R Pohmann

A 16-channel dual-row transmit array in combination with a 31-element receive array for human brain imaging at 9.4 T

Arranging transmit array elements in multiple rows provides an additional degree of freedom to correct B1+ field inhomogeneities and to achieve whole‐brain excitation at ultrahigh field strengths. Receive arrays shaped to the contours of the anatomy increase the signal‐to‐noise ratio of the image. In this work, the advantages offered by the transmit and receive array techniques are combined for human brain imaging at 9.4 T.

Optimization of k‐space trajectories for compressed sensing by Bayesian experimental design

The optimization of k‐space sampling for nonlinear sparse MRI reconstruction is phrased as a Bayesian experimental design problem. Bayesian inference is approximated by a novel relaxation to standard signal processing primitives, resulting in an efficient optimization algorithm for Cartesian and spiral trajectories. On clinical resolution brain image data from a Siemens 3T scanner, automatically optimized trajectories lead to significantly improved images, compared to standard low‐pass, equispaced, or variable density randomized designs. Insights into the nonlinear design optimization problem for MRI are given. Magn Reson Med, 2010.

Signal‐to‐noise ratio and MR tissue parameters in human brain imaging at 3, 7, and 9.4 tesla using current receive coil arrays

Relaxation times, transmit homogeneity, signal‐to‐noise ratio (SNR) and parallel imaging g‐factor were determined in the human brain at 3T, 7T, and 9.4T, using standard, tight‐fitting coil arrays.

Human imaging at 9.4 T using T2*-, phase-, and susceptibility-weighted contrast

The effect of susceptibility differences on an MR image is known to increase with field strength. Magnetic field inhomogeneities within the voxels influence the apparent transverse relaxation time T2*, while effects due to different precession frequencies between voxels caused by local field variations are evident in the image phase, and susceptibility‐weighted imaging highlights the veins and deep brain structures. Here, these three contrast mechanisms are examined at a field strength of 9.4 T. The T2* maps generated allow the identification of white matter structures not visible in conventional images. Phase images with in‐plane resolutions down to 130 μm were obtained, showing high gray/white matter contrast and allowing the identification of internal cortical structures. The susceptibility‐weighted images yield excellent visibility of small venous structures and attain an in‐plane resolution of 175 μm. Magn Reson Med, 2011.

Calcium-responsive paramagnetic CEST agents

The assessment of changes in the extracellular calcium concentration by magnetic resonance imaging would be a valuable biomedical research tool to monitor brain neuronal activity. In this perspective, we report here the synthesis of novel ligands consisting of tetraamide and bisamide derivatives of cyclen, L(1) and L(2), respectively, each bearing imino(diacetate) moieties for Ca(2+) binding. Yb(3+) and Eu(3+) complexes are investigated as chemical exchange saturation transfer (CEST) agents that respond to the presence of Ca(2+). A CEST effect is observed for both YbL(1) and EuL(1) complexes (B=11.7T), originating from the slow exchange of the amide protons and those of the coordinated water, respectively, whilst no CEST is detected for complexes of L(2). Upon calcium binding, the CEST effect decreases considerably (from 60% to 20% for YbL(1) and from 35% to 10% for EuL(1)). A similar variation is observed in the presence of Mg(2+). The affinity constants between the lanthanide complexes and the alkaline earth metal ions have been estimated from the variation of the CEST effect to be K(YbL(1)-Ca)(aff) = 8 ± 2M(-1), K(YbL(1)-Mg)(aff) = 23 ± 3M(-1) and K(EuL(1)-Ca)(aff) = 10 ± 3M(-1). These low values imply the coordination of the alkaline earth ions to a single iminodiacetate arm. Ca(2+)/Mg(2+) binding to the lanthanide complexes slows down the exchange of the amide protons on YbL(1) which is responsible for the diminished CEST effect. This has been evidenced by assessing the proton exchange rates from the dependency of the CEST effect on the saturation time and the saturation power, in the absence and in the presence of Ca(2+) and Mg(2+). The applicability of the PARACEST MRI agents for Ca(2+) detection has been evaluated on a 16T MRI scanner.

Cell-Penetrating Peptides and Peptide Nucleic Acid-Coupled MRI Contrast Agents: Evaluation of Cellular Delivery and Target Binding

Molecular imaging of cells and cellular processes can be achieved by tagging intracellular targets such as receptors, enzymes, or mRNA. Seeking to visualize the presence of specific mRNAs by magnetic resonance (MR) imaging, we coupled peptide nucleic acids (PNA) with gadolinium-based MR contrast agents using cell-penetrating peptides for intracellular delivery. Antisense to mRNA of DsRed2 protein was used as proof of principle. The conjugates were produced by continuous solid-phase synthesis followed by chelation with gadolinium. Their cellular uptake was confirmed by fluorescence microscopy and spectroscopy as well as by MR imaging of labeled cells. The cell-penetrating peptide D-Tat(57-49) was selected over two other derivatives of HIV-1 Tat peptide, based on its superior intracellular delivery of the gadolinium-based contrast agents. Further improved delivery of conjugates was achieved upon coupling peptide nucleic acids (antisense to mRNA of DsRed2 protein and nonsense with no natural counterpart). Significant enhancement in MR contrast was obtained in cells labeled with concentrations as low as 2.5 μM of these agents. Specific binding of the targeting PNA containing conjugate to its complementary oligonucleotide sequence was proven by in vitro cell-free assay. In contrast, a lack of specific enrichment was observed in transgenic cells containing the target due to nonspecific vesicular entrapment of contrast agents. Preliminary biodistribution studies showed conjugate-related fluorescence in several organs, especially the liver and bladder, indicating high mobility of the agent in spite of its high molecular weight. No conjugate related toxicity was observed. These results are encouraging, as they warrant further molecular optimization and consecutive specificity studies in vivo of this new generation of contrast agents.

