MAGMA Magnetic Resonance Materials in Physics Biology and Medicine (MAGN RESON MATER PHY)

Publications in this journal

• Article: High resolution morphologic imaging and T2 mapping of cartilage at 7 Tesla: comparison of cartilage repair patients and healthy controls.

  Gregory Chang, Ding Xia, Orrin Sherman, Eric Strauss, Laith Jazrawi, Michael P Recht, Ravinder R Regatte
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  ABSTRACT: OBJECT: Our objective was to use 7 T MRI to compare cartilage morphology (thickness) and collagen composition (T2 values) in cartilage repair patients and healthy controls. MATERIALS AND METHODS: We scanned the knees of 11 cartilage repair patients and 11 controls on a 7 T MRI scanner using a high-resolution, gradient-echo sequence to measure cartilage thickness and a multi-echo spin-echo sequence to measure cartilage T2 values. We used two-tailed t tests to compare cartilage thickness and T2 values in: repair tissue (RT) versus adjacent cartilage (AC); RT versus healthy control cartilage (HC); AC versus HC. RESULTS: Mean thickness in RT, AC, HC were: 2.2 ± 1.4, 3.6 ± 1.1, 3.3 ± 0.7 mm. Differences in thickness between RT-AC (p = 0.01) and RT-HC (p = 0.02) were significant, but not AC-HC (p = 0.45). Mean T2 values in RT, AC, HC were: 51.6 ± 7.6, 40.0 ± 4.7, 45.9 ± 3.7 ms. Differences in T2 values between RT-AC (p = 0.0005), RT-HC (p = 0.04), and AC-HC (p = 0.004) were significant. CONCLUSION: 7 T MRI allows detection of differences in morphology and collagen architecture in: (1) cartilage repair tissue compared to adjacent cartilage and (2) cartilage repair tissue compared to cartilage from healthy controls. Although cartilage adjacent to repair tissue may be normal in thickness, it can demonstrate altered collagen composition.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 05/2013;
• Article: Single spin-echo T 2 relaxation times of cerebral metabolites at 14.1 T in the in vivo rat brain.

  Lijing Xin, Giulio Gambarota, Cristina Cudalbu, Vladimír Mlynárik, Rolf Gruetter
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  ABSTRACT: OBJECT: To determine the single spin-echo T 2 relaxation times of uncoupled and J-coupled metabolites in rat brain in vivo at 14.1 T and to compare these results with those previously obtained at 9.4 T. MATERIALS AND METHODS: Measurements were performed on five rats at 14.1 T using the SPECIAL sequence and TE-specific basis-sets for LCModel analysis. RESULTS AND CONCLUSION: The T 2 of singlets ranged from 98 to 148 ms and T 2 of J-coupled metabolites ranged from 72 ms (glutamate) to 97 ms (myo-inositol). When comparing the T 2s of the metabolites measured at 14.1 T with those previously measured at 9.4 T, a decreasing trend was found (p < 0.0001). We conclude that the modest shortening of T 2 at 14.1 T has a negligible impact on the sensitivity of the (1)H MRS when performed at TE shorter than 10 ms.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 04/2013;
• Article: 3T Renal (23)Na-MRI: effects of desmopressin in patients with central diabetes insipidus.

