Rapid whole-brain quantitative MT imaging

PURPOSE To provide a robust whole-brain quantitative magnetization transfer (MT) imaging method that is not limited by long acquisition times. METHODS Two variants of a spiral 2D interleaved multi-slice spoiled gradient echo (SPGR) sequence are used for rapid quantitative MT imaging of the brain at 3 T. A dual flip angle, steady-state prepared, double-contrast method is used for combined B$_{1}$ and-T$_{1}$ mapping in combination with a single-contrast MT-prepared acquisition over a range of different saturation flip angles (50 deg to 850 deg) and offset frequencies (1 kHz and 10 kHz). Five sets (containing minimum 6 to maximum 18 scans) with different MT-weightings were acquired. In addition, main magnetic field inhomogeneities (ΔB$_{0}$) were measured from two Cartesian low-resolution 2D SPGR scans with different echo times. Quantitative MT model parameters were derived from all sets using a two-pool continuous-wave model analysis, yielding the pool-size ratio, F, their exchange rate, k$_{f}$, and their transverse relaxation time, T$_{2r}$. RESULTS Whole-brain quantitative MT imaging was feasible for all sets with total acquisition times ranging from 7:15 min down to 3:15 min. For accurate modeling, B$_{1}$-correction was essential for all investigated sets, whereas ΔB$_{0}$-correction showed limited bias for the observed maximum off-resonances at 3 T. CONCLUSION The combination of rapid B$_{1}$-T$_{1}$ mapping and MT-weighted imaging using a 2D multi-slice spiral SPGR research sequence offers excellent prospects for rapid whole-brain quantitative MT imaging in the clinical setting

Expression of aquaporins in Xenopus laevis oocytes and glial cells as detected by diffusion-weighted 1H NMR spectroscopy and photometric swelling assay

AbstractExpression of aquaporins (AQP) and water permeability were studied in Xenopus laevis oocytes and immobilized glial cells by a pulsed-field gradient spin echo NMR technique and a photometric swelling assay. Oocytes injected with poly(A) RNA from C6-BU-1 cells showed increased swelling behavior under hypoosmotic stress due to expressed water channels as compared to control oocytes. The swelling could be reversibly inhibited by HgCl2. Furthermore, the intracellular relaxation time and the apparent intracellular diffusion coefficient of water in oocytes were determined by diffusion-weighted 1H NMR experiments to be T2=36 ms and Dapp,intra=0.18×10−3 mm2/s. In immobilized C6 and F98 cells the mean exchange time of intracellular water was found to be 51 ms which increased to 75 ms upon chronic treatment (4 days) in hypertonic medium. Additional hybrid depletion experiments with antisense oligonucleotides directed against AQP1 were performed on oocytes and C6 cells. Moreover, different water channel subtypes of glial cells were assessed by a reverse transcriptase polymerase chain reaction assay. With this, the mRNA encoding AQP1 could be detected in primary cultures and glial cell lines, whereas AQP4 mRNA was found in astroglia-rich primary cultures, but not in F98 and C6 cells. Our results show that water permeability in glial cells is mainly mediated by water channels which play an important role in the regulation of water flow in brain under normal and pathological conditions

New Techniques and Instrumentation – TEXUS Service Module (TSM)

Until the TEXUS-42 (EML-1) project, successfully launched in Dec. 2005, the payload of the TEXUS and the MAXUS were equipped with the former Kayser-Threde 12 bit based PCM data acquisition system. To fulfill experimental requirements for higher data resolution and the intention to reduce weight and also to improve the performance of the service module, ESA has taken initiative to contract industry for the development and built up of a new data acquisition system and the new TEXUS Service Module (TSM) in 2004. For the design, manufacturing and qualification task sharing, a cooperation of DLR Moraba and the Kayser-Threde GmbH has been initialized. With respect to the compatibility of already existing experiment modules, Kayser-Threde has developed, manufactured and qualified the decentralized 16 bit CTS 3000 (Compact Telemetry System) data acquisition system and together with DLR Moraba the TSM. In order to improve existing systems and to comply with new requirements the DLR/Moraba has designed a new power distribution and a GPS system. The TSM is incorporating all known standard features, modern technologies and is capable of serving actual and future experiment requirements. The TSM provides flexibility for future implementation of up to two digital TV respectively TM down links besides the three standard analog TV down links. The design implies economic technical concepts consuming a minimum of service module mass and length. The service module acquires and transmits all experimental and service system housekeeping data via telemetry transmitter to ground. Commands to the service system and for experiment control are received with a dedicated diversity system from the ground station and distributed onboard. Furthermore three TV down links, 3-axis micro-g and acceleration measurement as well as a rate control (RCS) and a GPS system are incorporated. The TSM is integrated within a standard TEXUS cylindrical structure with Radax flanges on both ends. Most of the components are assembled on the instrumentation deck, which is fixated via shock mounts to the outer structure. All electronic boards for TM/TC, RCS, power switching, sequencing, μ-g measurement and housekeeping are integrated and wired within one

Abstract On the nature of the BOLD fMRI contrast mechanism

Since its development about 15 years ago, functional magnetic resonance imaging (fMRI) has become the leading research tool for mapping brain activity. The technique works by detecting the levels of oxygen in the blood, point by point, throughout the brain. In other words, it relies on a surrogate signal, resulting from changes in oxygenation, blood volume and flow, and does not directly measure neural activity. Although a relationship between changes in brain activity and blood flow has long been speculated, indirectly examined and suggested and surely anticipated and expected, the neural basis of the fMRI signal was only recently demonstrated directly in experiments using combined imaging and intracortical recordings. In the present paper, we discuss the results obtained from such combined experiments. We also discuss our current knowledge of the extracellularly measured signals of the neural processes that they represent and of the structural and functional neurovascular coupling, which links such processes with the hemodynamic changes that offer the surrogate signal that we use to map brain activity. We conclude by considering applications of invasive MRI, including injections of paramagnetic tracers for the study of connectivity in the living animal and simultaneous imaging and electrical microstimulation

Liebfritz D. Monitoring of cell volume and water exchange time in perfused cells by diffusion-weighted 1H NMR spectroscopy

ABSTRACT: Diffusion of intracellular water was measured in perfused cells embedded in basement membrane gel threads. F98 glioma cells, primary astrocytes, and epithelial KB cells were used and were exposed to osmotic stress, immunosuppressiva, the water channel blocker p-chloromercuriobenzenesulfonate (pCMBS), and apoptotic conditions. With diffusion-weighted 1 H NMR spectroscopy changes in the intracellular signal could be monitored and quantified with single signal (ss), constant diffusion time (ct), and constant gradient strength (cg) experiments. The temporal resolution of the ss monitoring was 3.5 s with a standard deviation of 0.5% of the signal intensity and 32 s (3%) with ct monitoring, respectively. A mean intracellular residence time of water was determined with the cg experiment to about 50 ms. Changes of this exchange time from (51.9 AE 1.0) to (59.0 AE 1.1) ms were observed during treatment with pCMBS. The changes in the diffusion attenuated signal could be simulated analytically varying the intracellular volume fraction and exchange time by combination of restricted diffusion (Tanner model) and water exchange (Kärger model). This sensitive and noninvasive NMR method on perfused cells allows to determine changes in the intracellular volume and residence time in a simple and accurate manner. Many further applications as anoxia, volume regulation, ischemia and treatment with various pharmaceuticals are conceiveable to follow up their effect on the cell volume and the exchange time of intracellular water