Assessment of skeletal structure by MR relaxometry

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.