  Stefan Haneder, Henrik J Michaely, Simon Konstandin, Lothar R Schad, John N Morelli, Bernhard K Krämer, Stefan O Schoenberg, Alexander Lammert
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  ABSTRACT: PURPOSE: The purpose of this prospective study was to assess physiologic changes in the renal corticomedullary (23)Na-concentration ([(23)Na]) gradient with (23)Na-MRI at 3.0T in patients with central diabetes insipidus (CDI) before and after intranasal administration of 20 μg desmopressin (DDAVP). METHODS AND MATERIALS: Four patients with CDI (all male, mean age 60.2 years) were included in this IRB-approved study. For (23)Na-imaging, a 3D density adapted, radial GRE-sequence (TE = 0.55 ms; TR = 120 ms; projections = 8,000; spatial resolution = 5 × 5 × 5 mm(3)) was used in combination with a dedicated (23)Na-coil and reference phantoms. The corticomedullary [(23)Na] gradient (in mmol/L/mm) was calculated pixel-by-pixel along a linear region-of-interest (ROI) spanning from the renal cortex in the direction of the medulla. Mean ± SDs of [(23)Na] were calculated for each patient as well as for the entire group. RESULTS: Mean [(23)Na] increased along the corticomedullary gradient from the cortex (pre-DDAVP 38.0 ± 6.3 mmol/L vs. post-DDAVP 30.7 ± 3.5 mmol/L) to the medulla (pre-DDAVP 71.6 ± 14.8 mmol/L vs. post-DDAVP 59.7 ± 10.8 mmol/L). The overall mean decrease of [(23)Na] after DDAVP administration was 17.1 ± 1.1 %. CONCLUSION: (23)Na-MRI with state-of-the-art techniques at 3T depicts the physiologic renal response to the administration of desmopressin in patients with central diabetes insipidus.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 04/2013;
• Article: Spatial analysis of diffusion tensor tractography statistics along the inferior fronto-occipital fasciculus with application in progressive supranuclear palsy.

  Johanna Mårtensson, Markus Nilsson, Freddy Ståhlberg, Pia C Sundgren, Christer Nilsson, Danielle van Westen, Elna-Marie Larsson, Jimmy Lätt
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  ABSTRACT: BACKGROUND: The purpose of the study was to develop a method for analysis of diffusion parameters along white matter (WM) tracts, using spatial normalization based on anatomical landmarks, and to introduce the apparent area coefficient (AAC). The method's applicability was tested in the inferior fronto-occipital fasciculus (IFO) in patients with progressive supranuclear palsy (PSP) and healthy controls (HCs). METHODS: A framework for analysis of diffusion parameters was developed. Spatial normalization of the tracts was performed using anatomical landmarks, to avoid deformations caused by cerebral atrophy. Initially, 38 HCs were used to optimize a threshold for the minimal size of regions that differ between groups. The fractional anisotropy, mean diffusivity, AAC, and the hemispheric asymmetry index (AI), were compared between 11 PSP patients and 15 HCs. RESULTS: The method was feasible for analysis of PSP patients and HCs. The AI showed that the observed hemispheric asymmetry of AAC was significantly larger in PSP patients compared with HCs in small regions of the IFO. CONCLUSIONS: The method was successfully employed for analysis of diffusion parameters along the IFO in a patient group. This method can be potentially useful in studies of WM diseases, with or without cerebral atrophy.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 03/2013;
• Article: 3D gradient system for two B 0 field directions in earth's field MRI.

  Steffen Lother, Uvo Hoelscher, Thomas Kampf, Peter Jakob, Florian Fidler
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  ABSTRACT: OBJECT: A new gradient system for earth's field magnetic resonance imaging (EFMRI) is presented that can be rotated relatively to the earth's field direction while maintaining the ability to encode images. Orthogonal components of the gradient field are exploited to reduce the number of gradient coils. MATERIALS AND METHODS: Two favorable orientations of the gradient system relative to the earth's magnetic field (parallel and perpendicular) are discussed. We introduce the theory for the magnetic fields of the new gradient system and illustrate the design of the coil geometries which were worked out with the help of simulations and a numerical optimization algorithm. Field mapping measurements and imaging experiments in the two different orientations of the gradient system were carried out. RESULTS: Orthogonal components of the gradient field take over the role of the additionally needed gradient fields when the gradient system is rotated relative to the earth's magnetic field. The results from the field mapping and imaging experiments verify the presented theory and show the functionality of the new gradient system. CONCLUSION: The presented system demonstrates that gradient coils can be used for image encoding in multiple directions. This fact can be exploited to realize an EFMRI setup for parallel and perpendicular prepolarization with a single set of gradient coils.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 03/2013;
• Article: Magnetic resonance imaging of solid dental restoration materials using 3D UTE sequences: visualization and relaxometry of various compounds.

