A quantitative study of water proton relaxation in packed beds of porous particles with varying water content

Effects of diffusion in magnetically inhomogeneous media on rotating frame spin-lattice relaxation

In an aqueous medium containing magnetic inhomogeneities, diffusion amongst the intrinsic susceptibility gradients contributes to the relaxation rate R_1ρ of water protons to a degree that depends on the magnitude of the local field variations ΔB_z, the geometry of the perturbers inducing these fields, and the rate of diffusion of water, D. This contribution can be reduced by using stronger locking fields, leading to a dispersion in R_1ρ that can be analyzed to derive quantitative characteristics of the material. A theoretical expression was recently derived to describe these effects for the case of sinusoidal local field variations of a well-defined spatial frequency q. To evaluate the degree to which this dispersion may be extended to more realistic field patterns, finite difference Bloch-McConnell simulations were performed with a variety of three-dimensional structures to reveal how simple geometries affect the dispersion of spin-locking measurements. Dispersions were fit to the recently derived expression to obtain an estimate of the correlation time of the field variations experienced by the spins, and from this the mean squared gradient and an effective spatial frequency were obtained to describe the fields. This effective spatial frequency was shown to vary directly with the second moment of the spatial frequency power spectrum of the ΔB_z field, which is a measure of the average spatial dimension of the field variations. These results suggest the theory may be more generally applied to more complex media to derive useful descriptors of the nature of field inhomogeneities. The simulation results also confirm that such diffusion effects disperse over a range of locking fields of lower amplitude than typical chemical exchange effects, and should be detectable in a variety of magnetically inhomogeneous media including regions of dense microvasculature within biological tissues.

Chemical exchange in knee cartilage assessed by R1ρ (1/T1ρ) dispersion at 3T

PURPOSE

To quantify the characteristics of proton chemical exchange in knee cartilage in vivo by R1ρ dispersion analysis.

MATERIALS AND METHODS

Six healthy subjects (one female and five males, age range 24 to 71 y) underwent T1ρ imaging of knee cartilage on a 3T MRI scanner. Quantitative estimates of R1ρ (=1/T1ρ) were made using 5 different spin-lock durations for each of 12 different spin-lock amplitudes over the range 0 to 550Hz. When the variations of R1ρ with spin-locking strength (the R1ρ dispersion) are dominated by chemical exchange contributions, R1ρ dispersion curves can be analyzed to derive quantitative characteristics of the exchange and provide information on tissue composition. In this work, in vivo R1ρ dispersion of human knee articular cartilage at 3T was analyzed, and the exchange rates of protons between water and macromolecular hydroxyls (mainly in glycosaminoglycans) were estimated based on a theoretical model.

RESULTS

R1ρ values showed marked dispersion in articular cartilage and varied by approximately 50% between low and high values of the locking field, a change much greater than in surrounding tissues, consistent with greater contributions from chemical exchange. From the theoretical model, the exchange rates in cartilage were estimated to be in the range of 1.0-3.0kHz, and varied within the tissue. Variations within a single knee appear to be larger with increasing age.

CONCLUSION

R1ρ dispersion analysis may provide more specific information for studying cartilage biochemical composition and form the basis for quantitative evaluation of cartilage disorders.

Exchange-mediated contrast in CEST and spin-lock imaging

PURPOSE

Magnetic resonance images of biological media based on chemical exchange saturation transfer (CEST) show contrast that depends on chemical exchange between water and other protons. In addition, spin-lattice relaxation rates in the rotating frame (R1ρ) are also affected by exchange, especially at high fields, and can be exploited to provide novel, exchange-dependent contrast. Here, we evaluate and compare the factors that modulate the exchange contrast for these methods using simulations and experiments on simple, biologically relevant samples.

METHODS

Simulations and experimental measurements at 9.4 T of rotating frame relaxation rate dispersion and CEST contrast were performed on solutions of macromolecules containing amide and hydroxyl exchanging protons.

RESULTS

The simulations and experimental measurements confirm that both CEST and R1ρ measurements depend on similar exchange parameters, but they manifest themselves differently in their effects on contrast. CEST contrast may be larger in the slow and intermediate exchange regimes for protons with large resonant frequency offsets (e.g. >2 ppm). Spin-locking techniques can produce larger contrast enhancement when resonant frequency offsets are small (<2 ppm) and exchange is in the intermediate-to-fast regime. The image contrasts scale differently with field strength, exchange rate and concentration.

