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  • Journal cover
    Springer
    Online: 1.1993 –
    Publisher: Springer
    Print ISSN: 0968-5243
    Electronic ISSN: 1352-8661
    Topics: Medicine , Physics
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  • Publication Date: 2016-04-07
    Description: Automatic or semi-automatic segmentation of tissue types or organs is well established for X-ray-based computed tomography, with its fixed grey-scale and tissue classes with well-established ranges of Hounsfield units. MRI is much more powerful with regard to soft tissue contrast and quantitative assessment of tissue properties (e.g., perfusion, diffusion, fat content), but the principle of signal generation and recording in MRI leads to inherent problems if simple threshold based segmentation procedures are applied. In this editorial in the special issue of MAGMA on tissue segmentation, a number of relevant methodical, scientific, and clinical aspects of reliable tissue segmentation using data recording by MRI are reported and discussed.
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  • Publication Date: 2016-06-14
    Description: Objectives Contrast agent (CA) relaxivities are generally not well established in vivo, and the relationship between frequency/phase shift and magnetic susceptibility might be a useful alternative for CA quantification. Materials and methods Twenty volunteers (25–84 years old) were investigated using test–retest pre-bolus dynamic susceptibility-contrast (DSC) magnetic resonance imaging (MRI). The pre-bolus phase-based venous output function (VOF) time integral was used for arterial input function (AIF) rescaling. Resulting cerebral blood flow (CBF) data for grey matter (GM) were compared with pseudo-continuous arterial spin labelling (ASL). During the main bolus CA passage, the apparent spatial shift (pixel shift) of the superior sagittal sinus (seen in single-shot echo-planar imaging (EPI)) was converted to CA concentration and compared with conventional ΔR2*-based data and with a predicted phase-based VOF from the pre-bolus experiment. Results The phase-based pre-bolus VOF resulted in a reasonable inter-individual GM CBF variability (coefficient of variation 28 %). Comparison with ASL CBF values implied a tissue R2*-relaxivity of 32 mM −1  s −1 . Pixel-shift data at low concentrations (data not available at peak concentrations) were in reasonable agreement with the predicted phase-based VOF. Conclusion Susceptibility-induced phase shifts and pixel shifts are potentially useful for large-vein CA quantification. Previous predictions of a higher R2*-relaxivity in tissue than in blood were supported.
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    Topics: Medicine , Physics
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  • Publication Date: 2016-04-09
    Description: Objectives For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B 1 + inhomogeneity. Previously, it was shown that a single k T -point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static k T -points). This work demonstrates that the B 1 + dependence of T 2 -weighted imaging can be further mitigated by designing a specific k T -point pulse for each pulse of a 3D TSE sequence (dynamic k T -points) even on single-channel transmit systems Materials and methods By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic k T -points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic k T -points were inserted into the TSE sequence to acquire in vivo images at 7T. Results The improvement provided by the dynamic k T -points over the static k T -point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic k T -points showed systematic improvement of signal and contrast at 7T over regular TSE—especially in cerebellar and temporal lobe regions without the need of parallel transmission. Conclusion Designing dynamic k T -points for a 3D TSE sequence allows the acquisition of T 2 -weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B 1 + inhomogeneity.
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  • Publication Date: 2016-04-09
    Description: Objective To demonstrate that high resolution 1 H semi-LASER MRSI acquired at 7 T permits discrimination of metabolic patterns of different thalamic nuclei. Materials and methods Thirteen right-handed healthy volunteers were explored at 7 T using a high-resolution 2D-semi-LASER 1 H-MRSI sequence to determine the relative levels of N -Acetyl Aspartate (NAA), choline (Cho) and creatine-phosphocreatine (Cr) in eight VOIs (volume <0.3 ml) centered on four different thalamic nuclei located on the Oxford thalamic connectivity atlas. Post-processing was done using the CSIAPO software. Chemical shift displacement of metabolites was evaluated on a phantom and correction factors were applied to in vivo data. Results The global assessment (ANOVA p  < 0.05) of the neurochemical profiles (NAA, Cho and Cr levels) with thalamic nuclei and hemispheres as factors showed a significant global effect ( F  = 11.98, p  < 0.0001), with significant effect of nucleus type ( p  < 0.0001) and hemisphere ( p  < 0.0001). Post hoc analyses showed differences in neurochemical profiles between the left and the right hemisphere ( p  < 0.05), and differences in neurochemical profiles between nuclei within each hemisphere ( p  < 0.05). Conclusion For the first time, using high resolution 2D-PRESS semi-LASER 1 H-MRSI acquired at 7 T, we demonstrated that the neurochemical profiles were different between thalamic nuclei, and that these profiles were dependent on the brain hemisphere.
