for Journals by Title or ISSN
for Articles by Keywords
help
  Subjects -> COMMUNICATIONS (Total: 308 journals)
    - AUDIO/VIDEO RECORDING AND REPRODUCTION (13 journals)
    - COMMUNICATIONS (258 journals)
    - DIGITAL AND WIRELESS COMMUNICATION (19 journals)
    - MEETINGS AND CONGRESSES (7 journals)
    - RADIO, TELEVISION AND CABLE (11 journals)

COMMUNICATIONS (258 journals)            First | 1 2 3     

The end of the list has been reached. Please navigate to previous pages.

  First | 1 2 3     

Journal Cover Magnetic Resonance Materials in Physics, Biology and Medicine
   Journal TOC RSS feeds Export to Zotero [3 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0968-5243 - ISSN (Online) 1352-8661
     Published by Springer-Verlag Homepage  [2210 journals]   [SJR: 0.821]   [H-I: 36]
  • Development and performance of a 129-GHz dynamic nuclear polarizer in an
           ultra-wide bore superconducting magnet
    • Abstract: Objective We sought to build a dynamic nuclear polarization system for operation at 4.6 T (129 GHz) and evaluate its efficiency in terms of 13C polarization levels using free radicals that span a range of ESR linewidths. Materials and methods A liquid helium cryostat was placed in a 4.6 T superconducting magnet with a 150-mm warm bore diameter. A 129-GHz microwave source was used to irradiate 13C enriched samples. Temperatures close to 1 K were achieved using a vacuum pump with a 453-m3/h roots blower. A hyperpolarized 13C nuclear magnetic resonance (NMR) signal was detected using a saddle coil and a Varian VNMRS console operating at 49.208 MHz. Samples doped with free radicals BDPA (1,3-bisdiphenylene-2-phenylallyl), trityl OX063 (tris{8-carboxyl-2,2,6,6-benzo(1,2-d:4,5-d)-bis(1,3)dithiole-4-yl}methyl sodium salt), galvinoxyl ((2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy), 2,2-diphenylpicrylhydrazyl (DPPH) and 4-oxo-TEMPO (4-Oxo-2,2,6,6-tetramethyl-1-piperidinyloxy) were assayed. Microwave dynamic nuclear polarization (DNP) spectra and solid-state 13C polarization levels for these samples were determined. Results 13C polarization levels close to 50 % were achieved for [1-13C]pyruvic acid at 1.15 K using the narrow electron spin resonance (ESR) linewidth free radicals trityl OX063 and BDPA, while 10–20 % 13C polarizations were achieved using galvinoxyl, DPPH and 4-oxo-TEMPO. Conclusion At this field strength free radicals with smaller ESR linewidths are still superior for DNP of 13C as opposed to those with linewidths that exceed that of the 1H Larmor frequency.
      PubDate: 2014-08-14
       