CONCLUSION

CEST and R1ρ measurements provide different and somewhat complementary information about exchange in tissues. Whereas CEST can depict exchange of protons with specific chemical shifts, appropriate R1ρ-dependent acquisitions can be employed to selectively portray protons of specific exchange rates.

Contributions of chemical and diffusive exchange to T1ρ dispersion

Variations in local magnetic susceptibility may induce magnetic field gradients that affect the signals acquired for MR imaging. Under appropriate diffusion conditions, such fields produce effects similar to slow chemical exchange. These effects may also be found in combination with other chemical exchange processes at multiple time scales. We investigate these effects with simulations and measurements to determine their contributions to rotating frame (R1ρ ) relaxation in model systems. Simulations of diffusive and chemical exchange effects on R1ρ dispersion were performed using the Bloch equations. Additionally, R1ρ dispersion was measured in suspensions of Sephadex and latex beads with varying spin locking fields at 9.4 T. A novel analysis method was used to iteratively fit for apparent chemical and diffusive exchange rates with a model by Chopra et al. Single- and double-inflection points in R1ρ dispersion profiles were observed, respectively, in simulations of slow diffusive exchange alone and when combined with rapid chemical exchange. These simulations were consistent with measurements of R1ρ in latex bead suspensions and small-diameter Sephadex beads that showed single- and double-inflection points, respectively. These observations, along with measurements following changes in temperature and pH, are consistent with the combined effects of slow diffusion and rapid -OH exchange processes.

Proton NMR relaxation study of swelling and gelatinisation process in rice starch-water samples

Proton transverse magnetization decay curves of rice flour starch-water samples were measured and analysed for the presence of four components in the relaxation curve. T2 values were interpreted on the basis of the diffusive and chemical exchange model that provided evidence for extra granular bulk water and three more water populations whose relaxation rate is governed by diffusive and chemical exchange with starch components. The analysis of relaxation data provided information on dynamics of water molecules as well as on the size and dispersion of diffusive domains. Furthermore, by measuring solid to liquid ratio, transverse and longitudinal relaxation curves of starch-water mixtures at increasing temperatures - from 20 to 77°C - swelling and gelatinisation processes were monitored.

MR Imaging Detection of Superparamagnetic Iron Oxide–loaded Tris-acryl Embolization Microspheres

PURPOSE

To assess by magnetic resonance (MR) imaging the detectability of superparamagnetic iron oxide (SPIO)-labeled microspheres (MSs) in vitro on gelose, ex vivo in kidneys from embolized sheep, and in vivo in kidneys from embolized pigs.

MATERIALS AND METHODS

With various sizes of SPIO-labeled MSs, common neck and pelvic spin-echo and gradient-echo sequences were acquired on a 1.5-T MR unit. SPIO-labeled MSs of four sizes were embedded in a hydrogel as single MSs or in multiple units, or multiplets. Detection rate on MR imaging was assessed according to the real size and number of MSs. SPIO-loaded and unloaded MSs of four sizes were injected into eight sheep kidneys, which underwent MR and pathologic examinations. For each size, the location of MSs in renal vasculature was determined and compared according to the technique used. Kidneys were embolized in pigs with various amounts of MSs in three sizes. MR was performed immediately after embolization and SPIO-labeled MS detection was assessed according to size, organ, and amount injected. Results SPIO-labeled MSs provide a low signal intensity on T1-weighted sequences, without distortion. In vitro, 28% of 100-300-microm single MSs were detected and more than 80% were detected for larger sizes. MS multiplets were all detected in all sizes. Ex vivo, all sizes of MSs were detected by MR imaging in kidneys, whereas control MSs were not observed. Histologic analysis showed that there was no difference in vascular distribution between SPIO-labeled MS and control MSs, and therefore for each caliber (P > .05). Arterial location of SPIO-labeled MSs was the same on MR imaging and histologic analysis. In vivo, SPIO-labeled MS were detected in the kidney vasculature when volumes greater than 1 mL of 100-300-microm or 500-700-microm MSs were injected. Volumes lower than 1 mL SPIO-labeled MSs were hardly detected in kidneys, regardless of MS size. Conclusions SPIO-labeled MSs are detected by MR imaging with common gradient-echo sequences in vitro in gelose and ex vivo and in vivo in kidneys. SPIO-labeled MSs could allow better control of embolization and thereby enhance efficacy and safety of the procedure.

