Ingolf Sack

Electromagnetic actuator for generating variably oriented shear waves in MR elastography

Magnetic resonance elastography (MRE) is a recently developed technique for determining the mechanical properties of biological tissue. In dynamic MRE, electromagnetic units (actuators) are widely used to generate shear waves in tissue. These actuators exploit the interaction between the static magnetic field B0 and an annular coil supplied with alternating currents. Therefore, coil movements are restricted to selected orientations to B0. Conventional actuators transfer this movement collinearly to B0 into the tissue. In this study, an electromagnetic actuator was introduced that overcomes this limitation. It is demonstrated that different directions of mechanical excitation can be generated and monitored by MRE. Different spatial components of the propagation of the shear waves were determined using agarose phantoms. The technique allows maximum contrast for MRE images of objects with anisotropic strain components such as muscle tissue. Magn Reson Med 50:220–222, 2003.

Conformational studies of polymorphic N-octyl-D-gluconamide with 15N (labeled) 13C (natural abundance) REDOR spectroscopy

Magnetic n dipolar couplings between the 15N atom (labeled) and neighboring 13C atoms (natural abundance) in three solid modifications of N-octyl-D-gluconamide are measured with rotational echo double resonance (REDOR). n A unique spectral assignment of 13C resonances is possible by means of their dipolar dephasing. While n in the monolayer crystal and in the fiber modification the assignment is amenable to the solution spectra, in the n bilayer crystallites a different assignment is found. The dipolar couplings in the range 45 to 1220 Hz n are converted into CN distances. These distances are employed in conjunction with the 13C chemical shieldings of the CP-MAS spectra to determine sets of possible torsion angles, which define the molecular conformation n in the neighborhood of the amide group. In contrast to the monolayer crystal, for the fiber and bilayer crystallite n modifications n a gauche bend n at the C2–C3 bond is found, n giving the molecules the shape of a 2G sickle.

A15N-1H dipolar CSA solid-state NMR study of polymorphous polyglycine (-CO-CD2-15NH-)n

The solid-state1H MAS (magic-angle spinning),2H static,15N CP (cross polarization)-MAS and15N-1H dipolar CSA (chemical shielding anisotropy) NMR (nuclear magnetic resonance) spectra of two different modifications of Cα-deuteratedl5N-polyglycine, namely PG I and PG II (-CO-CD2-l5NH-)n are measured. The data from these spectra are compared to previous NMR, infrared, Raman and inelastic neutron scattering work. The deuteration of Cα eliminates the largest intramolecular1H-1H dipolar coupling. The effect of the remaining (N)H-(N)H interaction (∼5 kHz) is not negligible compared to the15N-1H coupling (about 10 kHz). Its effect on the dipolar CSA spectra, described as a two-spin system, is analyzed analytically and numerically and it is shown that those parts of the powder spectrum, which correspond to orientations with a strong dipolar15N-1H interaction, can be described as an effective two-spin system, permitting the measurement of the strength of the15N-1H dipolar interaction and the orientation of the dipolar vector with respect to the15N CSA frame. While in the PG II system the15N CSA tensor is collinear with the amide plane, in the PG I system the CSA tensor is tilted ca. 16° with respect to the (δ11δ22) CSA plane.

Solid-state NMR study of the SH3 domain of ?-spectrin: application of13C-15N TEDOR and REDOR

A fully 13C–15N‐labeled and a selectively alanine‐13Cβ tryptophan‐15Nring‐labeled sample of the Src homology region 3 (SH3) domain of α‐spectrin (chicken), a 62 residue protein, were biosynthesized and studied by solid‐state cross‐polarization magic angle spinning (CP/MAS) NMR, 13C–15N rotational echo double resonance (REDOR) and 15N–13C transferred echo double resonance (TEDOR) spectroscopy. In the first part of the study it is shown that spectral editing with the TEDOR sequence leads to a drastic simplification of the 13C MAS spectrum of the fully labeled sample, allowing the resolved spectroscopy of groups of 13C nuclei, according to their distance to neighboring 15N nuclei. In the second part of the study the inter‐residual distance between the alanine residue Ala55 and the tryptophan residue Trp42 was determined by the measurement of the dipolar coupling between Ala‐13Cβ and Trp‐15Nring, yielding a dipolar coupling of 48 ± 8 Hz, which after correction for fast molecular vibrations gives a value of 53 ± 8 Hz, corresponding to a CN distance of 3.85 ± 0.25 Hz. The result is compared to the CN distances obtained by x‐ray diffraction and liquid‐state NMR. Copyright

