Download this article in .PDF format
This file type includes high resolution graphics and schematics.

Figure 11 shows an experimental setup for evaluating a field distribution inside a head coil. A cylindrical phantom filled with tissue simulation fluid was used, and a model with the same EM properties was constructed in CST MICROWAVE STUDIO (MWS) simulation software from CST. The phantom was exposed to a field from the head coil, and the magnitude of the electric and magnetic fields within the phantom was measured. Measured results (Fig. 12) are close quantitatively to the simulated values.5

11. These images show a simulated phantom in (a) a head coil and (b) a real phantom.

12. These plots represent s show H- and E-field distributions along (a) the x (left-right) and (b) z (superior-inferior) axes of a phantom.

Measurements can also be used to test the applicability of different voxel models. Most patients will not be perfect matches to voxel models, but much can be learned from comparisons between simulations and experiments. Using three different body models, a body coil was simulated, and three subjects similar in body shape to the models in the simulations were found and scanned. Figure 13 shows that the B1+ field distribution measured for an applied field agree closely with simulations. Although the models were not exact matches for patients, they were useful in predicting key features of the EM field, such as which fields would be homogeneous and which would not.

13. The images show (a) simulated field distributions in three patients from identical applied fields from a body coil and (b) measured field distributions in three similar subjects.

At present, these simulations are still most useful at the design stage of an MRI coil, to ensure that the coil will produce usable images from a wide range of body shapes. But it is hoped that the simulations may one day lead to look-up tables for improved MRI scanning performance, allowing radiographers to use the results from simulations to calibrate and adjust their coils without time-consuming prescans, as well as matching and tuning procedures.

Andreas Bitz, Senior Scientist

Erwin L. Hahn Institute for Magnetic Resonance Imaging, Arendahls Wiese 199, Tor 3, D-45141 Essen, Germany; (+49) 201-183-6070, FAX: (+49) 201-183-6073.

Jorg Felder, RF Engineer

Forschungszentrum Julich GmbH, 52425 Julich, Germany; (+49) 2461-61-0, FAX: (+49) 2461-61-8100.

Tilmann Wittig, Principal Engineer and Bio-EM Market Coordinator

Computer Simulation Technology (CST), Bad Nauheimer Strausse 19, 64289 Darmstadt, Germany; (+49) 6151-7303-0.

References

1. J.M. Theysohn, O. Kraff, S. Maderwald, M.U. Schlamann, A. de Greiff, M. Forsting, S.C. Ladd, M.E. Ladd, and E.R. Gizewski, “The Human Hippocampus at 7 T—In Vivo MRI,” Hippocampus, Vol. 19, 2009, pp. 1-7.

2. A. Christ, W. Kainz, E.G. Hahn, K. Honegger, M. Zefferer, E. Neufeld, W. Rascher, R. Janka, W. Bautz, J. Chen, B. Kiefer, P. Schmitt, H.P. Hollenbach, J. Shen, M. Oberle, D. Szczerba, A. Kam, J.W. Guag, and N. Kuster, “The Virtual Family - development of surface-based anatomical models of two adults and two children for dosimetric simulations,” Physical Medicine Biology Vol. 55, 2010, pp. N23-38.

3. S. Orzada, O. Kraff, L.C. Schäfer, I. Brote, A. Bahr, T. Bolz, S. Maderwald, M.E. Ladd, and A.K. Bitz, “8-Channel Transmit/receive Head Coil for 7 T Human Imaging Using Intrinsically Decoupled Strip Line Elements with Meanders,” Proceedings of the International Society of Magnetic Resonance Medicine (ISMRM), Vol. 17, April 2009, p. 3010.

4. T. Stöcker, K. Vahedipour, D. Pflugfelder, and N. J. Shah, “High Performance Computing MRI Simulations”, Magnetic Resonance Medicine, Vol. 64, 2010, pp. 186-193.

5. A.K. Bitz, O. Kraff, S. Orzada, S. Maderwald, I. Brote, S. Johst, and M. E. Ladd, “Assessment of RF Safety of Transmit Coils at 7 Tesla by Experimental  and Numerical Procedures,” Proceedings of the International Society of Magnetic Resonance Medicine(ISMRM), Vol. 19, May 2011, p. 490.

Download this article in .PDF format
This file type includes high resolution graphics and schematics.