  • Artefacts in 1H
           NMR-based metabolomic studies on cell cultures
    • Abstract: Object Metabolomic studies on cultured cells involve assays of cell extracts and culture medium, both of which are often performed by 1H NMR. Cell culture is nowadays performed in plastic dishes or flasks, and the extraction of metabolites from the cells is typically performed with perchloric acid, methanol–chloroform, or acetonitrile, ideally while the cells are still adherent to the culture dish. We conducted this investigation to identify contaminants from cell culture plasticware in metabolomic studies. Materials and methods Human diploid fibroblasts (IMR90) (n = 6), HeLa cells (n = 6), and transformed astrocytes with HIF-1 knockout (Astro-KO) (n = 6) were cultured. Cells were seeded in 100 mm Petri dishes with 10 ml complete growth medium (Dulbecco’s minimum essential medium) containing 10 % foetal bovine serum (FBS). Cell cultures were incubated at 37 °C in 5 % CO2 for approximately 3 days. Metabolites were extracted by use of a perchloric acid procedure. 1H NMR spectroscopy was used for metabolite analysis. “Null sample” (i.e. cell-free) experiments were performed by either rinsing dishes with medium or incubating the medium in Petri dishes from five different manufacturers for 72 h and then by performing a dummy “extraction” of each Petri dish by the perchloric acid, methanol–chloroform, or acetonitrile procedures. Principal components analysis was used for classification of samples and to determine the contaminants arising from plasticware. Results We found that even brief rinsing of cell culture plasticware with culture medium elutes artefactual chemicals, the 1H NMR signals of which could confound assays of acetate, succinate, and glycolate. Incubation of culture medium in cell-culture dishes for 72 h (as in a typical cell-culture experiment) followed by perchloric extraction in the dishes enhanced elution of the artefacts. These artefacts were present, but somewhat less pronounced, in the 1H NMR spectra of null samples extracted with methanol and acetonitrile. Ethanol, lactate, alanine, fructose, and fumarate signals that appear in the 1H NMR spectrum of the unused (pure) medium originate from FBS. Conclusions Plastic Petri dishes from five different manufacturers gave rise to essentially identical artefactual peaks. Use of a pH indicator to assist neutralisation introduced still more artefactual signals in the aromatic region, as well as methanol and ethanol signals. Methanol and acetonitrile extracts also contained artefacts arising from the plasticware, although the amounts were less than in the perchloric acid extracts. Finally, we provide suggestions for minimizing these artefacts. The best practice would be to run a “null” extraction with every batch of cellular metabolomics experiments to test for contamination and to provide a “background” spectrum.
      PubDate: 2014-08-10
       
  • An in vivo comparison of the DREAM sequence with current RF shim
           technology
    • Abstract: Object In the present study the performance of the dual refocusing echo acquisition mode (DREAM) B1 + mapping sequence is evaluated for RF shimming in the abdomen at 3 T and validated against existing RF shim technology. Materials and methods In vivo experiments were performed on 19 normal volunteers using a clinical 3 T dual channel MRI system. For each volunteer three different B1 + mapping techniques [DREAM, actual flip angle imaging (AFI) and saturated double angle method (SDAM)] were employed for RF shimming of the liver and to subsequently assess the quality of the obtained RF shim settings in terms of the achieved B1 + homogeneity and accuracy of the mean B1 +. Results DREAM-based B1 + calibration led to an average homogeneity improvement of 39.1 % (AFI = 38.7 %, SDAM = 38.1 %) and a mean B1 + of 90.9 % of the prescribed B1 + (AFI = 88.9 %, SDAM = 92.0 %). The duration of the B1 + calibration scan was reduced from 30 s (AFI) and 15 s (SDAM) to 2.5 s (DREAM). Conclusion DREAM accelerates RF shimming of the liver by an order of magnitude without compromising RF shimming performance.
      PubDate: 2014-08-10
       
  • Characterization of metabolites determined by means of        class="a-plus-plus">1H HR MAS NMR in
           intervertebral disc degeneration
    • Abstract: Object The objective of this study is the identification of metabolites by means of 1H high resolution magic angle spinning nuclear magnetic resonance (1H HR MAS NMR) spectroscopy and the evaluation of their applicability in distinguishing between healthy and degenerated disc tissues. Materials and methods Differences between the metabolic profiles of healthy and degenerated disc tissues were studied by means of 1H HR MAS NMR. Analysis was performed for 81 disc tissue samples (control samples n = 21, degenerated disc tissue samples n = 60). Twenty six metabolites (amino acids, carbohydrates, and alcohols) were identified and quantified. Results The results indicate that the metabolic profile of degenerated discs is characterized by the presence of 2-propanol and the absence of scyllo-inositol and taurine. The concentrations of 2-propanol and lactate increase with age. Conclusion PCA analysis of ex vivo 1H HR MAS NMR data revealed the occurrence of two groups: healthy and degenerative disc tissues. The effects of insufficient nutrient supply of discs, leading to their degeneration and back pain, are discussed.
      PubDate: 2014-08-10
       
