Estimating patient dielectric losses in NMR imagers

Dielectric losses in the patient may impair radiofrequency receiver coil sensitivity, and transmitter coil efficiency, in nuclear magnetic resonance (NMR) imagers. The frequency dependence of this loss mechanism is derived. Patient losses in a solenoidal head coil used for imaging heads were simulated by a cylindrical saline phantom. The frequency dependence of the loss introduced by the phantom can indicate whether dielectric losses in the patient will be significant compared to eddy current losses. The detuning caused by the phantom is used to calculate an upper limit for the distributed stray capacitance between coil and patient. Given the approximate conductivity of the patient, an upper limit for the dielectric loss can be estimated. Some methods of reducing patient dielectric losses are suggested.     

Synthesis and dielectric properties of Bi1−xNdxFeO3 perovskites

Single-phase polycrystalline samples of Bi_1−xNd_xFeO₃ were prepared by standard solid state reaction method. X-ray diffraction (XRD) patterns of the powder samples were recorded and analyzed for the confirmation of crystal structure lattice parameters. Further, these samples were characterized by IR technique to identify and understand the aspect of bonding in the present samples. The dielectric measurements were carried out on the samples as a function of frequency in the range 100 Hz to 1 MHz at room temperature and also as a function of temperature in the range 300–750 K at certain fixed frequencies. The DC and AC electrical resistivity studies were carried out in order to understand conduction mechanism in the samples.     

Investigations of backscattering peaks and of the nature of the confining potential in open quantum dots

The properties of open quantum dots are examined in magneto-transport. The quantum dots are prepared from a two-dimensional electron system (2DES) in AlGaAs/GaAs by lateral gate structures. These quantum dots are open, i.e. they are still connected to the surrounding 2DES regions. The low magnetic field magnetoresistance shows peak structures. These structures can be related to semi-classical ballistic trajectories in the confining potential of a dot. The calculations of different confining potentials (abrupt “hard-wall” and parabolic “soft-wall”) are compared with the experimental results. The experiments are better described by a soft-wall potential.     

Magnetic resonance imaging as an adjunct to sonography in the evaluation of the female pelvis

Magnetic resonance imaging (MRI) of the pelvis is generally considered to be most beneficial in those cases where the pelvic sonogram is limited or equivocal. All cases that underwent both sonographic and MRI examinations at our institution for the evaluation of the female pelvis in the past two years were retrospectively reviewed. We reviewed the sonographic and MRI reports and the subsequent clinical management in the 41 cases that had both studies to assess whether MRI contributed to the clinical management decision. Both studies were interpreted independently based upon the known clinical and laboratory data available at the time. MRI was obtained in 21 cases because the sonogram was suboptimal or inconclusive. In the other 20 cases it was obtained for additional information, even though the sonogram was diagnostic. Of the 21 inconclusive sonographic studies, MRI established or clarified the diagnosis in all cases. Of the 20 studies where MRI was obtained for additional information, MRI added useful data that helped contribute to the clinical management of 11 patients. MRI is an important adjunct to pelvic sonography. It established, clarified, or added significant data in 78% of cases.     

Cardiac lipoma: Six-year follow-up with MRI characteristics, and a review of the literature

A total of 63 cardiac lipomas have been reported to date. Although most of these rare tumors cause no symptoms, a few can have a detrimental effect on myocardial function as well as displacing and encasing the coronary arteries. This case of a cardiac lipoma was initially seen in 1982. The lipoma was found to be nonresectable at surgery due to involvement of the coronary arteries. This neoplasm was characterized with magnetic resonance imaging in 1989 on a follow-up visit. Magnetic resonance imaging is shown to be superior to computed tomography for identifying the relationship of the lipoma to the coronary arteries. This is a significant factor in determining resectability.     

An assessment of the safety of rapidly changing magnetic fields in the rabbit: Implications for NMR imaging

The effects of rapidly changing magnetic fields on the cardiac and respiratory functions of anaesthetised rabbits have been investigated. No changes were observed using pulsed fields with peak rates of change of 60 T/sec. The implications of these experiments for the safety of NMR imaging are discussed.     

Increasing the yield from volumetric MRI in patients with epilepsy

Volumetric magnetic resonance imaging has become a routine investigation in the management of patients with chronic partial epilepsy. However, even with the use of reformatted images, an underlying cause for epileps can-not be found in many of these patients. We show that further processing of the data, including three-dimensional reconstruction and quantitative analysis of the volume and complexity of the images in three dimensions, reveals additional positive information in up to 75% of patients. This may be useful for surgical planning, prognostication, and understanding of the structure and development of the human brain.     

