Proposal of flux‐lock‐type fault current limiter with high‐TC superconducting element

This paper proposes a new type of fault current limiter (FCL), which consists of a high‐T_C superconducting (HTS) element and two coils wound on the same core without any leakage magnetic flux. In this FCL, either the limiting impedance or the initial limiting current level can be controlled by adjusting the inductances and the winding direction of the coils. Therefore, this FCL could relax the material restrictions on high‐T_C superconducting FCL. A current‐limiting experiment by a model FCL was carried out, and the limiting performance was observed. The initial limiting current level of the model FCL was 1.7 times higher than the critical current of the HTS element, and the fault current is suppressed to 52% immediately after the short‐circuit in the test. Considering voltage–current characteristics of a high‐T_C superconductor in a computer simulation, the calculated results almost agreed with the experimental results. © 1999 Scripta Technica, Electr Eng Jpn, 127(1): 31–38, 1999     

Simulation-based evaluation of SAR and flip angle homogeneity for five transmit head arrays at 14 T

Introduction

Various research sites are pursuing 14 T MRI systems. However, both local SAR and RF transmit field inhomogeneity will increase. The aim of this simulation study is to investigate the trade-offs between peak local SAR and flip angle uniformity for five transmit coil array designs at 14 T in comparison to 7 T.

Methods

Investigated coil array designs are: 8 dipole antennas (8D), 16 dipole antennas (16D), 8 loop coils (8D), 16 loop coils (16L), 8 dipoles/8 loop coils (8D8L) and for reference 8 dipoles at 7 T. Both RF shimming and k_T-points were investigated by plotting L-curves of peak SAR levels vs flip angle homogeneity.

Results

For RF shimming, the 16L array performs best. For k_T-points, superior flip angle homogeneity is achieved at the expense of more power deposition, and the dipole arrays outperform the loop coil arrays.

Discussion and conclusion

For most arrays and regular imaging, the constraint on head SAR is reached before constraints on peak local SAR are violated. Furthermore, the different drive vectors in k_T-points alleviate strong peaks in local SAR. Flip angle inhomogeneity can be alleviated by k_T-points at the expense of larger power deposition. For k_T-points, the dipole arrays seem to outperform loop coil arrays.

     

Changes in proton transverse relaxation times of rat myocardium that has suffered a previous oxidative insult

An oxidative insult can induce severe damage, as in the phenomenon of myocardial ischemia and reperfusion. However, there are situations in which the damage is not so obvious (e.g., silent ischemia or aging), and the negative effects will be seen only in time. Our aim was to reveal these small changes in the myofilaments by using the nuclear magnetic resonance (NMR) technique. We used Wistar rat hearts in a constant-pressure Langendorff system, perfused with oxygenated Krebs-Henseleit buffer at 37°C. After 15 minutes of stabilization, the hearts were perfused with buffer supplemented with H₂O₂ at 50, 75, or 100 μmol/L for 15 or 30 minutes. Fifteen-minute and 45-minute perfusion controls and unperfused hearts were also collected. Heart rate (HR) and left ventricular developed pressure (LVDP) were determined with the help of a latex balloon, inserted in the left ventricle and connected with a pressure transducer. Proton transverse relaxation times (T ₂) were determined at the end of the experiment.T ₂ values were measured again in the same tissue fragments after they had been glycerinated and incubated in relaxation and rigor media. The functional parameters (HR, LVDP, coronary flow) were not significantly changed in control and 50 μmol/L H₂O₂ groups but were increased in the 75 μmol/L H₂O₂ group and significantly decreased in the 100 μmol/L H₂O₂ group.T ₂ is significantly decreased in rigor media starting with 50 μmol/L H₂O₂ administrated for 30 minutes and does not correlate with dose and duration of the oxidative insult.T ₂ in rigor is shorter than in relaxation media within the groups, and this difference is increased in the treated hearts.     

