Medium Field Strengths Research Articles

MRT des Pankreas - Wertigkeit von Standard- gegenüber fettunterdrückenden Pulssequenzen bei 0,5 T

To compare five pulse sequences in 60 patients with pancreatic disease by means of MRI at 0.5 T. T1 weighted Spin-Echo sequences (T1W SE) before and after i.v. contrast medium administration, fat-suppressed T1W SE sequences after i.v. contrast medium administration, T2 weighted Turbo-Spin-Echo sequences (T2W TSE), and fat-suppressed T2W TSE were evaluated within an ROC study. The standard sequences (T1W SE before and after i.v. contrast medium administration, T2W TSE) were superior to the fat-suppressed T1W and T2W (T)SE sequences in delineating the anatomy and for correct diagnosis. The T1W sequence with and without i.v. contrast media detected all anatomic structures best. In most instances the correct diagnosis and coexistent findings in the diseased pancreas were obtained with the T2W sequence. Fat-suppressed sequences are less useful in the diagnosis of pancreatic disease by MRI at medium field strength compared to the standard sequences.

MR-Mammographie bei 0,5 Tesla

To determine whether medium field strength (0.5 T) MR mammography is able to differentiate between benign and malignant lesions in the same way as a 1.5 T standard technique. In 40 consecutive female patients with nodular lesions, examinations were carried out at 1.5 T (2D-FFE, TR/TE/FA 200/3.9/80) and 0.5 T (3D-FFE, 24/3.4/40). There was wide spread of the speed of enhancement of malignant tumours for both techniques (44%-145% with signal increase in the first minute of 42%-189%). In 15 out of 17 carcinomas, the rapidity of uptake and final degree of enhancement after contrast was higher during 0.5 T/3D measurements than it was for 1.5 T/2D images. Fibroadenomas and mastopathies showed similar enhancement characteristics for both techniques. Sharply defined "wash-out" was seen only in malignant lesions; it appeared ten times more frequently in the 0.5 T/3D examinations than at 1.5 T/2D. The distinction between benign and malignant lesions can be made with more certainty using medium field strength and 3D-FFE sequence than using the 2D high-field standard technique.

Functional magnetic resonance imaging of primary visual processing using a 1.0 Tesla scanner.

Recent advances in functional magnetic resonance imaging (fMRI) at > or = 1.5 T magnetic field strength and with high speed single-shot echo planar imaging techniques have made it possible to monitor local changes in cerebral blood volume, cerebral blood flow, and blood oxygenation level in response to sensory stimulation, simple motor activity, and possibly also to more complex cognitive processing. However, fMRI has also been accomplished on conventional MR scanners of medium field strength (approximately 1.0 T) using special pulse sequences and appropriate methods for image analysis. We present results from six subjects on photic stimulation using a standard 1.0 T MR scanner together with special software for off-line image analysis. Continuous serial T2-weighted imaging were performed for 6 minutes in the plane of the calcarine fissure. There were 3 repetitions of 1 minute resting state of darkness (OFF) and 1 minute activated state (ON) with 8 Hz flicker stimulation. To directly map these functional images to the underlying anatomy we also acquired a high resolution T1-weighted image from the same axial slice. The results demonstrated that stimulus-related signals can be obtained from primary visual cortex with a conventional 1.0 T MR scanner. Further methodological improvements are discussed and related to present and future possibilities for the use of fMRI within psychophysiology.

Magnetic resonance imaging of pancreatic carcinoma

The authors have used MRI for investigation of 41 patients with pancreatic tumours. MR examinations were performed with 0.5 T superconductive equipment. Short TR, short TE spin echo (SE) sequences were obtained with 8 averages and 256×256 matrix. T2-weighted sequences were also acquired. T1-weighted SE sequences provided more detail, with high intrinsic contrast between the tumour and the normal pancreas; small lesions (19 smaller than 3 cm in diameter) were always detected on these images. T2-weighted SE sequences were not useful for lesion detection, due to the lower intrinsic contrast and the number of artefacts. T2-weighted sequences proved helpful for lesion characterisation in two cases of cystoadenocarcinoma only. Staging of the tumour was possible with MRI, with good assessment of local spread, lymphoadenopathy, vascular involvement and hepatic metastases. MRI of the pancreas at medium field strength can be an alternative to CT in selected cases.

Paranasal sinuses in pediatric patients by MRI: normal development and preliminary findings in disease

A two-part study using medium field strength magnetic resonance imaging (MRI) was designed to describe the normal paranasal sinus development and to provide preliminary criteria for clinical sinus disease. In part I of the study the paranasal sinuses were retrospectively evaluated in 80 infants and children aged 0–17 years undergoing brain MRI for indications both unrelated and related to sinus disease. We developed MRI criteria for independent grading of paranasal sinus development and ‘anatomical’ sinus disease, i.e., disease as seen by the radiologist. We quantified the variability in extent of sinus pneumatization (a measure of sinus development) in infants and young children. Part II of the study was a double-blind prospective study in 21 patients to correlate ‘anatomical’ disease with ‘clinical’ sinus disease. In this limited preliminary study, clinical sinus disease was only seen in the patients with moderate or severe anatomical disease (sensitivity 100%; specificity 100%).

