Biomedical subjects

P Börnert

Publications and source records attributed to P Börnert.

17 recordsLinked to original sources

PubMed2001

Improved 3D spiral imaging for coronary MR angiography.

Thin-slab 3D spiral imaging has been used for MR angiography to image selected coronary arteries. Improved scan efficiency was achieved using a train of multiple spiral interleaves within each single R-R interval acquired in the late diastole. Data acquisition was performed during free breathing, using navigator gating. Additionally, prospective slice tracking was applied to further reduce the sensitivity to motion. The application of a T(2)-preparation pulse and fat suppression increased the contrast between blood and myocardium. Experiments performed on healthy volunteers are presented to show the feasibility of this approach, which allows coronary artery imaging of selected vessels within a few minutes. Magn Reson Med 45:172-175, 2001.

Coronary Vessels↗

PubMed2000

Reversed spiral MR imaging.

Reversed spiral imaging is discussed as an approach that provides strong intrinsic T *(2) contrast without the need for long repetition times. In comparison to the conventional forward spiral method, the T *(2) contrast achieved by reversing the spiral k-space trajectory is similar and differs only for very fast relaxing species. The flow and motion sensitivity of the reversed approach is the same if flow compensation is applied, except for a flow-dependent voxel shift and the sign of the artifact pattern. By simulations as well as phantom and in vivo experiments, it is shown that the image quality in reversed spiral imaging is comparable to that obtained with the forward spiral method.

Blood Flow Velocity↗

PubMed1999

Motion pattern adapted real-time respiratory gating.

The information about the current respiratory motion state used in conventional gating to accept or to reject data can further be used to obtain motion statistics during an MR scan. This can serve to monitor changes in the respiratory pattern of the patient by comparison of motion statistics subsequently obtained during scanning. Two indicators are introduced: first a parameter that registers changes of the motion pattern, and second an indicator that expresses the quality of the data set already obtained. Based on these indicators, the gating algorithm decides to change gating parameters during scanning automatically. This new approach has the potential to increase the scan efficiency considerably without the need of operator interaction and/or significant patient cooperation. The basic principle is described and illustrated for the motion-adapted gating technique, and first in vivo results are presented to underline the feasibility of this concept.

Abdomen↗

PubMed1999

Improvements in spiral MR imaging.

The basic principles of spiral MR image acquisition and reconstruction are summarised with the aim to explain how high quality spiral images can be obtained. The sensitivity of spiral imaging to off-resonance effects, gradient system imperfections and concomitant fields are outlined and appropriate measures for corrections are discussed in detail. Phantom experiments demonstrate the validity of the correction approaches. Furthermore, in-vivo results are shown to demonstrate the applicability of the corrections under in-vivo conditions. The spiral image quality thus obtained was found to be comparable to that obtainable with robust spin warp sequences.

Brain↗

PubMed1999

On the performance and accuracy of 2D navigator pulses.

The purpose of this study was to investigate and to optimize the performance of two-dimensional spatially selective excitation pulses used for navigator applications on a clinical scanner. The influence of gradient imperfections, off-resonance effects, and incomplete k-space covering on the pencil beam-shaped spatial excitation profile of the 2D RF pulse was studied. The studies involved experiments performed on phantoms and in vivo. In addition, simulations were carried out by numerical integration of the Bloch equations. The accuracy of positioning of the pencil beam was increased by a factor of three by employing a simple correction scheme for the compensation of gradient distortions. The spatial selectivity of the 2D RF pulse was improved by taking sampling density corrections into account. The 2D RF pulse performance was found to be sufficient to monitor the diaphragm motion even at moderate gradient strength. For applications, where a high spatial resolution is required or a less characteristic contrast is present a strong gradient system is recommended.

Artifacts↗

PubMed1999

Resampling of data between arbitrary grids using convolution interpolation.

For certain medical applications resampling of data is required. In magnetic resonance tomography (MRT) or computer tomography (CT), e.g., data may be sampled on nonrectilinear grids in the Fourier domain. For the image reconstruction a convolution-interpolation algorithm, often called gridding, can be applied for resampling of the data onto a rectilinear grid. Resampling of data from a rectilinear onto a nonrectilinear grid are needed, e.g., if projections of a given rectilinear data set are to be obtained. In this paper we introduce the application of the convolution interpolation for resampling of data from one arbitrary grid onto another. The basic algorithm can be split into two steps. First, the data are resampled from the arbitrary input grid onto a rectilinear grid and second, the rectilinear data is resampled onto the arbitrary output grid. Furthermore, we like to introduce a new technique to derive the sampling density function needed for the first step of our algorithm. For fast, sampling-pattern-independent determination of the sampling density function the Voronoi diagram of the sample distribution is calculated. The volume of the Voronoi cell around each sample is used as a measure for the sampling density. It is shown that the introduced resampling technique allows fast resampling of data between arbitrary grids. Furthermore, it is shown that the suggested approach to derive the sampling density function is suitable even for arbitrary sampling patterns. Examples are given in which the proposed technique has been applied for the reconstruction of data acquired along spiral, radial, and arbitrary trajectories and for the fast calculation of projections of a given rectilinearly sampled image.

