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R Deichmann

Publications and source records attributed to R Deichmann.

At least 19 recordsLinked to original sources
PubMed2006

Fast structural brain imaging using an MDEFT sequence with a FLASH-EPI hybrid readout.

A sequence for the fast acquisition of T1 weighted structural brain images with whole brain coverage and isotropic resolution of 1mm is presented. It is based on MDEFT with a FLASH-EPI hybrid readout. Several techniques for artefact suppression are implemented, like echo time shifting, asymmetric k-space sampling, and navigator echo acquisition. It is shown experimentally that the hybrid MDEFT sequence with a total duration of 8min yields approximately the same signal-to-noise and contrast-to-noise ratios as a 12min standard MDEFT sequence based on a FLASH readout. Further experiments show that echo time shifting suppresses artefacts in the vicinity of the scalp and in areas suffering from field inhomogeneities and that the concept of asymmetric k-space sampling reduces the susceptibility to head movement.

Adult↗
PubMed2006

Removing the effects of CSF partial voluming on fitted CBF and arterial transit times using FAIR, a pulsed arterial spin labelling technique.

FAIR, an arterial spin labelling technique, provides non-invasive, quantitative CBF values and arterial transit times deltat. This paper focuses on the negative impact of CSF partial voluming on FAIR results. To understand and solve this problem, we performed a theoretical analysis and a range of simulations. We then acquired FAIR data from a volunteer to illustrate our findings. We found that the determinant effect of CSF is a delayed zero-crossing during inversion recovery. The subtraction of magnitude inversion recovery data in FAIR generates erroneous negative data and distorted fit results: we simulated that for CSF percentages of 0-40%, CBF and deltat will be progressively overestimated by up to 50%. For higher CSF percentages the errors were found to increase steeply. We explored a straightforward solution: taking the magnitude of the FAIR data before fitting. This provided a remarkably strong antidote against the effects of CSF partial voluming: for CSF percentages of 0-40%, simulations now gave CBF values accurate within 1%, and deltat within 5%. The fit remained robust for high CSF fractions. Our analysis and simulations demonstrate that using magnitude FAIR data minimises the detrimental effects of CSF partial voluming. Data from a healthy volunteer illustrate these results.

Artifacts↗
PubMed2004

Optimisation of the 3D MDEFT sequence for anatomical brain imaging: technical implications at 1.5 and 3 T.

An algorithm for the optimisation of 3D Modified Driven Equilibrium Fourier Transform (MDEFT) sequences for T1-weighted anatomical brain imaging is presented. Imaging parameters are optimised for a clinical whole body scanner and a clinical head scanner operating at 1.5 and 3 T, respectively. In vivo studies show that the resulting sequences allow for the whole brain acquisition of anatomical scans with an isotropic resolution of 1 mm and high contrast-to-noise ratio (CNR) in an acceptable scan time of 12 min. Typical problems related to the scanner-specific hardware configurations are discussed in detail, especially the occurrence of flow artefacts in images acquired with head transmit coils and the enhancement of scalp intensities in images acquired with phased array receive coils. It is shown both theoretically and experimentally that these problems can be avoided by using spin tagging and fat saturation.

Adult↗
PubMed2003

Optimized EPI for fMRI studies of the orbitofrontal cortex.

A common problem in gradient-echo echo planar imaging (EPI) is the occurrence of image distortions and signal losses caused by susceptibility gradients near air/tissue interfaces. Since EPI is frequently used for functional magnetic resonance imaging experiments based on the blood oxygenation level-dependent effect, functional studies of certain brain regions affected by susceptibility gradients, such as the temporal lobes and the orbitofrontal cortex, may be compromised. In this work a method for signal recovery in certain regions of the orbitofrontal cortex is presented. The influence of in-plane susceptibility gradients is reduced by optimization of the imaging slice orientation. Through-plane susceptibility gradients are partly compensated by means of a moderate preparation gradient pulse similar to z-shimming. In contrast to several other techniques proposed in the literature for reducing susceptibility effects, this method does not compromise the temporal resolution and is therefore applicable to event-related studies.

Algorithms↗
PubMed2002

RF inhomogeneity compensation in structural brain imaging.

Three-dimensional T(1)-weighted magnetization-prepared rapid gradient-echo (MP-RAGE) sequences with centric phase encoding (PE) in the inner loop provide structural brain images with a high spatial resolution and high tissue contrast. A disadvantage of this sequence type is the susceptibility to inhomogeneities of the radiofrequency (RF) coil, which may result in poor image contrast in some peripheral regions. A special excitation pulse is presented which compensates for these effects in both the head/foot and anterior/posterior directions. This pulse has a duration of only 1.3 ms and is thus compatible with the short repetition times (TRs) required for MP-RAGE imaging. It is shown experimentally that images acquired with the compensation pulse may be segmented without using intensity correction algorithms during data postprocessing.

Artifacts↗
PubMed2002

Compensation of susceptibility-induced BOLD sensitivity losses in echo-planar fMRI imaging.

