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COMMUNICATIONS (357 journals)

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Journal Cover Magnetic Resonance Materials in Physics, Biology and Medicine
  [SJR: 0.787]   [H-I: 46]   [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  [2354 journals]
  • Improved $$T_{2}^{*}$$ T 2 ∗ determination in 23 Na, 35 Cl, and 17 O MRI
           using iterative partial volume correction based on 1 H MRI segmentation
    • Authors: Sebastian C. Niesporek; Reiner Umathum; Thomas M. Fiedler; Peter Bachert; Mark E. Ladd; Armin M. Nagel
      Pages: 519 - 536
      Abstract: Objective Functional parameters can be measured with the help of quantitative non-proton MRI where exact relaxometry parameters are needed. Investigation of \(T_{2}^{*}\) is often biased by strong partial volume (PV) effects. Hence, in this work a PV correction algorithm approach was evaluated that uses iteratively adapted \(T_{2}^{*}\) -values and high-resolution structural 1H data to determine transverse relaxation in non-proton MRI more accurately. Materials and methods Simulations, a phantom study and in vivo 23Na, 17O and 35Cl MRI measurements of five healthy volunteers were performed to evaluate the algorithm. \(T_{2}^{*}\) values of grey matter (GM), white matter (WM) and cerebrospinal fluid (CSF) were obtained. Data were acquired at B 0  = 7T with nominal spatial resolutions of (4–7 mm)3 using a density-adapted radial sequence. The resulting transverse relaxation times were used for quantification of 17O data. Results The conducted simulations and phantom study verified the correction performance of the algorithm. For in vivo measured \(T_{2}^{*}\) values, the correction of PV effects leads to an increase in CSF and to a decrease in GM/WM (23Na MRI: long/short GM, WM \(T_{2}^{*}\) : 36.4 ± 3.1/5.4 ± 0.2, 23.3 ± 2.6/3.5 ± 0.1 ms; 35Cl MRI: 8.9 ± 1.4/1.0 ± 0.4, 5.9 ± 0.3/0.4 ± 0.1 ms; 17O MRI: 2.5 ± 0.1, 2.8 ± 0.1 ms). Iteratively corrected in vivo \(T_{2}^{*}\) values of the 17O study resulted in improved water content quantification. Conclusion The proposed iterative algorithm for PV correction leads to more accurate \(T_{2}^{*}\) values and, thus, can improve accuracy in quantitative non-proton MRI.
      PubDate: 2017-12-01
      DOI: 10.1007/s10334-017-0623-2
      Issue No: Vol. 30, No. 6 (2017)
       
  • Automatic frequency and phase alignment of in vivo J-difference-edited MR
           spectra by frequency domain correlation
    • Authors: Evita C. Wiegers; Bart W. J. Philips; Arend Heerschap; Marinette van der Graaf
      Pages: 537 - 544
      Abstract: Objective J-difference editing is often used to select resonances of compounds with coupled spins in 1H-MR spectra. Accurate phase and frequency alignment prior to subtracting J-difference-edited MR spectra is important to avoid artefactual contributions to the edited resonance. Materials and methods In-vivo J-difference-edited MR spectra were aligned by maximizing the normalized scalar product between two spectra (i.e., the correlation over a spectral region). The performance of our correlation method was compared with alignment by spectral registration and by alignment of the highest point in two spectra. The correlation method was tested at different SNR levels and for a broad range of phase and frequency shifts. Results In-vivo application of the proposed correlation method showed reduced subtraction errors and increased fit reliability in difference spectra as compared with conventional peak alignment. The correlation method and the spectral registration method generally performed equally well. However, better alignment using the correlation method was obtained for spectra with a low SNR (down to ~2) and for relatively large frequency shifts. Conclusion Our correlation method for simultaneously phase and frequency alignment is able to correct both small and large phase and frequency drifts and also performs well at low SNR levels.
      PubDate: 2017-12-01
      DOI: 10.1007/s10334-017-0627-y
      Issue No: Vol. 30, No. 6 (2017)
       