Huang–Rhys side-bands in the emission line of a single InAs quantum dot

We report the first direct observation of Huang–Rhys side-bands in the photoluminescence spectrum of a single InAs/GaAs quantum dot (QD). At low temperature (10 K) the single QD spectrum has a quasi-Lorentzian profile. At higher temperatures, we observe a strong deviation from a Lorentzian profile with the appearance of asymmetric side-bands which become symmetric above 70 K. We obtain an excellent agreement with theoretical calculations done in the framework of the Huang–Rhys formalism. We conclude that the zero-phonon linewidth is the relevant parameter for the observation of the low-energy acoustic phonon side-bands.     

Joint effect of dispersions of reflection-induced retardance and Verdet constant upon the sensitivity of an optical current sensor

The dispersion properties of the reflection-induced retardance of polarization-preserving reflecting medium layer and the Verdet constant of the ZF-7 optical glass and their joint effect on the sensitivity of a bulk glass optical current sensor are theoretically analyzed taking the Jones’ Matrix as a mathematical tool and using simulation with a computer. The investigation results show that the dispersions will significantly affect the scale factor of the system. Therefore, it is necessary to stabilize the driving current and the ambient temperature of the optical source by some technical means to ensure the wavelength stability. These results might be useful for the researchers and designers working in the optical current sensing techniques area.     

A rotating phantom for the study of flow effects in MR imaging

A common type of phantom used for the study of flow effects in MR imaging is the tube phantom, where a liquid passes through a set of tubes placed in the main magnetic field of an MR scanner. Among the disadvantages with this type of phantom are that a distribution of velocities is present in each tube, and that quantifications of flow effects using tube phantoms may be very time-consuming. In this work, we describe the design and the properties of a rotating wheel flow phantom used for quantification of the effects of flow through the imaging plane as well as in the imaging plane. The proposed phantom is constructed as a rotating gel-filled wheel, surrounded by static volumes filled with the same gel, and the evaluation of the information from rotating and static parts is made with a specially designed computer program. The phantom can be used as a plug flow phantom covering simultaneously an interchangeable velocity interval, which at present has the range −52 mm/s, +52 mm/s. It is shown that the phantom gives adequate information on the dependence of pixel content on first-order motion in MR modulus and phase images. Among the fields of application are rapid calibration of MR imaging units for flow determination using phase information, as well as testing of pulse sequence characteristics and verification of theoretical predictions concerning the flow dependence in MR images.     

Absolute quantification of the hepatic glycogen content in a patient with glycogen storage disease by C magnetic resonance spectroscopy

Using natural-abundance ¹³C magnetic resonance spectroscopy (MRS) on a conventional whole-body system operating at 1.5 T, the absolute hepatic glycogen concentration was noninvasively determined in a patient with type Ia glycogen storage disease. Furthermore, to assess the reliability of glycogen determination, hepatic glycogen content was assessed after an overnight fasting period in 35 healthy volunteers divided into two groups, one with a carbohydrate-rich diet, the other without any particular dietary preparation. In the patient, the glycogen concentration was found to be 458 mM. In the healthy subjects, average glycogen concentrations were 229 ± 34 mM (mean ± standard deviation) and 257 ± 31 mM for the group without and with dietary preparation, respectively. The ¹³C-MRS results are in agreement with those obtained by conventional liver biopsy. ¹³C MRS in natural abundance may thus serve as a straightforward, fast, and noninvasive tool for quantification of the liver glycogen content in patients.     

The measurement of the electrical field distribution by external electro-optic measuring system with submicrometre spatial resolution

A GaAs solid immersion lens has been applied to the external electric-optic measuring system. The spatial resolution of the system is improved to 0.7 μm because of the increased effective number aperture. We measured the electrical field distribution of ceramic microstrip lines transversely. The variation of electrical field with a submicrometre interval has been revealed experimentally and the results are in good agreement with the theoretical analysis.     

Real-time dynamics of the acoustically driven electron–hole transport in GaAs quantum wires

We have investigated the dynamics of photogenerated carriers in GaAs quantum wires during their transport by surface acoustic waves using spatially and time-resolved photoluminescence. This technique allows us to map in real time the spatial carrier distribution during the transport as well as to study the nature of radiative defect sites in the transport path.     