Microstructural factors controlling the survival of food-borne pathogens in porous media

The survival of Salmonella typhimurium LT2, Escherichia coli K-12 and Pseudomonas putida in several model porous media poised at a water activity of 0.94 is shown to depend critically on the microstructure of the particulate matrix and the microscopic water distribution. The porous media were made by randomly dispersing a liquid inoculum containing ca. 10(7) cells/ml throughout the pores and interparticle spaces of packed beds of silica particles and Sephadex microspheres. The purely "microstructural stress" effects were isolated by comparison with a homogeneous liquid growth medium having the same water activity. The possibility of exploiting similar microstructural stress effects in food preservation is discussed.

Water availability and the survival of Salmonella typhimurium in porous systems

The survival of Salmonella Typhimurium LT2 in randomly packed beds of glass beads, microporous silica particles and Sephadex microspheres is examined. It is shown that the decrease in the percentage cell recovery in these porous materials at reduced water content is not correlated with the global water activity as determined by conventional vapour pressure measurements but rather with the osmotic shock induced by the sudden redistribution of water and air among the microscopic pores in the matrix surrounding the cells. For this reason the bacterial survival and growth data correlates best with physical measurements, such as NMR and electrical conductivity, which are sensitive to the microscopic air-water distribution. The implications of this observation in food safety and preservation are discussed.

Detection of Cl- binding to band 3 by double-quantum-filtered 35Cl nuclear magnetic resonance

We have applied double-quantum-filtered (DQF) NMR of 35Cl to study binding of Cl- to external sites on intact red blood cells, including the outward-facing anion transport sites of band 3, an integral membrane protein. A DQF 35Cl NMR signal was observed in cell suspensions containing 150 mM KCl, but the DQF signal can be totally eliminated by adding 500 microM 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS), an inhibitor that interferes with Cl- binding to the band 3 transport site. Therefore, it seems that only the binding of Cl- to transport sites of band 3 can give rise to a 35Cl DQF signal from red blood cell suspensions. In accordance with this concept, analysis of the single quantum free induction decay (FID) revealed that signals from buffer and DNDS-treated cells were fitted with a single exponential function, whereas the FID signals of untreated control cells were biexponential. The DQF signal remained after the cells were treated with eosin-5-maleimide (EM), a noncompetitive inhibitor of chloride exchange. This result supports previous reports that EM does not block the external chloride binding site. The band 3-dependent DQF signal is shown to be caused at least in part by nonisotropic motions of Cl- in the transport site, resulting in incompletely averaged quadrupolar couplings.

Dynamic NMR Q-space studies of microstructure with the multigrade CPMG sequence

A new approach to q-space studies of microstructure is proposed, which exploits the combined information contained in the water proton transverse relaxation time distribution and the frequency dependence of the apparent water diffusivity in heterogeneous systems. Using an automated two-dimensional multigrade CPMG sequence, both the pulse spacing and the amplitude of the applied field gradient are varied systematically and used to measure the frequency and wave vector dependence of the multiple exponential echo decay constants and amplitudes. Undesirable crossterms in the applied and background field gradients are eliminated by a simple procedure involving a sign reversal in the applied gradient. Nonlinear, local susceptibility-induced field gradients are shown to lead to enhanced, frequency-dependent apparent water diffusivities that are sensitive to the local microstructure.

Quantitative radial imaging of porous particles beds with varying water contents

Radial imaging protocols suitable for monitoring water transport in biopolymer and food materials during processes such as drying and rehydration are developed and tested on a well-characterized model sample. This model consisted of a randomly packed bed of Sephadex beads with varying water content. The results are interpreted with theoretical models for the dependence of the initial water magnetization, transverse relaxation, and diffusive attenuation on water content for two slice-selective radial imaging pulse sequences. It is shown that volume shrinkage and changes in packing density complicate the dependence of the initial magnetization on water content, so that the transverse relaxation rate provides the most reliable monitor of water content. Radial imaging is shown to offer many advantages over conventional two-dimensional imaging whenever the sample can be made with cylindrical symmetry.