Stage substitution <Emphasis Type="Italic">E</Emphasis>-<Emphasis Type="Italic">H</Emphasis> circuits for calculating magnetoelectric induction heating plants

An approximate analytical method is proposed to calculate the electromagnetic field of a magnetoelectric induction heating plant using a stage substitution circuit formed based on a comparison of the general laws and methods of the computation of electromagnetic field in a conducting medium and methods of the circuit theory for the sinusoidal current.     

Intravascular contrast agent <Emphasis Type="Italic">T1</Emphasis> shortening: fast <Emphasis Type="Italic">T1</Emphasis> relaxometry in a carotid volunteer study

OBJECTIVE: To measure the T1 times in blood after the administration of the intravascular contrast agent gadofosveset trisodium in humans. MATERIALS AND METHODS: In a pilot study for parameter optimization, the T1-shortening induced by the injection of a single dose (0.03 mmol/kg body weight) of the MR contrast agent Vasovist (Bayer Schering Pharma AG) was measured at B (0) = 1.5 T as a function of time. In four sessions, T1 measurements were performed in the carotid vein of 9 volunteers up to 30 min after injection. T1 times were measured using a segmented saturation recovery turboFLASH (SSRTFL) pulse sequence with 7 different saturation recovery delay times in a total acquisition time of 20 s. RESULTS: The SSRTFL measurements showed T1 times of about 100 ms immediately after injection, which gradually increased to 175 ms at 30 min. The time curve of the R1 = 1/T1 averaged over all volunteers could be described with an exponential decay with a time constant T = 330 +/- 65 s and an amplitude DeltaR1 = 4.1 +/- 0.3 s(-1), and a constant offset of R1(0) = 5.7 +/- 0.2 s(-1). Mean relaxation values are in excellent agreement with theoretical predictions. CONCLUSION: An analytical expression for the initial T1-shortening of Vasovist was derived which can now be used for optimization of the pulse sequence parameters in clinical studies.     

Generalized 12<Emphasis Type="Italic">n</Emphasis>-phase converter

Topological features and calculation relationships are considered for a 12n-phase converter with a multistage configuration of six-phase asymmetric voltage systems of secondary transformer windings. An example of a synthesized circuit of the phase number and shift converter is given for a 24-pulse rectifier recommended for use in networks with distorting factors.     

Torque ripple minimization with optimized <Emphasis Type="Italic">i</Emphasis><Subscript><Emphasis Type="Italic">sx</Emphasis></Subscript> estimation in vector control of current source inverter induction motor

In variable speed applications of high-power induction motors, current source inverters are usually used instead of voltage source inverters. Power switches delay and switching losses in high-power ranges, prevents inverters to operate properly in high frequencies. So the voltage and current waveforms of the motor supply consist of high harmonic content causing a high-torque ripple. One of the parameters that influences the torque ripple most is the stator flux reference value. Motors do not usually operate in their nominal load. Therefore, the optimum value of the flux is not equal to the nominal value. In this paper, an optimal i _sx reference for vector control system of current source inverter fed induction motor is produced by applying a simple estimator. Simulation results show that torque ripple has been reduced, especially in the loads less than their nominal power, as significantly as power factor has been improved. Also flux reduction causes to noticeable reduction in core and switching losses.     

A novel alternative to classify tissues from <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> and <Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript> relaxation times for prostate MRI

Objective

To segment and classify the different attenuation regions from MRI at the pelvis level using the T ₁ and T ₂ relaxation times and anatomical knowledge as a first step towards the creation of PET/MR attenuation maps.

Materials and methods

Relaxation times were calculated by fitting the pixel-wise intensities of acquired T ₁- and T ₂-weighted images from eight men with inversion-recovery and multi-echo multi-slice spin-echo sequences. A decision binary tree based on relaxation times was implemented to segment and classify fat, muscle, prostate, and air (within the body). Connected component analysis and an anatomical knowledge-based procedure were implemented to localize the background and bone.

