Optimization of Resolution in Gas MRI

R. H. ACOSTA; L. AGULLES-PEDRÓS; P. BLÜMLER; H.W. SPIESS

Paris, Francia

Congreso; 7th International Conference of Magnetic Resonance in Porous Media; 2004

found an important clinical application in the diagnostics of lungs [1]. While many authorsemphasize the fact that the signal (hence the resolution) in such images is increased by 4-5 orders ofmagnitude, only a small number of publication considers the problem of the extremely highdiffusion coefficients in gases and their destructive influence on the MRI-signal.Typically the influence of diffusion on MRI-signals is minimized by reducing the timing of thesequence, but technical limits are quickly reached in the case of gases. On the other hand thediffusion time controls also the diffusion contrast by selecting structures, which then show restrictedor free diffusion.For imaging structures with various pore sizes, such as lungs, two very different approaches can befollowed. Firstly, the whole of all air spaces should be visible independent of their size andsecondly a certain size should be selectable, so that only structures smaller than this will contributeto the image (see Fig. 1). We demonstrate that this goal can only be reached if both the timing of thesequence and the diffusion coefficient is controlled. The latter can be realized by the use of buffergases of various molar weight.However, to understand the influence on the signal-to-noise of the resulting image, the theory ofdiffusion coefficients in gas mixtures has to be revised due to isotope specific measurements. Signalchanges by external excitation and radiation damping have to be addressed as well. Theseinfluences are discussed in theory and experiments on phantoms and animal lungs usinghyperpolarized 3He and 129Xe.