Versatile coil design and positioning of transverse-field RF surface coils for clinical 1.5-T MRI applications

Clinical MRI/MRS applications require radio frequency (RF) surface coils positioned at an arbitrary angle with respect to B₀. In these experimental conditions the standard circular loop (CL) coil, producing an axial RF field, shows a large signal loss in the central region of interest (ROI). We demonstrate that transverse-field figure-of-eight (FO8) RF surface coils design are not subject to the same amount of signal loss in the central ROI as loop coils when their orientations are changed. The 1.5-T CL and FO8 prototypes (diameter = 10 cm) were built on Plexiglas using copper strips (width = 4 mm, thickness = 100 m). The two linear elements of the FO8 coil were 1 cm apart. Axial spoiled gradient echo (SPGR) images of a phantom containing doped water were acquired with the coil plane at =0°, 45°, and 90°. As increases, the CL images show, in the central ROI, a signal that decreases from a maximum value to zero. Whereas the FO8 images show, in the same ROI, a signal that varies little from the maximum value (20%). Optimized FO8 coils can be oriented with the coil plane positioned along any direction with respect to B₀ without significant signal loss. Transverse RF coil design should be useful for clinical MRS studies and also for parallel imaging techniques where versatile RF coils disposed along arbitrary directions are required.     

Investigation of complex phased array coil designs for cardiac imaging

In this study we present a method to simulate complex phased array coil designs for cardiac imaging. It is based on the combination of numerically calculatedB ₁ field vectors for each coil of the array and a noise resistance data set, which is acquired only once with a set of test coils. This technique allowed fast assessment of the SNR performance of arbitrary geometries of single coils to be used as building blocks in complex array configurations. In addition, since clinical scanners usually provide only four receiver channels, we used this method to investigate the use of hardware combiners for different array configurations, consisting of up to eight coils. Simulated array geometries resulted in up to ≈30% gain in SNR for deep cardiac structures, compared to a conventional linear four coil array. This was confirmed by phantom experiments with implemented coils     

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.     

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.

     

Quantitative T 2 measurement of a single voxel with arbitrary shape using pinwheel excitation and CPMG acquisition

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.     

Towards an integrated neonatal brain and cardiac examination capability at 7 T: electromagnetic field simulations and early phantom experiments using an 8-channel dipole array

Objective

Neonatal brain and cardiac imaging would benefit from the increased signal-to-noise ratio levels at 7 T compared to lower field. Optimal performance might be achieved using purpose designed RF coil arrays. In this study, we introduce an 8-channel dipole array and investigate, using simulations, its RF performances for neonatal applications at 7 T.

Methods

The 8-channel dipole array was designed and evaluated for neonatal brain/cardiac configurations in terms of SAR efficiency (ratio between transmit-field and maximum specific-absorption-rate level) using adjusted dielectric properties for neonate. A birdcage coil operating in circularly polarized mode was simulated for comparison. Validation of the simulation model was performed on phantom for the coil array.

Results

The 8-channel dipole array demonstrated up to 46% higher SAR efficiency levels compared to the birdcage coil in neonatal configurations, as the specific-absorption-rate levels were alleviated. An averaged normalized root-mean-square-error of 6.7% was found between measured and simulated transmit field maps on phantom.

Conclusion

The 8-channel dipole array design integrated for neonatal brain and cardiac MR was successfully demonstrated, in simulation with coverage of the baby and increased SAR efficiency levels compared to the birdcage. We conclude that the 8Tx-dipole array promises safe operating procedures for MR imaging of neonatal brain and heart at 7 T.

     

Ultra-high field MRI for primate imaging using the travelling-wave concept

Object

Ultra-high field (UHF) neuroimaging is usually conducted with volume transmit (Tx) and phased array receive (Rx) coils, both tightly enclosing the object. The travelling-wave (TW) concept allows a remote excitation offering more flexible experimental setups. To investigate the feasibility of primate MRI in horizontal UHF MRI, we first compared the distribution of the electromagnetic fields in an oil phantom and then verified the concept with an in vivo experiment.

Materials and methods

In the phantom experiments an in-house circularly polarized hybrid birdcage coil and a self-developed patch antenna were used for Tx and an eight-element phased array antenna for Rx. B ₁ ⁺ fields were calculated and measured for both approaches. For in vivo experiments the Rx part was replaced with an optimized three-element phased array head coil. The SAR was calculated using field simulation.

Results

In the phantom the field distribution was homogenous in a central volume of interest of about 10 cm diameter. The TW concept showed a slightly better homogeneity. Examination of a female crab-eating macaque led to homogeneous high-contrast images with a good delineation of anatomical details.