  • K-t GRAPPA-accelerated 4D flow MRI of liver hemodynamics: influence of
           different acceleration factors on qualitative and quantitative assessment
           of blood flow
    • Abstract: Objective We sought to evaluate the feasibility of k-t parallel imaging for accelerated 4D flow MRI in the hepatic vascular system by investigating the impact of different acceleration factors. Materials and methods k-t GRAPPA accelerated 4D flow MRI of the liver vasculature was evaluated in 16 healthy volunteers at 3T with acceleration factors R = 3, R = 5, and R = 8 (2.0 × 2.5 × 2.4 mm3, TR = 82 ms), and R = 5 (TR = 41 ms); GRAPPA R = 2 was used as the reference standard. Qualitative flow analysis included grading of 3D streamlines and time-resolved particle traces. Quantitative evaluation assessed velocities, net flow, and wall shear stress (WSS). Results Significant scan time savings were realized for all acceleration factors compared to standard GRAPPA R = 2 (21–71 %) (p < 0.001). Quantification of velocities and net flow offered similar results between k-t GRAPPA R = 3 and R = 5 compared to standard GRAPPA R = 2. Significantly increased leakage artifacts and noise were seen between standard GRAPPA R = 2 and k-t GRAPPA R = 8 (p < 0.001) with significant underestimation of peak velocities and WSS of up to 31 % in the hepatic arterial system (p <0.05). WSS was significantly underestimated up to 13 % in all vessels of the portal venous system for k-t GRAPPA R = 5, while significantly higher values were observed for the same acceleration with higher temporal resolution in two veins (p < 0.05). Conclusion k-t acceleration of 4D flow MRI is feasible for liver hemodynamic assessment with acceleration factors R = 3 and R = 5 resulting in a scan time reduction of at least 40 % with similar quantitation of liver hemodynamics compared with GRAPPA R = 2.
      PubDate: 2014-08-07
       
  • Cylinders or walls' A new computational model to estimate the MR
           transverse relaxation rate dependence on trabecular bone architecture
    • Abstract: Objective Bone density is distributed in a complex network of interconnecting trabecular plates and rods that are interspersed with bone marrow. A computational model to assess the dependence of the relaxation rate on the geometry of bone can consider the distribution of bone material in the form of two components: cylinders and open walls (walls with gaps). We investigate whether the experimentally known dependence of the transverse relaxation rate on the trabecular bone structure can be usefully interpreted in terms of these two components. Materials and methods We established a computer model based on an elementary computational cell. The model includes a variable number of open walls and infinitely long cylinders as well as multiple geometric parameters. The transverse relaxation rate is computed as a function of these parameters. Within the model, increasing the trabecular spacing with a fixed trabecular radius is equivalent to thinning the trabeculae while maintaining constant spacing. Results Increasing the number of cylinder and wall gap elements beyond their nearest neighbors does not change the transverse relaxation rate. Although the absolute contribution to the relaxation due to open walls is on average more important than that due to cylinders, the latter drops off rapidly. The change on transverse relaxation rate is larger for changing cylinder geometry than for changing wall geometry, as it can be seen from the effect on the relaxation rate when trabecular spacing is varied, compared to varying the size of wall gaps. Conclusion Our results provide strong evidence that trabecular thinning, which is associated with increasing age, decreases the relaxation rates. The effect of thinning plates and rods on the transverse relaxation can be understood in terms of simple cylinders and open walls. A reduction in the relaxation rate can be seen as an indication of thinning cylinders, corresponding to reduced bone stability and ultimately, osteoporosis.
      PubDate: 2014-08-01
       
