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Journal Cover Magnetic Resonance Materials in Physics, Biology and Medicine     [SJR: 0.821]   [H-I: 36]
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   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]
  • Highly undersampled peripheral Time-of-Flight magnetic resonance
           angiography: optimized data acquisition and iterative image reconstruction
    • Abstract: Object The aim of this study was to investigate the acceleration of peripheral Time-of-Flight magnetic resonance angiography using Compressed Sensing and parallel magnetic resonance imaging (MRI) while preserving image quality and vascular contrast. Materials and methods An analytical sampling pattern is proposed that combines aspects of parallel MRI and Compressed Sensing. It is used in combination with a dedicated Split Bregman algorithm. This approach is compared with current state-of-the-art patterns and reconstruction algorithms. Results The acquisition time was reduced from 30 to 2.5 min in a study using ten volunteer data sets, while showing improved sharpness, better contrast and higher accuracy compared to state-of-the-art techniques. Conclusion This study showed the benefits of the proposed dedicated analytical sampling pattern and Split Bregman algorithm for optimizing the Compressed Sensing reconstruction of highly accelerated peripheral Time-of-Flight data.
      PubDate: 2015-01-22
  • Reproducibility of pharmacological ASL using sequences from different
           vendors: implications for multicenter drug studies
    • Abstract: Object The current study assesses the multicenter feasibility of pharmacological arterial spin labeling (ASL) by comparing a caffeine-induced relative cerebral blood flow decrease (%CBF↓) measured with two pseudo-continuous ASL sequences as provided by two major vendors. Materials and methods Twenty-two healthy volunteers were scanned twice with both a 3D spiral (GE) and a 2D EPI (Philips) sequence. The inter-session reproducibility was evaluated by comparisons of the mean and within-subject coefficient of variability (wsCV) of the %CBF↓, both for the total cerebral gray matter and on a voxel level. Results The %CBF↓ was larger when measured with the 3D spiral sequence (23.9 ± 5.9 %) than when measured with the 2D EPI sequence (19.2 ± 5.6 %) on a total gray matter level (p = 0.02), and on a voxel level in the posterior watershed area (p < 0.001). There was no difference between the gray matter wsCV of the 3D spiral (57.3 %) and 2D EPI sequence (66.7 %, p = 0.3), whereas on a voxel level, the wsCV was visibly different between the sequences. Conclusion The observed differences between ASL sequences of both vendors can be explained by differences in the employed readout modules. These differences may seriously hamper multicenter pharmacological ASL, which strongly encourages standardization of ASL implementations.
      PubDate: 2015-01-15
  • Incorporation of image data from a previous examination in 3D serial MR
    • Abstract: Object We aimed to demonstrate that follow-up scans in longitudinal examinations can be significantly accelerated by using images from previous scans as priors for constrained reconstruction. Materials and methods In this work, we propose a method for incorporating a prior image to improve the reconstruction of a new acquisition with considerable k-space undersampling, which contains a two-level registration scheme with non-parametric transformation, an adaptive synthesis procedure, and a constrained reconstruction with weighted total variation constraint. The performance of the method is evaluated using simulations, as well as results from volunteer and patient examinations. Results In vivo experiments with both volunteers and patients show that incorporating a prior image into the constrained reconstruction produces many fewer reconstruction errors compared to the conventional reconstruction using only the highly undersampled k-space data. Conclusion The redundant information in the prior image can be efficiently adopted to improve the reconstruction quality of the new acquisition. When maintaining the image quality, higher acceleration can be achieved with the incorporation of the prior image.
