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J W Carlson

Publications and source records attributed to J W Carlson.

16 recordsLinked to original sources
PubMed1992

Design and evaluation of shielded gradient coils.

Techniques are described for the design of shielded gradient coils for superconducting MRI systems. These design methods are suited for constructing the most efficient gradient coil that meets a specified homogeneity requirement. Tradeoffs in coil design of efficiency with coil size and gap size are discussed. Residual eddy currents from coils constructed with a finite number of wires are calculated and give guidelines for the construction of efficient, whole-body gradient coils.

Amplifiers, Electronic↗
PubMed1992

MR relaxometry imaging. Work in progress.

Acquisition of relaxation rate dispersion curves from magnetic resonance images was demonstrated on a clinical, whole-body imaging system. Study of the behavior of relaxation rates over a range of field strengths probes the structural environment of imaged hydrogen protons and reveals information about the composition of tissue. The authors determined relaxation rates in extremities and heads of healthy volunteers. The sensitivity of the measurement is sufficient to obtain a distinctive relaxation rate dispersion behavior for different tissues.

Adult↗
PubMed1991

Electromagnetic fields of surface coil in vivo NMR at high frequencies.

A high frequency solution of the electromagnetic field produced by a circular surface coil adjacent to a homogeneous conducting, dielectric sphere is used to predict the attainable signal to noise ratio (S/N) and specific absorption rate (SAR) for in vivo 1H NMR spectroscopy experiments from 200 to 430 MHz (4.7-10 T). Above 200 MHz the S/N increases more rapidly with frequency and the SAR increases less rapidly compared with the respective S/N and SAR frequency dependence below 200 MHz. The difference in frequency dependence is due to dielectric resonances of the magnetic field inside the sphere at frequencies above 200 MHz. It is predicted that surface coil 1H NMR experiments may be performed on a head-sized sphere, having conductivity and relative dielectric constant of brain, at frequencies up to 430 MHz without exceeding 8 W/kg local SAR and 3.2 W/kg SAR. The calculations of the S/N and SAR are used to determine optimum surface coil geometries for NMR experiments. The power radiated by the surface coil in the absence of shielding and asymmetries in the received signal with respect to the plane defined by the surface coil axis and the direction of the static magnetic field are significant at high frequency. Experimental measurements of the magnetic field inside a head-sized sphere verify the presence of dielectric resonances at frequencies above 200 MHz.

Brain↗
PubMed1991

A case report of massive vulvar edema during tocolysis of preterm labor.

Bilateral vulvar edema that is not associated with preeclampsia has been reported only rarely during pregnancy or the puerperium. A primiparous patient in premature labor at 34 weeks' gestation with no history of lymphatic or venous obstruction underwent combination tocolysis with intravenous ritodrine and magnesium sulfate. On the fifth day of tocolysis, edema developed in the right labium majus pudendi and gradually spread to the left labia during the ensuing 24 hours. The patient remained afebrile, normotensive, and without signs of localized infection or anasarca. A trial of triple intravenous antibiotics and local skin care proved ineffective. By the ninth hospital day, the edema and discomfort had progressed, thereby precluding vaginal examination without sedation. In spite of progressive cervical dilatation with tocolysis, cesarean delivery was performed. With no further treatment, the vulvar edema gradually resolved during the next week.

Adult↗
PubMed1990

Rapid radiofrequency calibration in MRI.

A collection of modified procedures for setting rf transmitter levels using a three-pulse sequence is described. Based on a geometrically weighted ratio of four signals, an estimate of the flip angle is calculated and used for an updating prescription. These techniques are designed to perform rapidly yet eschew systematic errors due to relaxation during the pulse sequence.

Algorithms↗
PubMed1989

Power deposition and noise correlation in NMR samples.

A general solution for the power deposited in an NMR sample by an rf coil is presented. The result is simplified in the long wavelength, long skin depth approximation. This formalism is used to derive an expression for noise correlation in two independent receiver coils.

Magnetic Resonance Spectroscopy↗
PubMed1989

Donum Vitae on homologous interventions: is IVF-ET a less acceptable gift than "GIFT"?

