Bio

Professional Education


  • Doctor Rerum Naturalium, University of Freiburg, Germany (2014)
  • Diplom Physiker, University of Freiburg, Germany (2008)

Stanford Advisors


Patents


  • Hans Weber, Maxim Zaitsev, Daniel Gallichan, Juergen Hennig. "Germany Patent DE 10 2011 007 501 B3 Method of Magnetic Resonance Imaging for the Selection and Recording of Curved Slices", University Medical Center Freiburg, Freiburg, Germany, May 31, 2012
  • Hans Weber, Maxim Zaitsev, Daniel Gallichan, Gerrit Schultz. "Germany Patent DE 10 2010 003 552 B4 Method for Homogenizing Resolution in Magnetic Resonance Tomography Experiments Using Non-Linear Encoding Fields", University Medical Center Freiburg, Freiburg, Germany, Mar 22, 2012

Publications

All Publications


  • Local Shape Adaptation for Curved Slice Selection MAGNETIC RESONANCE IN MEDICINE Weber, H., Haas, M., Kokorin, D., Gallichan, D., Hennig, J., Zaitsev, M. 2014; 72 (1): 112-123

    Abstract

    Nonlinear spatial encoding magnetic fields allow excitation and geometrically matched local encoding of curved slices. However, the nonlinearity of the fields results in a varying slice thickness. Within this study, the technique is combined with multidimensional RF excitation for local adaptation of the slice shape.A framework originally developed for nonlinear receive encoding is applied to multidimensional excitation with nonlinear spatial encoding magnetic fields for determination of dedicated target patterns and combined with a model for assessment of minimum transmit-resolution requirements for the design of efficient transmit k-space trajectories.Cross-sections of curved slices acquired in a phantom with both locally adapted slice thickness and curvature are evaluated. In addition, resulting voxel shapes are analyzed to investigate the range of applicability of the technique. Finally, slice-thickness adaptation is applied to in vivo curved slice imaging.Local adaptation of the slice thickness is feasible both in phantom and in vivo. The technique further allows local adaptation of the slice curvature. However, its range of applicability is limited by prolonged pulse duration and voxel shape distortion.Multidimensional excitation allows imaging of curved slices with constant thickness. It also has the potential for further modification of the slice shape for increased ability to adapt to the anatomy.

    View details for DOI 10.1002/mrm.24906

    View details for Web of Science ID 000337624400014

    View details for PubMedID 24006118

  • Stages: Sub-Fourier Dynamic Shim Updating Using Nonlinear Magnetic Field Phase Preparation MAGNETIC RESONANCE IN MEDICINE Witschey, W. R., Littin, S., Cocosco, C. A., Gallichan, D., Schultz, G., Weber, H., Welz, A., Hennig, J., Zaitsev, M. 2014; 71 (1): 57-66

    Abstract

    Heterogeneity of the static magnetic field in magnetic resonance imaging may cause image artifacts and degradation in image quality. The field heterogeneity can be reduced by dynamically adjusting shim fields or dynamic shim updating, in which magnetic field homogeneity is optimized for each tomographic slice to improve image quality. A limitation of this approach is that a new magnetic field can be applied only once for each slice, otherwise image quality would improve somewhere to its detriment elsewhere in the slice. The motivation of this work is to overcome this limitation and develop a technique using nonlinear magnetic fields to dynamically shim the static magnetic field within a single Fourier-encoded volume or slice, called sub-Fourier dynamic shim updating. However, the nonlinear magnetic fields are not used as shim fields; instead, they impart a strong spatial dependence to the acquired MR signal by nonlinear phase preparation, which may be exploited to locally improve magnetic field homogeneity during acquisition. A theoretical description of the method is detailed, simulations and a proof-of-principle experiment are performed using a magnet coil with a known field geometry. The method is shown to remove artifacts associated with magnetic field homogeneity in balanced steady-state free-precession pulse sequences. We anticipate that this method will be useful to improve the quality of magnetic resonance images by removing deleterious artifacts associated with a heterogeneous static magnetic field.