      PubDate: 2015-01-09
  • The separation of Gln and Glu in STEAM: a comparison study using short and
           long TEs/TMs at 3 and 7 T
    • Abstract: Objectives This study aimed to determine the optimal echo time (TE) and mixing time (TM) for in vivo glutamine (Gln) and glutamate (Glu) separation in stimulated-echo acquisition mode at 3 and 7 T. We applied a short TE/TM (20/10 ms) for a high signal-to-noise-ratio and a field-specific long TE/TM (3 T: 72/6 ms; 7 T: 74/68 ms) for optimal Gln and Glu separation of the Carbon-4 proton resonances. Materials and methods Corresponding Gln and Glu spectra were simulated using VeSPA software, and measured in a phantom and human brains at 3 and 7 T. Results Higher spectral separation for Gln and Glu was achieved at 7 than 3 T. At 7 T, short TE/TM provided comparable spectral separation and in vitro Gln and Glu quantification compared to long TE/TM. Moreover, it showed greater reliability in in vivo Gln and Glu detection and separation than long TE/TM, with significantly lower Cramer–Rao lower bounds (Gln: 14.9 vs. 75.8; Glu: 3.8 vs. 6.5) and correlation between Gln and Glu (p = 0.004). Conclusion Based on the optimal separation for Gln and Glu, a short TE/TM at 7 T is proposed for future in vivo Gln and Glu acquisition.
      PubDate: 2015-01-08
  • Comparing velocity and fluid shear stress in a stenotic phantom with
           steady flow: phase-contrast MRI, particle image velocimetry and
           computational fluid dynamics
    • Abstract: Object This study aims to validate phase-contrast magnetic resonance imaging (PC-MRI) measurements of a steady flow through a severe stenotic phantom using particle image velocimetry (PIV) and computational fluid dynamics (CFD). Materials and methods The study was performed in an axisymmetric 87 % area stenosis model using an inlet Reynolds number (Re) of 160, corresponding to a jet Re of 444. Velocity patterns and estimated fluid shear stresses from three modalities were analyzed and compared qualitatively and quantitatively. Results Visual analysis via contour subtraction and Bland–Altman plots showed good agreement for flow velocities and less agreement for maximum shear stress (MSS). The Pearson’s coefficients of correlation between PC-MRI and PIV were 0.97 for the velocity field and 0.82 for the MSS. The corresponding parameters between PC-MRI and CFD were 0.96 and 0.84, respectively. Conclusion Findings indicate that PC-MRI can be implemented to estimate velocity flow fields and MSS; however, this method is not sufficiently accurate to quantify the MSS at regions of high shear rate.
      PubDate: 2014-12-12
  • Measurement reproducibility of magnetic resonance imaging-based finite
           element analysis of proximal femur microarchitecture for in vivo
           assessment of bone strength
    • Abstract: Introduction Osteoporosis is a disease of weak bone. Our goal was to determine the measurement reproducibility of magnetic resonance assessment of proximal femur strength. Methods This study had institutional review board approval, and written informed consent was obtained from all subjects. We obtained images of proximal femur microarchitecture by scanning 12 subjects three times within 1 week at 3T using a high-resolution 3-D FLASH sequence. We applied finite element analysis to compute proximal femur stiffness and femoral neck elastic modulus. Results Within-day and between-day root-mean-square coefficients of variation and intraclass correlation coefficients ranged from 3.5 to 6.6 % and 0.96 to 0.98, respectively. Conclusion The measurement reproducibility of magnetic resonance assessment of proximal femur strength is suitable for clinical studies of disease progression or treatment response related to osteoporosis bone-strengthening interventions.
      PubDate: 2014-12-09
  • IVIM analysis of brain tumors: an investigation of the relaxation effects
           of CSF, blood, and tumor tissue on the estimated perfusion fraction
    • Abstract: Object We sought to investigate the dependence of intravoxel incoherent motion (IVIM)-related perfusion fraction (f) estimates on the transverse relaxation of brain tissue, blood, and cerebrospinal fluid (CSF), attempting to overcome the influence of CSF on conventional f maps. Materials and methods Eighteen patients with gliomas underwent DWI with 14 b-values (0–1,300 s/mm2) and two distinct echo times (TEs). Regions of interest representing tumour and normal brain tissue were analysed by calculating the f values for both TEs. A mask for pixels with relevant CSF partial volume was subsequently created. The f values were tested for significant differences. Results We found statistically significant differences between the two TEs in the f values for cortical and juxtacortical structures and non-enhancing areas of the tumour /oedema. Normal white matter and gadolinium-enhancing tumour tissue appeared insensitive to TE variation. In all tissue types examined, the masking of voxels with considerable CSF content was able to overcome issues of erroneous f estimation and calculation of f values insensitive to TE changes was feasible. Conclusion Due to the complex interaction in the relaxation rates of CSF, blood, and tumour tissue, the estimation of f values is affected by the choice of TE. Only f values in normal white matter and tumour tissue—which largely comprise blood voxels with minor CSF partial volume—may be clinically applicable in the present form of IVIM-based DWI analysis. Going a step further, and after removing voxels with heavily TE-susceptible f values, we were able to obtain accurate and TE-independent f values in contrast-enhancing tumour tissue, white matter, and grey matter, which were essentially consistent with those reported in the literature.