Results

Relaxation times at 3 T are reported for fat (T ₁ = 385 ms, T ₂ = 121 ms), muscle (T ₁ = 1295 ms, T ₂ = 40 ms), and prostate (T ₁ = 1700 ms, T ₂ = 80 ms). The relaxation times allowed the segmentation–classification of fat, prostate, muscle, and air, and combined with anatomical knowledge, they allowed classification of bone. The good segmentation–classification of prostate [mean Dice similarity score (mDSC) = 0.70] suggests a viable implementation in oncology and that of fat (mDSC = 0.99), muscle (mDSC = 0.99), and bone (mDSCs = 0.78) advocates for its implementation in PET/MR attenuation correction.

Conclusion

Our method allows the segmentation and classification of the attenuation-relevant structures required for the generation of the attenuation map of PET/MR systems in prostate imaging: air, background, bone, fat, muscle, and prostate.

     

Comparison of methods for estimation of the intravoxel incoherent motion (IVIM) diffusion coefficient (<Emphasis Type="Italic">D</Emphasis>) and perfusion fraction (<Emphasis Type="Italic">f</Emphasis>)

Objective

Intravoxel incoherent motion (IVIM) shows great potential in many applications, e.g., tumor tissue characterization. To reduce image-quality demands, various IVIM analysis approaches restricted to the diffusion coefficient (D) and the perfusion fraction (f) are increasingly being employed. In this work, the impact of estimation approach for D and f is studied.

Materials and methods

Four approaches for estimating D and f were studied: segmented IVIM fitting, least-squares fitting of a simplified IVIM model (sIVIM), and Bayesian fitting of the sIVIM model using marginal posterior modes or posterior means. The estimation approaches were evaluated in terms of bias and variability as well as ability for differentiation between tumor and healthy liver tissue using simulated and in vivo data.

Results

All estimation approaches had similar variability and ability for differentiation and negligible bias, except for the Bayesian posterior mean of f, which was substantially biased. Combined use of D and f improved tumor-to-liver tissue differentiation compared with using D or f separately.

Discussion

The similar performance between estimation approaches renders the segmented one preferable due to lower numerical complexity and shorter computational time. Superior tissue differentiation when combining D and f suggests complementary biologically relevant information.

     

Imaging lung function using rapid dynamic acquisition of <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript>-maps during oxygen enhancement

This paper describes imaging of lung function with oxygen-enhanced MRI using dynamically acquired T ₁ parameter maps, which allows an accurate, quantitative assessment of time constants of T ₁-enhancement and therefore lung function. Eight healthy volunteers were examined on a 1.5-T whole-body scanner. Lung T ₁-maps based on an IR Snapshot FLASH technique (TE = 1.4 ms, TR = 3.5 ms, FA = 7 ^∘) were dynamically acquired from each subject. Without waiting for full relaxation between subsequent acquisition of T ₁-maps, one T ₁-map was acquired every 6.7 s. For comparison, all subjects underwent a standard pulmonary function test (PFT). Oxygen wash-in and wash-out time course curves of T ₁ relaxation rate (R ₁)-enhancement were obtained and time constants of oxygen wash-in (w _in) and wash-out (w _out) were calculated. Averaged over the whole right lung, the mean w _out was 43.90 ± 10.47 s and the mean (w _in) was 51.20 ± 15.53 s, thus about 17% higher in magnitude. Wash-in time constants correlated strongly with forced expired volume in one second in percentage of the vital capacity (FEV₁ % VC) and with maximum expiratory flow at 25% vital capacity (MEF25), whereas wash-out time constants showed only weak correlation. Using oxygen-enhanced rapid dynamic acquisition of T ₁-maps, time course curves of R ₁-enhancement can be obtained. With w _in and w _out two new parameters for assessing lung function are available. Therefore, the proposed method has the potential to provide regional information of pulmonary function in various lung diseases.     

<Emphasis Type="Italic">K</Emphasis>-space data processing for magnetic resonance elastography (MRE)

Objective

Magnetic resonance elastography (MRE) requires substantial data processing based on phase image reconstruction, wave enhancement, and inverse problem solving. The objective of this study is to propose a new, fast MRE method based on MR raw data processing, particularly adapted to applications requiring fast MRE measurement or high elastogram update rate.

Materials and methods

The proposed method allows measuring tissue elasticity directly from raw data without prior phase image reconstruction and without phase unwrapping. Experimental feasibility is assessed both in a gelatin phantom and in the liver of a porcine model in vivo. Elastograms are reconstructed with the raw MRE method and compared to those obtained using conventional MRE. In a third experiment, changes in elasticity are monitored in real-time in a gelatin phantom during its solidification by using both conventional MRE and raw MRE.