  Ulrich Grosse, Roland Syha, Dimitrios Papanikolaou, Petros Martirosian, Gerd Grözinger, Christoph Schabel, Fritz Schick, Fabian Springer
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  ABSTRACT: OBJECT: Due to an increasing scientific interest in MR-imaging of carious lesions and teeth, an accurate signal characterization of dental restoration materials is necessary for optimization of MR sequence protocols and evaluation of material degradation. Therefore, signal yield and relaxation behavior of common dental restoration materials in comparison to those of dentine of extracted human teeth were assessed in vitro by ultrashort echo time (UTE) sequences. MATERIALS AND METHODS: Eighteen material samples and dentine of two freshly extracted human teeth were investigated on a 3T whole-body clinical MR-scanner. Transverse (T2*) and longitudinal relaxation times (T1) were quantified using a recently published modified Ernst equation that takes relevant in-pulse relaxation effects into account. RESULTS: All investigated samples could be successfully visualized but maximum signal yield was highly variable between samples. T1-values of the investigated dental restoration materials ranged between 28 and 365 ms, whereas T2*-values ranged between 96 and 917 μs. In contrast, T1-values of dentine (T1 = 545 ms ± 299 ms) were higher, while T2*-values (T2* = 478 μs ± 271 μs) showed similar values. CONCLUSIONS: Dental restoration materials and dentine of extracted human teeth can be visualized by UTE sequences and show a broad range of signal yield and relaxation times.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 03/2013;
• Article: A modified oddball paradigm for investigation of neural correlates of attention: a simultaneous ERP-fMRI study.

  Mateusz Rusiniak, Monika Lewandowska, Tomasz Wolak, Agnieszka Pluta, Rafał Milner, Małgorzata Ganc, Andrzej Włodarczyk, Andrzej Senderski, Lech Sliwa, Henryk Skarżyński
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  ABSTRACT: INTRODUCTION: The objective of the presented study was to develop and evaluate a P300 experimental protocol for simultaneous registration of event-related potentials (ERPs) and functional MRI (fMRI) data with continuous imaging. It may be useful for investigating attention and working memory processes in specific populations, such as children and neuropsychiatric patients. MATERIALS AND METHODS: Eleven children were investigated with simultaneous ERP-fMRI. To fulfill requirements of both BOLD and electroencephalographic signal registration, a modified oddball task was used. To verify the ERP-fMRI protocol we also performed a study outside the scanner using a typical two-stimuli oddball paradigm. RESULTS: Localization of the P300 component of ERPs partially corresponded with fMRI results in the frontal and parietal brain regions. FMRI activations were found in: middle frontal gyrus, insula, SMA, parietal lobule, thalamus, and cerebellum. Our modified oddball task provided ERP-fMRI results with high level of significance (EEG SNR = 35, fMRI p < 0.05-Bonf.). ERPs obtained in the scanner were comparable with those registered outside the scanner, although some differences in the amplitude were noticed, mainly in the N100 component. CONCLUSION: In our opinion the presented paradigm may be successfully applied for simultaneous ERP-fMRI registration of neural correlates of attention in vulnerable populations.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 03/2013;
• Article: Quantitative liver ADC measurements using diffusion-weighted MRI at 3 Tesla: evaluation of reproducibility and perfusion dependence using different techniques for respiratory compensation.