Quantitative assessment of regional myocardial function in a rat model of myocardial infarction using tagged MRI

We characterized global and regional left ventricular (LV) function during post myocardium infarction (MI) remodeling in rats, which has been incompletely described by previous MRI studies. To assess regional wall motion, four groups of infarcted animals corresponding to 1–2, 3–4, 6–8 and 9–12 weeks post-MI respectively were imaged using a fast gradient echo sequence with a 2D spatial modulation of magnetization (SPAMM) tagging preparation. An additional group was serially imaged (1–2 and 6–7 weeks post-MI) to assess the global function. Regional and global functional parameters of infarcted rats were compared to non-infarcted normal rats. Compared to normal rats, a decrease in ejection fraction (70 ±7 vs. 40 ± 8%, p<0.05) was observed in rats with MI. Maximal and minimal principal stretches (₁, ₂) and strains (E₁, E₂), principal angle () and displacement varied regionally in normal rats but deviated significantly from the normal values in rats with MI particularly in the infarcted and adjacent zones. Not only was strain magnitude reduced segmentally post-MI, but strain direction became more circumferentially oriented, particularly in rats with larger infarctions. We report the first regional myocardial strain values in normal and infarcted rats. These results parallel findings in humans, and provide a unique tool to examine regional mechanical influences on the remodeling process.     

The effect of temperature on MR relaxation times and signal intensities for human tissues

T ₁ andT ₂ relaxation times were measured on human tissue samples of adipose tissue, muscle, bone marrow and osteolytic skeletal metastases at temperatures ranging from +37°C to –10°C. Relative signal intensities forT ₁, proton density andT ₂-weighted imaging sequences were also calculated.T ₁ andT ₂ of adipose tissue decreased almost linearly with decreasing temperature while for muscle, bone marrow and metastasesT ₁ andT ₂ decreased slightly to moderately, with temperature reduction to about –5 °C at which temperature a sudden marked decrease occurred. Calculated signal intensities showed a decrease in image contrast with temperature reduction and reversal of contrast between adipose tissue and the other tested tissues with all imaging sequences at temperatures around 0°C.     

Magnetization transfer by simple sequences of rectangular pulses

On-resonant radio frequency (RF) sequences composed of a train of short rectangular pulses of the same kind were optimized in order to obtain selective saturation of protons with short transverse relaxation times for magnetization transfer purposes. It is demonstrated that the sequences regarded allow a good adaptation to different requirements for magnetization transfer examinations on whole-body imagers. The sequences presented here provide relatively strong saturation of protons with very short transverse relaxation timesT ₂50 µs, whereas signals from protons with longT ₂ to be recorded are hardly influenced in a broad frequency range. The sequences are especially advantageous for applications in pulse files with limited numbers of support points.     

Estimation of metabolite <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> relaxation times using tissue specific analysis,signal averaging and bootstrapping from magnetic resonance spectroscopic imaging data

Object A novel method of estimating metabolite T ₁ relaxation times using MR spectroscopic imaging (MRSI) is proposed. As opposed to conventional single-voxel metabolite T ₁ estimation methods, this method investigates regional and gray matter (GM)/white matter (WM) differences in metabolite T ₁ by taking advantage of the spatial distribution information provided by MRSI. Material and methods The method, validated by Monte Carlo studies, involves a voxel averaging to preserve the GM/WM distribution, a non-linear least squares fit of the metabolite T ₁ and an estimation of its standard error by bootstrapping. It was applied in vivo to estimate the T ₁ of N-acetyl compounds (NAA), choline, creatine and myo-inositol in eight normal volunteers, at 1.5 T, using a short echo time 2D-MRSI slice located above the ventricles. Results WM-T _1,NAA was significantly (P < 0.05) longer in anterior regions compared to posterior regions of the brain. The anterior region showed a trend of a longer WM T ₁ compared to GM for NAA, creatine and myo-Inositol. Lastly, accounting for the bootstrapped standard error estimate in a group mean T ₁ calculation yielded a more accurate T ₁ estimation. Conclusion The method successfully measured in vivo metabolite T ₁ using MRSI and can now be applied to diseased brain.     

A comparison of the proton relaxation in human epithelial tumors and associated uninvolved tissue

Longitudinal and transverse proton relaxation from several sets of paired surgical samples of human epithelial tumors and associated histologically uninvolved tissues were studied at 37°C and at 20 MHz. Broad distributions of exponential terms fit allT ₁ andT ₂ decays well. The tumor times showed over twice the scatter of the associated tissue times but no clear trends. Although the nontumor relaxation show little variability, the tumor curves can have either longer or shorter times and different shapes.     