Results

The raw MRE method shows promising results by providing similar elasticity values to the ones obtained with conventional MRE methods while decreasing the number of processing steps and circumventing the delicate step of phase unwrapping. Limitations of the proposed method are the influence of the magnitude on the elastogram and the requirement for a minimum number of phase offsets.

Conclusion

This study demonstrates the feasibility of directly reconstructing elastograms from raw data.

     

Acoustic noise reduction in <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript>- and proton-density-weighted turbo spin-echo imaging

Objective

To reduce acoustic noise levels in T ₁-weighted and proton-density-weighted turbo spin-echo (TSE) sequences, which typically reach acoustic noise levels up to 100 dB(A) in clinical practice.

Materials and methods

Five acoustic noise reduction strategies were combined: (1) gradient ramps and shapes were changed from trapezoidal to triangular, (2) variable-encoding-time imaging was implemented to relax the phase-encoding gradient timing, (3) RF pulses were adapted to avoid the need for reversing the polarity of the slice-rewinding gradient, (4) readout bandwidth was increased to provide more time for gradient activity on other axes, (5) the number of slices per TR was reduced to limit the total gradient activity per unit time. We evaluated the influence of each measure on the acoustic noise level, and conducted in vivo measurements on a healthy volunteer. Sound recordings were taken for comparison.

Results

An overall acoustic noise reduction of up to 16.8 dB(A) was obtained by the proposed strategies (1–4) and the acquisition of half the number of slices per TR only. Image quality in terms of SNR and CNR was found to be preserved.

Conclusions

The proposed measures in this study allowed a threefold reduction in the acoustic perception of T ₁-weighted and proton-density-weighted TSE sequences compared to a standard TSE-acquisition. This could be achieved without visible degradation of image quality, showing the potential to improve patient comfort and scan acceptability.

     

The Applicability of Sr-deficient <Emphasis Type="Italic">n</Emphasis>-type SrTiO<Subscript>3</Subscript> for SOFC Anodes

Here we discuss the effect of preparation conditions on structural stability and electrical properties of Sr-deficient n-type SrTiO₃. In particular, an explanation of a wide scatter of conductivity values in Y- and Nb-doped SrTiO₃. reported in the literature is proposed, based on the existing defect chemistry model of n-doped SrTiO₃. It was confirmed that when sintered in air, Sr-deficient SrTiO₃ doped with Nb and/or Y, remains single phase until the solubility limit (e.g., 30% for Nb or 4% for Y). However, when sintered at low po₂, the material transforms from a vacancy compensated to an electronically compensated compound with a strontium deficient second phase. Measured at 800°C in low po₂, the maximum conductivity of these multi-phase compounds was 340 S/cm and 100 S/cm for the Nb-doped and Y-doped sample, respectively. However, the conductivity dropped dramatically to less than 10 S/cm when samples of the same compositions were sintered in air, again measured in reducing atmosphere.     

Dielectric properties and defect mechanisms of (1-<Emphasis Type="Italic">x</Emphasis>)Ba(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> -<Emphasis Type="Italic">x</Emphasis>BiYbO<Subscript>3</Subscript> ceramics

(1-x)Ba(Fe_0.5Nb_0.5)O₃ -xBiYbO₃ (BFN-xBY) ceramics were prepared by a conventional solid-state reaction method. The dielectric properties and relaxation behavior of BFN-xBY ceramics were analyzed according to dielectric and impedance spectroscopy. Dielectric permittivity of the ceramics increases with increasing temperature below 500 K then remains unchanged up to 700 K, while corresponding loss factor decreases with the increase of temperature below 500 K then increase slowly. Defect compensation mechanism of this system was analyzed in detail. The giant dielectric behavior of the ceramics arises from the internal barrier layer capacitor (IBLC) effect. Polarization effect at insulating grain boundaries between semiconducting grains accompanied by a strong Maxwell-Wagner (MW) relaxation mode. The characteristic of grain boundaries was revealed using impedance spectroscope and the universal dielectric response law.     