Conclusion

The TW concept opens up a new approach for MRI of medium-sized animals in horizontal UHF scanners.     

Potential pitfall in the determination of free [Mg2+] by31P NMR when using the β/α-ATP peak height ratio method

Recently, Clarke et al., (Clarke K, Kashiwaya Y, King MT, Gates D, Keon CA, Cross HR, Radda GK, Veech RL. The β/α peak height ratio of ATP. A measure of free [Mg _free ²⁺ ] using³¹ P NMR, J. Biol. Chem. 1996;271:21142–21150.) reported a new method to noninvasively determine the concentration of intracellular free magnesium ([Mg _free ²⁺ ]) based on the measurement of the peak height ratioh _β/α of the β- and α-ATP signals in³¹P NMR spectra.h _β/α varies with Mg_free ²⁺], however, the study presented here shows thath _β/α also strongly depends on the homogeneity of the static magnetic field. For this reason, we performed at a magnetic field strength of 1.5 T³¹P NMR measurements of solutions that mimic intracellular medium. The magnetic field homogeneity was varied by changing the currents in the shim coils, and the effect onh _β/α is demonstrated with and without proton decoupling. In both cases,h _β/α strongly depends on the magnetic field homogeneity and can therefore lead to a pitfall in the determination of [Mg_free ²⁺].     

Multi-feed,loop-dipole combined dielectric resonator antenna arrays for human brain MRI at 7 T

Objective

To determine whether a multi-feed, loop-dipole combined approach can be used to improve performance of rectangular dielectric resonator antenna (DRA) arrays human brain for MRI at 7 T.

Materials and methods

Electromagnetic field simulations in a spherical phantom and human voxel model “Duke” were conducted for different rectangular DRA geometries and dielectric constants ε_r. Three types of RF feed were investigated: loop-only, dipole-only and loop-dipole. Additionally, multi-channel array configurations up to 24-channels were simulated.

Results

The loop-only coupling scheme provided the highest B₁⁺ and SAR efficiency, while the loop-dipole showed the highest SNR in the center of a spherical phantom for both single- and multi-channel configurations. For Duke, 16-channel arrays outperformed an 8-channel bow-tie array with greater B₁⁺ efficiency (1.48- to 1.54-fold), SAR efficiency (1.03- to 1.23-fold) and SNR (1.63- to 1.78). The multi-feed, loop-dipole combined approach enabled the number of channels increase to 24 with 3 channels per block.

Discussion

This work provides novel insights into the rectangular DRA design for high field MRI and shows that the loop-only feed should be used instead of the dipole-only in transmit mode to achieve the highest B₁⁺ and SAR efficiency, while the loop-dipole should be the best suited in receive mode to obtain the highest SNR in spherical samples of similar size and electrical properties as the human head.

     

Water proton T 1 measurements in brain tissue at 7, 3, and 1.5T using IR-EPI, IR-TSE, and MPRAGE: results and optimization

Method This paper presents methods of measuring the longitudinal relaxation time using inversion recovery turbo spin echo (IR-TSE) and magnetization-prepared rapid gradient echo (MPRAGE) sequences, comparing and optimizing these sequences, reporting T ₁ values for water protons measured from brain tissue at 1.5, 3, and 7T. T ₁ was measured in cortical grey matter and white matter using the IR-TSE, MPRAGE, and inversion recovery echo planar imaging (IR-EPI) pulse sequences. Results In four subjects the T ₁ of white and grey matter were found to be 646±32 and 1,197±134ms at 1.5T, 838±50 and 1,607±112ms at 3T, and 1,126±97, and 1,939±149ms at 7T with the MPRAGE sequence. The T ₁ of the putamen was found to be 1,084±63ms at 1.5T, 1,332±68ms at 3T, and 1,644±167ms at 7T. The T ₁ of the caudate head was found to be 1,109± 66ms at 1.5T, 1,395±49ms at 3T, and 1,684±76ms at 7T. Discussion There was a trend for the IR-TSE sequence to underestimate T ₁ in vivo. The sequence parameters for the IR-TSE and MPRAGE sequences were also optimized in terms of the signal-to-noise ratio (SNR) in the fitted T ₁. The optimal sequence for IR-TSE in terms of SNR in the fitted T ₁ was found to have five readouts at TIs of 120, 260, 563, 1,221, 2,647, 5,736ms and TR of 7 s. The optimal pulse sequence for MPRAGE with readout flip angle = 8° was found to have five readouts at TIs of 160, 398, 988, 2,455, and 6,102ms and a TR of 9 s. Further optimization including the readout flip angle suggests that the flip angle should be increased, beyond levels that are acceptable in terms of power deposition and point-spread function.     