  • Diffusional kurtosis imaging (DKI) incorporation into an intravoxel
           incoherent motion (IVIM) MR model to measure cerebral hypoperfusion
           induced by hyperventilation challenge in healthy subjects
    • Authors: Aude Pavilla; Giulio Gambarota; Alessandro Arrigo; Mehdi Mejdoubi; Régis Duvauferrier; Hervé Saint-Jalmes
      Pages: 545 - 554
      Abstract: Objectives The objectives were to investigate the diffusional kurtosis imaging (DKI) incorporation into the intravoxel incoherent motion (IVIM) model for measurements of cerebral hypoperfusion in healthy subjects. Materials and methods Eight healthy subjects underwent a hyperventilation challenge with a 4-min diffusion weighted imaging protocol, using 8 b values chosen with the Cramer-Rao Lower Bound optimization approach. Four regions of interest in gray matter (GM) were analyzed with the DKI–IVIM model and the bi-exponential IVIM model, for normoventilation and hyperventilation conditions. Results A significant reduction in the perfusion fraction (f) and in the product fD* of the perfusion fraction with the pseudodiffusion coefficient (D*) was found with the DKI–IVIM model, during the hyperventilation challenge. In the cerebellum GM, the percentage changes were f: −43.7 ± 40.1, p = 0.011 and fD*: −50.6 ± 32.1, p = 0.011; in thalamus GM, f: −47.7 ± 34.7, p = 0.012 and fD*: −47.2 ± 48.7, p = 0.040. In comparison, using the bi-exponential IVIM model, only a significant decrease in the parameter fD* was observed for the same regions of interest. In frontal-GM and posterior-GM, the reduction in f and fD* did not reach statistical significance, either with DKI–IVIM or the bi-exponential IVIM model. Conclusion When compared to the bi-exponential IVIM model, the DKI–IVIM model displays a higher sensitivity to detect changes in perfusion induced by the hyperventilation condition.
      PubDate: 2017-12-01
      DOI: 10.1007/s10334-017-0629-9
      Issue No: Vol. 30, No. 6 (2017)
       
  • Assessment of MRI contrast agent concentration by quantitative
           susceptibility mapping (QSM): application to estimation of cerebral blood
           volume during steady state
    • Authors: Emelie Lind; Linda Knutsson; Robin Kämpe; Freddy Ståhlberg; Ronnie Wirestam
      Pages: 555 - 566
      Abstract: Objective One major issue in dynamic susceptibility contrast MRI (DSC-MRI) is to accurately determine contrast agent (CA) concentration, since T2* relaxivity in vivo is generally unknown and varies between blood and tissue. In this study, quantitative susceptibility mapping (QSM) was used for quantification of CA concentration. Materials and methods A DSC-MRI protocol, including phase data acquisition, was applied to 20 healthy volunteers in a test–retest study. By selecting a CSF reference region of interest (ROI), the values of all QSM images were shifted to show no CA-induced change in CSF. CA concentration and cerebral blood volume (CBV) were estimated using shifted QSM data. CSF reference ROI optimization was evaluated by investigation of CBV repeatability. The CBV age dependence was analysed and tissue T2* relaxivity was estimated. Results The best repeatability of CBV, using an optimal CSF reference ROI, showed test-versus-retest correlations of r = 0.81 and r = 0.91 for white and grey matter, respectively. A slight CBV decrease with age was observed, and the estimated in vivo T2* relaxivity was 85 mM−1s−1. Conclusion Provided that a carefully selected CSF reference ROI is used to shift QSM image values, susceptibility information can be used to estimate concentration of contrast agent and to calculate CBV.
      PubDate: 2017-12-01
      DOI: 10.1007/s10334-017-0637-9
      Issue No: Vol. 30, No. 6 (2017)
       