Self-consistent calculation for valence subband structure and hole mobility in <Emphasis Type="Italic">p</Emphasis>-channel inversion layers

Using six- and eight-band k⋅p models—with parameters calibrated against the bulk band structure obtained using non-local empirical pseudopotentials—we have employed a new hybrid self-consistent method to calculate the valence subband structure in p-channel inversion layers of InAs, InSb, GaAs, In_0.53Ga_0.47As, and GaSb. This method involves two separate stages: first, density-of-states (DOS) of the three lowest-energy subbands (heavy, light, and split-off holes) is calculated using the triangular-well approximation. Then, the self-consistent calculation is performed using the DOS previously obtained, but shifting each subband by the amount obtained from the self-consistent eigenvalues obtained during the self-consistent iteration. Finally, we present results regarding the hole mobility in Ge p-channel inversion layers. The results are compared to those obtained employing the subband structure computed with the triangular-well approximation and also with experimental data.     

Controlled<Emphasis Type="Bold"> E</Emphasis>-field gradient coils

Peripheral neural stimulation is a major problem in current gradient coil designs. Induced current problems in patients relate directly to gradient strength and modulation frequency. Current designs of gradient coil tend to limit ultra-high-speed imaging methods such as echo-planar imaging through the effect of induced currents which produce tingling sensations and involuntary muscle twitch. Neural stimulation could also trigger epileptic fits and/or cardiac fibrillation. For reduction of induced currents, an important aspect is the coil geometry. It is desirable to design the gradient coil in such a way as to prevent closed loop circulating currents within the body. Preliminary results using a four-sector gradient coil with rectangular geometry, operating in a low mutual coupling mode, indicate significant reduction in the E-field within the subject volume of the coil. Reduction in induced currents in the patient allows safer operation at higher magnetic field strengths together with faster scans currently prohibited through neural stimulation effects in standard coil geometries.     

Estimation of pancreatic <Emphasis Type="Italic">R</Emphasis>2* for iron overload assessment in the presence of fat: a comparison of different approaches

Objectives

To propose a method for estimating pancreatic relaxation rate, R2*, from conventional multi-echo MRI, based on the nonlinear fitting of the acquired magnitude signal decay to MR signal models that take into account both the signal oscillations induced by fat and the different R2* values of pancreatic parenchyma and fat.

Materials and methods

Single-peak fat (SPF) and multi-peak fat (MPF) models were introduced. Single-R2* and dual-R2* assumptions were considered as well. Analyses were conducted on simulated data and 20 thalassemia major patients.

Results

Simulations revealed the ability of the MPF model to correctly estimate the R2* value in a large range of fat fractions and R2* values. From the comparison between the results obtained with a single R2* value for water and fat and the dual-R2* approach, the latter is more accurate in both water R2* and fat fraction estimation. In patient’s data analysis, a strong concordance was found between SPF and MPF estimated data with measurements done with manual signal correction and from fat-saturated images. The MPF method showed better reproducibility.

Conclusion

The MPF dual-R2* approach improves reproducibility and reduces image analysis time in the assessment of pancreatic R2* value in patients with iron overload.

     

Correction of <Emphasis Type="Italic">B</Emphasis><Subscript>0</Subscript>-induced geometric distortion variations in prospective motion correction for 7T MRI

Objective

Prospective motion correction can effectively fix the imaging volume of interest. For large motion, this can lead to relative motion of coil sensitivities, distortions associated with imaging gradients and B ₀ field variations. This work accounts for the B ₀ field change due to subject movement, and proposes a method for correcting tissue magnetic susceptibility-related distortion in prospective motion correction.

Materials and methods

The B ₀ field shifts at the different head orientations were characterized. A volunteer performed large motion with prospective motion correction enabled. The acquired data were divided into multiple groups according to the object positions. The correction of B ₀-related distortion was applied to each group of data individually via augmented sensitivity encoding with additionally integrated gradient nonlinearity correction.

Results

The relative motion of the gradients, B ₀ field and coil sensitivities in prospective motion correction results in residual spatial distortion, blurring, and coil artifacts. These errors can be mitigated by the proposed method. Moreover, iterative conjugate gradient optimization with regularization provided superior results with smaller RMSE in comparison to standard conjugate gradient.

Conclusion

The combined correction of B ₀-related distortion and gradient nonlinearity leads to a reduction of residual motion artifacts in prospective motion correction data.

