Centric scan SPRITE for spin density imaging of short relaxation time porous materials

The single-point ramped imaging with T1 enhancement (SPRITE) imaging technique has proven to be a very robust and flexible method for the study of a wide range of systems with short signal lifetimes. As a pure phase encoding technique, SPRITE is largely immune to image distortions generated by susceptibility variations, chemical shift and paramagnetic impurities. In addition, it avoids the line width restrictions on resolution common to time-based sampling, frequency encoding methods. The standard SPRITE technique is however a longitudinal steady-state imaging method; the image intensity is related to the longitudinal steady state, which not only decreases the signal-to-noise ratio, but also introduces many parameters into the image signal equation. A centric scan strategy for SPRITE removes the longitudinal steady state from the image intensity equation and increases the inherent image intensity. Two centric scan SPRITE methods, that is, Spiral-SPRITE and Conical-SPRITE, with fast acquisition and greatly reduced gradient duty cycle, are outlined. Multiple free induction decay (FID) points may be acquired during SPRITE sampling for signal averaging to increase signal-to-noise ratio or for T2* and spin density mapping without an increase in acquisition time. Experimental results show that most porous sedimentary rock and concrete samples have a single exponential T2* decay due to susceptibility difference-induced field distortion. Inhomogeneous broadening thus dominates, which suggests that spin density imaging can be easily obtained by SPRITE.     

SPRITE MRI of bubbly flow in a horizontal pipe

Bubble flow is characterised by numerous phase interfaces and turbulence, leading to fast magnetic resonance signal decay and artefacts in spin-warp imaging. In this paper, the SPRITE MRI pulse sequence, with its potential for very short encoding times, is demonstrated as an ideal technique for studying such dynamic systems. It has been used to acquire liquid velocity and relative intensity maps of two-phase gas–liquid dispersed bubble flow in a horizontal pipe at a liquid Reynolds number of 14,500. The fluids were air and water and a turbulence grid was used to generate a dispersed bubble flow pattern. The SPRITE technique shows promise for future research in gas–liquid flow.     

Magnetic resonance imaging of rigid polymers at elevated temperatures with SPRITE

Magnetic resonance imaging has rarely been applied to rigid polymeric materials, due primarily to the strong dipolar coupling and short signal lifetimes inherent in these materials. SPRITE (single point ramped imaging withT ₁ enhancement) (B. J. Balcom, R. P. MacGregor, S. D. Beyea, D. P. Green, R. L. Armstrong, T. W. Bremner: J. Magn. Reson. A123, 131–134, 1996) is particularly well suited to imaging solid materials. With SPRITE, the only requirement is thatT ₂^* be long enough so that the signal can be phase-encoded. The minimum phase encoding time is limited by the maximum gradient strength available and by the instrument deadtime. At present this is usually tens of microseconds and will only improve with refinements in technology. We have used the SPRITE sequence in conjunction with raising the sample temperature to obtain images of rigid polymers that have largely frustrated conventional imaging methods. This approach provides a straightforward and reliable method for imaging a class of samples that, up until now, have been very difficult to image.     

SPRITE MRI with Prepared Magnetization and Centrick-Space Sampling

A technique for imaging materials with short transverse relaxation times and prepared longitudinal magnetization is proposed. The technique is single-point ramped imaging withT₁-enhancement (SPRITE) MRI with centrick-space sampling. The effects of transient state behavior on image resolution and signal/noise are estimated. Centric sampling in the basic SPRITE sequence gives increased signal-to-noise and permits a quantitative determination of the MR parameters associated with longitudinal spin preparation. Spin-lock and inversion recovery preparation experiments are presented.     

Repetition time and flip angle variation in SPRITE imaging for acquisition time and SAR reduction

Single point imaging methods such as SPRITE are often the technique of choice for imaging fast-relaxing nuclei in solids. Single point imaging sequences based on SPRITE in their conventional form are ill-suited for in vivo applications since the acquisition time is long and the SAR is high. A new sequence design is presented employing variable repetition times and variable flip angles in order to improve the characteristics of SPRITE for in vivo applications. The achievable acquisition time savings as well as SAR reductions and/or SNR increases afforded by this approach were investigated using a resolution phantom as well as PSF simulations. Imaging results in phantoms indicate that acquisition times may be reduced by up to 70% and the SAR may be reduced by 40% without an appreciable loss of image quality.     

Centric-scan SPRITE magnetic resonance imaging with prepared magnetisation

The combination of contrast preparation with centric-scan SPRITE imaging readout is investigated. The main benefit of SPRITE, its ability to image objects with short T2, is retained. We demonstrate T1 and T2 mapping as examples of magnetisation preparation followed by magnetisation storage and spatially resolved encoding. A strategy for selection of the most advantageous imaging parameters for contrast mapping is presented.     