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  • Publication Date: 2016-04-16
    Description: Objective Arterial spin labelling (ASL) techniques benefit from the increased signal-to-noise ratio and the longer T 1 relaxation times available at ultra-high field. Previous pulsed ASL studies at 7 T concentrated on the superior regions of the brain because of the larger transmit radiofrequency inhomogeneity experienced at ultra-high field that hinders an adequate inversion of the blood bolus when labelling in the neck. Recently, researchers have proposed to overcome this problem with either the use of dielectric pads, through dedicated transmit labelling coils, or special adiabatic inversion pulses. Materials and methods We investigate the performance of an optimised time-resampled frequency-offset corrected inversion (TR-FOCI) pulse designed to cause inversion at much lower peak B 1 + . In combination with a PICORE labelling, the perfusion signal obtained with this pulse is compared against that obtained with a FOCI pulse, with and without dielectric pads. Results Mean grey matter perfusion with the TR-FOCI was 52.5 ± 10.3 mL/100 g/min, being significantly higher than the 34.6 ± 2.6 mL/100 g/min obtained with the FOCI pulse. No significant effect of the dielectric pads was observed. Conclusion The usage of the B 1 + -optimised TR-FOCI pulse results in a significantly higher perfusion signal. PICORE–ASL is feasible at ultra-high field with no changes to operating conditions.
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  • Publication Date: 2016-04-20
    Description: Objective Diffusion-weighted magnetic resonance imaging (DW-MRI) combined with intravoxel incoherent motion (IVIM) analysis may be applied for assessment of organ lesions, diffuse parenchymal pathologies, and therapy monitoring. The aim of this study was to determine IVIM reference parameters of abdominal organs for translational research in a large cohort of C57Bl/6 laboratory mice. Materials and methods Anesthetized mice ( n  = 29) were measured in a 4.7 T small-animal MR scanner with a diffusion-weighted echo-planar imaging sequence at the \(b\) -values 0, 13, 24, 55, 107, 260, 514, 767, 1020 s/mm 2 . IVIM analysis was conducted on the liver, spleen, renal medulla and cortex, pancreas, and small bowel with computation of the true tissue diffusion coefficient \(D_{\text{t}}\) , the perfusion fraction \(f_{\text{p}}\) , and the pseudodiffusion coefficient \(D_{\text{p}}\) . Microvessel density (MVD) was assessed by immunohistochemistry (IHC) against panendothelial cell antigen CD31. Results Mean values of the different organs [ \(D_{\text{t}}\) (10 −3  mm 2 /s); \(f_{\text{p}}\) (%); \(D_{\text{p}}\) (10 −3  mm 2 /s); MVD (MV/mm 2 )]: liver 1.15 ± 0.14; 14.77 ± 6.15; 50.28 ± 33.21, 2008.48 ± 419.43, spleen 0.55 ± 0.12; 9.89 ± 5.69; 24.46 ± 17.31; n.d., renal medulla 1.50 ± 0.20; 14.63 ± 4.07; 35.50 ± 18.01; 1231.88 ± 290.61, renal cortex 1.34 ± 0.18; 10.83 ± 3.70; 16.74 ± 6.74; 810.09 ± 193.50, pancreas 1.23 ± 0.22; 20.12 ± 7.46; 29.35 ± 17.82, 591.15 ± 86.25 and small bowel 1.06 ± 0.13; 16.48 ± 3.63; 15.31 ± 7.00; 420.50 ± 168.42. Unlike \(D_{\text{t}}\) and \(f_{\text{p}}\) , \(D_{\text{p}}\) correlates significantly with MVD ( r  = 0.90, p  = 0.037). Conclusion This systematic evaluation of murine abdominal organs with IVIM and MVD analysis allowed to establish reference parameters for future DW-MRI translational research studies on small-animal disease models.