3D magnetic resonance fingerprinting on a low-field 50 mT point-of-care system prototype: evaluation of muscle and lipid relaxation time mapping and comparison with standard techniques

Objective

To implement magnetic resonance fingerprinting (MRF) on a permanent magnet 50 mT low-field system deployable as a future point-of-care (POC) unit and explore the quality of the parameter maps.

Materials and methods

3D MRF was implemented on a custom-built Halbach array using a slab-selective spoiled steady-state free precession sequence with 3D Cartesian readout. Undersampled scans were acquired with different MRF flip angle patterns and reconstructed using matrix completion and matched to the simulated dictionary, taking excitation profile and coil ringing into account. MRF relaxation times were compared to that of inversion recovery (IR) and multi-echo spin echo (MESE) experiments in phantom and in vivo. Furthermore, B₀ inhomogeneities were encoded in the MRF sequence using an alternating TE pattern, and the estimated map was used to correct for image distortions in the MRF images using a model-based reconstruction.

Results

Phantom relaxation times measured with an optimized MRF sequence for low field were in better agreement with reference techniques than for a standard MRF sequence. In vivo muscle relaxation times measured with MRF were longer than those obtained with an IR sequence (T₁: 182 ± 21.5 vs 168 ± 9.89 ms) and with an MESE sequence (T₂: 69.8 ± 19.7 vs 46.1 ± 9.65 ms). In vivo lipid MRF relaxation times were also longer compared with IR (T₁: 165 ± 15.1 ms vs 127 ± 8.28 ms) and with MESE (T₂: 160 ± 15.0 ms vs 124 ± 4.27 ms). Integrated ΔB₀ estimation and correction resulted in parameter maps with reduced distortions.

Discussion

It is possible to measure volumetric relaxation times with MRF at 2.5 × 2.5 × 3.0 mm³ resolution in a 13 min scan time on a 50 mT permanent magnet system. The measured MRF relaxation times are longer compared to those measured with reference techniques, especially for T₂. This discrepancy can potentially be addressed by hardware, reconstruction and sequence design, but long-term reproducibility needs to be further improved.

     

Experimental31p nmr study of the influence of ionic strength on the apparent dissociation constant of mgatp

The classical method for³¹P NMR determination of intracellular free magnesium concentration ([Mg _free ²⁺ ]) requires an accurate knowledge of the apparent dissociation constant (K _D ) of MgATP. There is a large difference between the previously determined values ofK _D . Although the value of 50 µM, determined by a³¹P NMR method, is now largely accepted, a value of 86 µM has more recently been measured with a fitting method derived from the original one, and with a different ionic strength. The purpose of our study was to assess if the cause of the difference between these two previously reportedK _D values was due to the measuring method or to the ionic strength value used.Working at pH=7.2,T=37°C, and [KCl]=0.25 M, we performedK _D measurements with the original³¹P NMR method and with the fitting method. The results (67±13 µM and 61±20 µM, respectively) were not significantly different. Then, with the first method, we measured K_D at [KCl]=0.12 M and found a value of 19±5 µM. We conclude that the main cause of difference between theK _D values measured by³¹P NMR reside in the disparity of ionic strength values used for their measurement. OurK _D measurements at [KCl]=0.25 and 0.12 M demonstrate the importance of the ionic strength value used for imitating the intracellular medium on the absolute value of ([Mg _free ²⁺ ]) measured by³¹P NMR spectroscopy.Address for correspondence: Université Catholique de Louvain, Unité CPMC, Bâtiment Lavoisier, Place Louis Pasteur n°1, B-1348 Louvain-la-Neuve, Belgium. Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Penn Plaza, New York, NY 10119.     

Measurement of T1 and T2 relaxation times of the pancreas at 7 T using a multi-transmit system

Objective

To determine T₁ and T₂ relaxation times of healthy pancreas parenchyma at 7 T using a multi-transmit system.