  • Adaptive step size LMS improves ECG detection during MRI at 1.5 and
           3 T
    • Authors: André Guillou; Jean-Marc Sellal; Sarah Ménétré; Grégory Petitmangin; Jacques Felblinger; Laurent Bonnemains
      Pages: 567 - 577
      Abstract: Objective We describe a new real-time filter to reduce artefacts on electrocardiogram (ECG) due to magnetic field gradients during MRI. The proposed filter is a least mean square (LMS) filter able to continuously adapt its step size according to the gradient signal of the ongoing MRI acquisition. Materials and methods We implemented this filter and compared it, within two databases (at 1.5 and 3 T) with over 6000 QRS complexes, to five real-time filtering strategies (no filter, low pass filter, standard LMS, and two other filters optimized within the databases: optimized LMS, and optimized Kalman filter). Results The energy of the remaining noise was significantly reduced (26 vs. 68%, p < 0.001) with the new filter vs. standard LMS. The detection error of our ventricular complex (QRS) detector was: 11% with our method vs. 25% with raw ECG, 35% with low pass filter, 17% with standard LMS, 12% with optimized Kalman filter, and 11% with optimized LMS filter. Conclusion The adaptive step size LMS improves ECG denoising during MRI. QRS detection has the same F1 score with this filter than with filters optimized within the database.
      PubDate: 2017-12-01
      DOI: 10.1007/s10334-017-0638-8
      Issue No: Vol. 30, No. 6 (2017)
       
  • Temperature dependence of 1 H NMR chemical shifts and its influence on
           estimated metabolite concentrations
    • Authors: Felizitas C. Wermter; Nico Mitschke; Christian Bock; Wolfgang Dreher
      Pages: 579 - 590
      Abstract: Objectives Temperature dependent chemical shifts of important brain metabolites measured by localised 1H MRS were investigated to test how the use of incorrect prior knowledge on chemical shifts impairs the quantification of metabolite concentrations. Materials and methods Phantom measurements on solutions containing 11 metabolites were performed on a 7 T scanner between 1 and 43 °C. The temperature dependence of the chemical shift differences was fitted by a linear model. Spectra were simulated for different temperatures and analysed by the AQSES program (jMRUI 5.2) using model functions with chemical shift values for 37 °C. Results Large differences in the temperature dependence of the chemical shift differences were determined with a maximum slope of about ±7.5 × 10−4 ppm/K. For 32–40 °C, only minor quantification errors resulted from using incorrect chemical shifts, with the exception of Cr and PCr. For 1–10 °C considerable quantification errors occurred if the temperature dependence of the chemical shifts was neglected. Conclusion If 1H MRS measurements are not performed at 37 °C, for which the published chemical shift values have been determined, the temperature dependence of chemical shifts should be considered to avoid systematic quantification errors, particularly for measurements on animal models at lower temperatures.
      PubDate: 2017-12-01
      DOI: 10.1007/s10334-017-0642-z
      Issue No: Vol. 30, No. 6 (2017)
       
  • Variable slice thickness (VAST) EPI for the reduction of susceptibility
           artifacts in whole-brain GE-EPI at 7 Tesla
    • Authors: Sascha Brunheim; Sören Johst; Viktor Pfaffenrot; Stefan Maderwald; Harald H. Quick; Benedikt A. Poser
      Pages: 591 - 607
      Abstract: Objective A new technique for 2D gradient-recalled echo echo-planar imaging (GE-EPI) termed ‘variable slice thickness’ (VAST) is proposed, which reduces signal losses caused by through-slice susceptibility artifacts, while keeping the volume repetition time (TR) manageable. The slice thickness is varied across the brain, with thinner slices being used in the inferior brain regions where signal voids are most severe. Materials and methods Various axial slice thickness schemes with identical whole-brain coverage were compared to regular EPI, which may either suffer from unfeasibly long TR if appropriately thin slices are used throughout, or signal loss if no counter-measures are taken. Evaluation is based on time-course signal-to-noise (tSNR) maps from resting state data and a statistical group-level region of interest (ROI) analysis on breath-hold fMRI measurements. Results The inferior brain region signal voids with static B0 inhomogeneities could be markedly reduced with VAST GE-EPI in contrast to regular GE-EPI. ROI-averaged event-related signal changes showed 48% increase in VAST compared to GE-EPI with regular “thick” slices. tSNR measurements proved the comparable signal robustness of VAST in comparison to regular GE-EPI with thin slices. Conclusion A novel acquisition strategy for functional 2D GE-EPI at ultrahigh magnetic field is presented to reduce susceptibility-induced signal voids and keep TR sufficiently short for whole-brain coverage.
      PubDate: 2017-12-01
      DOI: 10.1007/s10334-017-0641-0
      Issue No: Vol. 30, No. 6 (2017)
       