Gradient-echo echo-planar imaging is a standard technique in functional magnetic resonance imaging (fMRI) experiments based on the blood oxygenation level-dependent (BOLD) effect. A major problem is the occurrence of susceptibility gradients near air/tissue interfaces. As a consequence, the detection of neuronal activation may be greatly compromised in certain brain areas, especially in the temporal lobes and in the orbitofrontal cortex. Common approaches to overcome this problem, such as z-shimming or the use of tailored radio frequency pulses, usually compensate only for susceptibility gradients in the slice selection direction. In the present study, the influence of susceptibility gradients in the phase encoding direction is investigated both theoretically and experimentally. It is shown that these gradients influence the effective echo time TE and may reduce considerably the local BOLD sensitivity, even in the case of acceptable image intensities. A compensation method is proposed and tested in an fMRI experiment based on a hypercapnic challenge. The results suggest that the compensation method allows for the detection of activation in brain areas which are usually unavailable for BOLD studies.

Brain↗
PubMed2000

Optimization of 3-D MP-RAGE sequences for structural brain imaging.

An optimized MR sequence for structural three-dimensional brain scans is presented, giving good T(1) contrast and excellent white matter/gray matter segmentation. Modification of the usual linear phase encoding order to centric phase encoding restores the contrast loss, which usually occurs after magnetization preparation during the acquisition process when large volumes are imaged. The deleterious effects on the point-spread function are compensated by means of an appropriate k-space filter. RF coil inhomogeneities are corrected by means of shaped excitation pulses. High contrast-to-noise images of the entire brain with 1 mm isotropic resolution can be obtained in 12 min. The contrast-to-noise-ratio is about 100% higher than for sequences based on linear phase encoding.

Anatomy, Cross-Sectional↗
PubMed1999

Fast T1 mapping on a whole-body scanner.

A method for the fast acquisition of quantitative T1 maps is presented. It is based on the acquisition of a series of snapshot fast low-angle shot (FLASH) images after inversion of the magnetization. A drawback of this method is the necessity for a sufficiently long relaxation delay before each inversion pulse, leading to long experimental times if averaging or segmentation is required. Thus implementation of this method on a whole-body scanner is problematic, as the longer gradient rise times provide prolonged repetition times, which makes segmentation indispensable to provide a sufficient temporal resolution. We present a modification of the method that allows for reduction in the intermediate relaxation delays and thus for considerably reduced experimental durations.

Artifacts↗
PubMed1999

Signal intensities in FLASH-EPI-hybrid sequences.

Theoretical considerations on the signal-to-noise ratio (SNR) in FLASH-EPI-Hybrid imaging were published previously. The purpose of this work was to investigate in vivo the signal intensities in Hybrid images as a function of sequence specific parameters. In detail, the SNR as a function of the number of echoes m per RF excitation, the excitation flip angle alpha, and the dependence on the tissue relaxation times T1 and T2* were studied. In eight healthy subjects brain and abdominal Hybrid images were acquired where m and alpha were changed independently. Signal intensities in human brain, liver, and kidney were evaluated for each Hybrid experiment. Additionally, T1 and T2* values of these tissue types were quantified to allow for a comparison with the theory. An excellent agreement between calculated and measured signal behavior was found. The theory was therefore validated in vivo and can thus be used to optimize the signal-to-noise in Hybrid experiments.

Brain↗
PubMed1998

Functional magnetic resonance imaging in intact plants--quantitative observation of flow in plant vessels.

Quantitative magnetic resonance (MR) images of flow velocities in intact corn plants were acquired using magnetization-prepared MR microscopy. A phase contrast flow imaging technique was used to quantitate water flow velocities and total volume flow rates in small xylem vessels. The simultaneous measurement of the transpiration of the whole plant was achieved by using a closed climate chamber within the MR magnet. The total volume flow rate and the transpiration values were in close correlation. Functional magnetic resonance imaging in intact plants was performed by light stimulation of the transpiration inside of the magnet. The change in the flow velocities in the xylem vessels of single vascular bundles was in correlation with the changes in the transpiration. Significant differences were observed between the xylem vessels in different vascular bundles. Furthermore, flow velocity measurements were performed on excised plant stems and visualized by the uptake of the MR contrast agent, gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA). A comparison between the phase contrast flow imaging and the contrast media uptake showed to be in good agreement with each other.

Hydroponics↗
PubMed1997

Quantitative magnetic resonance imaging of capillary water permeability and regional blood volume with an intravascular MR contrast agent.

A novel method is presented to simultaneously measure the permeability surface area product of water (PS), also known as capillary diffusion capacity, and the regional blood volume (RBV). It is based on magnetic resonance imaging of the longitudinal relaxation times of tissue and blood at different concentrations of an intravascular MR contrast agent. PS and RBV were measured in vivo in different regions of the brain and the skeletal muscle of the rat. The average PS values (n = 5) obtained in cerebral cortex, corpus callosum, hippocampus, thalamus, jaw muscle, and tongue muscle were 3.31 +/- 0.20, 1.81 +/- 0.25, 3.37 +/- 0.36, 3.68 +/- 0.44, 10.6 +/- 1.1, and 14.1 +/- 2.51 ml x min(-1) x g(-1), respectively. The corresponding average RBV values were 1.63 +/- 0.18, 1.22 +/- 0.25, 3.30 +/- 0.37, 3.03 +/- 0.36, 1.66 +/- 0.30, and 1.38 +/- 0.33 ml x 100 g(-1). These results are in good agreement with previously reported literature values obtained by means of autoradiography.