Algorithms↗

PubMed1998

Effects of gradient anisotropy in MRI.

A gradient system is anisotropic if the impulse responses of at least two of the gradient channels, x, y, or z, differ from each other. Such an undesired condition may arise, for example, from differences between the gradient channels with respect to eddy currents or from unbalanced time delays in the electronic components. Depending on the degree of anisotropy, the actual gradient then deviates from the nominal, desired gradient under certain oblique orientations during the transient periods of gradient switching. The adverse consequence is degradation of image quality, such as distortion, ghosting, and blurring. In this paper, a theoretical analysis is given of the basic effects. Furthermore, the implications for the MRI process and possible correction methods are described. The effects of anisotropy are shown experimentally for echo-planar imaging and two-dimensional selective RF excitation with spiral gradient pulses.

Anisotropy↗

PubMed1998

Fast 1H spectroscopic imaging using a multi-element head-coil array.

Fast proton magnetic resonance spectroscopic imaging (MRSI) using a multi-element head-coil array is examined with respect to three aspects: the coil design, the use of an appropriate signal combination method, and the design of the MRSI pulse sequence itself. An eight-element head-coil array has been developed to increase the signal-to-noise ratio (SNR) of MRSI in the human brain. The flexible wraparound design optimally fits different head sizes and thus provides high sensitivity. The signal combination of the individual coil elements is based on the approach proposed by Roemer et al. (Magn. Reson. Med. 16, 192 (1990)). An additional short prescan is performed to provide a good estimate of the complex coil sensitivity profiles, which are used in the signal combination procedure to correct the spectroscopic imaging data for the spatially varying intensity. The use of coil arrays in MRSI has some effect on the requirements for both water and lipid suppression. These techniques and a MRSI pulse sequence that provides a high spectroscopic resolution are described and discussed. Experimental results at 1.5 T show that metabolite maps of N-acetylaspartate (NAA), choline (Cho), phosphocreatine (PCr)/creatine (Cr) can be obtained within a 5-min acquisition time.

Aspartic Acid↗

PubMed1998

On spatially selective RF excitation and its analogy with spiral MR image acquisition.

The basic principles of the design of spatially selective RF pulses are described, and their analogy with MR image acquisition and reconstruction is shown. The paper focuses on RF-pulse design and imaging schemes in which spiral k-space trajectories are used. The sensitivity of RF excitation to gradient-system imperfections and to spatially varying off-resonance are analyzed, and suitable measures of correction are discussed. The spatial resolution obtainable with selective RF pulses and the consequences of the linearity of the pulse-design problem are examined. Phantom experiments showing the performance of multidimensional spatially selective RF pulses further illustrate the analogy with MR image acquisition.

Fourier Analysis↗

PubMed1997

Motion-adapted gating based on k-space weighting for reduction of respiratory motion artifacts.

A new modified type of gating is presented that shows the ability to reduce the total scan time with almost conserved image quality compared with conventional gating. This new motion-adapted gating approach is based on a k-space-dependent gating threshold function. MR data acquired are only accepted if the motion-induced displacements measured from a reference position are below the chosen gating threshold function. During the MR measurement the scanner analyses respiratory motion decides in real-time which data in k-space could be measured according to the gating threshold function and performs data acquisition. In the present paper the approach will be described and discussed. Simulations based on in vivo data and initial in vivo experiments are presented to compare different variants of the new approach mutually and to the conventional technique. The analysis given is focused on spin warp type sequences, which are the best candidates for this approach.

Abdomen↗

PubMed1996

Curved slice imaging.

Curved slice imaging based on multidimensional RF pulses is introduced and discussed. This new approach makes it possible to image curved anatomical structures by using MRI. The 2D RF or 3D RF pulses used can be tailored to excite or refocus transverse magnetization of a previously defined arbitrarily curved slice profile in a 3D space. These RF pulses can be integrated into all standard MRI sequences to perform slice selection. The final curved slice image is obtained as a projection of the curved slice magnetization onto a selected imaging plane. The problem of ambiguities arising due to this projection process is addressed. Phantom and in vivo experiments were performed to illustrate the advantages and limitations of this approach.