Tertiary structure of conotoxin GIIIA in aqueous solution

The three-dimensional structure of conotoxin GIIIA, an important constituent of the venom from the marine hunting snail Conus geographus L., was determined in aqueous solution by two-dimensional proton nuclear magnetic resonance and simulated annealing based methods. On the basis of 162 assigned nuclear Overhauser effect (NOE) connectivities obtained at the medium field strength frequency of 400 MHz, 74 final distance constraints of sequential and tertiary ones were derived and used together with 18 torsion angle (phi, chi 1) constraints and 9 distance constraints derived from disulfide bridges. A total of 32 converged structures were obtained from 200 runs of calculations. The atomic root-mean-square (RMS) difference about the mean coordinate positions (excluding the terminal residues 1 and 22) is 0.8 A for backbone atoms (N, C alpha, C). Conotoxin GIIIA is characterized by a particular folding of the 22 amino acid peptidic chain, which is stabilized by three disulfide bridges arranged in cage at the center of a discoidal structure of approximately 20-A diameter. The seven cationic side chains of lysine and arginine residues project radially into the solvent and form potential sites of interaction with the skeletal muscle sodium channel for which the toxin is a strong inhibitor. The present results provide a molecular basis to elucidate the remarkable physiological properties of this neurotoxin.

(3) An Experimental Study on MR Imaging Artifacts by Air and Fat : Comparison of 2.0 T and 0.5 T Images(2. Tomographic Imaging)

Very low magnetic susceptibility of the air causes local distortion of magnetic gradient and results in spatial mismapping and signal change, chemical shift artifacts are produced in the frequency-encoding axis by lipid molecules because of difference of resonance frequencies of contained protons with those of protons in water. A test tube phantom study was performed in 2.0 T and 0.5 T superconducting magnetic resonance imaging systems for the investigation of natures of susceptibility and chemical shift artifacts encountered in high and medium field strength and in various imaging sequences. Both artifacts were prominent in high field MR and showed characteristic appearances. Several signal voids were noted around the air-colums in gradients echo images and MRI at 0.5 T showed more severe artifacts in T2-weighted image than those in the other sequences. Familiarity with causes, appearances and mechanisms of various artifacts helps Radiotechnologist for interpretation of MRI.

Normal penile anatomy and abnormal penile conditions: evaluation with MR imaging.

The appearance of the normal penis and of a variety of penile abnormalities on magnetic resonance (MR) images was studied in 55 patients with either medium (0.35 T) or high (1.5 T) magnetic field strengths. Penile morphologic characteristics with anatomic detail of the corpora cavernosa and corpus spongiosum were demonstrated in each patient. MR images clearly displayed congenital anomalies (n = 6), penile prostheses (n = 7), fibrous tissue or hematoma due to trauma (n = 8), and fibrous plaque in Peyronie disease (n = 3). MR imaging also demonstrated urethral (n = 6) and penile (n = 5) neoplasms and allowed tumor staging, thus facilitating the surgical approach.

The field dependence of NMR imaging. II. Arguments concerning an optimal field strength

Some of the factors involved in the choice of field strength for NMR imaging are examined. The influences of relaxation times and chemical shift upon image quality and signal-to-noise ratio are highlighted, and power deposition is introduced as a significant factor which may limit the flexibility and information available at higher fields as long as 180 degrees echo pulses continue to be necessary. Chemical-shift imaging is examined and found wanting as a means of coping with chemical-shift artifacts, and the use of multiple echoes (albeit with research) in conjunction with multiple-slice techniques is advocated as representing an efficient data-gathering scheme which can improve image signal-to-noise ratio. With such use, a medium field strength (0.5-1 T) is presented as representing, for general purpose imaging of head and torso, the best current compromise when imaging time is of major importance, with the important caveat that new techniques may always invalidate this conclusion.

Electrical conduction in reactively sputtered silicon nitride films at medium field strengths

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Field dependence of the electrophoretic mobility of particles suspended in low-conductivity liquids

An experimental arrangement is described for the determination of the electrophoretic mobility of suspended particles as a function of particle size. While sedimenting between plane vertical electrodes the particles were deflected to and fro by short rectangular field pulses. Results of polystyrene spheres suspended in isodecane with oil-soluble ionic additives as stabilizers are reported. The field dependence of the mobility at higher conductivity (10 −10 … 10 −9 Ω −1 m −1) and medium field strength (5 × 10 4 … 5 × 10 5 V/m) is interpreted as a partial decoupling of the particles from their diffuse electric double layer. At low conductivity and high field strength, when the double layer is completely stripped off, the charge of the particle can be calculated directly by equating the Coulomb and Stokes forces. The ratio between constant low-field (intact double layer) and constant high-field mobility (1:8) is in good agreement with theoretical calculations.

Polarized monolayers formed by epithelial cells on a permeable and translucent support.