Measurement of arterial input function of <Superscript>17</Superscript>O water tracer in rat carotid artery by using a region-defined (REDE) implanted vascular RF coil

A method of determining arterial input function (AIF) by continuously detecting the ¹⁷O MR signal changes of ¹⁷O-labeled water tracer in the rat carotid artery using a region-defined (REDE) implanted vascular RF coil at 9.4 Tesla is reported. This coil has a compact physical size of 1 mm inner diameter, 3 mm outer diameter and 11 mm in length. It can be readily implanted into the rat neck and wrapped around the rat carotid artery for achieving adequate MR detection sensitivity for determining AIF with minimal surgical trauma. Water phantom and in vivo MR experiments were conducted for validating the coil's performance. A signal-to-noise ratio of ~20:1 was achieved for the ¹⁷O signal acquired from naturally abundant H₂ ¹⁷O in a small amount of blood (~7 μl) inside the rat carotid artery with an acquisition time of 11 s. The REDE RF coil design electromagnetically isolates the rat carotid artery from surrounding tissues and ensures that the MR signal detected by the RF coil is only attributable to the artery blood. It also minimizes the electromagnetic coupling between the implanted RF coil and a head surface coil tuned at the same operating frequency (two-coil configuration). This configuration allowed simultaneous measurements of dynamic changes of ¹⁷O MR signal of the H₂ ¹⁷O tracer in both rat carotid artery and brain. Compared to most contemporary MR approaches, the REDE implanted RF provides a simple, accurate, and promising solution for determination of AIF in small experimental animals.     

Dielectric Properties of Bi4−x La x Ti3O12 (0 ≤ x ≤ 2) Ceramics

The dielectric properties of the Bi_4–x La _x Ti₃O₁₂ (0 x 2) ceramics were characterized and discussed together with the P-E relation (polarization vs. electric field). With increasing x, the P-E relation changed from normal ferroelectric hysteresis loops to pure linear relation, which indicated that La³⁺ substitution for Bi³⁺ in Bi₄Ti₃O₁₂ induced a phase transition from ferroelectric to paraelectric state at ambient temperature. Low loss dielectric ceramics with temperature stable dielectric constant were obtained for x > 1.2 in Bi_4–x La _x Ti₃O₁₂ at 1 MHz. And the loss increased in all the compositions when the ceramics were measured at microwave frequencies.     

Effect of Sb2O3 addition on the varistor characteristics of ZnO-Bi2O3-ZrO2-MtrO (Mtr = Mn, Co)

Effect of Sb₂O₃ addition on the varistor characteristics of pyrochlore-free ZnO-Bi₂O₃-ZrO₂-M_trO (M_tr = Mn, Co) system previously proposed has been studied. With Sb₂O₃ up to 0.1 mol%, a gradual enhancement of densification and the grain growth inhibition were seen in the system sintered between 900 and 1200^∘C. In X-ray diffraction patterns, small amount of pyrochlore appeared in the specimens doped with Sb₂O₃ (>0.06 mol%), which is thought responsible for the sintering behavior. Enhanced values of non linear coefficient (α) were obtained in ZnO-Bi₂O₃-ZrO₂ (ZBZ) doped with 0.001 mol% Sb₂O₃, but was leveled off at higher concentrations. In ZBZ added with M_trO (M_tr = Mn, Co), significant increase of nonlinear coefficient (α > 30) along with low leakage current (I _L ≪ 100 μA/cm²) was attained. The α-enhancement effect of Sb₂O₃, however, was not observed in high-α ZBZ added with M_trO. As for degradation, addition of a trace amount (0.001 mol%) of Sb₂O₃ to ZBZM_tr was efficient, especially in I _L.     

An 8-channel Tx/Rx dipole array combined with 16 Rx loops for high-resolution functional cardiac imaging at 7 T

Objective

To demonstrate imaging performance for cardiac MR imaging at 7 T using a coil array of 8 transmit/receive dipole antennas and 16 receive loops.

Materials and methods

An 8-channel dipole array was extended by adding 16 receive-only loops. Average power constraints were determined by electromagnetic simulations. Cine imaging was performed on eight healthy subjects. Geometrical factor (g-factor) maps were calculated to assess acceleration performance. Signal-to-noise ratio (SNR)-scaled images were reconstructed for different combinations of receive channels, to demonstrate the SNR benefits of combining loops and dipoles.