      PubDate: 2014-12-05
  • Direct 17 O MRI with partial volume correction: first experiences in a
           glioblastoma patient
    • Abstract: Object In tumor cells the energy production is shifted from aerobic to anaerobic metabolization of glucose, which makes the cerebral metabolic rate of oxygen consumption (CMRO2) a diagnostic parameter for tissue viability. Direct oxygen-17 (17O) MRI during inhalation of 17O gas allows for a non-invasive determination of the CMRO2. However, the low spatial resolution and the fast transverse relaxation of 17O lead to partial volume effects that severely bias the quantification of signal intensities. The aim of this work was to determine the CMRO2 in a tumor patient by 17O MRI in combination with a partial volume correction (PVC) scheme. Materials and methods Direct 17O MRI was performed in a glioblastoma patient (F, 51 years) prior to surgery at 7 T. The ‘geometric transfer matrix’ algorithm for volume of interest based PVC was adapted to 17O MRI to recover the true signal intensities. We determined the CMRO2 values of gray matter (GM), white matter (WM), cerebrospinal fluid (CSF) and the tumor areas of the contrast enhancing rim (CE), the necrotic center (NE), and the perifocal edema (PE) using a three-phase metabolic model. Results Large differences in the signal increase during 17O2 inhalation were obtained ranging from less than 2 % in the tumor center up to more than 20 % in GM areas. After PVC of the signal time curves, we determined CMRO2 values of 0.67 ± 0.08 μmol/g/min (WM), 3.57 ± 0.67 μmol/g/min (GM), 0.35 ± 0.09 μmol/g/min (CE), and 0.42 ± 0.05 μmol/g/min (PE). In CSF and NE no oxygen uptake (i.e. CMRO2 = 0) was determined from the corrected signals, well in accordance with the underlying physiology in these regions. Conclusion The results show that PVC has a strong effect on the resulting CMRO2 values obtained by 17O MRI. We found substantial differences—especially in GM tissue—between corrected and non-corrected CMRO2 values. Additionally, we demonstrated the feasibility of CMRO2 assessment in a glioblastoma patient by 17O MRI.
      PubDate: 2014-12-01
  • Absolute quantification of perfusion by dynamic susceptibility contrast
           MRI using Bookend and VASO steady-state CBV calibration: a comparison with
           pseudo-continuous ASL
    • Abstract: Objective Dynamic susceptibility contrast MRI (DSC-MRI) tends to return elevated estimates of cerebral blood flow (CBF) and cerebral blood volume (CBV). In this study, subject-specific calibration factors (CFs), based on steady-state CBV measurements, were applied to rescale the absolute level of DSC-MRI CBF. Materials and methods Twenty healthy volunteers were scanned in a test–retest approach. Independent CBV measurements for calibration were accomplished using a T1-based contrast agent steady-state method (referred to as Bookend), as well as a blood-nulling vascular space occupancy (VASO) approach. Calibrated DSC-MRI was compared with pseudo-continuous arterial spin labeling (pCASL). Results For segmented grey matter (GM) regions of interests (ROIs), pCASL-based CBF was 63 ± 11 ml/(min 100 g) (mean ± SD). Nominal CBF from non-calibrated DSC-MRI was 277 ± 61 ml/(min 100 g), while calibrations resulted in 56 ± 23 ml/(min 100 g) (Bookend) and 52 ± 16 ml/(min 100 g) (VASO). Calibration tended to eliminate the overestimation, although the repeatability was generally moderate and the correlation between calibrated DSC-MRI and pCASL was low (r < 0.25). However, using GM instead of WM ROIs for extraction of CFs resulted in improved repeatability. Conclusion Both calibration approaches provided reasonable absolute levels of GM CBF, although the calibration methods suffered from low signal-to-noise ratio, resulting in weak repeatability and difficulties in showing high degrees of correlation with pCASL measurements.