  Nis Elbrønd Larsen, Søren Haack, Lars Peter Skovgaard Larsen, Erik Morre Pedersen
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  ABSTRACT: OBJECT: Diffusion weighted imaging (DWI) of the liver suffers from low signal to noise making 3 Tesla (3 T) an attractive option, but 3 T data is scarce. It was the aim to study the influence of different b values and respiratory compensation methods (RCM) on the apparent diffusion coefficient (ADC) level and on ADC reproducibility at 3 T. MATERIALS AND METHODS: Ten healthy volunteers and 12 patients with malignant liver lesions underwent repeated (2-22 days) breathhold, free-breathing and respiratory triggered DWI at 3 T using b values between 0 and 1,000 s/mm2. RESULTS: The ADCs changed up to 150 % in healthy livers and up to 48 % in malignant lesions depending on b value combinations. Best ADC reproducibility in healthy livers were obtained with respiratory triggering (95 % limits of agreement: ±0.12) and free-breathing (±0.14). In malignant lesions equivalent reproducibility was obtained with less RCM dependence. The use of a lower maximum b value (b = 500) decreased reproducibility (±0.14 to ±0.32) in both normal liver and malignant lesions. CONCLUSION: Large differences in absolute ADC values and reproducibility caused by varying combinations of clinically realistic b values were demonstrated. Different RCMs caused smaller differences. Lowering maximum b value to 500 increased limits of agreement up to a factor of two. Serial ADC changes larger than approximately 15 % can be detected confidently on an individual basis in both malignant lesions and normal liver parenchyma at 3 T using appropriate b values and respiratory compensation.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 03/2013;
• Article: Iterative separation of transmit and receive phase contributions and B (1) (+) -based estimation of the specific absorption rate for transmit arrays.

  Stefanie Buchenau, Martin Haas, Daniel Nicolas Splitthoff, Jürgen Hennig, Maxim Zaitsev
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  ABSTRACT: OBJECT: The specific absorption rate (SAR) can be determined from radiofrequency transmit fields measured via magnetic resonance imaging. MATERIALS AND METHODS: The proposed method estimates the SAR solely from the complex transmit field (B (1) (+) ) by taking into account the particular properties of the electromagnetic field generated by an 8-channel transmit array. It is further based on an iterative consistency check between the measured B (1) (+) magnitude and an appropriate field estimate fulfilling Maxwell's equations. For testing the method, simulations and phantom experiments were performed for a multi-transmit array at 3T using a cylindrical phantom. RESULTS: The method's robustness with respect to the assumptions made about electric tissue properties as well as its stability under different initial conditions regarding the signal phase was shown. A high sensitivity to signal noise was found. Robust reconstruction results were achieved including information from more than two transmit elements. The validity of the experimental results was confirmed by a qualitative comparison to simulated electromagnetic fields. CONCLUSIONS: The method allows the determination of the SAR as well as the transmit phase of the individual channels of a multi-transmit array. With additional B(0) inhomogeneity measurements, a reconstruction of the receive phase is feasible independent of the receive coil type in use.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 02/2013;
• Article: Alleviating artifacts in (1)H MRI thermometry by single scan spatiotemporal encoding.

  Rita Schmidt, Lucio Frydman
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  ABSTRACT: OBJECTIVE: Recent years have seen an increased interest in combining MRI thermometry with devices capable of destroying malignancies by heat ablation. Expected from the MR protocols are accurate and fast thermal characterizations, providing real time feedback on restricted tissue volumes and/or rapidly moving organs like liver. This article explores the potential advantages of relying on spatiotemporally encoded (SPEN) sequences for retrieving real-time thermometric images based on the water's proton resonance frequency (PRF) shifts. MATERIALS AND METHODS: Hybrid spatiotemporal/k-space encoding single-scan MRI experiments were implemented on animal and human scanners, and their abilities to deliver single- and multi-slice real-time thermometric measurements based on PRF-derived phase maps in phantoms and in vivo, were compared against echo planar imaging (EPI) and gradient-echo counterparts. RESULTS: Under comparable acquisition conditions, SPEN exhibited advantages vis-à-vis EPI in terms of dealing with inhomogeneous magnetic field distortions, with shifts arising due to changes in the central frequency offsets, with PRF distributions, and for zooming into restricted fields-of-view without special pulse sequence provisions. CONCLUSION: This work confirms the ability of SPEN sequences, particularly when implemented under fully-refocused conditions, to exploit their built-in robustness to shift- and field-derived inhomogeneities for monitoring thermal changes in real-time under in vitro and in vivo conditions.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 02/2013;
• Article: The role of tissue microstructure and water exchange in biophysical modelling of diffusion in white matter.