     

The impact of fibre orientation on <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript>-relaxation and apparent tissue water content in white matter

Objective

Recent MRI studies have shown that the orientation of nerve fibres relative to the main magnetic field affects the R₂*(= 1/T₂*) relaxation rate in white matter (WM) structures. The underlying physical causes have been discussed in several studies but are still not completely understood. However, understanding these effects in detail is of great importance since this might serve as a basis for the development of new diagnostic tools and/or improve quantitative susceptibility mapping techniques. Therefore, in addition to the known angular dependence of R₂*, the current study investigates the relationship between fibre orientation and the longitudinal relaxation rate, R₁ (= 1/T₁), as well as the apparent water content.

Materials and methods

For a group of 16 healthy subjects, a series of gradient echo, echo-planar and diffusion weighted images were acquired at 3T from which the decay rates, the apparent water content and the diffusion direction were reconstructed. The diffusion weighted data were used to determine the angle between the principle fibre direction and the main magnetic field to examine the angular dependence of R₁ and apparent water content.

Results

The obtained results demonstrate that both parameters depend on the fibre orientation and exhibit a positive correlation with the angle between fibre direction and main magnetic field.

Conclusion

These observations could be helpful to improve and/or constrain existing biophysical models of brain microstructure by imposing additional constraints resulting from the observed angular dependence R₁ and apparent water content in white matter.

     

On high suppression of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and CO emissions in gas-turbine plants with combined gas-and-steam cycles

In this work are given results of analyzing processes of production of nitrogen oxides (NO _x ) and afterburning of CO when firing natural gas at combined-cycle gas-turbine plants. It is shown that for suppressing emissions of the said microcomponents it is necessary to lower temperature in hot local zones of the flame in which NO_x is formed, and, in so doing, to avoid chilling of cold flame zones that prevents afterburning of CO. The required lowering of the combustion temperature can be provided by combustion of mixtures of methane with steam, with high mixing uniformity that ensures the same and optimum fraction of the steam “ballast” in each microvolume of the flame. In addition to chilling, the steam ballast makes it possible to maintain a fairly high concentration of hydroxil radicals in the flame zone as well, and this provides high burning out of fuel and reduction in carbon monoxide emissions (active steam ballast). Due to this fact the fraction of steam when firing its mixtures with methane in a gas-turbine plant can be increased up to the weight ratio 4: 1. In this case, the concentrations of NO _x and CO in emissions can be reduced to ultra-low values (less than 3 ppm).     

Hyperpolarized <Superscript>3</Superscript>He lung ventilation imaging with <Emphasis Type="Italic">B</Emphasis><Subscript>1</Subscript>-inhomogeneity correction in a single breath-hold scan

The signal-to-noise ratio (SNR) of hyperpolarized noble gas MR images is sensitive to the flip angle used. Variations in flip angle due to B ₁-inhomogeneity of the RF coil cause intensity variation artifacts in lung ventilation images which may mask or mimic disease. We show these artifacts can be minimized by using the optimal flip angle and corrected if the local flip angle is known. Hyperpolarized ³He lung images were obtained in ten healthy subjects using both a conventional gradient-echo sequence and a new hybrid pulse sequence designed to simultaneously acquire lung ventilation images and corresponding flip-angle maps in comparable imaging time. Flip-angle maps and corrected images were calculated from the hybrid scan and compared with conventional images. The qualitative theoretical dependence of flip angle on SNR was verified. Ventilation images and flip-angle maps were successfully obtained with the hybrid sequence. Corrections to image intensity calculated from the flip-angle maps appeared reasonable for images acquired using an average flip angle near optimal. Use of the optimal flip angle is crucial to the quality of lung ventilation images. Artifactual intensity variations due to RF-coil inhomogeneity may be identified and potentially corrected using our hybrid sequence. Acknowledgement The authors thank John M. Christopher, RT(R)(MR) for valuable assistance performing the studies, and Jaime F. Mata, MS, Jing Cai, MS, and Andrew G. Reish, BS, for excellent operation of the ³He polarization system. We gratefully acknowledge support for this research from the Commonwealth of Virginia Technology Research Fund (Grant No. IN2002-01), Siemens Medical Solutions and Amersham Health.