New approach for analyzing Magnetic Resonance Elastography images

A new method for evaluating Magnetic Resonance Elastography (MRE) wave images is introduced, which consists of both local frequency estimation (LFE) and simulation of wave patterns by a coupled harmonic oscillator (CHO) approach. It is shown that i) LFE performs improved reconstruction by use of Gauss filters and ii) CHO calculations can help to refine the resulting wave speed or elasticity map by taking local attenuation, reflection, and diffraction into account. The performance of new LFE and CHO calculations is demonstrated by MRE experiments on a gel phantom as well as by simulations of shear waves in a brain phantom, such as for potential in-vivo MRE-experiments.

Ortsaufgelöste Quantifizierung frequenzabhängiger Kenngrößen aus MR-Bilddaten

Als Erweiterung und Erganzung bisheriger kernspintomographischer Aufnahmetechniken liefert die neu entwickelte Magnetresonanzelastographie (MRE) Bildinformationen, die eine Bestimmung der mechanischen Eigenschaften des Untersuchungsobjektes ermoglichen. Die MRE erlaubt erstmals, Veranderungen biomechanischer Gewebeeigenschaften, die als Folge pathologischer Veranderungen auftreten, nichtinvasiv zu bestimmen. Dies erfordert die Analyse lokaler Frequenzverteilungen, die sich nach mechanischer Anregung im Untersuchungsobjekt ausbilden. hn folgenden wird ein neuer Ansatz zur Analyse der Frequenzverteilungsmuster vorgestellt und anhand von Test- und experimentellen Bildern validiert. Vor allem bei Geweben, die von ausen einer Tastuntersuchung (Palpation) nicht zuganglich sind, wie das Hirnparenchym, hat die ‘MR-Palpation’ ein hohes diagnostisches Potential.

Prospective registration of inter-examination MR images

The monitoring of the development of cerebral diseases such as stroke or brain tumors with MRI requires high-precision comparison of initial and follow-up images. Retrospective registration often produces artifacts, especially at boundariesbetween different tissue structures. However, by manipulating the gradients, MRI scanners offer the possibility of shifting and rotating image planes fast and without removing the patient. Two approaches for prospective registration were implemented and tested on phantoms and healthy volunteers. To speed up calculation, both registration algorithms used the three orthogonal two-dimensional localizer images that were acquired prior to each measurement. In the first approach, the image is projected onto one axis to determine the rotation between initial and follow-up examination. The second algorithm uses cross-correlation for rotational correction. Both algorithms maximize the cross-correlation for correction of the shifts. After 2-D registration in each orientation, the gradients of the tomograph are adapted according to the calculated transformation matrix. The results were evaluated with a 3-D rigid-body registration using Automated Image Registration. The cross-correlation method was found to be very robust, while the 1-D projection algorithm was sufficiently fast but registration results depended on the shape of the head.

Simulation von Kernspinelastographie-Experimenten zur Beurteilung der Machbarkeit und Optimierung potentieller Anwendungen

Ein neues bildgebendes Verfahren, die Magnetresonanzelastographie (MRE), erlaubt erstmals die nichtinvasive Quantifizierung biomechanischer Kenngrosen. Vorteile der MRE sind die Untersuchung manuell nicht zuganglicher Gewebe sowie die hohe Ortsauflosung. Eine Beurteilung der Eignung hinsichtlich neuer Anwendungen ware vorteilhaft, insbesondere noch vor der Durchfuhrung erster Experimente. Um dieses Ziel zu erreichen, wurde ein physikalisches Modell aufgestellt, das elastische Kenngrosen und typische Phanomene des MRE-Experimentes, wie zum Beispiel Anregungscharakteristik, Reflexion, Dampfung und Beugung, berucksichtigt.