  Markus Nilsson, Danielle van Westen, Freddy Ståhlberg, Pia C Sundgren, Jimmy Lätt
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  ABSTRACT: Biophysical models that describe the outcome of white matter diffusion MRI experiments have various degrees of complexity. While the simplest models assume equal-sized and parallel axons, more elaborate ones may include distributions of axon diameters and axonal orientation dispersions. These microstructural features can be inferred from diffusion-weighted signal attenuation curves by solving an inverse problem, validated in several Monte Carlo simulation studies. Model development has been paralleled by microscopy studies of the microstructure of excised and fixed nerves, confirming that axon diameter estimates from diffusion measurements agree with those from microscopy. However, results obtained in vivo are less conclusive. For example, the amount of slowly diffusing water is lower than expected, and the diffusion-encoded signal is apparently insensitive to diffusion time variations, contrary to what may be expected. Recent understandings of the resolution limit in diffusion MRI, the rate of water exchange, and the presence of microscopic axonal undulation and axonal orientation dispersions may, however, explain such apparent contradictions. Knowledge of the effects of biophysical mechanisms on water diffusion in tissue can be used to predict the outcome of diffusion tensor imaging (DTI) and of diffusion kurtosis imaging (DKI) studies. Alterations of DTI or DKI parameters found in studies of pathologies such as ischemic stroke can thus be compared with those predicted by modelling. Observations in agreement with the predictions strengthen the credibility of biophysical models; those in disagreement could provide clues of how to improve them. DKI is particularly suited for this purpose; it is performed using higher b-values than DTI, and thus carries more information about the tissue microstructure. The purpose of this review is to provide an update on the current understanding of how various properties of the tissue microstructure and the rate of water exchange between microenvironments are reflected in diffusion MRI measurements. We focus on the use of biophysical models for extracting tissue-specific parameters from data obtained with single PGSE sequences on clinical MRI scanners, but results obtained with animal MRI scanners are also considered. While modelling of white matter is the central theme, experiments on model systems that highlight important aspects of the biophysical models are also reviewed.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 02/2013;
• Article: A battery-driven, low-field NMR unit for thermally and hyperpolarized samples.

  Robert Borowiak, Niels Schwaderlapp, Frank Huethe, Thomas Lickert, Elmar Fischer, Sébastien Bär, Jürgen Hennig, Dominik von Elverfeldt, Jan-Bernd Hövener
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  ABSTRACT: OBJECT: The design of a multinuclear low-field NMR unit with variable field strength <6 mT providing accurate spin manipulations and sufficient sensitivity for direct detection of samples in thermal equilibrium to aid parahydrogen-based hyperpolarization experiments. MATERIALS AND METHODS: An optimized, resistive magnet connected to a battery or wall-power driven current source was constructed to provide a magnetic field <6 mT. A digital device connected to a saddle-shaped transmit- and solenoid receive-coil enabled MR signal excitation and detection with up to 10(6) samples/s, controlled by a flexible pulse-programming software. RESULTS: The magnetization of thermally polarized samples at 1.8 and 5.7 mT is detected in a single acquisition with a SNR ≈10(1) and ≈10(2) and a line width of 42 and 32 Hz, respectively. Nuclear spins are manipulated to an uncertainty of ±1° by means of pulses, which can be arranged in an arbitrary combination. As a demonstration, standard experiments for the measurement of relaxation parameters of thermally polarized samples were implemented. The detection of much stronger hyperpolarized signal was exemplified employing parahydrogen. CONCLUSION: Direct detection of thermal and hyperpolarized (1)H-MR signal in a single acquisition and accurate spin manipulations at 1.8 and 5.5 mT were successfully demonstrated.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 02/2013;
• Article: Highly efficient 3D motion-compensated abdomen MRI from undersampled golden-RPE acquisitions.