Materials and methods

Twenty-six healthy subjects were scanned with a 7 T MR system using eight parallel transceiver antennas, each with two additional receive loops. A Look-Locker sequence was used to obtain images for T₁ determination, while T₂ was obtained from spin-echo images and magnetic resonance spectroscopy measurements with different echo times. T₁ and T₂ times were calculated using a mono-exponential fit of the average magnitude signal from a region of interest in the pancreas and were tested for correlation with age.

Results

The age range of the included subjects was 21–72 years. Average T₁ and T₂ relaxation times in healthy pancreas were 896 ± 149 ms, and 26.7 ± 5.3 ms, respectively. No correlation with age was found.

Conclusion

T₁ and T₂ relaxation times of the healthy pancreas were reported for 7 T, which can be used for image acquisition optimization. No significant correlations were found between age and T₁ or T₂ relaxation times of the pancreas. Considering their low standard deviation and no observable age dependence, these values may be used as a baseline to study potentially pancreatic tissue affected by disease.

     

Quantitative in vivo 1H spectroscopic imaging of metabolites in the early postnatal mouse brain at 17.6 T

Object  Early postnatal brain maturation is closely connected to local changes of metabolite levels. Spatially resolved in vivo ¹H NMR spectroscopic imaging is applied to follow absolute changes of brain metabolites in early postnatal mouse brain. Materials and methods  A short echo time semi LASER (localization by adiabatic selective refocusing) chemical shift imaging (CSI) sequence incorporating weighted k-space averaging was implemented at high magnetic field (17.6 T). In vivo measurements were carried out on postnatal days 5, 8, 12, 16, and 20. In vivo relaxation times T ₁ and T ₂ were measured using variable repetition times or a CPMG sequence, respectively, combined with LASER single voxel localization. Results  Spectra were obtained with a spatial resolution of (1 × 1) mm² in a 1.5 mm slice as early as postnatal day 5. Maturational changes of absolute metabolite concentrations of major metabolites were calculated in four different brain regions. A significant increase of N-acetylaspartate (NAA), total creatine (tCr), and glutamate/glutamine (Glx) concentration was paralleled by a decrease of taurine (Tau) concentration with age (P < 0.05). Differences between brain regions were found for NAA, tCr, and Tau (P < 0.05). Furthermore, in vivo T ₁ and T ₂ of the four major brain metabolites in adult mice are reported. Conclusion  The implemented semi LASER CSI sequence allows following regional changes of metabolite levels. It is suitable for investigation of local differences in brain metabolism and development.     

Proton MR spectroscopy of human pancreas allografts

Objective

To estimate pancreas graft relaxation times and concentrations of total fat, and the intracellular lipids of non-adipose pancreatic cells (NAPC) using proton (¹H) magnetic resonance spectroscopy (MRS) during cold preservation.

Materials and methods

Grafts from 11 human donors were investigated. Each pancreas was perfused in situ with histidine-tryptophan-ketoglutarate (HTK) or with University of Wisconsin solution and placed into a transport container. Temperature of the grafts was maintained at 4 ± 2 °C during transport to our hospital and MR scanning. A 1.5 T clinical scanner was used for the measurements. Single-voxel PRESS spectra were acquired using transmit–receiver head coil.

Results

Relaxation times were measured for lipid (–CH₂–)_n (T₁, 287 ± 60 ms; T₂, 27 ± 4 ms), and tissue water (T₁, 670 ± 69 ms; T₂, 77 ± 17 ms). Average total fat, and intracellular lipids of NAPC concentrations were 79.2 ± 100.8 (range 2.4–304.4), and 2.9 ± 1.2 mmol/kg ww, respectively.

Conclusion

We have shown that ¹H-MRS is a useful tool for the estimation of pancreas graft lipid concentrations. Total pancreatic fat and especially content of intracellular lipids of NAPC are valuable measures for inspection of graft quality prior to transplantation or islet of Langerhans isolation.

     

Impact-ionization coefficient in silicon at high fields— a parametric approach

The impact-ionization coefficient α _n at high fields is derived in terms of the electric field ℰ and lattice temperature T _L , without introducing a priori relations among the parameters. An asymptotic analysis leads to simplifications that validate closed-form expressions of α _n . The role of the relaxation times in determining the slope of α _n (ℰ) is discussed, along with the meaning of the critical field.     