  • Measuring liver triglyceride content in mice: non-invasive magnetic
           resonance methods as an alternative to histopathology
    • Abstract: Object Quantitative assessment of liver fat is highly relevant to preclinical liver research and should ideally be performed non-invasively. This study aimed to compare three non-invasive Magnetic Resonance (MR) and two histopathological methods against the reference standard of biochemically determined liver triglyceride content (LTC). Materials and methods A total of 50 mice [21 C57Bl/6OlaHsd mice (C57Bl/6), nine low-density lipoprotein (LDL) receptor knock-out −/− (LDL −/−) mice and 20 C57BL/6 mice] received either a high-fat, high-fat-high-cholesterol or control diet, respectively. Mice were examined 4, 8 or 12 weeks into the diet using MR [1H-MR Spectroscopy, Proton Density Fat Fraction (PDFF), mDixon] and histopathological methods (visual scoring or digital image analysis (DIA) of Oil-Red-O (ORO) stained liver sections). Correlations [Pearson’s coefficient (r)] were studied with respect to LTC. Results Microvesicular steatosis was seen in 42/50 mice. 1H-MRS values showed normal to moderately elevated liver fat content. Visual scoring and DIA of ORO-sections correlated moderately with LTC at r = 0.59 and r = 0.49 (P < 0.001), respectively. 1H-MRS, PDFF and mDixon correlated significantly better, at r = 0.74, r = 0.75 and r = 0.82, respectively. Conclusion Non-invasively determined MR measures of normal to moderately elevated liver fat in mice had a higher correlation with LTC than invasive histopathological measures. Where available, MR is the preferred method for fat quantification.
      PubDate: 2014-08-01
       
  • A preliminary study on the effects of acute ethanol ingestion on default
           mode network and temporal fractal properties of the brain
    • Abstract: Object To study the effect of acute alcohol intoxication on the functional connectivity of the default mode network (DMN) and temporal fractal properties of the healthy adult brain. Materials and methods Eleven healthy male volunteers were asked to drink 0.59 g/kg of ethanol. Resting state blood oxygen level dependent (rsBOLD) MRI scans were obtained before consumption, 60 min post-consumption and 90 min post-consumption. Before each rsBOLD scan, pointed-resolved spectroscopy (PRESS) 1H-MRS (magnetic resonance spectroscopy) scans were acquired to measure ethanol levels in the right basal ganglia. Results Significant changes in DMN connectivity were found following alcohol consumption (p < 0.01). Both increased and decreased regional connectivity were found after 60 min, whereas mostly decreased connectivity was found after 90 min. The fractal behaviour of the rsBOLD signal, which is believed to help reveal complexity of small-scale neuronal circuitry, became more ordered after both 60 and 90 min of alcohol consumption (p < 0.01). Conclusion The DMN has been linked to personal identity and social behavior. As such, our preliminary findings may provide insight into the neuro-functional underpinnings of the cognitive and behavioral changes observed during acute alcohol intoxication. The reduced fractal dimension implies a change in function of small-scale neural networks towards less complex signaling.
      PubDate: 2014-08-01
       
  • Longitudinal sensitivity to change of MRI-based muscle cross-sectional
           area versus isometric strength analysis in osteoarthritic knees with and
           without structural progression: pilot data from the Osteoarthritis
           Initiative
    • Abstract: Object Biomechanical measurement of muscle strength represents established technology in evaluating limb function. Yet, analysis of longitudinal change suffers from relatively large between-measurement variability. Here, we determine the sensitivity to change of magnetic resonance imaging (MRI)-based measurement of thigh muscle anatomical cross sectional areas (ACSAs) versus isometric strength in limbs with and without structural progressive knee osteoarthritis (KOA), with focus on the quadriceps. Materials and methods Of 625 “Osteoarthritis Initiative” participants with radiographic KOA, 20 had MRI cartilage and radiographic joint space width loss in the right knee isometric muscle strength measurement and axial T1-weighted spin-echo acquisitions of the thigh. Muscle ACSAs were determined from manual segmentation at 33 % femoral length (distal to proximal). Results In progressor knees, the reduction in quadriceps ACSA between baseline and 2-year follow-up was −2.8 ± 7.9 % (standardized response mean [SRM] = −0.35), and it was −1.8 ± 6.8 % (SRM = −0.26) in matched, non-progressive KOA controls. The decline in extensor strength was more variable than that in ACSAs, both in progressors (−3.9 ± 20 %; SRM = −0.20) and in non-progressive controls (−4.5 ± 28 %; SRM = −0.16). Conclusion MRI-based analysis of quadriceps muscles ACSAs appears to be more sensitive to longitudinal change than isometric extensor strength and is suggestive of greater loss in limbs with structurally progressive KOA than in non-progressive controls.
      PubDate: 2014-08-01
       