An epithelial cell line (MDCK) was used to prepare monolayers which, in vitro, develop properties of transporting epithelia. Monolayers were formed by plating cells at high densities (10(6) cells/cm2) on collagen-coated nylon cloth disks to saturate the area available for attachment, thus avoiding the need for cell division. An electrical resistance developed within 4-6 h after plating and achieved a steady-state value of 104 +/- 1.8 omega-cm2 after 24 h. Mature monolayers were morphologically and functionally polarized. They contained junctional complexes composed of desmosomes and tight junctions with properties similar to those of "leaky" epithelia. Monolayers were capable of maintaining a spontaneous electrical potential sensitive to amiloride, produced a net water flux from the apical to basal side, and discriminated between Na+ and Cl- ions. The MDCK permeability barrier behaves as a "thin" membrane with negatively charged sites. It has: (a) a linear conductance/concentration relationship; (b) an asymmetric instantaneous current/voltage relationship; (c) a reduced ability to discriminate between Na+ and Cl- caused by lowering the pH; and (d) a characteristic pattern of ionic selectivity which suggests that the negatively charged sites are highly hydrates and of medium field strength. Measurements of Na+ permeability of electrical and tracer methods ruled out exchange diffusion as a mechanism for ion permeation and the lack of current saturation in the I/deltapsi curves does not support the involvement of carriers. The discrimination between Na+ and Cl- was severely but reversibly decreased at low pH, suggesting that Na+-specific channels which exclude Cl- contain acidic groups dissociated at neutral pH. Bound Ca++ ions are involved in maintaining the integrity of the junctions in MDCK monolayers as was shown by a reversible drop of resistance and opening of the junctions in Ca++-free medium containing EGTA. Several other epithelial cell lines are capable of developing a significant resistance under the conditions used to obtain MDCK monolayers.

Resistivity of transformer oil at low and medium field strengths

In certain terminal equipment used in high-voltage d.c. transmission schemes, transformer oil is subjected to a d.c. stress. The behaviour of the oil under this stress is important in the design of equipment. The volume resistivity ρ of five samples of transformer oil, ranging from new oil to oil after 10 years' service in a transformer, was measured over a range of stress levels E and electrode spacings d. The resistivity increased with increasing field strength up to a critical field strength, above which it fell sharply. Below this critical value, the resistivity was found to be a function of the ratio E/d. The curve of ρ against E/d consisted of two parts: (a) for low values of E/d, ρ was constant, (b) for higher values, the resistivity followed the equation ρ = α ρ = α (E/d)n where α and n are constants. Some potential-distribution measurements were also made, and the stress in the oil was found to be uniform and approximately equal to the applied stress over the bulk of the oil in the interelectrode space. The results of the resistivity measurements are interpreted in terms of electrolytic conduction due to dissociation of weak electrolytes in the oil.

Cooling by means of anisotropic hyperfine structure coupling; Orientation and relaxation of nuclear spin systems in strong magnetic fields

A new method is given for obtaining low temperatures in paramagnetic crystals in the presence of a large magnetic field by utilizing anisotropic hyper fine structure coupling. The method consists essentially of reduction of h.f.s. splittings by rotation of a magnetic field under adiabatic conditions. A discussion of the requirements on the properties of the usable paramagnetic ions is given. The usefulness of the method was demonstrated with crystals of CoCs 2(SO 4) 2.6D 2O in which 54Mn was incorporated in order to serve as a thermometer. The ratio between initial and final temperature was 4 to 5, resulting in a lowest temperature of 0.006°K, which could be maintained for at least one hour. In high magnetic fields establishment of thermal equilibrium between the 54Mn and stable 59Co nuclei could not be obtained, indicating extremely long relaxation times in the interaction of nuclear spins of unlike paramagnetic ions. However, for medium field strengths relaxation time measurements could be carried out as a function of field strength, of the direction of the field and of the temperature. It was found that the relaxation time was very strongly dependent on the field strength when the field was in a fixed direction, but was approximately constant for a fixed value of the sum of electronic Zeeman splittings of Mn and Co-ions, when varying both the direction and magnitude of the field. Gamma ray intensity measurements of 54Mn at a very high degree of nuclear polarization (> 97%) resulted in the establishment of an upper limit to the possible disturbance of the 54Mn nuclear polarization due to K-capture or to its after-effects. The rotational cooling procedure was further investigated by measurements of anisotropies of 60Co gamma radiations; the main features of the experimental results, obtained for various field directions, were in agreement with theoretical predictions. However, at the lowest temperatures significant deviations were found. 137Cs-nuclei were oriented by electric h.f.s. coupling; peculiar relaxation phenomena were observed which led to the conclusion that 137mBa has a negative sign for its nuclear electric quadrupole moment. The magnitude of the crystalline electric field gradient was found to be at least a factor of 30 larger than that found by Kiddle et Proctor by means of nuclear quadrupole resonance at room temperature. The relaxation time for establishment of thermal equilibrium between Cs and Co-nuclei was found to be a few orders of magnitude shorter than that between Co- and Mn-nuclear spins. Negative results concerning attempts to orient 134Cs, 86Rb are reported.