  • Utilization of MR angiography in perfusion imaging for identifying
           arterial input function
    • Authors: Bora Buyuksarac; Mehmed Ozkan
      Pages: 609 - 620
      Abstract: Objective This research utilizes magnetic resonance angiography (MRA) to identify arterial locations during the parametric evaluation of concentration time curves (CTCs), and to prevent shape distortions in arterial input function (AIF). Materials and methods We carried out cluster analysis with the CTC parameters of voxels located within and around the middle cerebral artery (MCA). Through MRA, we located voxels that meet the AIF criteria and those with distorted CTCs. To minimize partial volume effect, we re-scaled the time integral of CTCs by the time integral of venous output function (VOF). We calculated the steady-state value to area under curve ratio (SS:AUC) of VOF and used it as a reference in selecting AIF. CTCs close to this reference value (selected AIF) and those far from it were used (eliminated AIF) to compute cerebral blood flow (CBF). Results Eliminated AIFs were found to be either on or anterior to MCA, whereas selected AIFs were located superior, inferior, posterior, or anterior to MCA. If the SS:AUC of AIF was far from the reference value, CBF was either under- or over-estimated by a maximum of 41.1 ± 14.3 and 36.6 ± 19.2%, respectively. Conclusion MRA enables excluding voxels on the MCA during cluster analysis, and avoiding the risk of shape distortions.
      PubDate: 2017-12-01
      DOI: 10.1007/s10334-017-0643-y
      Issue No: Vol. 30, No. 6 (2017)
       
  • Arterial input function in a dedicated slice for cerebral perfusion
           measurements in humans
    • Authors: Elias Kellner; Irina Mader; Marco Reisert; Horst Urbach; Valerij Gennadevic Kiselev
      Abstract: Object We aimed to modify our previously published method for arterial input function measurements for evaluation of cerebral perfusion (dynamic susceptibility contrast MRI) such that it can be applied in humans in a clinical setting. Materials and methods Similarly to our previous work, a conventional measurement sequence for dynamic susceptibility contrast MRI is extended with an additional measurement slice at the neck. Measurement parameters at this slice were optimized for the blood signal (short echo time, background suppression, magnitude and phase images). Phase-based evaluation of the signal in the carotid arteries is used to obtain quantitative arterial input functions. Results In all pilot measurements, quantitative arterial input functions were obtained. The resulting absolute perfusion parameters agree well with literature values (gray and white matter mean values of 46 and 24 mL/100 g/min, respectively, for cerebral blood flow and 3.0% and 1.6%, respectively, for cerebral blood volume). Conclusions The proposed method has the potential to quantify arterial input functions in the carotid arteries from a direct measurement without any additional normalization.
      PubDate: 2017-12-09
      DOI: 10.1007/s10334-017-0663-7
       
  • 3D true-phase polarity recovery with independent phase estimation using
           three-tier stacks based region growing (3D-TRIPS)
    • Authors: Haining Liu; Gregory J. Wilson; Niranjan Balu; Jeffrey H. Maki; Daniel S. Hippe; Wei Wu; Hiroko Watase; Jinnan Wang; Martin L. Gunn; Chun Yuan
      Abstract: Objectives A postprocessing technique termed 3D true-phase polarity recovery with independent phase estimation using three-tier stacks based region growing (3D-TRIPS) was developed, which directly reconstructs phase-sensitive inversion-recovery images without acquisition of phase-reference images. The utility of this technique is demonstrated in myocardial late gadolinium enhancement (LGE) imaging. Materials and methods A data structure with three tiers of stacks was used for 3D-TRIPS to directly achieve reliable region growing for successful background-phase estimation. Fifteen patients undergoing postgadolinium 3D phase-sensitive inversion recovery (PSIR) cardiac LGE magnetic resonance imaging (MRI) were recruited, and 3D-TRIPS LGE reconstructions were compared with standard PSIR. Objective voxel-by-voxel comparison was performed. Additionally, blinded review by two radiologists compared scar visibility, clinical acceptability, voxel polarity error, or groups and blurring. Results 3D-TRIPS efficiently reconstructed postcontrast phase-sensitive myocardial LGE images. Objective analysis showed an average 95% voxel-by-voxel agreement between 3D-TRIPS and PSIR images. Blinded radiologist review demonstrated similar image quality between 3D-TRIPS and PSIR reconstruction. Conclusion 3D-TRIPS provided similar image quality to PSIR for phase-sensitive myocardial LGE MRI reconstruction. 3D-TRIPS does not require acquisition of a reference image and can therefore be used to accelerate phase-sensitive LGE imaging.
      PubDate: 2017-12-07
      DOI: 10.1007/s10334-017-0666-4
       