      PubDate: 2014-12-01
  • Quantitative lung ventilation using Fourier decomposition MRI; comparison
           and initial study
    • Abstract: Objective The Fourier decomposition (FD) method is a noninvasive method for assessing ventilation and perfusion-related information in the lungs, but the lack of quantifiable values is a drawback. We demonstrate a novel technique for quantification of the FD ventilation maps, compare it to two published methods, and show results from both healthy volunteers and patients diagnosed with lung cancer. Materials and methods We quantified the standard FD ventilation images by utilizing additional information, i.e., the zero-frequency component image, which is also obtained from the Fourier analysis. This image acts as a baseline for the changes recorded in the FD ventilation image and can therefore be used to calculate the ventilation. Using this technique, we compared the ventilation values from ten healthy volunteers and ten patients to two previously published methods for quantitative ventilation assessment. Results All methods showed good overall agreement (mean difference between the methods was 14–38 ml/min). The mean minute ventilation for the FD method was calculated to be 693 ml/min for a 2D slice, which is in the expected range. Conclusion The zero-frequency component image can be used as a baseline to quantify the FD ventilation maps. Our initial study showed good agreement with published methods in healthy volunteers, but less so in patients with lung cancer.
      PubDate: 2014-12-01
  • Delayed hepatic signal recovery on ferucarbotran-enhanced magnetic
           resonance images in a rat model with regional liver irradiation
    • Abstract: Object To determine whether superparamagnetic iron oxide (SPIO)-enhanced magnetic resonance (MR) imaging could demonstrate signal recovery delay in irradiated areas of rat livers. We also investigated the relationship between MR imaging and histological findings. Materials and methods Twelve rats received 20 μmol iron/kg of SPIO followed by X-irradiation to the right upper abdomen 4 h later. Radiation doses were 0, 50 and 70 Gy. Hepatic signals were assessed on unenhanced T 2 * -weighted images for up to 7 days using a 9.4-Tesla scanner. The livers were excised on day 7 and examined histologically. Results Normalized relative signal intensity of 70 Gy-irradiated right liver lobe (2.36 ± 0.22) and 50 Gy-irradiated right liver lobe (2.37 ± 0.46) was significantly lower than that of the non-irradiated right liver lobe (4.04 ± 0.28) on day 7, respectively (p < 0.05). Pearson product-moment correlation coefficient between relative intensity of the liver and the number of hepatic iron deposits was −0.588 (p < 0.01). Conclusion Superparamagnetic iron oxide-enhanced MR imaging could demonstrate signal recovery delay in irradiated areas of rat livers. It seems that the signal recovery delay in irradiated areas was due to SPIO-derived iron deposition. Hepatic signal recovery could be a novel diagnostic marker for delineation of irradiated areas.