  • Quantitative biodistribution and pharmacokinetics of multimodal
           gadolinium-based nanoparticles for lungs using ultrashort TE MRI
    • Abstract: Objective To study the biodistribution and lung pharmacokinetics of tracheally administered gadolinium-based contrast agents [gadoteric acid and multimodal ultra-small rigid platforms (USRPs)], to validate their pharmacokinetics against optical imaging of fluorescent USRPs, and to test their short-term toxicity. Materials and methods Ultrashort echo-time (UTE) lung proton magnetic resonance imaging (MRI) was performed at 4.7-Tesla (T) after the intratracheal instillation of different concentrations of contrast agent solutions in mice. Pharmacokinetic models were implemented on the absolute concentration calculated from the MRI signal enhancement measurements. Fluorescent USRPs were used to obtain optical images with the same protocol. Bronchoalveolar lavage inflammatory cell count and serum creatinine measurement were performed on four groups of instilled mice (sham, saline, USRPs, lipopolysaccharide). Results MR and optical imaging showed similar kinetics of the USRPs, passing from the airways to the lung tissue and to the kidneys, with negligible hepatic clearance. No significant increase of lung and renal inflammation markers were observed in USRP-instilled animals. Conclusion A T 1-weighted radial UTE sequence was found to be valuable in quantitatively monitoring the biodistribution and pharmacokinetics of nanoparticles in the lungs of mice. The observed favorable pharmacokinetics, which was validated by fluorescence imaging, ensures the negligible toxicity of the nanoprobes, making the USRPs and the developed protocol good candidates for applications on selected lung diseases.
      PubDate: 2014-08-01
       
  • Association of white matter deficits with clinical symptoms in
           antipsychotic-naive first-episode schizophrenia: an optimized VBM study
           using 3T
    • Abstract: Object To examine the whole brain white matter morphology in antipsychotic-naive patients with first-episode schizophrenia (FES) and its correlations with symptom severity. Materials and methods High-resolution T1-weighted images of 64 drug-naive FES patients and 64 matched healthy controls were acquired using a 3 T MR imaging system. Then, optimized voxel-based morphometry was performed to compare the group differences. Finally, correlation analyses were conducted between the white matter volume (WMV) changes and clinical symptoms. Results The FES showed significantly decreased WMV in the bilateral posterior limb of the internal capsule (PLIC) and right subgyral frontal white matter. The volume of the bilateral PLIC was negatively correlated with the Positive and Negative Syndrome Scale positive scores. Positive correlations were observed between all of the changed WMV measures and the Global Assessment of Functioning scores. Conclusion The current findings provide further evidence to support internal capsule and subgyral frontal white matter deficits at the early stage of schizophrenia that are potentially related to the core pathophysiology of the disease. Furthermore, these anatomical alterations were related to the clinical symptoms but not the untreated illness duration, suggesting that these deficits are related to aberrations in the neurodevelopmental process and may be relatively stable during the early course of schizophrenia.
      PubDate: 2014-08-01
       