Magnetic resonance evaluation of autonomous thyroid nodules treated by percutaneous ethanol injection

Magnetic resonance (MR) imaging was used to evaluate the effect of ultrasound-guided percutaneous ethanol injection (PEI) of autonomous thyroid nodules (ATNs). Nine patients affected with ATN (3.7–32.2 mL volume) underwent PEI (4–19 mL ethanol, subdivided in 3–6 weekly procedures). MR imaging (1.5 T) was performed before each alcoholization and 1 month after the last PEI procedure with the following parameters:T ₁-(550/12) andT ₂-weighted (2200/160) spin-echo images; 4-mm slices, 10% gap; coronal planes. A further seven patients with normal thyroid function, who had undergone PEI 6–18 months before, underwent an MR examination with the same parameters. The signal-to-noise ratio (S/N) of ATN and extranodular gland, as well as ATN volume, were evaluated on theT ₂-weighted images. OnT ₁-weighted images, ATNs appeared mostly hypointense before treatment, with hyperintense areas during treatment, and were lightly hyperintense 6–18 months after treatment. S/N onT ₂-weighted images: extranodular gland 3.5–9.2; ATNs, before treatment 13.2–19.7, before the last procedure 7.7–11.6, 1 month after the last procedure 5.6–10.9; previously treated ATNs, 4.3–8.2. No significant volume reduction was observed with MR 1 month after the last procedure. The MR examination time was about 15 minutes. In conclusion, the effect of PEI on ATNs can be evaluated with an MR examination that is not very time consuming.     

Assessment of inhibitory potency of antibiotics by MRI: apparent T 2 as a marker of cell growth

A new method to assess the antibiotic potency by MRI has been developed. Correlating ¹H NMR spectra of bacterial cultures with the extracellular parameters T ₂, OD₆₀₀, and pH, a relationship between cell growth and T ₂ variations was established. T ₂ is influenced by chemical exchange that depends on pH, composition, and concentration of the medium. Changes in the medium from bacterial metabolism are reflected in alternating T ₂ values. At 17.6 T, growth curves based on T ₂ values were measured simultaneously of several cultures of Streptococcus vestibularis. From T ₂ growth curves in the presence of varying concentrations of vancomycin, the minimum inhibitory concentration of the antibiotic could be determined to be 0.33 ± 0.08 μM. This value was in good agreement with the result obtained by the conventional broth microdilution. In principle, T ₂ growth curves can be determined on a large number of cultures simultaneously and may potentially be used as a novel tool in high through-put screening of novel anti-infective substances.     

Selective 3D ultrashort TE imaging: comparison of “dual-echo” acquisition and magnetization preparation for improving short-<Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript> contrast

Objective The objective of this study was to compare two different schemes for long-T ₂ component suppression in ultrashort echo-time (UTE) imaging. The aim was to increase conspicuity of short-T ₂ components accessible by the UTE technique. Materials and methods A “dual-echo” and a magnetization-preparation approach for long-T ₂ and fat suppression were implemented on clinical scanners. Both techniques were compared in 3D UTE exams on healthy volunteers regarding short-T ₂ Signal-to-noise ratio (SNR), long-T ₂ suppression quality, and scan efficiency. A quantitative SNR evaluation was performed using ankle scans of six volunteers. T ₂ suppression profiles were simulated for both approaches to facilitate interpretation of the observations. Results At 1.5 T, both techniques perform equally well in suppressing long-T ₂ components and fat. Magnetization preparation requires more shimming effort due to the use of narrow-band pulses, while the “dual-echo” technique requires a post-processing step to form a subtraction image. For scans with a short repetition time (TR), the “dual-echo” approach is much faster than the magnetization preparation, which depends on slow T ₁ recovery between preparation steps. The SNR comparison shows slightly higher short-T ₂ SNR for the “dual-echo” approach. At 3.0 T, magnetization preparation becomes more challenging due to stronger off-resonance effects. Conclusion Both techniques are well suited for long-T ₂ suppression and offer comparable short-T ₂ SNR. However, the “dual-echo” approach has strong advantages in terms of scan efficiency and off-resonance behavior.     