      PubDate: 2014-12-01
  • Endometrial cancer: diagnostic value of quantitative measurements of
           microvascular changes with DCE-MR imaging
    • Abstract: Object To assess the diagnostic value of dynamic contrast-enhanced (DCE) perfusion-magnetic resonance imaging (MRI) in detection, characterization and grading of endometrial cancer, using histopathological analysis as the standard of reference. Materials and methods Eighty patients with histologically proven endometrial carcinoma who underwent MRI (1.5 T magnet) of the pelvis for staging purposes were enrolled in the study. Each MR examination consisted of multiplanar T2 and T1-weighted turbo spin echo (TSE) sequences and T1-weighted gradient echo sequences before, during and after the administration of contrast medium. For each patient colour perfusion maps were derived from the dynamic sequences using a dedicated workstation. On the maps a region of interest was manually drawn both on normal myometrium and on the endometrial lesion. Then the following perfusion parameters were automatically calculated: relative enhancement (RE, %), maximum enhancement (ME, %), maximum relative enhancement (MRE, %) and time to peak (TTP, s). Results All patients underwent total hysterectomy. Histopathological analysis documented: G1 tumour in 21 patients, G2 tumour in 44 patients, G3 tumour in 14 patients and one squamous cell carcinoma. The following mean value perfusion parameters, with corresponding mean standard deviation, were obtained for endometrial cancer: RE (%) = 59.3 ± 36.3; ME (%) = 862.7 ± 475.9; MRE (%) = 75.3 ± 37.6 and TTP (s) = 164.7 ± 78. RE, ME and MRE were lower in tumour lesions than in normal myometrium (p < 0.001) and significantly higher values (p < 0.001) of perfusion parameters were obtained for G1 (well-differentiated) tumours as compared to those in G2 and G3 (moderately and poorly differentiated) lesions. Conclusion DCE perfusion-MRI can provide quantitative information on tissue vascularity, which may be of help in detecting endometrial cancer and in the assessment of tumour grading.
      PubDate: 2014-12-01
  • Pilot study of Iopamidol-based quantitative pH imaging on a clinical 3T MR
    • Abstract: Objective The objective of this study was to show the feasibility to perform Iopamidol-based pH imaging via clinical 3T magnetic resonance imaging (MRI) using chemical exchange saturation transfer (CEST) imaging with pulse train presaturation. Materials and methods The pulse train presaturation scheme of a CEST sequence was investigated for Iopamidol-based pH measurements using a 3T magnetic resonance (MR) scanner. The CEST sequence was applied to eight tubes filled with 100-mM Iopamidol solutions with pH values ranging from 5.6 to 7.0. Calibration curves for pH quantification were determined. The dependence of pH values on the concentration of Iopamidol was investigated. An in vivo measurement was performed in one patient who had undergone a previous contrast-enhanced computed tomography (CT) scan with Iopamidol. The pH values of urine measured with CEST MRI and with a pH meter were compared. Results In the measured pH range, pH imaging using CEST imaging with pulse train presaturation was possible. Dependence between the pH value and the concentration of Iopamidol was not observed. In the in vivo investigation, the pH values in the human bladder measured by the Iopamidol CEST sequence and in urine were consistent. Conclusion Our study shows the feasibility of using CEST imaging with Iopamidol for quantitative pH mapping in vitro and in vivo on a 3T MR scanner.
      PubDate: 2014-12-01
  • An RF-induced voltage sensor for investigating pacemaker safety in MRI
    • Abstract: Object Magnetic resonance imaging (MRI) is inadvisable for patients with pacemakers, as radiofrequency (RF) voltages induced in the pacemaker leads may cause the device to malfunction. Our goal is to develop a sensor to measure such RF-induced voltages during MRI safety tests. Materials and methods A sensor was designed (16.6 cm2) for measuring voltages at the connection between the pacemaker lead and its case. The induced voltage is demodulated, digitized, and transferred by optical fibres. The sensor was calibrated on the bench using RF pulses of known amplitude and duration. Then the sensor was tested during MRI scanning at 1.5 T in a saline gel filled phantom. Results Bench tests showed measurement errors below 5 % with a (−40 V; +40 V) range, a precision of 0.06 V, and a temporal resolution of 24.2 μs. In MRI tests, variability in the measured voltages was below 3.7 % for 996 measurements with different sensors and RF exposure. Coupling between the sensor and the MRI electromagnetic environment was estimated with a second sensor connected and was below 6.2 %. For a typical clinical MRI sequence, voltages around ten Vp were detected. Conclusion We have built an accurate and reproducible tool for measuring RF-induced voltages in pacemaker leads during MR safety investigations. The sensor might also be used with other conducting cables including those used for electrocardiography and neurostimulation .