  • In vivo visualization of cells labeled with superparamagnetic iron oxides
           by a sub-millisecond gradient echo sequence
    • Abstract: Object In vivo magnetic resonance imaging (MRI) of iron-labeled pancreatic islets (PIs) transplanted into the liver is still challenging in humans. The aim of this study was to develop and evaluate a double contrast method for the detection of PIs labeled with superparamagnetic iron oxide (SPIO) nanoparticles. Materials and methods A double-echo three-dimensional (3D) spoiled gradient echo sequence was adapted to yield a sub-millisecond first echo time using variable echo times and highly asymmetric Cartesian readout. Positive contrast was achieved by conventional and relative image subtraction. Experiments for cell detection efficiency were performed in vitro on gelatin phantoms, in vivo on a Lewis rat and on a patient 6 months after PI transplantation. Results It was demonstrated that the proposed method can be used for the detection of transplanted PIs with positive contrast in vitro and in vivo. For all experiments, relative subtraction yielded comparable and in some cases better contrast than conventional subtraction. For the first time, positive contrast imaging of transplanted human PIs was performed in vivo in patients. Conclusion The proposed method allows 3D data acquisition within a single breath-hold and yields enhanced contrast-to-noise ratios of transplanted SPIO labeled pancreatic islets relative to negative contrast images, therefore providing improved identification.
      PubDate: 2014-08-01
       
  • An MR-compatible stereoscopic in-room 3D display for MR-guided
           interventions
    • Abstract: Background and methods A commercial three-dimensional (3D) monitor was modified for use inside the scanner room to provide stereoscopic real-time visualization during magnetic resonance (MR)-guided interventions, and tested in a catheter-tracking phantom experiment at 1.5 T. Brightness, uniformity, radio frequency (RF) emissions and MR image interferences were measured. Results and discussion Due to modifications, the center luminance of the 3D monitor was reduced by 14 %, and the addition of a Faraday shield further reduced the remaining luminance by 31 %. RF emissions could be effectively shielded; only a minor signal-to-noise ratio (SNR) decrease of 4.6 % was observed during imaging. During the tracking experiment, the 3D orientation of the catheter and vessel structures in the phantom could be visualized stereoscopically.
      PubDate: 2014-08-01
       
  • Influence of blood/tissue differences in contrast agent relaxivity on
           tracer-based MR perfusion measurements
    • Abstract: Purpose Perfusion assessment by monitoring the transport of a tracer bolus depends critically on conversion of signal intensity into tracer concentration. Two main assumptions are generally applied for this conversion; (1) contrast agent relaxivity is identical in blood and tissue, (2) change in signal intensity depends only on the primary relaxation effect. The purpose of the study was to assess the validity and influence of these assumptions. Materials and methods Blood and cerebral tissue relaxivities r1, r2, and r2* for gadodiamide were measured in four pigs at 1.5 T. Gadolinium concentration was determined by inductively coupled plasma atomic emission spectroscopy. Influence of the relaxivities, secondary relaxation effects and choice of singular value decomposition (SVD) regularization threshold was studied by simulations. Results In vivo relaxivities relative to blood concentration [in s−1 mM−1 for blood, gray matter (GM), white matter (WM)] were for r1 (2.614 ± 1.061, 0.010 ± 0.001, 0.004 ± 0.002), r2 (5.088 ± 0.952, 0.091 ± 0.008, 0.059 ± 0.014), and r2* (13.292 ± 3.928, 1.696 ± 0.157, 0.910 ± 0.139). Although substantial, by a nonparametric test for paired samples, the differences were not statistically significant. The GM to WM blood volume ratio was estimated to 2.6 ± 0.9 by r1, 1.6 ± 0.3 by r2, and 1.9 ± 0.2 by r2*. Secondary relaxation was found to reduce the tissue blood flow, as did the SVD regularization threshold. Conclusion Contrast agent relaxivity is not identical in blood and tissue leading to substantial errors. Further errors are introduced by secondary relaxation effects and the SVD regularization.
      PubDate: 2014-06-28
       
  • Tracking metabolite dynamics in plants via indirect        class="a-plus-plus">13C chemical shift imaging
           with an interleaved variable density acquisition weighted sampling pattern
           
