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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.

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.

Object: To optimize strategies and measurement parameters for quantification of small fat and water fractions (<10%) in mixtures of both components by 4-point in-phase and opposed-phase gradient-echo imaging and to compare theoretical results with in-vitro experiments using emulsions., Materials and Methods: Theoretical analysis was based on steady-state signal equations for spoiled GRE-sequences and on relaxation properties of water and fat components. For quantification, signals were corrected for T2*-decay, T1-decay, and signal contributions from double bonds. Theoretical results were exemplarily compared to measurements at 1.5 T on emulsions with either low water or fat fractions (0.5-10%) using spoiled 2D- and 3D-GRE-sequences. Excitation flip angle was varied in order to determine suitable values for sensitive detection of small fat/water fractions., Results: Theoretical results and measurements correlated well, especially for 3D-sequences. Maximal sensitivity to a small signal fraction (S (fat) and S (water), respectively), was provided at the Ernst angle of the lower concentrated component. For 2D-sequences, the nominal flip angle had to be increased for compensation of slice profile effects and B(1) inhomogeneities. IP- and OP-echoes are recommended to be acquired in separate measurements with smallest possible receiver bandwidth to increase SNR/unit-time. Lowest detectable fat/water concentration in emulsions under typical conditions regarding spatial resolution and measuring time was approximately 1%., Conclusion: Using IP/OP-imaging with optimized parameters and post-processing, a sensitive and reliable detection of small fat/water fractions larger than 1% is possible in emulsions.

Object: To reduce acquisition time and increase signal-to-noise ratio (SNR) of the wavelet-encoding gradient-echo (GE-WE) sequence used in small field of view dynamic magnetic resonance imaging., Materials and Methods: A GE-WE imaging sequence acquires wavelet lines from a 3-Tesla scanner. Those with intensity below a given threshold are skipped in real time, improving SNR and reducing acquisition time., Results: The SNR of GE-WE images increases by approximately three times in simulation, while the acquisition time reduction ranges from 5 to 10% on phantom tests., Conclusion: This technique improves the SNR of GE-WE images, while reducing acquisition time.

Objective: Single-shot echo planar imaging (EPI) acquisitions at 7T are challenging due to increased distortions, signal dropouts, RF-power requirements, and reduced T2*. This study developed and tested pulse sequence and protocol modifications required to allow high resolution EPI for whole brain functional neuroimaging., Materials and Methods: Using geometric distortion correction methods, modified fat saturation, and parallel imaging, we acquired high resolution single-shot gradient-echo EPI data at 7T with different spatial resolution. The BOLD sensitivity was evaluated and quantified in a breath hold experiment., Results: Single-shot EPI data with isotropic resolution from 3 to 1.1 mm were acquired in human subjects. The RF-power deposition has been reduced to allow up to 22 slices per second. In addition, acoustic noise and helium boil-off have been reduced. A reduction of the fat saturation flip angle resulted in up to 20% signal gain without compromising the fat suppression quality. For the coil used, the BOLD sensitivity is highest for 2 or 1.4 mm isotropic resolution., Conclusion: High resolution single-shot EPI in the whole brain can be performed at 7T with high efficiency, low signal dropout, and without major geometric distortions.

Introduction: The need for a homogeneous magnetic field in magnetic resonance imaging is well established, especially at high static magnetic field strengths where susceptibility-induced image distortions and signal losses become excessively large. Dynamic shim updating, where the optimal set of shim currents is applied for each slice during a multi-slice acquisition, has been shown to improve magnetic field homogeneity to a greater extent than conventional global shimming., Methods: Here, in an initial feasibility study, we show via simulation that improved efficacy of shimming can be achieved by using the novel parcellated dynamic shimming method., Results: The results of these simulations indicate that parcellated dynamic shimming based on just linear shim terms can perform approximately as well as slice-based dynamic shimming with up to third-order shim terms., Conclusions: This work shows that the effective magnetic field inhomogeneity can be further reduced if shimming and image data acquisition are sequentially performed over a series of compact, cuboidal sub-volumes rather than planes. Further work is needed to develop an imaging approach that can be used for the optimal implementation of parcellated dynamic shimming.