  • Accelerated whole brain intracranial vessel wall imaging using black blood
           fast spin echo with compressed sensing (CS-SPACE)
    • Authors: Chengcheng Zhu; Bing Tian; Luguang Chen; Laura Eisenmenger; Esther Raithel; Christoph Forman; Sinyeob Ahn; Gerhard Laub; Qi Liu; Jianping Lu; Jing Liu; Christopher Hess; David Saloner
      Abstract: Objective Develop and optimize an accelerated, high-resolution (0.5 mm isotropic) 3D black blood MRI technique to reduce scan time for whole-brain intracranial vessel wall imaging. Materials and methods A 3D accelerated T1-weighted fast-spin-echo prototype sequence using compressed sensing (CS-SPACE) was developed at 3T. Both the acquisition [echo train length (ETL), under-sampling factor] and reconstruction parameters (regularization parameter, number of iterations) were first optimized in 5 healthy volunteers. Ten patients with a variety of intracranial vascular disease presentations (aneurysm, atherosclerosis, dissection, vasculitis) were imaged with SPACE and optimized CS-SPACE, pre and post Gd contrast. Lumen/wall area, wall-to-lumen contrast ratio (CR), enhancement ratio (ER), sharpness, and qualitative scores (1–4) by two radiologists were recorded. Results The optimized CS-SPACE protocol has ETL 60, 20% k-space under-sampling, 0.002 regularization factor with 20 iterations. In patient studies, CS-SPACE and conventional SPACE had comparable image scores both pre- (3.35 ± 0.85 vs. 3.54 ± 0.65, p = 0.13) and post-contrast (3.72 ± 0.58 vs. 3.53 ± 0.57, p = 0.15), but the CS-SPACE acquisition was 37% faster (6:48 vs. 10:50). CS-SPACE agreed with SPACE for lumen/wall area, ER measurements and sharpness, but marginally reduced the CR. Conclusion In the evaluation of intracranial vascular disease, CS-SPACE provides a substantial reduction in scan time compared to conventional T1-weighted SPACE while maintaining good image quality.
      PubDate: 2017-12-05
      DOI: 10.1007/s10334-017-0667-3
       
  • An 8-channel Tx/Rx dipole array combined with 16 Rx loops for
           high-resolution functional cardiac imaging at 7 T
    • Authors: Bart R. Steensma; Ingmar J. Voogt; Tim Leiner; Peter R. Luijten; Jesse Habets; Dennis W. J. Klomp; Cornelis A. T. van den Berg; Alexander J. E. Raaijmakers
      Abstract: Objective To demonstrate imaging performance for cardiac MR imaging at 7 T using a coil array of 8 transmit/receive dipole antennas and 16 receive loops. Materials and methods An 8-channel dipole array was extended by adding 16 receive-only loops. Average power constraints were determined by electromagnetic simulations. Cine imaging was performed on eight healthy subjects. Geometrical factor (g-factor) maps were calculated to assess acceleration performance. Signal-to-noise ratio (SNR)-scaled images were reconstructed for different combinations of receive channels, to demonstrate the SNR benefits of combining loops and dipoles. Results The overall image quality of the cardiac functional images was rated a 2.6 on a 4-point scale by two experienced radiologists. Imaging results at different acceleration factors demonstrate that acceleration factors up to 6 could be obtained while keeping the average g-factor below 1.27. SNR maps demonstrate that combining loops and dipoles provides a more than 50% enhancement of the SNR in the heart, compared to a situation where only loops or dipoles are used. Conclusion This work demonstrates the performance of a combined loop/dipole array for cardiac imaging at 7 T. With this array, acceleration factors of 6 are possible without increasing the average g-factor in the heart beyond 1.27. Combining loops and dipoles in receive mode enhances the SNR compared to receiving with loops or dipoles only.
      PubDate: 2017-11-24
      DOI: 10.1007/s10334-017-0665-5
       