    • Abstract: Objective Developing and evaluating an improved sampling pattern to track the dynamics of labeled substances in plants using indirect 13C chemical shift imaging. Materials and methods An algorithm to split an acquisition weighted sampling pattern into several undersampled sub-images is presented. The sampling patterns are used in CSI moving phantom experiments as well as in in vivo POCE-CSI experiments on barley stem and grain. Reconstruction is performed traditionally or by compressed sensing. Results The moving phantom experiments show that the sampling pattern can reduce motion artifacts at the cost of an increased overall noise. The in vivo experiments demonstrate the feasibility of extracting a time series from a single imaging experiment. Conclusion The sampling pattern is suitable for tracking the uptake of label substances into plant material. The use of compressed sensing allows an increased spatial and temporal resolution.
      PubDate: 2014-06-22
       
  • Effect of acute hyperglycemia on moderately hypothermic GL261 mouse glioma
           monitored by T1-weighted DCE MRI
    • Abstract: Objective We sought to evaluate the effects of acute hyperglycemia induced by intraperitoneal injection of glucose (2.7 g/kg) on vascular delivery to GL261 mouse gliomas kept at moderate hypothermia (~30 °C). Materials and methods Seven GL261 glioma-bearing mice were studied by T1-weighted DCE MRI before and after an injection of glucose (n = 4) or saline (n = 3). Maximum relative contrast enhancement (RCE) and initial area under the enhancement curve (IAUC) were determined in each pixel. Results The mean tumor parameter values showed no significant changes after injecting either saline (RCE −5.9 ± 5.0 %; IAUC −3.7 ± 3.6 %) or glucose (RCE −1.6 ± 9.0 %; IAUC +0.6 ± 6.4 %). Pixel-by-pixel analysis revealed small post-injection changes in RCE and IAUC between the glucose and saline groups, all within 13 % range of their baseline values. Conclusion Perturbing the metabolism of GL261 tumors kept at moderate hypothermia with hyperglycemia did not induce significant changes in the permeability/perfusion of these tumors. This is relevant for future studies with this model since regional differences in glucose accumulation could thus reflect basal heterogeneities in vasculature and/or metabolism of GL261 tumors.
      PubDate: 2014-06-11
       
  • Fast water concentration mapping to normalize        class="a-plus-plus">1H MR spectroscopic imaging
    • Abstract: Object To propose a fast and robust acquisition and post-processing pipeline that is time-compatible with clinical explorations to obtain a proton density (ρ) map used as a reference for metabolic map normalization. This allows inter-subject and inter-group comparisons of magnetic resonance spectroscopic imaging (MRSI) data and longitudinal follow-up for single subjects. Materials and methods A multi-echo T 2 * mapping sequence, the XEP sequence for B 1 + -mapping and Driven Equilibrium Single Pulse Observation of T 1—an optimized variable flip angle method for T 1 mapping used for both B 1 − -mapping and M 0 calculation—were used to determine correction factors leading to quantitative water proton density maps at 3T. Normalized metabolite maps were obtained on a phantom and nine healthy volunteers. To show the potential use of this technique at the individual level, we also explored one patient with low-grade glioma. Results Accurate ρ maps were obtained both on phantom and volunteers. After signal normalization with the generated ρ maps, metabolic concentrations determined by the present method differed from theory by <7 % in the phantom and were in agreement with data from the literature for the healthy controls. Using these normalized metabolic values, it was possible to demonstrate in the patient with brain glioma, metabolic abnormalities in normalized N-acetyl aspartate, choline and creatine levels; illustrating the potential for direct use of this technique in clinical studies. Conclusion The proposed combination of sequences provides a robust ρ map that can be used to normalize metabolic maps in clinical MRSI studies.
      PubDate: 2014-06-08
       