Characterization of water mobility and distribution in low- and intermediate-moisture food systems

The mechanism of water uptake in low moisture cereals and cookies has been studied by NMR relaxometry and solid imaging technology implemented on a low-resolution benchtop NMR spectrometer. A comparison between classical MRI and SPRITE imaging are also presented to highlight the benefits of each technology. The spin lattice (T(1)) and spin spin (T(2)) relaxation times, the 1D and 2D SPRITE imaging, were determined on Smacks, corn flakes, chocolate chips cookies, soft caramel candies with a chocolate crème filler, and corn starch/water systems. The Smacks and corn flakes were studied based on the soaking time in milk, and the results showed that T(1) and T(2) decreased in the first 20 sec of soaking and then increased with the soaking time. For Smacks stored at different relative humidity, T(1) decreased during the first day of storage and then was relatively constant over storage time indicating that the system reached an equilibrium. 1D SPRITE profiles indicated an increase in signal intensity over storage time for cookies in 58% RH. However, the moisture uptake was insignificant indicating that the water mobility (and not the amount of water) changed due to various chemical interactions in the system (hydrogen bonding, starch retrogradation, glassy/rubbery equilibrium). The T(1) and T(2) of corn starch/water systems decreased as the concentration in starch increased and temperature increased from 30 degrees C to 60 degrees C. However, for temperatures higher than 60 degrees C, the relaxation times increased showing more mobility and flexibility of the polymer chains during gelatinization.     

Centric scan SPRITE magnetic resonance imaging: optimization of SNR, resolution, and relaxation time mapping

Two strategies for the optimization of centric scan SPRITE (single point ramped imaging with T1 enhancement) magnetic resonance imaging techniques are presented. Point spread functions (PSF) for the centric scan SPRITE methodologies are numerically simulated, and the blurring manifested in a centric scan SPRITE image through PSF convolution is characterized. Optimal choices of imaging parameters and k-space sampling scheme are predicted to obtain maximum signal-to-noise ratio (SNR) while maintaining acceptable image resolution. The point spread function simulation predictions are verified experimentally. The acquisition of multiple FID points following each RF excitation is described and the use of the Chirp z-Transform algorithm for the scaling of field of view (FOV) of the reconstructed images is illustrated. Effective recombination of the rescaled images for SNR improvement and T*2 mapping is demonstrated.     

Effect of high signal photon fluxes on the responsivity of SPRITE detectors

The response of a SPRITE infrared detector is normally calculated and measured under small signal conditions, such that the signal photon flux is very much less than the background photon flux. However, with very hot targets, the signal flux may be comparable with or significantly greater than the background flux, and the small signal theory will no longer be appropriate. This paper reports calculations to determine the detector response in such large signal situations.     

Sectoral sampling in centric-scan SPRITE magnetic resonance imaging

A new approach to the construction of k-space trajectories for centric-scan SPRITE in both 2D and 3D is presented. All benefits of previous SPRITE methods are retained, most importantly the ability to image objects with short T*(2). This new approach gives more flexibility in the choice of number of interleaves with points more evenly distributed across k-space. All these improvements positively contribute to image quality and resolution, which can be also traded off against experimental speed. Sectoral sampling will have significant benefits for magnetisation preparation contrast imaging.     

Imaging of water distribution in thermally fractured granites by SPRITE

Water distribution in thermally fractured granite samples was visualized by using SPRITE sequences. Networks of intergranular fractures were observed in the coarse-grained Inada granite after heating at 873 K or above. On the other hand, bright spots were observed in the fine-grained Okazaki granite, which may be due to pore water in feldspar grains. D2O diffusion into samples saturated with H2O was also observed by 2D-projected SPRITE imaging.     

CZT和ZFFT频谱细化性能分析及FPGA实现

快速傅里叶变换在信号的频谱分析中应用广泛,而在工程实际中往往只对信号频谱中一段区间感兴趣,需要对频谱进行细化分析。常用的频谱细化方法有线性调频Z变换(Chirp-Z Transform, CZT)算法和基于复调制的细化(Zoom-FFT, ZFFT)算法,在给出了两种频谱细化方法的计算流程后,通过MATLAB仿真说明两种算法都能提高信号频率的测量精度,最后给出了这两种算法在FPGA具体工程实现中的实现步骤。     

Single point imaging with suppressed sound pressure levels through gradient-shape adjustment

Acoustic noise produced during single point imaging (SPI) experiments was modulated by changes in the spatial encoding gradients. Parameters of both linear and sine-shaped gradient ramps were modified to minimize the acoustic noise levels. Acoustic noise measurements during SPI were measured on three different gradient systems and revealed that for small gradient-bore systems a considerable acoustic noise reduction of more than 20 dB can easily be achieved. SPI in conjunction with an optimized gradient waveform can be a superb alternative to the previously introduced single point ramped imaging with T(1) enhancement (SPRITE) method when sound levels and overheating of gradients are a concern.     