  • Automatic gallbladder segmentation using combined 2D and 3D shape features
           to perform volumetric analysis in native and secretin-enhanced MRCP
           sequences
    • Authors: Oliver Gloger; Robin Bülow; Klaus Tönnies; Henry Völzke
      Abstract: Objectives We aimed to develop the first fully automated 3D gallbladder segmentation approach to perform volumetric analysis in volume data of magnetic resonance (MR) cholangiopancreatography (MRCP) sequences. Volumetric gallbladder analysis is performed for non-contrast-enhanced and secretin-enhanced MRCP sequences. Materials and methods Native and secretin-enhanced MRCP volume data were produced with a 1.5-T MR system. Images of coronal maximum intensity projections (MIP) are used to automatically compute 2D characteristic shape features of the gallbladder in the MIP images. A gallbladder shape space is generated to derive 3D gallbladder shape features, which are then combined with 2D gallbladder shape features in a support vector machine approach to detect gallbladder regions in MRCP volume data. A region-based level set approach is used for fine segmentation. Volumetric analysis is performed for both sequences to calculate gallbladder volume differences between both sequences. Results The approach presented achieves segmentation results with mean Dice coefficients of 0.917 in non-contrast-enhanced sequences and 0.904 in secretin-enhanced sequences. Conclusion This is the first approach developed to detect and segment gallbladders in MR-based volume data automatically in both sequences. It can be used to perform gallbladder volume determination in epidemiological studies and to detect abnormal gallbladder volumes or shapes. The positive volume differences between both sequences may indicate the quantity of the pancreatobiliary reflux.
      PubDate: 2017-11-24
      DOI: 10.1007/s10334-017-0664-6
       
  • A note from Patrick J. Cozzone, MAGMA Editor-in-Chief
    • Authors: Patrick J. Cozzone
      PubDate: 2017-11-17
      DOI: 10.1007/s10334-017-0662-8
       
  • Segmental biventricular analysis of myocardial function using high
           temporal and spatial resolution tissue phase mapping
    • Authors: Marius Menza; Daniela Föll; Jürgen Hennig; Bernd Jung
      Abstract: Objective Myocardial dysfunction of the right ventricle (RV) is an important indicator of RV diseases, e.g. RV infarction or pulmonary hypertension. Tissue phase mapping (TPM) has been widely used to determine function of the left ventricle (LV) by analyzing myocardial velocities. The analysis of RV motion is more complicated due to the different geometry and smaller wall thickness. The aim of this work was to adapt and optimize TPM to the demands of the RV. Materials and methods TPM measurements were acquired in 25 healthy volunteers using a velocity-encoded phase-contrast sequence and kt-accelerated parallel imaging in combination with optimized navigator strategy and blood saturation. Post processing was extended by a 10-segment RV model and a detailed biventricular analysis of myocardial velocities was performed. Results High spatio-temporal resolution (1.0 × 1.0 × 6 mm3, 21.3 ms) and the optimized blood saturation enabled good delineation of the RV and its velocities. Global and segmental velocities, as well as time to peak velocities showed significant differences between the LV and RV. Furthermore, complex timing of the RV could be demonstrated by segmental time to peak analysis. Conclusion High spatio-temporal resolution TPM enables a detailed biventricular analysis of myocardial motion and might provide a reliable tool for description and detection of diseases affecting left and right ventricular function.
      PubDate: 2017-11-15
      DOI: 10.1007/s10334-017-0661-9
       