Donum Vitae argues that, by failing to respect the connection between the conjugal act and procreation, in vitro fertilization-even in the homologous or "simple case", where both gametes come from a married couple and the resulting embryo is transferred to the wife-shows itself to be morally unacceptable. On the other hand, the document refers approvingly to other technological interventions which "facilitate" or "assist" the conjugal act in achieving its objective. Although none of the latter interventions are mentioned by name, the recently developed gamete intrafallopian transfer (GIFT) and certain associated techniques have found favor with many orthodox Roman Catholic thinkers, as well with some church authorities. The present article explores this situation in the Catholic moral tradition, and offers reasons for believing that, given relevantly similar conditions, if GIFT is morally acceptable so also is homologous IVF-ET.

Catholicism↗
PubMed1989

Physics of thin-section MR imaging at low field strength.

Thin-section magnetic resonance imaging at low field strengths requires analysis of the relative merits of data collection techniques for direct three-dimensional versus multisection two-dimensional imaging. This analysis was done with specific emphasis on shortened relaxation times and the use of reduced magnetic field gradients. Three-dimensional Fourier transform techniques can provide thin-section images of good diagnostic quality when combined with partial flip-angle gradient-reversal techniques.

Humans↗
PubMed1987

Partial flip angle MR imaging.

Theoretical analysis predicts that performing magnetic resonance (MR) imaging with partial (less than 90 degrees) flip angles can reduce imaging times two- to fourfold when lesions with elevated T1 values are being examined. This time savings occurs because repetition time (TR) is reduced when imaging is performed with partial flips. Partial flip MR imaging can also improve signal-to-noise ratio (S/N) in fast body imaging. For this study, analytical tools were used to predict image contrast and S/N for short TR, partial flip sequences. Experimental implementation of the short TR, partial flip sequences that analytical work had predicted would be optimal supported the analytical predictions and demonstrated their validity. Partial flip MR imaging is applicable to reducing imaging time only when the ratio of signal differences to noise exceeds threshold values in conventional MR images. Partial flip sequences can be used to advantage in MR imaging of both the head and the body, and the observed effects are predictable through theoretical analysis.

Brain↗
PubMed1987

Depth-focused radio frequency coils for MR imaging.

Surface coils improve signal-to-noise ratios by the simple expedient of reducing field of view. The uniformity of response of these coils is poor, since signal reception is weighted toward the surface of the object, which generally is of least interest. Furthermore, whole-section transmitter coils excite the subject uniformly, so that weak signals are detected from far regions. A dual-coil arrangement permits avoidance of these problems by combining the nonuniform excitation and reception characteristics of two surface coils.

Equipment Design↗
PubMed1986

Currents and fields of thin conductors in rf saddle coils.

The current distribution on thin conductors and rf field homogeneity for rf coils is described theoretically. After a pedagogical introduction to the techniques and an exact solution for the current or an isolated strip conductor, this article describes current distribution and field uniformity for a variety of conventional and quadrature rf coil designs.

Humans↗
PubMed1976

Abnormal adolescent primiparous pregnancy: association of race, human chorionic somatomammotropin production, and smoking.

Abnormal pregnancies (33 per cent) were studied in 417 consecutive, unmarried, primiparous women, age 12 to 18 years (black, 54 per cent; white, 46 per cent). Black patients had no increase in abnormal pregnancies but had more infants weighting less than 2,500 grams (p less than 0.01). Black patients with pre-eclampsia gained less weight than white patients (24 versus 34 pounds, p less than 0.05) and had smaller infants (2,834 versus 3,316 grams). In white patients, the mean age at menarche was earlier in women who lost their infants (11.0 versus 12.7 years; p less than 0.01). Smoking was more prevalent in normal pregnancies (47 per cent) than abnormal pregnancies (38 per cent). Spontaneous abortion and fetal loss occurred more often in smokers (p less than 0.01). White women smoked more than blacks (60 versus 37 per cent). In mothers of infants weighting less than 2,500 grams, there were fewer smokers. Unexpectedly, smokers had less hypertension and pre-eclampsia. Human chorionic somatomammotropin values correlated with infant size. Levels of less than 4 mug per milliliter after 30 weeks' gestation were identical in normal and abnormal pregnancies (4.4 per cent). They were associated with small infants but no fetal deaths and did not predict outcome of pregnancy.

Abortion, Spontaneous↗