  • Measuring short-term liver metabolism non-invasively: postprandial and
           post-exercise 1H and
           31P MR spectroscopy
    • Abstract: Object The objective of this study was to determine the effects of a standardized fat rich meal and subsequent exercise on liver fat content by 1H MRS and on liver adenosine triphosphate (ATP) content by 31P MRS in healthy subjects. Materials and methods Hepatic 1H and proton decoupled 31P MRS were performed on nine healthy subjects on a clinical 3.0 T MR imager three times during a day: after (1) an overnight fast, (2) a following standardized fat rich meal and (3) a subsequent exercise session. Blood parameters were followed during the day to serve as a reference to MRS. Results Liver fat content increased gradually over the day (p = 0.0001) with an overall increase of 30 %. Also γ-NTP changed significantly over the day (p = 0.005). γ-NTP/tP decreased by 9 % (p = 0.019, post hoc) from the postprandial to the post-exercise state. Conclusion Our study shows that in vivo MRS can depict short lived physiological changes; entering of fat into liver cells and consumption of ATP during exercise can be measured non-invasively in healthy subjects. The physiological state may have an impact on fat and energy metabolite levels. Hepatic 1H and 31P MRS studies should be performed under standardized conditions.
      PubDate: 2014-06-04
       
  • Hemi-spectrum substitution after water signal fitting (HESWAF): an
           improvement of the modulus post-processing of MR spectra
    • Abstract: Objective In a previous study, we have shown that modulus post-processing is a simple and efficient tool to both phase correct and frequency align magnetic resonance (MR) spectra automatically. Furthermore, this technique also eliminates sidebands and phase distortions. The advantages of the modulus technique have been illustrated in several applications to brain proton MR spectroscopy. Two possible drawbacks have also been pointed out. The first one is the theoretical decrease in signal-to-noise ratio (SNR) by a factor up to √2 when comparing the spectrum obtained after modulus versus conventional post-processing. The second pitfall results from the symmetrization of the spectrum induced by modulus post-processing, since any resonance or artifact located at the left of the water resonance is duplicated at the right of the water resonance, thus contaminating the region of the spectrum containing the resonances of interest. Herein, we propose a strategy in order to eliminate these two limitations. Materials and methods Concerning the SNR issue, two complementary approaches are presented here. The first is based on the application of modulus post-processing before spatial apodization, and the second consists in substituting the left half of the spectrum by the fit of the water resonance before applying modulus post-processing. The symmetrization induced by modulus post-processing then combines the right half of the original spectrum containing the resonances of interest with the left half of the water fit, free of noise and artifacts. Consequently, the SNR is improved when compared to modulus post-processing alone. As a bonus, any artifact or resonance present in the left half of the original spectrum is removed. This solves the second limitation. Results After validation of the technique on simulations, we demonstrated that this improvement of the modulus technique is significantly advantageous for both in vitro and in vivo applications. Conclusion By improving the SNR of the spectra and eliminating eventual contaminations, the new strategies proposed here confer an additional competitive advantage to the modulus post-processing technique.
      PubDate: 2014-06-03
       
  • Purely phase-encoded MRI of turbulent flow through a dysfunctional
           bileaflet mechanical heart valve
    • Abstract: Object We have used a purely phase-encoded magnetic resonance imaging (MRI) technique, single-point ramped imaging with T1 enhancement (SPRITE), to investigate the steady, turbulent flow dynamics through a bileaflet mechanical heart valve (BMHV). Materials and methods We have measured in vitro the turbulent diffusivity and velocity downstream of the valve in two configurations (fully opened and partially opened), which mimic normal and dysfunctional operation. Our constant-time implementation of the MRI measurement is unusually robust to fast turbulent flows, and to artefacts caused by the pyrolytic carbon construction of the valve. Results Turbulent diffusivity downstream of the normally functioning valve peaks at 1.05 × 10−6 m2/s, while the turbulent diffusivity is higher downstream of the dysfunctional valve (peaking at 3.15 × 10−6 m2/s) and is accompanied by a high-velocity fluid jet and re-circulating flow. The fluid jet is not along the centreline of the valve, as might be anticipated in conventional Doppler echocardiography measurements. Conclusion The nature of motion-sensitized SPRITE makes it unusually capable in turbulent flows and near to boundaries between different magnetic susceptibilities. These qualities have allowed us to compare the three-dimensional flow fields through normal and dysfunctional BMHVs.
      PubDate: 2014-06-01
       
 
 
JournalTOCs
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Email: journaltocs@hw.ac.uk
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
 
About JournalTOCs
API
Help
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2014