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  • Publication Date: 2016-04-27
    Description: Objectives We aimed to evaluate the feasibility of triple-echo steady state (TESS) T2 mapping as an alternative to conventional multi-echo-spin-echo (CPMG) T2 mapping for the quantitative assessment of hip joint cartilage at 7 T. Materials and methods A total of eight healthy volunteers and three patients were included. Reproducibility of both techniques was evaluated in five volunteers in five scans each. T2 relaxation times were measured by manually drawing regions of interest in multiple regions of the hip joint. Data from both methods were compared using Pearson correlation coefficient, intra-class correlation coefficient, and coefficient of repeatability. The overall image quality and presence of artifacts was assessed. Results Cartilage transplant and surrounding fluid were well depicted by both methods. Compared to CPMG, TESS provided systematically reduced T2 values (43.3 ± 7.3 vs. 19.2 ± 5.5 ms for acetabular cartilage, and 41.4 ± 5.6 vs. 21.7 ± 5.2 ms for femoral cartilage), in line with previously reported values. No correlation between both methods was found. TESS yielded a slightly better reproducibility than CPMG, while CPMG showed pronounced sensitivity to B1 inhomogeneities. Conclusion TESS seems to be an attractive alternative to CPMG for improvements in quantitative hip joint imaging at 7 T, allowing shortening of the total acquisition time paired with insensitivity to B1, while rendering comparable image quality with good repeatability.
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  • Publication Date: 2016-01-07
    Description: Segmentation of human brain using structural MRI is a key step of processing in imaging neuroscience. The methods have undergone a rapid development in the past two decades and are now widely available. This non-technical review aims at providing an overview and basic understanding of the most common software. Starting with the basis of structural MRI contrast in brain and imaging protocols, the concepts of voxel-based and surface-based segmentation are discussed. Special emphasis is given to the typical contrast features and morphological constraints of cortical and sub-cortical grey matter. In addition to the use for voxel-based morphometry, basic applications in quantitative MRI, cortical thickness estimations, and atrophy measurements as well as assignment of cortical regions and deep brain nuclei are briefly discussed. Finally, some fields for clinical applications are given.
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  • Publication Date: 2016-01-07
    Description: Objective We assess inter- and intra-subject variability of magnetic resonance (MR)-based attenuation maps (MRμMaps) of human subjects for state-of-the-art positron emission tomography (PET)/MR imaging systems. Materials and methods Four healthy male subjects underwent repeated MR imaging with a Siemens Biograph mMR, Philips Ingenuity TF and GE SIGNA PET/MR system using product-specific MR sequences and image processing algorithms for generating MRμMaps. Total lung volumes and mean attenuation values in nine thoracic reference regions were calculated. Linear regression was used for comparing lung volumes on MRμMaps. Intra- and inter-system variability was investigated using a mixed effects model. Results Intra-system variability was seen for the lung volume of some subjects, ( p  = 0.29). Mean attenuation values across subjects were significantly different ( p  < 0.001) due to different segmentations of the trachea. Differences in the attenuation values caused noticeable intra-individual and inter-system differences that translated into a subsequent bias of the corrected PET activity values, as verified by independent simulations. Conclusion Significant differences of MRμMaps generated for the same subjects but different PET/MR systems resulted in differences in attenuation correction factors, particularly in the thorax. These differences currently limit the quantitative use of PET/MR in multi-center imaging studies.
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