Animals↗
PubMed1997

Perfluoro-15-crown-5-ether labelled macrophages in adoptive transfer experimental allergic encephalomyelitis.

In this serial in vivo study, macrophages labelled with perfluoro-15-crown-5-ether (15C5) were monitored in rats after inducing adoptive transfer experimental allergic encephalomyelitis (AT-EAE). AT-EAE is an animal model of multiple sclerosis and is characterized by inflammatory infiltrates in the central nervous system (CNS) and breakdown of the blood-brain-barrier. A particular feature of AT-EAE are macrophage infiltrates. Purpose of this study was to monitor the invasive and evasive phase of the macrophages in AT-EAE by using 3-dimensional 19F magnetic resonance imaging (3D 19F-MRI). In the early stage of the disease, a much stronger 19F-signal intensity was observed in AT-EAE-rats than in healthy control rats in the tissue adjacent to CNS regions severely affected by inflammatory infiltrates, and thereafter the 19F-signal intensity was decreasing over the time. However, no 19F-signal could be observed in the CNS itself neither in AT-EAE-rats nor in control rats. According to these findings it is assumed that we monitored the evasion of the macrophages from the region of inflammation.

Animals↗
PubMed1996

Partial inhibition of AT-EAE by an antibody to ICAM-1: clinico-histological and MRI studies.

The role of quantitative proton magnetic resonance imaging (MRI) for the evaluation of immunopathological lesions in the CNS was studied in adoptively transferred experimental allergic encephalomyelitis (AT-EAE). We utilized a recently established treatment model, inhibition of the cell adhesion molecule ICAM-1 by the monoclonal antibody 1A-29. The animals were scanned on days 3, 5 and 7 after injection of encephalitogenic T-cells, before and after bolus injection of Gd-DTPA by performing T1-measurements to assess the integrity of the blood-brain barrier (BBB). On day 7, immunohistochemistry was performed looking for T-cells, activated macrophages, and albumin staining. There was clinical evidence of partial inhibition of AT-EAE in rats treated with antibodies against ICAM-1. This finding was in line with a significantly reduced number of T-cells in the medulla. However, the number of activated macrophages and the distribution of albumin did not differ from untreated AT-EAE animals. The histological findings are in agreement with the MRI data before and after Gd-DTPA injection which were similar in treated and untreated AT-EAE rats on day 3 and 5. On day 7 after Gd-DTPA injection there was evidence of a delayed breakdown of the BBB in the treated rats. The observation of a dissociation of clinical and MRI findings, especially evidence of Gd-enhancement despite clinical improvement, may be important in the context of interpreting MRI studies in MS patients in treatment trials.

Animals↗
PubMed1995

Compensation of diffusion effects in T2 measurements.

Measurements of the transverse relaxation time T2 are usually conducted with the Carr Purcell Meiboom Gill (CPMG) pulse sequence, which causes T2-weighted magnetization. Diffusion effects are a common source of error in measurements of this kind, because the incoherent motion of spins in external magnetic field gradients distorts T2 weighting of the transverse magnetization. As a result, inaccurate T2-values are obtained. In this work, we present a method which completely compensates for the effect of diffusion.

Diffusion↗
PubMed1995

Calculation of signal intensities in hybrid sequences for fast NMR imaging.

A number of techniques that recently have been used for fast NMR-imaging are based on a hybrid sequence of echo planar imaging (EPI) and FLASH imaging: after each NMR excitation several k-space lines are measured. The complete k-space is covered by performance of several excitations. It has been observed that there is usually an optimal hybrid sequence that maximizes the signal-to-noise ratio. In this work, a method is presented that allows a determination of the optimal sequence as a function of the relaxation times T1 and T2*.

Echo-Planar Imaging↗
PubMed1995

In vivo measurement of partial oxygen pressure in large vessels and in the reticuloendothelial system using fast 19F-MRI.

Quantitative in vivo 19F-MRI was performed in a rat model to monitor partial oxygen pressure (pO2) using a perfluorocarbon (PFC) emulsion as contrast agent. On Days 1, 4, and 8 postinjection of the PFC emulsion, transaxial T1 and pO2 maps were acquired of the abdomen of rats that were consecutively ventilated with pure oxygen, air, and a mixture of 10% oxygen and 90% nitrogen. The images had a resolution of 0.75 mm x 0.75 mm x 2 mm and a total acquisition time of 24 min. In these images it was possible to distinguish between different vessels and hepatic and splenic tissue in the selected imaging plane. Serial 19F-MRI measurements on the different days postinjection of the PFC allowed to determine separately the pO2 of arterial and venous blood and the intracellular pO2 in macrophages of the liver and spleen.

Animals↗