Contact

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

 

Picture of Pfrommer, Andreas

Andreas Pfrommer

Position: PhD Student  Unit: Henning Scheffler

Motivation

The research on Ultra-high field Magnetic Resonance focuses on the nuclear spin and its clinical applications at ultra high magnetic field strength (B0≥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 B0. 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.

 

Optimization of Array Coils for Parallel Transmit and Receive

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 expansion1 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 functions2 (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.

 

Optimal Current Patterns

The following figure shows an optimized current pattern for maximal SNR at the center of the sphere at 9.4T:

 

Optimal current pattern for maximal SNR at central voxel position

 

Contribution of electric modes to SNR

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:

 

 

Mutual resistive coupling between two circular loops

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:

 

 

Optimal loop arrangement of finite element loop arrays

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:

 

 

 


 

1F. Wiesinger, P. Boesinger, K. P. Pruessmann: Electrodynamics and ultimate SNR in parallel MR imaging, MRM 52, p. 376-390, 2004

2R. 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

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Show abstracts

Articles (2):

Avdievich NI, Hoffmann J, Shajan G, Pfrommer A, Giapitzakis IA, Scheffler K and Henning A (February-2017) Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4 T NMR in Biomedicine 30(2) 1-12.
Pfrommer A and Henning A (February-2017) On the Contribution of Curl-Free Current Patterns to the Ultimate Intrinsic Signal-to-Noise Ratio at Ultra-High Field Strength NMR in Biomedicine Epub ahead.

Conference papers (1):

Pfrommer A and Henning A (May-2015) Optimal Arrangement of Finite Element Loop Arrays for Parallel Magnetic Resonance Imaging in the Human Head at 400 MHz, IEEE MTT-S International Microwave Symposium (IMS 2015), IEEE, Piscataway, NJ, USA, 1-4.

Posters (4):

Pfrommer A, Avdievich NI and Henning A (May-11-2016): Effect of the RF Shield on the Mutual Coupling Between Adjacent and Non-Adjacent Array Elements, 24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016), Singapore.
Avdievich NI, Pfrommer A, Giapitzakis IA and Henning A (May-10-2016): Analytical Modeling of the Coupling within a Human Head Surface Loop Transmit Phased Array at Ultra-High Fields, 24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016), Singapore.
Pfrommer A and Henning A (June-1-2015): Optimal Arrangement of Finite Element Loop Arrays for Parallel Imaging in a Spherical Geometry at 9.4 T, 23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015), Toronto, Canada.
Pfrommer A, Avdievich N and Henning A (May-12-2014): Four Channel Transceiver Array for Functional Magnetic Resonance Spectroscopy in the Human Visual Cortex at 9.4 T, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy.

Talks (5):

Pfrommer A and Henning A (October-1-2016) Abstract Talk: Comparison of the ultimate intrinsic SNR in a spherical phantom vs a realisitc human head model at 9.4 T, 33rd Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2016), Wien, Austria, Magnetic Resonance Materials in Physics, Biology and Medicine, 29(Supplement 1) S308-S309.
Pfrommer A, Avdievich NI and Henning A (May-9-2016) Abstract Talk: About the Ultimate SNR for Cylindrical and Spherical RF Arrays in a Realistic Human Head Model, 24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016), Singapore(0175).
Avdievich NI, Giapitzakis IA, Pfrommer A and Henning A (May-9-2016) Abstract Talk: Optimization of the Transceiver Phased Array for Human Brain Imaging at 9.4T: Loop Overlapping Rediscovered, 24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016), Singapore(0169).
Pfrommer A and Henning A (June-4-2015) Abstract Talk: On the Contribution of Electric-Type Current Patterns to UISNR for a Spherical Geometry at 9.4 T, 23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015), Toronto, Canada(0856).
Avdievich NI, Pfrommer A, Hoffmann J, Chadzynski GL, Scheffler K and Henning A (May-14-2014) Abstract Talk: Tranceive Phased Array with High Transmit Performance for Human Brain Application at 9.4 T, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy(0622).

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Last updated: Tuesday, 18.11.2014