    View details for DOI 10.1002/mrm.24625

    View details for Web of Science ID 000328580300008

    View details for PubMedID 23440677

  • Image reconstruction in k-space from MR data encoded with ambiguous gradient fields. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine Schultz, G., Gallichan, D., Weber, H., Witschey, W. R., Honal, M., Hennig, J., Zaitsev, M. 2014

    Abstract

    In this work, the limits of image reconstruction in k-space are explored when non-bijective gradient fields are used for spatial encoding.The image space analogy between parallel imaging and imaging with non-bijective encoding fields is partially broken in k-space. As a consequence, it is hypothesized and proven that ambiguities can only be resolved partially in k-space, and not completely as is the case in image space.Image-space and k-space based reconstruction algorithms for multi-channel radiofrequency data acquisitions are programmed and tested using numerical simulations as well as in vivo measurement data.The hypothesis is verified based on an analysis of reconstructed images. It is found that non-bijective gradient fields have the effect that densely sampled autocalibration data, used for k-space reconstruction, provide less information than a separate scan of the receiver coil sensitivity maps, used for image space reconstruction. Consequently, in k-space only the undersampling artifact can be unfolded, whereas in image space, it is also possible to resolve aliasing that is caused by the non-bijectivity of the gradient fields.For standard imaging, reconstruction in image space and in k-space is nearly equivalent, whereas there is a fundamental difference with practical consequences for the selection of image reconstruction algorithms when non-bijective encoding fields are involved. Magn Reson Med, 2014. 2014 Wiley Periodicals, Inc.

    View details for DOI 10.1002/mrm.25152

    View details for PubMedID 24777559

  • Development and Characterization of An Unshielded PatLoc Gradient Coil for Human Head Imaging CONCEPTS IN MAGNETIC RESONANCE PART B-MAGNETIC RESONANCE ENGINEERING Welz, A., Cocosco, C., Dewdney, A., Gallichan, D., Jia, F., Lehr, H., Liu, Z., Post, H., Schmidt, H., Schultz, G., Testud, F., Weber, H., Witschey, W., Korvink, J., Hennig, J., Zaitsev, M. 2013; 43 (4): 111-125
  • Excitation and geometrically matched local encoding of curved slices MAGNETIC RESONANCE IN MEDICINE Weber, H., Gallichan, D., Schultz, G., Cocosco, C. A., Littin, S., Reichardt, W., Welz, A., Witschey, W., Hennig, J., Zaitsev, M. 2013; 69 (5): 1317-1325

    Abstract

    In this work, the concept of excitation and geometrically matched local in-plane encoding of curved slices (ExLoc) is introduced. ExLoc is based on a set of locally near-orthogonal spatial encoding magnetic fields, thus maintaining a local rectangular shape of the individual voxels and avoiding potential problems arising due to highly irregular voxel shapes. Unlike existing methods for exciting curved slices based on multidimensional radiofrequency-pulses, excitation and geometrically matched local encoding of curved slices does not require long duration or computationally expensive radiofrequency-pulses. As each encoding field consists of a superposition of potentially arbitrary (spatially linear or nonlinear) magnetic field components, the resulting field shape can be adapted with high flexibility to the specific region of interest. For extended nonplanar structures, this results in improved relevant volume coverage for fewer excited slices and thus increased efficiency. In addition to the mathematical description for the generation of dedicated encoding fields and data reconstruction, a verification of the ExLoc concept in phantom experiments and examples for in vivo curved single and multislice imaging are presented.

    View details for DOI 10.1002/mrm.24364

    View details for Web of Science ID 000318026400014

    View details for PubMedID 22711656

  • Local field of view imaging for alias-free undersampling with nonlinear spatial encoding magnetic fields. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine Weber, H., Schultz, G., Gallichan, D., Hennig, J., Zaitsev, M. 2013

    Abstract

    PURPOSE: Nonlinear spatial encoding magnetic fields result in an inhomogeneous image resolution. Within this study, this characteristic property of nonlinear encoding is investigated with regard to its potential to accelerate MRI acquisitions. THEORY: A dependency between k-space coverage and local resolvability of the image causes k-space samples to have a spatially localized contribution to the reconstruction of the spin density. On the basis of this observation, a concept for alias-free data undersampling is developed, which is referred to as the local field of view concept. METHODS: On the basis of this concept, a fast sampling trajectory is developed. It is evaluated with simulations and experiments (both using a phantom and in vivo) for MRI with, as an example, pure quadrupolar encoding fields. To demonstrate that the concept is only applicable to (spatially) nonlinear encoding, a comparison with linear encoding is provided. RESULTS: Application of the local field of view concept results in a localized adaptation of the image resolution by undersampling higher frequency k-space samples without introducing aliasing. CONCLUSIONS: A new effect of nonlinear spatial encoding magnetic fields was found, which allows more efficient data sampling and at the same time counterbalancing the natural variation in image resolution. Magn Reson Med, 2013. 2013 Wiley Periodicals, Inc.