      PubDate: 2014-12-01
  • Automatic brain segmentation using fractional signal modeling of a
           multiple flip angle, spoiled gradient-recalled echo acquisition
    • Abstract: Object The aim of this study was to demonstrate a new automatic brain segmentation method in magnetic resonance imaging (MRI). Materials and methods The signal of a spoiled gradient-recalled echo (SPGR) sequence acquired with multiple flip angles was used to map T1, and a subsequent fit of a multi-compartment model yielded parametric maps of partial volume estimates of the different compartments. The performance of the proposed method was assessed through simulations as well as in-vivo experiments in five healthy volunteers. Results Simulations indicated that the proposed method was capable of producing robust segmentation maps with good reliability. Mean bias was below 3 % for all tissue types, and the corresponding similarity index (Dice’s coefficient) was over 95 % (SNR = 100). In-vivo experiments yielded realistic segmentation maps, with comparable quality to results obtained with an established segmentation method. Relative whole-brain cerebrospinal fluid, grey matter, and white matter volumes were (mean ± SE) respectively 6.8 ± 0.5, 47.3 ± 1.1, and 45.9 ± 1.3 % for the proposed method, and 7.5 ± 0.6, 46.2 ± 1.2, and 46.3 ± 0.9 % for the reference method. Conclusion The proposed approach is promising for brain segmentation and partial volume estimation. The straightforward implementation of the method is attractive, and protocols that already rely on SPGR-based T1 mapping may employ this method without additional scans.
      PubDate: 2014-12-01
  • Spatio-temporal wavelet regularization for parallel MRI reconstruction:
           application to functional MRI
    • Abstract: Background Parallel magnetic resonance imaging (MRI) is a fast imaging technique that helps acquiring highly resolved images in space/time. Its performance depends on the reconstruction algorithm, which can proceed either in the k-space or in the image domain. Objective and methods To improve the performance of the widely used SENSE algorithm, 2D regularization in the wavelet domain has been investigated. In this paper, we first extend this approach to 3D-wavelet representations and the 3D sparsity-promoting regularization term, in order to address reconstruction artifacts that propagate across adjacent slices. The resulting optimality criterion is convex but nonsmooth, and we resort to the parallel proximal algorithm to minimize it. Second, to account for temporal correlation between successive scans in functional MRI (fMRI), we extend our first contribution to 3D +  \(t\) acquisition schemes by incorporating a prior along the time axis into the objective function. Results Our first method (3D-UWR-SENSE) is validated on T1-MRI anatomical data for gray/white matter segmentation. The second method (4D-UWR-SENSE) is validated for detecting evoked activity during a fast event-related functional MRI protocol. Conclusion We show that our algorithm outperforms the SENSE reconstruction at the subject and group levels (15 subjects) for different contrasts of interest (motor or computation tasks) and two parallel acceleration factors ( \(R=2\) and \(R=4\) ) on \(2\times 2\times 3\,\hbox{mm}^3\) echo planar imaging (EPI) images.
      PubDate: 2014-12-01
  • Simple recipe for accurate T 2 quantification with multi spin-echo
    • Abstract: Objective The quantification of magnetic resonance relaxation parameters T 1 and T 2 have the potential for improved disease detection and classification over standard clinical weighted imaging. Performing a mono-exponential fit on multi spin-echo (MSE) data provides quantitative T 2 values in a clinically acceptable scan-time. However, due to technical imperfections of refocusing pulses, stimulated echo contributions to the signals lead to significant deviations in the resulting T 2 values. In this work, a simple auto-calibrating correction procedure is presented, allowing the accurate estimation of T 2 from MSE acquisitions. Materials and methods Correction factors for T 2 values obtained from MSE acquisitions with a mono-exponential fit are derived from simulations following the extended phase graph formulation. A closed formula is given for the calculation of the required correction factors directly from the measured data itself. Results Simulations and phantom experiments show high accuracy of corrected T 2 values for a wide range of clinically relevant T 2 values and for different nominal refocusing flip angles. In addition, corrected T 2 maps of the human brain are presented. Conclusion A simple recipe is provided to correct T 2 values obtained from MSE acquisitions via a mono-exponential fit for the influence of stimulated echoes. Since all required parameters are extracted from the data themselves, no additional acquisitions are required.