  • T 2 mapping of cerebrospinal fluid: 3 T versus 7 T
    • Authors: Jolanda M. Spijkerman; Esben T. Petersen; Jeroen Hendrikse; Peter Luijten; Jaco J. M. Zwanenburg
      Abstract: Object Cerebrospinal fluid (CSF) T 2 mapping can potentially be used to investigate CSF composition. A previously proposed CSF T 2–mapping method reported a T 2 difference between peripheral and ventricular CSF, and suggested that this reflected different CSF compositions. We studied the performance of this method at 7 T and evaluated the influence of partial volume and B 1 and B 0 inhomogeneity. Materials and methods T 2-preparation-based CSF T 2-mapping was performed in seven healthy volunteers at 7 and 3 T, and was compared with a single echo spin-echo sequence with various echo times. The influence of partial volume was assessed by our analyzing the longest echo times only. B 1 and B 0 maps were acquired. B 1 and B 0 dependency of the sequences was tested with a phantom. Results T 2,CSF was shorter at 7 T compared with 3 T. At 3 T, but not at 7 T, peripheral T 2,CSF was significantly shorter than ventricular T 2,CSF. Partial volume contributed to this T 2 difference, but could not fully explain it. B 1 and B 0 inhomogeneity had only a very limited effect. T 2,CSF did not depend on the voxel size, probably because of the used method to select of the regions of interest. Conclusion CSF T 2 mapping is feasible at 7 T. The shorter peripheral T 2,CSF is likely a combined effect of partial volume and CSF composition.
      PubDate: 2017-11-06
      DOI: 10.1007/s10334-017-0659-3
       
  • Relative enhanced diffusivity: noise sensitivity, protocol optimization,
           and the relation to intravoxel incoherent motion
    • Authors: Peter T. While; Jose R. Teruel; Igor Vidić; Tone F. Bathen; Pål Erik Goa
      Abstract: Objective To explore the relationship between relative enhanced diffusivity (RED) and intravoxel incoherent motion (IVIM), as well as the impact of noise and the choice of intermediate diffusion weighting (b value) on the RED parameter. Materials and methods A mathematical derivation was performed to cast RED in terms of the IVIM parameters. Noise analysis and b value optimization was conducted by using Monte Carlo calculations to generate diffusion-weighted imaging data appropriate to breast and liver tissue at three different signal-to-noise ratios. Results RED was shown to be approximately linearly proportional to the IVIM parameter f, inversely proportional to D and to follow an inverse exponential decay with respect to D*. The choice of intermediate b value was shown to be important in minimizing the impact of noise on RED and in maximizing its discriminatory power. RED was shown to be essentially a reparameterization of the IVIM estimates for f and D obtained with three b values. Conclusion RED imaging in the breast and liver should be performed with intermediate b values of 100 and 50 s/mm2, respectively. Future clinical studies involving RED should also estimate the IVIM parameters f and D using three b values for comparison.
      PubDate: 2017-11-06
      DOI: 10.1007/s10334-017-0660-x
       
  • Manipulating transmit and receive sensitivities of radiofrequency surface
           coils using shielded and unshielded high-permittivity materials
    • Authors: Manushka V. Vaidya; Cem M. Deniz; Christopher M. Collins; Daniel K. Sodickson; Riccardo Lattanzi
      Abstract: Objective To use high-permittivity materials (HPM) positioned near radiofrequency (RF) surface coils to manipulate transmit/receive field patterns. Materials and methods A large HPM pad was placed below the RF coil to extend the field of view (FOV). The resulting signal-to-noise ratio (SNR) was compared with that of other coil configurations covering the same FOV in simulations and experiments at 7 T. Transmit/receive efficiency was evaluated when HPM discs with or without a partial shield were positioned at a distance from the coil. Finally, we evaluated the increase in transmit homogeneity for a four-channel array with HPM discs interposed between adjacent coil elements. Results Various configurations of HPM increased SNR, transmit/receive efficiency, excitation/reception sensitivity overlap, and FOV when positioned near a surface coil. For a four-channel array driven in quadrature, shielded HPM discs enhanced the field below the discs as well as at the center of the sample as compared with other configurations with or without unshielded HPM discs. Conclusion Strategically positioning HPM at a distance from a surface coil or array can increase the overlap between excitation/reception sensitivities, and extend the FOV of a single coil for reduction of the number of channels in an array while minimally affecting the SNR.
      PubDate: 2017-11-06
      DOI: 10.1007/s10334-017-0657-5
       