    View details for DOI 10.1002/mrm.24754

    View details for PubMedID 23649975

  • Practical considerations for in vivo MRI with higher dimensional spatial encoding MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE Gallichan, D., Cocosco, C. A., Schultz, G., Weber, H., Welz, A. M., Hennig, J., Zaitsev, M. 2012; 25 (6): 419-431

    Abstract

    This work seeks to examine practical aspects of in vivo imaging when spatial encoding is performed with three or more encoding channels for a 2D image.The recently developed 4-Dimensional Radial In/Out (4D-RIO) trajectory is compared in simulations to an alternative higher-order encoding scheme referred to as O-space imaging. Direct comparison of local k-space representations leads to the proposal of a modification to the O-space imaging trajectory based on a scheme of prephasing to improve the reconstructed image quality. Data were collected using a 4D-RIO acquisition in vivo in the human brain and several image reconstructions were compared, exploiting the property that the dense encoding matrix, after a 1D or 2D Fourier transform, can be approximated by a sparse matrix by discarding entries below a chosen magnitude.The proposed prephasing scheme for the O-space trajectory shows a marked improvement in quality in the simulated image reconstruction. In experiments, 4D-RIO data acquired in vivo in the human brain can be reconstructed to a reasonable quality using only 5 % of the encoding matrix--massively reducing computer memory requirements for a practical reconstruction.Trajectory design and reconstruction techniques such as these may prove especially useful when extending generalized higher-order encoding methods to 3D images.

    View details for DOI 10.1007/s10334-012-0314-y

    View details for Web of Science ID 000311675100002

    View details for PubMedID 22484820

  • Localization by nonlinear phase preparation and k-space trajectory design MAGNETIC RESONANCE IN MEDICINE Witschey, W. R., Cocosco, C. A., Gallichan, D., Schultz, G., Weber, H., Welz, A., Hennig, J., Zaitsev, M. 2012; 67 (6): 1620-1632

    Abstract

    A technique is described to localize MR signals from a target volume using nonlinear pulsed magnetic fields and spatial encoding trajectories designed using local k-space theory. The concept of local k-space is outlined theoretically, and this principle is applied to simulated phantom and cardiac MRI data in the presence of surface and quadrupolar gradient coil phase modulation. Phantom and in vivo human brain images are obtained using a custom, high-performance quadrupolar gradient coil integrated with a whole-body 3-T MRI system to demonstrate target localization using three-dimensional T 2*-weighted spoiled gradient echo, two-dimensional segmented, multiple gradient encoded spin echo, and three-dimensional balanced steady-state free precession acquisitions. This method may provide a practical alternative to selective radiofrequency excitation at ultra-high-field, particularly for steady-state applications where repetition time (TR) must be minimized and when the amount of energy deposited in human tissues is prohibitive. There are several limitations to the approach including the spatial variation in resolution, high frequency aliasing artifacts, and spatial variation in echo times and contrast.

    View details for DOI 10.1002/mrm.23146

    View details for Web of Science ID 000304086000014

    View details for PubMedID 22127679

  • Radial Imaging With Multipolar Magnetic Encoding Fields IEEE TRANSACTIONS ON MEDICAL IMAGING Schultz, G., Weber, H., Gallichan, D., Witschey, W. R., Welz, A. M., Cocosco, C. A., Hennig, J., Zaitsev, M. 2011; 30 (12): 2134-2145

    Abstract

    We present reconstruction methods for radial magnetic resonance imaging (MRI) data which were spatially encoded using a pair of orthogonal multipolar magnetic fields for in-plane encoding and parallel imaging. It is shown that a direct method exists in addition to iterative reconstruction. Standard direct projection reconstruction algorithms can be combined with a previously developed direct reconstruction for multipolar encoding fields acquired with Cartesian trajectories. The algorithm is simplified by recasting the reconstruction problem into polar coordinates. In this formulation distortion and aliasing become separate effects. Distortion occurs only along the radial direction and aliasing along the azimuthal direction. Moreover, aliased points are equidistantly distributed in this representation, and, consequently, Cartesian SENSE is directly applicable with highly effective unfolding properties for radio-frequency coils arranged with a radial symmetry. The direct and iterative methods are applied to simulated data to analyze basic properties of the algorithms and for the first time also measured in vivo data are presented. The results are compared to linear spatial encoding using a radial trajectory and quadrupolar encoding using a Cartesian trajectory. The direct reconstruction gives good results for fully sampled datasets. Undersampled datasets, however, show star-shaped artifacts, which are significantly reduced with the iterative reconstruction.

