from a hip, wrist or vertebral fracture. Most of these fractures
are related to bone fragility, osteoporosis. If the disease is diagnosed
and treated at an early stage the risk of fracture can be decreased.
Traditional methods in clinical use primarily use the absorption of X-ray
energy to evaluate bone quality. However, these methods do not evaluate
the micro-structure of the bone, which has a major influence on
biomechanical strength. This thesis evaluates a method to study the micro
structure of trabecular bone by magnetic resonance (MR) relaxometry.
Methods. Magnetic inhomogeneity arise at the interface between trabecular
bone and bone marrow. The magnetic inhomogeneity results in decreased
signal intensity in gradient echo images. The rate of the signal decrease
can be used as an indirect measure of the trabecular bone structure
called R2* (= 1/T2*).
The trabecular bone is embedded in bone marrow containing fat and water.
To evaluate the influence of fat and water content on T2* measurements
the signal behaviour as a function of TE was studied in porcine lumbar
vertebrae and fat emulsions. T2* values were related to biomechanical
strength tested experimentally.
A new method to study trabecular bone orientation by MR relaxometry was
developed. MR relaxometric values of human vertebrae were related to the
amount of bone mineral in vitro. The obtained MR relaxometric values of
spine, hip and calcaneus were related to values obtained by dual X-ray
absorptiometry (DXA) and by quantitative ultrasound QUS).
Results. There was a relationship between fat content and T2*. As fat and
water have different spin-lattice relaxation times (Tl) TR will
theoretically also affect T2* measurements. The sagittal imaging plane is
advantageous when performing MR relaxometry of the spine. Under
experimental conditions MR relaxometry can predict biomechanical
strength. Trabecular bone orientation studied by MR relaxometry in vitro
can be calculated by rotation of the studied object in three orientations
orthogonal to the external magnetic field. A relationship between results
obtained by MR relaxometry and by bone mineral absorptiometry
measurements was observed in vitro. Also in vivo MR relaxometry was
related to absorptiometry of hip and to QUS of calcaneus.
Conclusion: MR relaxometry is an accurate method to evaluate trabecular
bone structure in vivo. The technique will be of interest not only for
research purposes, but also for diagnosis and for treatment control at
various diseases affecting the skeleton.
