Dielectric inserts for sensitivity and RF magnetic field enhancement in NMR volume coils.

A method for enhancing the signal to noise ratio (SNR) in NMR volume coils is described. By introducing inserts made of low-loss, high dielectric constant material into specific locations in the coil, the SNR can often be enhanced by up to 20%, while B(1) homogeneity is hardly affected. A model for predicting the limit of the SNR improvement is also presented. The model accurately predicts the SNR gain obtained in both numerical simulations and experiment. An experiment was conducted on a mini-MRI system. Experimental results are in very good agreement with the simulations in regard to both SNR improvement and B(1) enhancement in transmission. Inserts made of ultra high dielectric constant materials can be as thin as few millimeters, thus, conveniently fitting into existing coil-sample gaps in volume coils.

Diffusion tensor imaging of the median nerve in healthy and carpal tunnel syndrome subjects.

PURPOSE: To determine if diffusion tensor imaging (DTI) of the median nerve could allow identification of patients with carpal tunnel syndrome (CTS). MATERIALS AND METHODS: A total of 13 healthy subjects and 9 CTS patients were scanned on a 3T magnetic resonance imaging (MRI) scanner. The MRI protocol included a DTI sequence from which the fractional anisotropy (FA), apparent diffusion coefficient (ADC), and the parallel and radial diffusivities could be extracted. Those parameters were quantified at different locations along the median nerve (proximal to the carpal tunnel, within the carpal tunnel, and distal to the carpal tunnel). RESULTS: At the carpal tunnel, the FA, radial diffusivity, and ADC differed significantly between healthy subjects and CTS patients (P<0.0002). This highly significant difference between the two groups was due to an opposite trend of changes in the DTI indices between the proximal to the carpal tunnel and within the carpal tunnel locations. In healthy subjects the FA increased (+20%, P<0.001) and the radial diffusivity and ADC decreased (by -15% and -8%, respectively, P<0.05) between the proximal to the carpal tunnel and within the carpal tunnel locations. In CTS subjects the FA decreased (by -21%, P<0.05) and the radial diffusivity increased (by +23%, P<0.01) between the proximal to the carpal tunnel and within the carpal tunnel locations. CONCLUSION: DTI enables visualization and characterization of the median nerve in healthy subjects and CTS patients. DTI indices show clear-cut discrimination between the two groups and in fact enables the of use DTI in the diagnosis of CTS.

MRI of oriental hornet head.

The head of the Oriental hornet in situ, detached from a live sample was imaged using Magnetic Resonance Imaging (MRI). This non-invasive method enabled us to visualize the three-dimensional structure of the hornet's brain and intracerebral organs, as based on cubic voxels of 23 microm3. From these images, we could identify various cephalic structures in both supra-esophageal and sub-esophageal locations. In the former location, we identified and visualized the ocelli, ommatidia, mushroom body, lobula, medulla and the compound eyes in the protocerebrum, as well as the olfactory lobe and bases of the antenna in the deutocerebrum, while in the sub-esophageal region we visualized organs such as the mouthparts, the esophagus, the gnathal pouch and the salivary ducts that empty into the region. Additionally, we identified various muscles, the aorta, cuticular thickenings lending support to the interior of the head and also the cuticular skeleton providing support on the outside. All the mentioned structures and organs were visualized in their relative, normal proportions, without touching or dislocating them.

Susceptibility-matched envelope for the correction of EPI artifacts.

Fast gradient echo sequences, such as echo planer imaging (EPI) and spiral imaging, are vulnerable to artifacts resulting from B(0) inhomogeneities. A major contribution to these artifacts is the susceptibility variation across the head, which is most severe in regions adjacent to air-tissue interfaces, such as the mouth, nasal sinuses, ears and the cortex. Susceptibility artifacts can cause geometrical distortions in the image as well as loss of signal due to T(2)* dephasing. The extent of these artifacts increases with the main field, thus compromising the signal-to-noise ratio (SNR) benefit gained in higher fields. In the current work, inhomogeneity caused by susceptibility variations at the external boundary of the human body has been corrected by surrounding the organs with a liquid without hydrogen atoms and whose susceptibility is similar to that of the imaged organ. EPI experiments were conducted on head-sized phantom, human brain, hand and legs. This method causes minimal patient inconvenience and no interference with any function of the scanner, thus yielding a simple and efficient solution for the correction of B(0) variation.

Subcuticular microstructure of the hornet's gaster: Its possible function in thermoregulation.

The present study set out to elucidate the structure and function of the large subcuticular air sacs encountered in the gaster of the Oriental hornet Vespa orientalis (Hymenoptera, Vespinae). Gastral segments I, II, III, together with the anterior portion of segment IV, comprise the greater volume of the gaster, and inside them, beneath the cuticle, are contained not only structures that extend throughout their entire length, like the alimentary canal, and the nerve cord with its paired abdominal ganglia, situated near the cuticle in the ventral side, but also the heart, which is actually a muscular and dorsally located blood vessel that pumps blood anteriorly, toward the head of the hornet. The mentioned structures take up only a small volume of the gaster, while the rest is occupied by air sacs and tracheal ducts that also extend longitudinally. Interposed between the two air sacs, there is a hard partition and above it, at the center - a paired tracheal duct that extends the entire length of the air sacs. The endothelium of the air sacs is very anfractuous, thereby enlarging and strengthening the surface area. In each gastral segment there is an aperture for the entry of air, namely, a spiracle. Additionally, in each segment, in the antero-lateral aspect of its tergum and situated between two successive segments, there is an intersegmental conjunctive bearing parallel slits of 1-2 microM in width and 10-30 microM in length. The latter are arranged concentrically around bundles of tracheae that traverse the cuticle from segment to segment. From the upper rims of the slits are suspended downward fringe-like structures or "shutters" ranging between 3-10 microM in length. We discuss the possibility that the Oriental hornet resorts to internal circulation of air, along with a thermoelectric heat pump mechanism, in order to achieve cooling and thermoregulation of its body.

New MRI method with contrast based on the macromolecular characteristics of tissues.

A new MRI method with a contrast that is derived from the macromolecular composition and spin dynamics in the tissue is described and demonstrated on excised mouse brain and rat spinal cord. In the method, magnetization is selectively excited in the macromolecules by using a double quantum filter and subsequently transferred to water. The new imaging method differs from previous methods that rely on magnetization transfer contrast (MTC) in that it enables a separate and independent control of the effect of the macromolecule characteristics, chemical exchange, and water-related parameters on the images.