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At least 73 records · Page 4Linked to original sources
PubMedDate not supplied

Thermodynamics of movable inductively heated seeds for the treatment of brain tumors.

A thermodynamic study is presented of temperature distributions created by an inductively heated 6-mm-diam Ni sphere imbedded in vivo and in vitro into porcine brain tissue. This study was performed in support of the development of a system that creates localized heat-induced lesions in deep-seated brain tumors. In this system, a magnetic "seed" will be remotely repositioned within the brain by an externally produced magnetic field. Convective effects of a hot moving seed will produce a different thermodynamic situation than that arising from an array of static implants. In this work, a study is presented of part of the expected change, in which a static sphere is heated to high temperature. Measurements were made of the temporal and spatial dependence of the temperature rise in the vicinity of the heated sphere, in vivo in four animals and in one that was euthanized immediately prior to experimentation. These results are used for parameter estimation with a theoretical model based on a point source solution to a form of the thermal diffusion equation, i.e., the "bioheat transfer equation." With this model thermal distributions from a power source of arbitrary geometry can be found using appropriate integration methods, and the method has widespread applicability. Estimates of blood flow rates, tissue thermal conductivity, and seed power absorption were found using the parameter estimation algorithm. The estimated blood perfusion exhibits a step increase following the first heating in multiple heating experiments. Thermal conductivity estimated using data from the nonperfused (in vitro) animal is 0.6 W/m degrees C. Seed power absorption is estimated correspondingly to be 0.9 W, a result confirmed independently with calorimetry. Statistical uncertainty is established for the radial decrease of the tissue temperature rise created by this method. This result allows estimation of a cell death boundary uncertainty of 0.6 mm, caused by fluctuations in power delivered to the seed, uncertainty in the temperature probe placements, and thermal properties such as blood perfusion and tissue thermal conductivity.

Animals↗
PubMed1990

Single-exposure dual-energy computed radiography: improved detection and processing.

Recent reports have emphasized the potential for dual-energy computed radiographic applications. An improved method for single-exposure material-selective imaging with a photostimulable phosphor computed radiography system was investigated. The essential elements of the technique are (a) prefiltration with gadolinium, which divides the incident broad-beam x-ray spectrum into low-energy and high-energy peaks; (b) a cassette consisting of four photostimulable phosphor plates that record images of increasing mean energies, with a computed energy separation of 23 keV from the front to the rear plate; (c) spatially dependent scatter and beam-hardening corrections; and (d) a noise-reduction algorithm based on noise correlations between bone-selective and soft-tissue-selective dual-energy images. These elements result in improved material cancellation and signal-to-noise ratio throughout the image.

Algorithms↗
PubMed1991

Non invasive measurement of myocardial motion using magnetic resonance tagging.

Magnetic resonance imaging has become a key modality within the modern radiology department. In addition to the conventional slice images, three dimensional angiographic images, and quantitative flow measurements can be obtained. More recently methods have been developed which enable one to visualize the motion of objects through the application of magnetic resonance markers or tags. Generally the myocardium appears homogeneous with respect to magnetic resonance imaging. Thus at best the displacement of the myocardial surfaces can be observed, but rotational and shear motions are not discernable. With magnetic resonance tagging this all changing. Tagging involves modifying the ability of the tissue to produce a nuclear magnetic resonance signal, in a spatially dependent manner. Typically the pattern is a grid of reduced signal intensity. The tagging grid is applied prior to the heart contraction. Then images are made during systole and diastole. The rotation and shear of the myocardium can be inferred from the distortions of the tagging grid. Here our work in this field is reviewed.

Heart Diseases↗
PubMed1991

Digital imaging and picture archiving and communication systems.