      PubDate: 2014-12-01
  • Automatic segmentation of subcutaneous mouse tumors by multiparametric MR
           analysis based on endogenous contrast
    • Abstract: Object Contrast-enhanced T1-weighted imaging is usually included in MRI procedures for automatic tumor segmentation. Use of an MR contrast agent may not be appropriate for some applications, however. We assessed the feasability of automatic tumor segmentation by multiparametric cluster analysis that uses intrinsic MRI contrast only. Materials and methods Multiparametric MRI consisting of quantitative T1, T2, and apparent diffusion coefficient (ADC) mapping was performed in mice bearing subcutaneous tumors (n = 21). k-means and fuzzy c-means clustering with all possible combinations of MRI parameters, i.e. feature vectors, and 2–7 clusters were performed on the multiparametric data. Clusters associated with tumor tissue were selected on the basis of the relative signal intensity of tumor tissue in T2-weighted images. The optimum segmentation method was determined by quantitative comparison of automatic segmentation with manual segmentation performed by three observers. In addition, the automatically segmented tumor volumes from seven separate tumor data sets were quantitatively compared with histology-derived tumor volumes. Results The highest similarity index between manual and automatic segmentation (SImanual,automatic = 0.82 ± 0.06) was observed for k-means clustering with feature vector {T2, ADC} and four clusters. A strong linear correlation between automatically and manually segmented tumor volumes (R 2 = 0.99) was observed for this segmentation method. Automatically segmented tumor volumes also correlated strongly with histology-derived tumor volumes (R 2 = 0.96). Conclusion Automatic segmentation of mouse subcutaneous tumors can be achieved on the basis of endogenous MR contrast only.
      PubDate: 2014-11-27
  • Single-channel, box-shaped, monopole-type antenna for B1+ field
           manipulation in conjunction with the traveling-wave concept in 9.4 T
    • Abstract: Object We have developed a single-channel, box-shaped, monopole-type antenna which, if used in two different configurations, excites complementary B1+ field distributions in the traveling-wave setup. Materials and methods A new monopole-type, single-channel antenna for RF excitation in 9.4 T magnetic resonance imaging is proposed. The antenna is entirely made of copper without lumped elements. Two complementary B1+ field distributions of two different antenna configurations were measured and combined as a root sum of squares. B1+ field inhomogeneity of the combined maps was calculated and compared with published results. Results By combining B1+ field distributions generated by two antenna configurations, a “no voids” pattern was achieved for the entire upper brain. B1+ inhomogeneity of approximately 20 % was achieved for sagittal and transverse slices; it was <24 % for coronal slices. The results were comparable with those from CP, with “no voids” in slice B1+ inhomogeneity of multichannel loop arrays. The efficiency of the proposed antenna was lower than that of a multichannel array but comparable with that of a patch antenna. Conclusion The proposed single-channel antenna is a promising candidate for traveling-wave brain imaging. It can be combined with the time-interleaved acquisition of modes (TIAMO) concept if reconfigurability is obtained with a single-antenna element.
      PubDate: 2014-11-20
  • Phase-corrected bipolar gradients in multi-echo gradient-echo sequences
           for quantitative susceptibility mapping
    • Abstract: Objective Large echo spacing of unipolar readout gradients in current multi-echo gradient-echo (GRE) sequences for mapping fields in quantitative susceptibility mapping (QSM) can be reduced using bipolar readout gradients thereby improving acquisition efficiency. Materials and methods Phase discrepancies between odd and even echoes in the bipolar readout gradients caused by non-ideal gradient behaviors were measured, modeled as polynomials in space and corrected for accordingly in field mapping. The bipolar approach for multi-echo GRE field mapping was compared with the unipolar approach for QSM. Results The odd–even-echo phase discrepancies were approximately constant along the phase encoding direction and linear along the readout and slice-selection directions. A simple linear phase correction in all three spatial directions was shown to enable accurate QSM of the human brain using a bipolar multi-echo GRE sequence. Bipolar multi-echo acquisition provides QSM in good quantitative agreement with unipolar acquisition while also reducing noise. Conclusion With a linear phase correction between odd–even echoes, bipolar readout gradients can be used in multi-echo GRE sequences for QSM.
      PubDate: 2014-11-20
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