Synthesis and Characterization of a Cell-Permeable Bimodal Contrast Agent Targeting β-Galactosidase

Noninvasive monitoring of intracellular targets such as enzymes, receptors, or mRNA by means of magnetic resonance imaging (MRI) is increasingly gaining relevance in various research areas. A vital prerequisite for their visualization is the development of cell-permeable imaging probes, which can specifically interact with the target that characterizes the cellular or molecular process of interest. Here, we describe a dual-labeled probe, Gd-DOTA-k(FR)-Gal-CPP, designed to report the presence of intracellular β-galactosidase (β-gal) enzyme by MRI. This conjugate consists of a galactose based core serving as cleavable spacer, incorporated between the cell-penetrating peptide D-Tat(49-57) and reporter moieties (Gd-DOTA, fluorescein (FR)). We employed a facile building block approach to obtain our bimodal probe, Gd-DOTA-k(FR)-Gal-CPP. This strategy involved the preparation of the building blocks and their subsequent assembly using Fmoc-mediated solid phase synthesis, followed by the complexation of ligand 14 with GdCl(3). Gd-DOTA-k(FR)-Gal-CPP showed a considerably higher relaxivity enhancement (16.8±0.6 mM(-1)s(-1), 123 MHz, ∼21°C) relative to the commercial Gd-DOTA (4.0±0.12 mM(-1)s(-1), 123MHz, ∼21 °C). The activation of Gd-DOTA-k(FR)-Gal-CPP was based on a cellular retention strategy that required enzymatic cleavage of the delivery vector from galactose moiety following the cell internalization to achieve a prolonged accumulation of the reporter components (Gd-DOTA/FR) in the β-gal expressing cells. Cellular uptake of Gd-DOTA-k(FR)-Gal-CPP in β-gal expressing C6/LacZ and enzyme deficient parental C6 rat glioma cells was confirmed by fluorescence spectroscopy, MR imaging and ICP-AES measurements. All methods showed higher accumulation of measured reporters in C6/LacZ cells compared to enzyme deficient parental C6 cells. Fluorescence microscopy of cells labeled with Gd-DOTA-k(FR)-Gal-CPP indicated a predominantly vesicular localization of the green fluorescent conjugate around cell nuclei. This cellular distribution was most likely responsible for the observed non-specific background signal in the enzyme deficient C6 cells. Even though the specific accumulation of our bimodal probe has to be further improved, it could be already used for cell imaging by MRI and optical modalities.

Interleaved TMS/CASL: Comparison of different rTMS protocols

Continuous Arterial Spin Labeling (CASL) offers the possibility to quantitatively measure the regional cerebral blood flow (rCBF). We demonstrate, for the first time, the feasibility of interleaving Transcranial Magnetic Stimulation (TMS) with CASL at 3 T. Two different repetitive TMS (rTMS) protocols were applied to the motor cortex in 10 subjects and the effect on rCBF was measured using a CASL sequence with separate RF coils for labeling the inflowing blood. Each subject was investigated, using a block design, under 7 different conditions: continuous 2 Hz rTMS (3 intensities: 100%, 110% and 120% resting motor threshold [MT]), short 10 Hz rTMS trains at 110% MT (8 pulses per train; 3 different numbers of trains per block with 2, 4 and 12 s intervals between trains) and volitional movement (acoustically triggered by 50% MT stimuli). We show robust rCBF increases in motor and premotor areas due to rTMS, even at the lowest stimulation intensity of 100% MT. RCBF exhibited a linear positive dependency on stimulation intensity (for continuous 2 Hz rTMS) and the number of 10 Hz trains in the stimulated M1/S1 as well as in premotor and supplementary motor areas. Interestingly, the 2 different rTMS protocols yielded markedly different rCBF activation time courses, which did not correlate with the electromyographic recordings of the muscle responses. In future, this novel combination of TMS with ASL will offer the possibility to investigate the immediate and after-effects of rTMS stimulation on rCBF, which previously was only possible using PET.

New coil positioning method for interleaved transcranial magnetic stimulation (TMS)/functional MRI (fMRI) and its validation in a motor cortex study

To develop and test a novel method for coil placement in interleaved transcranial magnetic stimulation (TMS)/functional MRI (fMRI) studies.

Functional MRI in human subjects with gradient‐echo and spin‐echo EPI at 9.4 T

The increased signal‐to‐noise ratio and blood oxygen level dependent signal at ultra‐high field can only help to boost the resolution in functional MRI studies if the spatial specificity of the activation signal is improved. At a field strength of 9.4 T, both gradient‐echo and spin‐echo based echo‐planar imaging were implemented and applied to investigate the specificity of human functional MRI. A finger tapping paradigm was used to acquire functional MRI data with scan parameters similar to standard neuroscientific applications.