Brain↗

PubMed1995

Coronary artery imaging at 0.5 T using segmented 3D echo planar imaging.

The application of segmented 3D gradient echo EPI at 0.5 T for coronary artery imaging is described. Experiments were performed using fat suppression, ECG triggering, and a patient-controlled breath-holding scheme. This approach provides a sufficient signal-to-noise ratio for thin contiguous slices in conjunction with a phased array cardiac receive coil. Wide 3D volumes, covering the proximal branches of the coronary tree, were measured with a high spatial resolution. Such data sets can be used for subsequent vessel segmentation. Furthermore, data out of narrow 3D volumes were obtained containing fewer slices angulated in the direction of a selected coronary artery. This provides a good visualization of the selected vessel over several centimeters without the need for segmentation.

Coronary Vessels↗

PubMed1994

Single-shot-double-echo EPI.

Echo planar imaging (EPI) was extended for the acquisition of both, the spin-echo (SE) and the stimulated echo in a stimulated echo sequence after one excitation from thermal equilibrium. The two independent EPI images reflect different states of the transverse magnetization excited and measured almost simultaneously. This single-shot-double-echo EPI approach offers a variety of useful applications such as varying image contrast, obtaining flow or diffusion information, improving image resolution or circumventing T*2 requirements in single shot EPI. Benefits and limits of this technique are discussed.

Artifacts↗

PubMed1993

Fast perfluorocarbon imaging using 19F U-FLARE.

The application of an ultra-fast low angle RARE technique for the 19F imaging of perfluorocarbons (PFCs) used as temporary blood substitutes is described. This sequence is attractive for fast 19F imaging studies that measure the biodistribution of PFCs in vivo, due to its high signal-to-noise ratio. Extensions of this technique for the chemical shift selective measurement of fluorine T1 values are presented. Using the linear dependence between the oxygen partial pressure (pO2) and the T1 relaxation rate of PFC resonances this technique makes possible the fast in vivo measurement of oxygen tension. Using the sequence in a diffusion sensitized form 19F measurements of the diffusion constants of PFCs are also presented. Phantom experiments to test the methods, and in vivo images obtained in rat studies are given and discussed.

Animals↗

PubMed1992

On the application of ultra-fast RARE experiments.

The ultra-fast application of the RARE experiment is described in detail, with special emphasis on its multifarious applications with preparation experiments that produce transverse magnetization. The factors affecting the temporal evolution of the magnetization during the experiment are described, and the implications for the slice profile when using a Gaussian refocusing pulse are experimentally examined. The choice of phase-encoding scheme for use with preparation experiments is discussed, as is the use of various phase-encoding schemes to reduce line broadening in the phase-encoding direction if a number of averages are acquired. An explanation for the decomposition of the echo are into two components if the read gradient is imbalanced is given, and the experimental conditions necessary for the coherent addition of these two echo groups are described. An alternative sequence that removes one of these groups from the acquisition window is proposed. The sensitivity of the sequence to flow and motion is investigated, and the drastic loss of signal in this situation explained. The in vivo and in vitro application of preparation experiments leading to the accurate measurement of T1, T2, diffusion constant, and magnetization transfer characteristics is presented. The implementation of zoom-imaging using spin- and stimulated-echo preparation is described, and 3D in vivo spin-echo zoom images are presented. Simple phantom experiments demonstrating the feasibility of chemical-shift selective and spectroscopic imaging are also given.

Animals↗

PubMed1989

19F chemical shift imaging in perfluorocarbons.

The use of 19F chemical shift sensitive imaging techniques to monitor the biodistribution of perfluorocarbons (PFC) is discussed. For these experiments one has to study the spectroscopic properties of the PFC to be mapped for choosing high performance NMR imaging sequences. Three techniques used in our laboratory, a chemical shift selective approach, a method using spectrum simplification and a chemical shift sensitive NMR imaging method using adjusted phase encoding are discussed and illustrated by experiments.

Animals↗

PubMed1988

Pulse sequence and parameter choice in NMR imaging as a problem of constrained multidimensional nonlinear optimization.

The advantage of the multiparametric nature of NMR imaging is connected with the problem of finding optimum pulse sequences and sequence parameters, which ensure a high contrast-to-noise ratio. For a given imaging task with m (m greater than or equal to 2) regions characterized by sets of NMR parameters (e.g., rho, T1, T2) two functions are proposed, which can be used to transform the search for an optimum pulse sequence into a problem of constrained multidimensional nonlinear optimization. The numerical algorithm is described and the results of two examples are presented and discussed. A short description of useful extensions of the proposed optimization approach, which are currently implemented, is given.

Magnetic Resonance Spectroscopy↗