  • Rapid measurement of intravoxel incoherent motion (IVIM) derived perfusion
           fraction for clinical magnetic resonance imaging
    • Authors: Emma M. Meeus; Jan Novak; Hamid Dehghani; Andrew C. Peet
      Abstract: Objective This study aimed to investigate the reliability of intravoxel incoherent motion (IVIM) model derived parameters D and f and their dependence on b value distributions with a rapid three b value acquisition protocol. Materials and methods Diffusion models for brain, kidney, and liver were assessed for bias, error, and reproducibility for the estimated IVIM parameters using b values 0 and 1000, and a b value between 200 and 900, at signal-to-noise ratios (SNR) 40, 55, and 80. Relative errors were used to estimate optimal b value distributions for each tissue scenario. Sixteen volunteers underwent brain DW-MRI, for which bias and coefficient of variation were determined in the grey matter. Results Bias had a large influence in the estimation of D and f for the low-perfused brain model, particularly at lower b values, with the same trends being confirmed by in vivo imaging. Significant differences were demonstrated in vivo for estimation of D (P = 0.029) and f (P < 0.001) with [300,1000] and [500,1000] distributions. The effect of bias was considerably lower for the high-perfused models. The optimal b value distributions were estimated to be brain500,1000, kidney300,1000, and liver200,1000. Conclusion IVIM parameters can be estimated using a rapid DW-MRI protocol, where the optimal b value distribution depends on tissue characteristics and compromise between bias and variability.
      PubDate: 2017-10-26
      DOI: 10.1007/s10334-017-0656-6
       
  • Two-dimensional XD-GRASP provides better image quality than conventional
           2D cardiac cine MRI for patients who cannot suspend respiration
    • Authors: Eve Piekarski; Teodora Chitiboi; Rebecca Ramb; Larry A. Latson; Puneet Bhatla; Li Feng; Leon Axel
      Abstract: Objectives Residual respiratory motion degrades image quality in conventional cardiac cine MRI (CCMRI). We evaluated whether a free-breathing (FB) radial imaging CCMRI sequence with compressed sensing reconstruction [extradimensional (e.g. cardiac and respiratory phases) golden-angle radial sparse parallel, or XD-GRASP] could provide better image quality than a conventional Cartesian breath-held (BH) sequence in an unselected population of patients undergoing clinical CCMRI. Materials and methods One hundred one patients who underwent BH and FB imaging in a midventricular short-axis plane at a matching location were included. Visual and quantitative image analysis was performed by two blinded experienced readers, using a five-point qualitative scale to score overall image quality and visual signal-to-noise ratio (SNR) grade, with measures of noise and sharpness. End-diastolic and end-systolic left ventricular areas were also measured and compared for both BH and FB images. Results Image quality was generally better with the BH cines (overall quality grade for BH vs FB images 4 vs 2.9, p < 0.001; noise 0.06 vs 0.08 p < 0.001; SNR grade 4.1 vs 3, p < 0.001), except for sharpness (p = 0.48). There were no significant differences between BH and FB images regarding end-diastolic or end-systolic areas (p = 0.35 and p = 0.12). Eighteen of the 101 patients had poor BH image quality (grade 1 or 2). In this subgroup, the quality of the FB images was better (p = 0.0032), as was the SNR grade (p = 0.003), but there were no significant differences regarding noise and sharpness (p = 0.45 and p = 0.47). Conclusion Although FB XD-GRASP CCMRI was visually inferior to conventional BH CCMRI in general, it provided improved image quality in the subgroup of patients with respiratory-motion-induced artifacts on BH images.
      PubDate: 2017-10-24
      DOI: 10.1007/s10334-017-0655-7
       
 
 
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