The application of proton nuclear magnetic resonance imaging for the in vivo characterisation of chemically induced renal lesions in rats over a prolonged time study

Renal cortical and medullary spin-lattice (T₁) relaxation times were measured at various time points over a period of 56 days following the administration of a single i.p. injection of 100 mg/kg 2-bromoethanamine hydrobromide (BEA), 200 mg/kg hexachloro-1,3-butadiene (HCBD) or 100 mg/kg puromycin aminonucleoside (PAN) to male Wistar rats. Administration of a single injection of HCBD caused a dramatic, immediate rise in the cortical T₁ values above control values, and these levels remained elevated until, by Day 28 postinjection the levels were back to control values. Administration of BEA also caused an elevation in cortical T₁ values, but in this case these values remained above control values for the rest of the study. The administration of PAN did not produce any significant increases in cortical T₁ values until 14 days postinjection. The elevated T₁ values remained above control values for the rest of the study. These increases observed in cortical T₁ values appeared to be mirrored by decreases in medullary T₁ values. Increases in cortical T₁ values were accompanied by visual changes in the NMR images and enlargement of the kidneys. The histological findings were consistent with the NMR data, confirming that morphologically the tissues did show a full recovery by Day 28 in the HCBD-treated animals. This was not the case following injection of both BEA and PAN, where necrosis was not reversible and there was no recovery of the tissues.     

Vector magnetometer studies of the easy magnetisation direction in epitaxially grown DyFe2 film

A vector vibrating sample magnetometer has been used to determine the direction of easy magnetisation in a thin film of DyFe₂. The 4000 Å thick film, with a [1 1 0] growth direction, was prepared using molecular beam epitaxy. The results show that the direction of easy magnetisation is temperature dependent. At 4 K this direction corresponds to the [0 0 1] type axes. But at 290  the easy axis is close to a direction, but pointing out of the plane of the film at angle of (14±3)°. On the basis of the vector magnetometer results at 290 K we find two easy magnetisation axes close to or (and opposite directions). Finally, it is shown that DyFe₂ films are unique in that the remanent magnetisation can be manipulated to point either in or out of the plane of the film.     

Application of high field spectroscopic imaging in the evaluation of temporal lobe epilepsy

Previous spectroscopic imaging studies of temporal lobe epilepsy have used comparisons of metabolite content or ratios to lateralize the seizure focus. Although highly successful, these studies have shown significant variations within each of the groups of healthy subjects and patients. This variation may arise from the natural differences seen in metabolite concentration in gray and white matter, the complex anatomy seen about the hippocampus, and the large voxels typically employed at 1.5 T. Using a 4.1 T whole body system, we have acquired spectroscopic images with 0.5 cc nominal voxels (1 cc after filtering) to evaluate the regional variation in metabolite content of the hippocampus, temporal gray and white matter, midbrain, and cerebellar vermis. Using a threshold value of 0.90 for CR/NAA, a value 90% of all normal hippocampal voxels lay below, we have correctly identified the presence of epileptogenic tissue in patients with unilateral as well as bilateral seizures. By using comparisons to healthy values of the CR/NAA ratio, this method enables the visualization of bilateral disease and provides information on the extent of gray matter involvement.     

On the use of exchange biased top electrodes in magnetic tunnel junctions

We have investigated the magnetic behavior and the structural properties of ferromagnetic–antiferromagnetic systems (NiFe–IrMn and Co–IrMn) deposited directly on a thin tantalum buffer layer (bottom configuration) or above a thin Al₂O₃ tunnel barrier layer (top configuration). In the bottom configuration, the bilayer system exhibits higher magnetic performances than in the top configuration in terms of thermal stability. We have performed a detailed structural study by X-ray diffraction and high-resolution transmission electron microscopy which allow us to establish a clear correlation between the situation of the bilayer with respect to the tunnel barrier, its texture and its magnetic properties.     

MRI contrast-dose relationship of manganese(III)tetra(4-sulfonatophenyl) porphyrin with human xenograft tumors in nude mice at 2.0 T

Previously we reported that Mn(III)tetra(4-sulfonatophenyl) porphyrin, MnTPPS₄, is a contrast agent which can effectively enhance tumor detection by MRI. By imaging 30 additional athymic nude mice bearing subcutaneous MCF-7 WT human breast carcinoma xenografts, we have extended dose-contrast relationships over a wide range of intraperitoneal (IP) doses ranging from 0.025 to 0.50 mmol/kg. The benefits of IP injection are higher possible doses on a volume basis and a reduction in toxicity versus IV administration. Full coronal cross-section images have been obtained on a 2-T spectrometer. Although individual tumor masses displayed different distribution patterns, reflective of their internal morphology, single doses of 0.10 mmol/kg or greater were necessary to produce a detectable effect. At a dose of 0.50 mmol/kg, marked enhancement was produced. Multiple small dosages administered over the course of several days before imaging did not produce increased enhancement. Preliminary results on the new porphyrin derivative, MnTPPS₃, indicate that the ratio of the toxic dose to the effective dose may be adjustable to render this class of agents clinically useful.