  Christian Buerger, Claudia Prieto, Tobias Schaeffter
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  ABSTRACT: OBJECT: A common approach to compensate for respiratory motion in free-breathing 3D magnetic resonance imaging (MRI) is navigator gating where MRI data is only acquired when the respiratory signal coincides within a small predefined acceptance window. However, this leads to poor scan efficiency and prolonged scan times. Here, we propose a method to reconstruct motion-compensated 3D MRI of the abdomen acquired during free-breathing with nearly 100 % scan efficiency without increasing scan time. MATERIALS AND METHODS: The approach is based on a self-gated golden-radial phase encoding sampling scheme that allows for the reconstruction of multiple undersampled 3D images at different respiratory positions. Non-rigid image registrations and time-wise motion field interpolations are employed to form 3D motion models that combine all low-quality images into one high-quality motion-compensated image. RESULTS: Our highly efficient technique allows reconstruction of 3D liver MRI with a high isotropic resolution of 1.75 mm from a short acquisition of 1.1 min. The approach is validated in 10 healthy volunteers by comparing image quality to data sets acquired with a self-gating approach. CONCLUSION: Our method reduces scan time by 56 % compared to the gating technique while similar image quality is preserved.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 02/2013;
• Article: Combined acquisition technique (CAT) for high-field neuroimaging with reduced RF power.

  Morwan Choli, Martin Blaimer, Felix A Breuer, Philipp Ehses, Oliver Speck, Andreas J Bartsch, Peter M Jakob
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  ABSTRACT: OBJECT: Clinical 3 T MRI systems are rapidly increasing and MRI systems with a static field of 7 T or even more have been installed. The RF power deposition is proportional to the square of the static magnetic field strength and is characterized by the specific absorption rate (SAR). Therefore, there exist defined safety limits to avoid heating of the patient. Here, we describe a hybrid method to significantly reduce the SAR compared to a turbo-spin-echo (TSE) sequence. MATERIALS AND METHODS: We investigate the potential benefits of a combined acquisition technique (CAT) for high-field neuroimaging at 3 and 7 T. The TSE/EPI CAT experiments were performed on volunteers and patients and compared with standard TSE and GRASE protocols. Problems and solutions regarding T2 weighted CAT imaging are discussed. RESULTS: We present in vivo images with T2 and proton density contrast obtained on 3 and 7 T with significant SAR reduction (up to 60 %) compared with standard TSE. Image quality is comparable to TSE but CAT shows fewer artifacts than a GRASE sequence. CONCLUSION: CAT is a promising candidate for neuroimaging at high fields up to 7 T. The SAR reduction allows one to shorten the waiting time between two excitations or to image more slices thereby reducing the overall measurement time.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 01/2013;
• Article: Automatic slice alignment method for cardiac magnetic resonance imaging.

  Shuhei Nitta, Tomoyuki Takeguchi, Nobuyuki Matsumoto, Shigehide Kuhara, Kenichi Yokoyama, Rieko Ishimura, Toshiaki Nitatori
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  ABSTRACT: OBJECTIVES: Automatic slice alignment is important for easier operation and shorter examination times in cardiac magnetic resonance imaging (MRI) examinations. We propose a new automatic slice alignment method for six cardiac planes (short-axis, vertical long-axis, horizontal long-axis, 4-chamber, 2-chamber, and 3-chamber views). MATERIALS AND METHODS: ECG-gated 2D steady-state free precession axial multislice images were acquired using a 1.5-T MRI scanner during a single breath-hold. The scanning time was set to <20 s in 23 volumes from 23 healthy volunteers. In this method, the positions of the mitral valve, cardiac apex, left ventricular outflow tract, tricuspid valve, anterior wall of the heart, and right ventricular corner are detected to determine the positions of six reference planes by combining knowledge-based recognition and image processing techniques. In order to evaluate the results of automatic slice alignment for the short-axis, 4-chamber, 2-chamber, and 3-chamber views, the angular and positional errors between the results obtained by our proposed method and by manual annotation were measured. RESULTS: The average angular errors for the short-axis, 4-chamber, 2-chamber, and 3-chamber views were 3.05°, 4.52°, 7.28°, and 5.79°, respectively. The average positional errors for the short-axis (base), short-axis (apex), 4-chamber, 2-chamber, and 3-chamber views were 6.61°, 3.80°, 1.55°, 1.52°, and 1.48°, respectively. CONCLUSION: The experimental results showed that our proposed method can detect the cardiac planes quickly and accurately. Our method is therefore beneficial to both patients and operators.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 01/2013;
• Article: T (2) relaxometry measurements in low spatial frequency brain regions differ between fast spin-echo and multiple-echo spin-echo sequences.