    View details for DOI 10.1109/TMI.2011.2164262

    View details for Web of Science ID 000297584400010

    View details for PubMedID 21843982

  • Microcoil-based MRI: feasibility study and cell culture applications using a conventional animal system MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE Weber, H., Baxan, N., Paul, D., Maclaren, J., Schmidig, D., Mohammadzadeh, M., Hennig, J., von Elverfeldt, D. 2011; 24 (3): 137-145

    Abstract

    The aim of this study was to demonstrate the feasibility of MR microimaging on a conventional 9.4 T horizontal animal MRI system using commercial available microcoils in combination with only minor modifications to the system, thereby opening this field to a larger community.Commercially available RF microcoils designed for high-resolution NMR spectrometers were used in combination with a custom-made probehead. For this purpose, changes within the transmit chain and modifications to the adjustment routines and image acquisition sequences were made, all without requiring expensive hardware. To investigate the extent to which routine operation and high-resolution imaging is possible, the quality of phantom images was analysed. Surface and solenoidal microcoils were characterized with regard to their sensitive volume and signal-to-noise ratio. In addition, the feasibility of using planar microcoils to achieve high-resolution images of living glioma cells labelled with MnCl(2) was investigated.The setup presented in this work allows routine acquisition of high-quality images with high SNR and isotropic resolutions up to 10?m within an acceptable measurement time.This study demonstrates that MR microscopy can be applied at low cost on animal MR imaging systems, which are in widespread use. The successful imaging of living glioma cells indicates that the technique promises to be a useful tool in biomedical research.

    View details for DOI 10.1007/s10334-011-0244-0

    View details for Web of Science ID 000291041900003

    View details for PubMedID 21331647

  • Characterization of a 3D MEMS fabricated micro-solenoid at 9.4 T JOURNAL OF MAGNETIC RESONANCE Mohmmadzadeh, M., Baxan, N., Badilita, V., Kratt, K., Weber, H., Korvink, J. G., Wallrabe, U., Hennig, J., von Elverfeldt, D. 2011; 208 (1): 20-26

    Abstract

    We present for the first time a complete characterization of a micro-solenoid for high resolution MR imaging of mass- and volume-limited samples based on three-dimensional B(0), B(1) per unit current (B(1)(unit)) and SNR maps. The micro-solenoids are fabricated using a fully micro-electromechanical systems (MEMS) compatible process in conjunction with an automatic wire-bonder. We present 15 ?m isotropic resolution 3D B(0) maps performed using the phase difference method. The resulting B(0) variation in the range of [-0.07 ppm to -0.157 ppm] around the coil center, compares favorably with the 0.5 ppm limit accepted for MR microscopy. 3D B(1)(unit) maps of 40 ?m isotropic voxel size were acquired according to the extended multi flip angle (ExMFA) method. The results demonstrate that the characterized microcoil provides a high and uniform sensitivity distribution around its center (B(1)(unit) = 3.4 mT/A 3.86%) which is in agreement with the corresponding 1D theoretical data computed along the coil axis. The 3D SNR maps reveal a rather uniform signal distribution around the coil center with a mean value of 53.69 19%, in good agreement with the analytical 1D data along coil axis in the axial slice. Finally, we prove the microcoil capabilities for MR microscopy by imaging Eremosphaera viridis cells with 18 ?m isotropic resolution.

    View details for DOI 10.1016/j.jmr.2010.09.021

    View details for Web of Science ID 000286774300003

    View details for PubMedID 21071246

  • Extended Multi-Flip-Angle B-1 Mapping: A 3D Mapping Method for Inhomogeneous B-1 Fields CONCEPTS IN MAGNETIC RESONANCE PART B-MAGNETIC RESONANCE ENGINEERING Weber, H., Paul, D., Elverfeldt, D. V., Hennig, J., Zaitsev, M. 2010; 37B (4): 203-214
  • On-chip three dimensional microcoils for MRI at the microscale LAB ON A CHIP Badilita, V., Kratt, K., Baxan, N., Mohmmadzadeh, M., Burger, T., Weber, H., v Elverfeldt, D., Hennig, J., Korvink, J. G., Wallrabe, U. 2010; 10 (11): 1387-1390

    Abstract

    We present for the first time a fully MEMS-integrated technology to manufacture 3D geometrically perfect solenoidal microcoils for microscale MRI applications. We report 25 microm isotropic resolution MR images of a copper sulfate aqueous phantom. These images are acquired using microcoils with 5 windings of insulated 25 microm diameter Au wire and with quality factors as high as 46 at the operating frequency (400 MHz).

    View details for DOI 10.1039/c000840k

    View details for Web of Science ID 000277832800004

    View details for PubMedID 20407728

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