Quantification of Choline-containing Compounds in Malignant Breast Tumors by 1H MR Spectroscopy Using Water as an Internal Reference at 1.5 T

The quantification of choline-containing compounds (Cho) in breast tumors by proton MR spectroscopy (¹H-MRS) has been of great interest because such compounds have been linked to malignancy. In this study, an internal reference method for the absolute quantification of Cho metabolite in malignant breast tumors was presented using a clinical 1.5 T scanner. We performed in vitro measurements to examine the accuracy of absolute quantification using four phantoms of known choline chloride concentrations. There was a high correlation between the calculated concentrations by MRS and the known concentrations (r ² > 0.98). We applied the technique to in vivo breast study conducted on 45 patients with biopsy-confirmed breast cancer. After T ₁ and T ₂ relaxation times were corrected, the Cho levels in this work had a range of 0.76 – 21.20 mmol/kg from 34 MR spectra of 32 patients with malignant breast lesions. This result was rather consistent with the previously published value (i.e., 1.38 – 10 mmol/kg, Bolan et al. in Magn Reson Med 50:1134–1143, 2003). Therefore, we conclude that the internal method using the fully relaxed water as a reference could be used for quantifying Cho metabolite accurately in breast cancer patients using a clinical 1.5 T scanner.     

Low-molecular-weight paramagnetic 19F contrast agents for fluorine magnetic resonance imaging

Objective

¹⁹F MRI requires biocompatible and non-toxic soluble contrast agents with high fluorine content and with suitable ¹⁹F relaxation times. Probes based on a DOTP chelate with 12 magnetically equivalent fluorine atoms (DOTP-tfe) and a lanthanide(III) ion shortening the relaxation times were prepared and tested.

Methods

Complexes of DOTP-tfe with trivalent paramagnetic Ce, Dy, Ho, Tm, and Yb ions were synthetized and characterized. ¹⁹F relaxation times were determined and compared to those of the La complex and of the empty ligand. In vitro and in vivo ¹⁹F MRI was performed at 4.7 T.

Results

¹⁹F relaxation times strongly depended on the chelated lanthanide(III) ion. T₁ ranged from 6.5 to 287 ms, T₂ from 3.9 to 124.4 ms, and T₂* from 1.1 to 3.1 ms. All complexes in combination with optimized sequences provided sufficient signal in vitro under conditions mimicking experiments in vivo (concentrations 1.25 mM, 15-min scanning time). As a proof of concept, two contrast agents were injected into the rat muscle; ¹⁹F MRI in vivo confirmed the in vivo applicability of the probe.

Conclusion

DOTP-based ¹⁹F probes showed suitable properties for in vitro and in vivo visualization and biological applications. The lanthanide(III) ions enabled us to shorten the relaxation times and to trim the probes according to the actual needs. Similar to the clinically approved Gd³⁺ chelates, this customized probe design ensures consistent biochemical properties and similar safety profiles.

     

B1 field-insensitive transformers for RF-safe transmission lines

Objective: Integration of transformers into transmission lines suppresses radiofrequency (RF)-induced heating. New figure-of-eight-shaped transformer coils are compared to conventional loop transformer coils to assess their signal transmission properties and safety profile. Materials and methods: The transmission properties of figure-of-eight-shaped transformers were measured and compared to transformers with loop coils. Experiments to quantify the effect of decoupling from the B₁ field of the MR system were conducted. Temperature measurements were performed to demonstrate the effective reduction of RF-induced heating. The transformers were investigated during active tracking experiments. Results: Coupling to the B₁ field was reduced by 18 dB over conventional loop-shaped transformer coils. MR images showed a significantly reduced artifact for the figure-of-eight- shaped coils generated by local flip-angle amplification. Comparable transmission properties were seen for both transformer types. Temperature measurements showed a maximal temperature increase of 30K/3.5 K for an unsegmented/ segmented cable. With a segmented transmission line a robotic assistance system could be successfully localized using active tracking. Conclusion: The figure-of-eight-shaped transformer design reduces both RF field coupling with the MR system and artifact sizes. Anatomical structure close to the figure-of-eight-shaped transformer may be less obscured as with loop-shaped transformers if these transformers are integrated into e.g. intravascular catheters.