Results are presented for a model three-axis gradient coil incorporating active acoustic control which is applied to the switched read gradient during a single-shot rapid echo-planar imaging (EPI) sequence at a field strength of 3.0 T. The total imaging acquisition time was 10.6 ms. Substantial noise reduction is achieved both within the magnet bore and outside the magnet. Typical internal noise reduction over the specimen area is 40 dB(A) whereas outside the acoustic chamber the noise level is reduced by 60-67 dB(A). However these results are relative to a control winding which is switched in phase, adding 6 dB(A) in its non-optimized mode, which is included in the quoted figures.

Objective: The aim of this study is to present a new approach for making quantitative single-voxel T (2) measurements from an arbitrarily shaped region of interest (ROI), where the advantage of the signal-to-noise ratio (SNR) per unit time of the single-voxel approach over conventional imaging approach can be achieved., Materials and Methods: Two-dimensional (2D) spatially selective radiofrequency (RF) pulses are proposed in this work for T (2) measurements based on using interleaved spiral trajectories in excitation k-space (pinwheel excitation pulses), combined with a summed Carr-Purcell Meiboom-Gill (CPMG) echo acquisition. The technique is described and compared to standard multi-echo imaging methods, on a two-compartment water phantom and an excised brain tissue., Results: The studies show good agreement between imaging and our method. The measured improvement factors of SNR per unit time of our single-voxel approach over imaging approach are close to the predicted values., Conclusion: Measuring T (2) relaxation times from a selected ROI of arbitrary shape using a single-voxel rather than an imaging approach can increase the SNR per unit time, which is critical for dynamic T (2) or multi-component T (2) measurements.

Signal void artifacts in gradient echo imaging are caused by the intra-voxel dephasing of the spins. Intra-voxel dephasing can be estimated by computing the field distribution on a sub-grid inside each picture element, followed by integration of all magnetization components. The strategy of computing the artifacts based on the integration of the sub-voxel signal components is presented here for different sub-grids. The coarseness of the sub-grid is directly related to computational effort. The possibility to save memory space and computing time for the dipole model by computing the field only on a sub-grid is addressed in the presented article. It is investigated as to how far computational time and memory space can be reduced by using an appropriate sub-grid. Numerical results for a model of a partially diamagnetically coated needle shaft are compared to experimental findings. In the case of a pure titanium needle, it is shown as being sufficient to compute the field distribution on a sub-grid that is at least four times coarser in each direction than the grid used to discretize the object in the related MR image. Due to three nested loops over the 3D grid, the need for memory space and time is saved by a factor 64. Deviations between measurements and simulations for the broad side of the artifact (uncompensated) and for the small side of the artifact (compensated) were 15.5%, respectively, 19.1% for orientation parallel to the exterior field, and 22.7%, respectively, 23.1% for orientation perpendicular to the exterior field.

Objectives: In ultra-short echo time (UTE) imaging, the transverse magnetization experiences significant decay during data acquisition, which gives rise to a loss of signal intensity and spatial resolution in conventionally reconstructed images. The present work proposes an iterative algorithm to correct these adverse effects., Materials and Methods: The algorithm involves solving a large linear system of equations and requires a separate reference scan to map the spatially variant transverse relaxation time. It was implemented and applied to simulations and to experiments with custom-built resolution phantoms. The evaluation focused on its ability to improve the actual point-spread function (PSF) and on its influence upon the local signal-to-noise ratio (SNR)., Results: The algorithm is demonstrated to virtually restore the ideal PSF of the acquisition. It proves to provide images with better signal intensity and spatial resolution but reduced SNR, if the transverse relaxation time is known with sufficient accuracy., Conclusions: The present work shows the basic feasibility of correcting signal decay effects in UTE imaging.

The fast spin echo (FPE) sequence is sensitive to the phase of the magnetization, hindering its use in procedures such as diffusion imaging. The current solutions to this problem reduce the available signal by one half. We present the first volunteer study of a sequence which does not suffer from this loss of signal while measuring diffusion coefficients.

A new interface combining phased arrays and echo-planar imaging (EPI) technologies was developed for two channel breast MR EPI applications. A detailed design for a dual-channel, EPI-compatible, phased array breast coil is described. EPI digital data multiplexing, signal controlling and sampling schemes are also presented. Results from breast phantoms and patients demonstrate a 55% improvement in signal-to-noise ratio when compared to a conventional two-loop, single channel coil configuration. This method can be easily expanded to a four or more channel, EPI-compatible, phased array system to improve field-of-view coverage and signal-to-noise ratio.