Results

The overall image quality of the cardiac functional images was rated a 2.6 on a 4-point scale by two experienced radiologists. Imaging results at different acceleration factors demonstrate that acceleration factors up to 6 could be obtained while keeping the average g-factor below 1.27. SNR maps demonstrate that combining loops and dipoles provides a more than 50% enhancement of the SNR in the heart, compared to a situation where only loops or dipoles are used.

Conclusion

This work demonstrates the performance of a combined loop/dipole array for cardiac imaging at 7 T. With this array, acceleration factors of 6 are possible without increasing the average g-factor in the heart beyond 1.27. Combining loops and dipoles in receive mode enhances the SNR compared to receiving with loops or dipoles only.

     

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.

     

Investigation on the design and synthesis of new systems of BNT-based lead-free piezoelectric ceramics

Three primary differences between BNT- and PZT-based ceramics were analyzed from the composition and the active component of the materials. Based on the analysis the authors’ group developed the new idea of the design of the multiple complex in the A-site ions of BNT compounds. (Bi_0.5Na_0.5)²⁺, Bi³⁺ and Na⁺ in the ABO₃ structure are defined as A-site, A₁-site and A₂-site ions, respectively, and A, A₁ and A₂-site ions can be simultaneously or singly substituted partially by alkaline-earth metal ions, metal ions with +3 valence and metal ions with +1 valence, respectively. Under this consideration, Several new systems of Bi_0.5Na_0.5TiO₃ (abbreviated as BNT)-based lead-free piezoelectric ceramics were proposed. These ceramics can be prepared by conventional ceramic techniques and have excellent piezoelectric performance. Among these materials, Bi_0.5(Na_1−x−y K _x Li _y )_0.5TiO₃ possesses higher piezoelectric constant (d ₃₃ = 230 pC/N), higher electromechanical couple factor (k _p = 0.40), larger remanent polarization (P _r = 38.9 μC/cm²) and a better P-E hysteresis loop until about 200^∘C. This work was supported by the projects of NSFC (50410179), (50572066), and (59972020), and NAMMC (2001-AA325060).     

Sintering and compositional effects on the microwave dielectric characteristics of Mg(Ta1−x Nb x )2O6 ceramics with 0.25 ≦ x ≦ 0.35

1,500 °C−sintered MgTa₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 30.5, Q × f = 56,900 GHz, and τ _f = 28.3 ppm/°C, whereas 1,400 °C-sintered MgNb₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 21.7, Q × f = 89,900 GHz, and τ _f = −68.5 ppm/°C. In order to find the dielectric resonators with τ _f value close to 0 ppm/°C, the effects of sintering condition and composition on the microwave dielectric characteristics of Mg(Ta_1−x Nb _x )₂O₆ ceramics (0.25 ≦ x ≦ 0.35) prepared under sintering temperature of 1,300–1,450 °C are investigated. The results show that as the sintering temperature increases from 1,300 to 1,450 °C, the ɛ _r, Q × f and τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics all increase and saturate at 1,450 °C. On the other hand, as the Nb₂O₅ content decreases, the τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics will shift to near 0 ppm/°C. The optimized sintering conditions and composition to obtain the Mg(Ta_1−x Nb _x )₂O₆ dielectrics with τ _f close to 0 ppm/°C are sintering temperature of 1,450 °C, sintering duration of 4 h, and composition of x = 0.25, which exhibits the microwave dielectric characteristics of ɛ _r = 27.9, Q × f = 33,100 GHz, and τ _f = −0.7 ppm/°C.     

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.     

Temperature-Stable Super High Permittivity Dielectric Ceramics Based on (Ag1 − x Na x )(Nb1 − y Ta y )O3 System

The dielectric properties of (Ag_{1 –x} Na _x )(Nb_{1 –y} Ta _y )O₃ were studied in this paper. The molar ratios of Ag/Na and Nb/Ta were quite important to adjust dielectric properties of the system. The ceramic material with high permittivity and low dielectric loss can be obtained in the cases where Ag/Na ratio is 3/2 and Nb/Ta ratio is 3/2. In addition, the dielectric loss was reduced by preparing the precursor in advance.     

Gas sensors: New materials and processing approaches

To satisfy demands for detecting chemicals in the environment, versatile sensors are required to detect a rapidly growing range of chemical species. In this paper, focus is directed on progress being made to develop temperature-independent oxygen sensors based on the perovskite solid solution system SrTi_1−x Fe _x O_3−δ,and on improving the sensitivity of thin film gas sensors integrated on Si-based self heated substrates. A novel strategy to produce macroporous films with high surface area for enhanced chemical activity is described, and how this processing strategy results in markedly improved sensitivity of gas sensors based on a novel material, CaCu₃Ti₄O₁₂, is demonstrated.