Great progress has been made in digital imaging of the chest. Most studies are dealing with computed radiography. Chest radiography in the intensive care unit may, in most cases, be performed using computed radiography. However, subtle pulmonary interstitial disease can be demonstrated less confidently using computed radiography. Significantly better detection of calcified lung nodules can be obtained by using simplified single-exposure dual-energy technique that uses storage phosphor. The wide latitude of computed radiography permits images of high quality in areas other than chest radiography. Encouraging results are presented especially in the diagnostic evaluation of scoliosis and other musculoskeletal abnormalities. An important technical innovation in digital radiography is an improved method for single-exposure dual-energy digital imaging using prefiltration with gadolinium, a cassette consisting of four photostimulable phosphor plates, spatially dependent scatter and beam hardening corrections, and noise reduction algorithm. Other groups tested algorithms for enhancement of digital images that allowed significant data compression. The implementation of picture archiving and communication systems (PACS) is inevitable; the question concerning PACS implementation is not why, but when. A comparison of the cost-effectiveness of PACS with conventional film archiving and communication systems shows that PACS should provide indirect savings when regarding the hidden costs of conventional systems. Much more experience will be needed before there is general agreement on the best design for the radiologist's workstation. Teleradiology should contribute to radiologic consultation for remote locations, because it improves the efficacy of management of patients in such locations.

Child↗
PubMed1990

Panoramic zonography of the midface: a clinician's guide to vertical magnification.

Surgical procedures involving the maxilla require details regarding the topography of the nasal floor and that of the maxillary sinus. Panoramic zonography can provide this information with a minimum of radiation exposure and may be used in certain instances when other imaging modalities cannot. Spatially dependent variations in vertical magnification inherent to panoramic zonography using the midface program were determined. Radiographic lengths of marker wires attached at seven locations along the maxillary arch of dry skulls were measured. Radiographs were obtained at 0, -10 and -20 mm shifts. Analysis of variance and ranking did not reveal any significant differences in magnification between skulls. However, a dichotomy between the anterior and posterior portions of the maxillary arch in terms of vertical magnification was found at all three shift positions. In addition, at all marker locations, the magnification diminished as a function of decreasing shift position. These variations are due to the fact that teeth within the arch are not all located at the central plane of the focal trough generated by the unit. The results provide the clinician with vertical magnification factors from which actual measurements can be derived from zonographs.

Alveolar Process↗
PubMed1986

Magnetic resonance angiography in the head and neck.

Magnetic resonance angiography (MRA) provides a means to visualize vessel structure without the use of contrast agents and consequently without the risks and discomfort associated with contrast media. We have developed a flow encoding procedure which exploits the spatial dependence in the phase of spin magnetization generated by a gradient pulse. This technique yields image information for only those spins having macroscopic motion (i.e. flowing blood). Excellent suppression of non-moving spins can be obtained despite the overwhelming abundance of stationary spins. Data are presented in a projection format similar to that of traditional subtraction angiography.

Blood Vessels↗
PubMed1991

Spatial frequency difference thresholds depend on stimulus area.

Spatial frequency difference thresholds were estimated for vertical, high contrast 5.0 cpd sinewave gratings that had been multiplied by a two dimensional Gaussian window function of contrast. A range of different elliptical window configurations were tested that varied in their relative height and width. Spatial frequency acuity improved as the area of the target was increased. The data are compatible with a model of spatial frequency discrimination that is based on statistical averaging of spatial intervals interpolated between local retinal signs.

Adult↗
PubMed1991

The ability of cationic amphiphilic compounds to depress the transition temperature of dipalmitoylphosphatidic acid liposomes depends on the spatial arrangement of the lipophilic moiety.