CdZnTe <Emphasis Type="BoldItalic">γ</Emphasis> detector for deep inelastic neutron scattering on the VESUVIO spectrometer

In this paper it is shown that solid-state cadmium-zinc-telluride (CZT) is a promising photon detector for neutron spectroscopy in a wide energy interval, ranging from thermal (25 meV) to epithermal (70 eV) neutron energies. In the present study two CZT detectors were tested as part of the inverse-geometry neutron spectrometer VESUVIO operating at the ISIS pulsed neutron source. The response of the CZT detector to photon emission from radiative neutron capture in ²³⁸U was determined by biparametric measurements of neutron time of flight and photon energy. The scattering response function F(y) from a Pb sample has been derived using both CZT and conventional ⁶Li-glass scintillator detectors. The former showed both an improved signal to background ratio and higher efficiency as compared to ⁶Li glass, allowing us to measure F(y) up to the fourth ²³⁸U absorption energy (E_r=66.02 eV). Due to the small size of CZT detectors, their use is envisaged in arrays, with high spatial resolution, for neutron-scattering studies at high energy (>1 eV) and low wavevector (q <10 Å^-1) transfers. PACS 29.30.Hs; 29.30.Kv; 29.40.Kw; 61.12.Ek     

XPS and SRUPS study of oxygen adsorption on Cd0.9Zn0.1Te (1 1 1)A surface

Synchrotron radiation ultraviolet photoemission spectroscopy (SRUPS) and X-ray photoelectron spectroscopy (XPS) have been applied to investigate oxygen adsorption on a cadmium zinc telluride (CZT) (1 1 1)A surface. The surface chemical composition and the surface oxidation process were monitored by recording the Te 3d, O 1s, Zn 2p, Cd 4d core level peaks, and the Cd MNN Auger peak. The CZT (1 1 1)A surface was effectively oxidized by dosing oxygen directly. The typical surface state of the clean CZT (1 1 1)A surface was identified. After oxygen exposure, this surface state disappeared and a signal due to the formation of O–CZT appeared. In addition, the work function of CZT decreased with the increasing oxygen exposure.     

Magnetic Resonance Imaging of Gases: A Single-Point Ramped Imaging with T1 Enhancement (SPRITE) Study

A pure phase-encoding MRI technique, single-point ramped imaging withT₁enhancement, SPRITE, is introduced for the purpose of gas phase imaging. The technique utilizes broadband RF pulses and stepped phase encode gradients to produce images, substantially free of artifacts, which are sensitive to the gasT₁andT^*:₂relaxation times. Images may be acquired from gas phase species with transverse relaxation times substantially less than 1 ms. Methane gas images,¹H, were acquired in a phantom study. Sulfur hexafluoride,¹⁹F, images were acquired from a gas-filled porous coral sample. High porosity regions of the coral are observed in both the MRI image and an X-ray image. Sensitivity and resolution effects due to signal modulation during the time-efficient acquisition are discussed. A method to increase the image sensitivity is discussed, and the predicted improvement is shown through 1D images of the methane gas phantom.     

Centric scan SPRITE magnetic resonance imaging

Two rapid, pure phase encode, centric scan, Single Point Ramped Imaging with T₁-Enhancement (SPRITE) MRI methods are described. Each retains the benefits of the standard SPRITE method, most notably the ability to image short T₂^* systems, while increasing the sensitivity and generality of the technique. The Spiral-SPRITE method utilizes a modified Archimedean spiral k-space trajectory. The Conical-SPRITE method utilizes a system of spirals mapped to conical surfaces to sample the k-space cube. The sampled k-space points are naturally Cartesian grid points, eliminating the requirement of a re-gridding procedure prior to image reconstruction. The effects of transient state behaviour on image resolution and signal/noise are explored.     

Chemically selective NMR imaging of a 3-component (solid-solid-liquid) sedimenting system

A novel magnetic resonance imaging (MRI) technique which resolves the separate components of the evolving vertical concentration profiles of 3-component non-colloidal suspensions is described. This method exploits the sensitivity of MRI to chemical differences between the three phases to directly image the fluid phase and one of the solid phases, with the third phase obtained by subtraction. 19F spin-echo imaging of a polytetrafluoroethylene (PTFE) oil was interlaced with 1H SPRITE imaging of low-density polyethylene (LDPE) particles. The third phase was comprised of borosilicate glass spheres, which were not visible while imaging the PTFE or LDPE phases. The method is demonstrated by performing measurements on 2-phase materials containing only the floating (LDPE) particles, with the results contrasted to the experimental behaviour of the individual phases in the full 3-phase system. All experiments were performed using nearly monodisperse particles, with initial suspension volume fractions, phi(i), of 0.1.     

CdZnTe平面探测器对低能X/γ射线的光谱响应

基于3片不同条件下生长的CdZnTe晶片制备出平面电极(Planar)探测器CZT1、CZT2及CZT3.分析室温下3个探测器在不同场强作用下对低能X/γ射线的光谱响应,并结合相应晶体材料的载流子迁移特性和掺杂剂的浓度以及存在状态,归纳影响探测器分辨率的原因.掺杂In浓度高的探测器CZT1,由于材料中存在的深能级缺陷Cd2+i,作为电子的俘获中心,影响了载流子的收集效率,进而降低了探测器的能量分辨率;掺杂In浓度低的探测器CZT2对不同能量X/γ射线均具有较好的能量分辨率;而Al掺杂探测器CZT3,由于Al间隙原子Ali的存在作为电子的散射中心,最终影响了收集效率及能量分辨率.