Roger J Ordidge — Page 2

PubMed2002

Translational and rotational forces on heart valve prostheses subjected ex vivo to a 4.7-T MR system.

PURPOSE: To assess the magnetic field interactions on 60 heart valve prostheses subjected to a 4.7 T MR system. It addresses the question of whether heart valves deemed safe at 1.5 T may pose safety hazards as patients are exposed to increased static magnetic fields. MATERIALS AND METHODS: Ex vivo testing was performed to evaluate translational and rotational forces on 60 heart valves using previously described techniques. RESULTS: Translational forces were detected on 58 heart valves ranging from 0.5 degrees to 7.5 degrees. Seven valves exhibited paramagnetic/weakly ferromagnetic behavior, and 51 valves exhibited diamagnetic behavior. Rotational forces were observed for 46 valves. CONCLUSIONS: Criteria previously used for safety assessment of heart valve prostheses and expressed in terms of magnetic forces suggest the forces observed in this study are compatible with the safe use of these valves in magnetic resonance (MR) systems with static fields up to 4.7 T.

Heart Valve Prosthesis↗

PubMed2002

Rapid simultaneous mapping of T2 and T2* by multiple acquisition of spin and gradient echoes using interleaved echo planar imaging (MASAGE-IEPI).

A new MRI sequence for the rapid simultaneous measurement of T2 and T2* is presented. The technique uses the multiple acquisition of spin and gradient echoes with interleaved echo planar imaging (MASAGE-IEPI). IEPI data sets are sampled during and between a pair of short and long echo time spin echoes, allowing the reconstruction of a set of images with different combinations of T2 and T2* weighting and the calculation of T2 and T2* maps. In the context of neuroimaging, these maps can provide information on cerebral hemodynamics and oxygenation status, either via the deoxyhemoglobin-based BOLD signal or by the effect of exogenous paramagnetic contrast agents. MASAGE-IEPI benefits from the inherent advantages of the IEPI approach, i.e., high time resolution and minimal image distortion, and also has good time efficiency due to the acquisition of multiple image data sets following each excitation pulse. The accuracy of the sequence for the measurement of T2 and T2* is verified on phantoms, and the technique is applied to monitor changing hemodynamics in the rat brain during episodes of hypoxia. Data for the generation of maps of T2 and T2* are acquired with a time resolution of 12 s to accurately define the rapidly changing time course. As increasing emphasis is placed on the role of T2 and T2* in the direct measurement of physiological parameters such as cerebral metabolic rate of oxygen consumption and blood vessel sizes, MASAGE-IEPI offers an efficient method for the measurement of these two important MRI parameters.

Animals↗