Practical design of a 4 Tesla double-tuned RF surface coil for interleaved 1H and 23Na MRI of rat brain.

MRI is proving to be a very useful tool for sodium quantification in animal models of stroke, ischemia, and cancer. In this work, we present the practical design of a dual-frequency RF surface coil that provides (1)H and (23)Na images of the rat head at 4 T. The dual-frequency RF surface coil comprised of a large loop tuned to the (1)H frequency and a smaller co-planar loop tuned to the (23)Na frequency. The mutual coupling between the two loops was eliminated by the use of a trap circuit inserted in the smaller coil. This independent-loop design was versatile since it enabled a separate optimisation of the sensitivity and RF field distributions of the two coils. To allow for an easy extension of this simple double-tuned coil design to other frequencies (nuclei) and dimensions, we describe in detail the practical aspects of the workbench design and MRI testing using a phantom that mimics in vivo conditions. A comparison between our independent-loop, double-tuned coil and a single-tuned (23)Na coil of equal size obtained with a phantom matching in vivo conditions, showed a reduction of the (23)Na sensitivity (about 28 %) because of signal losses in the trap inductance. Typical congruent (1)H and (23)Na rat brain images showing good SNR ((23)Na: brain 7, ventricular cerebrospinal fluid 11) and spatial resolution ((23)Na: 1.25 x 1.25 x 5mm(3)) are also reported. The in vivo SNR values obtained with this coil were comparable to, if not better than, other contemporary designs in the literature.

Application of the chirp z-transform to MRI data.

A version of the chirp z-transform (CZT) enabling signal intensity and phase-preserving field-of-view scaling has been programmed. The algorithm is important for all single-point imaging sequences such as SPRITE when used with multiple data acquisition for T2* mapping or signal averaging. CZT has particular utility for SPRITE imaging of nuclei with short relaxation times such as sodium at high field. Here, a complete theory of the properties of CZT is given. This method operates entirely in k-space. It is compared with a conventional interpolation approach that works in image space after the application of a fast Fourier transformation.

A comparison of three SPRITE techniques for the quantitative 3D imaging of the 23Na spin density on a 4T whole-body machine.

Sodium density maps acquired with three SPRITE-based methods have been compared in terms of the resulting quantitative information as well as image quality and acquisition times. Consideration of factors relevant for the clinical implementation of SPRITE shows that the Conical-SPRITE variant is preferred because of a 20-fold reduction in acquisition time, slightly improved image quality, and no loss of quantitative information. The acquisition of a 3D data set (32x32x16; FOV=256x256x160 mm) for the quantitative determination of sodium density is demonstrated. In vivo Conical-SPRITE 23Na images of the brain of a healthy volunteer were acquired in 30 min with a resolution of 7.5x7.5x7.5 mm and a signal-to-noise ratio of 23 in cerebrospinal fluid and 17 in brain tissue.

Phase-contrast microtomography of thin biomaterials.

Phase-contrast microtomography, performed at the beamline ID 22 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France), is demonstrated for high-resolution 3-D imaging of a hydroxyapatite sample. The technique, which relies on phase contrast imaging, gives the possibility to observe features inside samples with negligible absorption contrast. The positive results obtained suggest a possible future investigation of the influence of the distribution of pores and defects inside biomaterial coatings, on the growth of osteoblast cells.

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.

Phase-contrast imaging of thin biomaterials.

The necessity of information about the inner microscopical features of low absorbing materials is one of the most important goals in the structural research field. So far, non destructive analysis have been performed using contact radiography giving the scope for great advances in the production and application of new materials. However, the nature of interaction, namely X-ray absorption, limited the observations only to materials having sufficient heavy elements content. The adoption of a different X-ray interaction with matter which involves refractive properties of materials is at the basis of phase-contrast imaging. The novel method allows the use of high X-ray energies, for a deeper penetration and a lower released dose, without losing any information on the nature of the sample. A demonstration study, performed at the third generation European Synchrotron Radiation Facility (ESRF)-Grenoble, to show the potential of the new technique applied to biomaterials characterization is presented here. The test samples are a commercial matrix barrier (GUIDOR) intended to aid the healing process after periodontal surgery and a hydroxyapatite thin slab originally deposited by plasma spray technique on a TA6V alloy substrate. Phase-contrast images showed significant advantages revealing features that have negligible absorption contrast. The technique can be successfully used for the characterization of biomaterials.

Curved optics for x-ray phase contrast imaging by synchrotron radiation.

Conventional radiographic techniques have strong limitations when low-absorption contrast samples are imaged. Phase contrast radiography has been shown to produce high-quality images of soft tissues. In this technique the recorded intensity patterns are related to gradients in the refractive index of the sample. A critical point of this new technique is the need to employ crystal analysers, which results in an appreciable reduction in the beam intensity and consequently in rather long exposure times. In this paper the use of focused beams is suggested to overcome this aspect. Biological samples with small structures and low absorption variations were imaged using both flat and curved monochromator crystals, demonstrating that the use of curved optics leads to a decrease in the exposure time with only a limited degradation of the spatial resolution. This opens up the possibility of using the phase contrast technique with laboratory sources.

Ligand-induced conformational changes in tissue transglutaminase: Monte Carlo analysis of small-angle scattering data.

Small-angle neutron and x-ray scattering experiments have been performed on type 2 tissular transglutaminase to characterize the conformational changes that bring about Ca(2+) activation and guanosine triphosphate (GTP) inhibition. The native and a proteolyzed form of the enzyme, in the presence and in the absence of the two effectors, were considered. To describe the shape of transglutaminase in the different conformations, a Monte Carlo method for calculating small-angle neutron scattering profiles was developed by taking into account the computer-designed structure of the native transglutaminase, the results of the Guinier analysis, and the essential role played by the solvent-exposed peptide loop for the conformational changes of the protein after activation. Although the range of the neutron scattering data is rather limited, by using the Monte Carlo analysis, and because the structure of the native protein is available, the distribution of the protein conformations after ligand interaction was obtained. Calcium activation promotes a rotation of the C-terminal with respect to the N-terminal domain around the solvent-exposed peptide loop that connects the two regions. The psi angle between the longest axes of the two pairs of domains is found to be above 50 degrees, larger than the psi value of 35 degrees calculated for the native transglutaminase. On the other hand, the addition of GTP makes possible conformations characterized by psi angles lower than 34 degrees. These results are in good agreement with the proposed enzyme activity regulation: in the presence of GTP, the catalytic site is shielded by the more compact protein structure, while the conformational changes induced by Ca(2+) make the active site accessible to the substrate.