  Venkateswaran Rajagopalan, Mark J Lowe, Erik B Beall, Guang H Yue, Erik P Pioro
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  ABSTRACT: OBJECT: Dual-echo fast spin-echo (FSE) sequences are used in T (2) relaxometry studies of neurological disorders because of shorter clinical scanning times and protocol simplicity. However, FSE sequences have possible spatial frequency-dependent effects, and derived T (2) values may include errors that depend on the spatial frequency characteristics of the brain region of interest. MATERIALS AND METHODS: Dual-echo FSE and multi-echo spin-echo (MESE) sequences were acquired in nine subjects. The T (2) decay curves for FSE and MESE sequences were estimated and percent error maps were generated. T (2) error values were obtained along each patient's corticospinal tract (CST). Whole-brain white matter (WM) and gray matter (GM) T (2) error values were also obtained. The paired t test was performed to evaluate differences in T (2) values in the CST between FSE and MESE sequences. RESULTS: Histograms of error values in CST and in whole-brain WM and GM structures revealed systematic errors in FSE sequences. Significant differences (P < 0.001) in CST T (2) values were also observed between FSE and MESE sequences. CONCLUSION: Our findings indicate that T (2) values derived from FSE sequences are prone to large errors, even in low spatial frequency regions such as the CST, when compared to MESE sequences. Future studies should be aware of this limitation of FSE sequences.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 01/2013;
• Article: Impact of number of channels on RF shimming at 3T.

  Alexander S Childs, Shaihan J Malik, Declan P O'Regan, Joseph V Hajnal
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  ABSTRACT: OBJECT: At high-field strengths (≥3T) inhomogeneity of the radio frequency (RF) field and RF power deposition become increasingly problematic. Parallel Transmission (PTx)-the use of segmented transmission arrays with independently driven elements-affords the ability to combat both of these issues. There are a variety of existing designs for PTx coils, ranging from systems with two channels to systems with eight or more. In this work, we have investigated the impact of the number of independent channels on the achievable results for both homogeneity improvement and power reduction in vivo. MATERIALS AND METHODS: A 3T Philips Achieva MRI system fitted with an 8-channel PTx body coil was driven so as to emulate configurations with 1, 2 4 and 8 independent channels. RF shimming was used in two different anatomies in order to assess improvements in RF homogeneity. RESULTS: Significant homogeneity improvements were observed when increasing from 1 to 2, 2 to 4, and 4 to 8 channel configurations. Reductions in RF power requirements and local SAR were predicted for increasing numbers of channels. CONCLUSION: Increasing the number of RF transmit channels adds extra degrees of freedom which can be used to benefit homogeneity improvement or power reduction for body imaging at 3T.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 01/2013;
• Article: Does PET/MR in human brain imaging provide optimal co-registration? A critical reflection.

  Uwe Pietrzyk, Hans Herzog
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  ABSTRACT: The introduction of hybrid positron emission/magnetic resonance tomography (PET/MR) in diagnostic clinical imaging was a major step in the evolution of ever-more sophisticated imaging systems combining two strategies formerly regarded as technically incompatible in a single device. The advent of PET/MR opened up many new avenues in clinical and research environments, mainly by providing multi-modality images obtained during a single examination. Ideally, simultaneous data acquisition with hybrid PET/MR should warrant exact image co-registration of all multi-modality image volumes provided by both systems. This assumes that there is negligible mutual electronic, technical and logistical interference on the respective simultaneous measurements. Recently, such hybrid dedicated head and whole-body systems were successfully applied in an increasing number of cases. When employed for brain imaging, PET/MR has the potential to provide high-resolution multi-modality datasets. However, it also demands careful consideration of the multitude of features offered, as well as the limitations. There are open issues that have to be considered, such as the handling of patient motion during extended periods of data acquisition, optimized sampling of derived images to ease the visual interpretation and quantitative evaluation of co-registered images. This paper will briefly summarize the current status of PET/MR within the framework of developments for image co-registration and discuss current limitations and future perspectives.
  MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 01/2013;