Andreas Pfrommer |
Address: | Spemannstr. 41 72076 Tübingen |
Room number: | 4.B.05 |
Phone: | +49 7071 601 918 |
Fax: | +49 7071 601 702 |
E-Mail: | andreas.pfrommer |
The research on Ultra-high field Magnetic Resonance focuses on the nuclear spin and its clinical applications at ultra high magnetic field strength (B_{0}≥7T). Such high field strength increases the intrinsic signal to noise ratio. This allows a higher resolution for imaging procedures (MRI) and thereby can improve clinical diagnostics. Moreover spectroscopy (MRSI) benefits directly from the enlarged spectral resolution which faciliates the detection of new metabolites. To excite the nuclear spins an RF magnetic field is applied, whose frequency is proportional to the strength of the static magneic field B_{0}. For the 1H nuclei the corresponding Lamor frequency (at 9.4T) is in the VHF range at roughly 400 MHz. The wavelength in human tissue becomes very short, approx. 10-11 cm. As the dimensions of the dielectric human resonator (e.g. brain) are much larger than the wavelength, there is the occurence of standing wave behavior such as partiell destructive interference and field inhomogeneities. My research interest focuses on the development of new RF coils for transmitting and receiving of the NMR signals at ultra high field strength. The transmit coil is to be optimized for good efficiency and safety issues (SAR). Multiple transmit channels in combination with particular pulse sequences might reduce field inhomogeneities to an extent that can be tolerated by the corresponding application. The design criteria for the receive coil is a maximal signal to noise ratio at the local region of interest.
My interest of research concerns the optimization of multi-channel array coils for parallel MRI/MRSI. Herein an important aspect deals with analytical models for the calculation of ultimate intrinsic (viz. independant of the practical coil geometry) SNR/SAR values. Within these models the human brain is simply assumed as a homogeneous, isotropic and spherical dielectric. The method of multipole expansion^{1} uses all electromagnetic field modes, which fulfill the Maxwell equations, for calculating UISNR/UISAR. An especially powerful tool is the concept of dyadic Green's functions^{2} (DGFs): If the DGF is known (it depends on the geometry and the boundary conditions of the particular electromagnetical field problem) the electromagnetic field can directly be calculated from its sources (volume current density, volume charge density). The charme of the DGF method is the fact, that the optimized source distribution is directly available.
The following figure shows an optimized current pattern for maximal SNR at the center of the sphere at 9.4T:
A complete set of vector solutions to the Helmholtz wave equation consists on the one hand of curl-free (electric modes) and on the other hand of divergence-free fields (magnetic modes). Both of them contribute to total UISNR. Conventional elements in receive arrays consit of closed loops only and therefore they are not able to reach maximum possible SNR in all areas in the human head at 9.4T:
Normally only the reactive component of the mutual impedance between adjacent loops is compensated by either geometrical overlapping or external circuitry. However there also exists a resistive component which degrades image SNR and parallel imaging performance. The analytical analysis for a spherical phantom with tissue-equivalent properties shows that for certain inclination angles the resistive component is zero:
The limiting factor for parallel imaging is the decrease in SNR which is proportional to 1/(g√χ), where χ accounts for reduced intrinsic signal averaging due to k-space undersampling and g is the geometry factor of the array. It reflects the ability of the array to unfold an aliased voxel. If the number of receive channels is close to the undersampling rate in k-space positioning of circular surface loops with regard to the acceleration directions has significant influence on parallel Imaging performance. Knowing the phase encoding direction(s) and undersampling rate in advance, one can formulate an optimization approach to fully exploit the unfolding potential for a given number of receive channels: min |g(r)|_{∞}. Some exemplary results for 16 channels and 3x3 acceleration in x- and y-direction are given below:
^{1}F. Wiesinger, P. Boesinger, K. P. Pruessmann: Electrodynamics and ultimate SNR in parallel MR imaging, MRM 52, p. 376-390, 2004
^{2}R. Lattanzi, D.K. Sodickson: Ideal current patterns yielding optimal signal-to-noise ratio and specific absorption rate in magnetic resonance imaging: Computational methods and physical insights, MRM 68, p.286-304, 2012
Education | |
02.2013-now |
MPI Tübingen Graduation |
09.2007-12.2012 |
University of Stuttgart Studies in Electrical Engineering and Information Technology Specialization in RF Engineering and Electronic Systems Diploma Degree |
06.2007 | A-level at Christophorus-Gymnasium Altensteig |
Awards | |
06.2015 | ISMRM Merit Award Summa Cum Laude |
10.2010 |
Anton-und-Klara-Röser-Stiftungpreis |
06.2007 |
Preis der Deutschen Physikalischen Gesellschaft Ferry-Porsche Preis |