The hypothesis was tested with the help of model compounds that the ability of cationic amphiphilic drugs to depress the phase-transition temperature Tt of dipalmitoylphosphatidic acid (DPPA) liposomes depends on the spatial arrangement of the lipophilic moiety. The main structure of the compounds with identical cationic side chain was 1-dimethylamino-3-phenylpropane (compound I). A further phenyl ring was introduced either at C3 of the propane chain (compound II) to broaden the lipophilic moiety, or in para-position of the phenyl ring (compound III) to elongate it. As shown by differential scanning calorimetry, the reduction of Tt (control 64 degrees) amounted for compound I to 29 degrees, for compound II to 28 degrees and for compound III to 53 degrees. In order to assess the binding affinity of the compounds to DPPA, their inhibitory effect on 45Ca(2+)-binding to DPPA films was measured. The IC50 values were 2100 microM for compound I, 40 microM for compound II, and 9 microM for compound III. Thus, binding affinity corresponded with the hydrophobicity of the compounds. In contrast, the depressing effect on the transition temperature was only augmented by the additional phenyl ring when substituted in the elongating position.

Calcium↗
PubMed1988

Spatial frequency-dependent abnormalities of the pattern electroretinogram and visual evoked potentials in a parkinsonian monkey model.

The pattern visual evoked potential (PVEP) and pattern electroretinogram (PERG) were studied in 5 cynomolgus monkeys before and during the development of a parkinsonian syndrome induced by MPTP. The stimuli were vertical bars of four spatial frequencies (0.5, 1.2, 2.5 and 3.5 cycles/degree (cpd) modulated at temporal rates of 1, 4, 6, and 8 Hz. Following MPTP administration, all monkeys developed parkinsonian signs accompanied by changes in the amplitude and latency of the PVEP and PERG. Sinemet L-dopat carbi olopa administration produced temporary recovery of both PVEP and PERG. Two of the monkeys were followed for a prolonged period: 30-40 days after MPTP, the parkinsonian signs showed partial recovery; the PVEP latency and amplitude to 2.5 and 3.5 cpd stimuli and the latency to 1.2 cpd showed improvement but remained abnormal. The latencies of PERGs were normal, but the amplitudes were significantly reduced when stimuli of 2.5 and 3.5 cpd were used. Both PVEP and PERG to 0.5 cpd stimuli returned to normal. No further modifications were seen in the recordings performed 6 months and 1 year later. This study demonstrates (1) that spatial frequency-dependent electrophysiological abnormalities occur in the MPTP-treated monkey, a result previously found in human Parkinson's disease, and (2) that dopamine has a specific function in neurotransmission in the visual system of primates.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMedDate not supplied

The spatial and energy dependence of bremsstrahlung production about beta point sources in H2O.

A Monte Carlo simulation was performed to characterize the spatial and energy distribution of bremsstrahlung radiation from beta point sources important to radioimmunotherapy (RIT). Using the EGS4 Monte Carlo code, the isotropic emission and transport of monoenergetic 0.1-, 0.5-, 1.0-, 2.0-, and 3.0-MeV electrons and 32P and 90Y beta particles was simulated in an infinite, homogeneous H2O phantom. The probability of bremsstrahlung production for each Monte Carlo-simulated electron step was accumulated in energy intervals not exceeding 5 keV and stored as a function of radial position. To validate this scheme, the EGS4 code was tested in the continuous slowing down approximation (csda) mode, with resulting radiation yields seen to agree with values in ICRU Report No. 37 (ICRU, Bethesda, MD, 1984) to better than 1.6%. The radiation yield calculated with the simulation of secondary particles is seen to be 3%-5% greater than the csda yield. The photon energy distributions are characterized by a typically broad bremsstrahlung spectrum with the probability of photon generation decreasing with radial distance. In the energy range 0.05-0.511 MeV, the probability for bremsstrahlung production from 90Y (2.76 x 10(-2) decay-1) is twice that from 32P (1.35 x 10(-2) decay-1). When passed through 10 cm of H2O and put upon a standard NaI scintillation camera, count rates of 2.3 x 10(-6) and 1.2 x 10(-6) counts s-1 Bq-1 are estimated from point sources of 90Y and 32P. These results predict the inherent spatial resolution limitation and provide the initial data required for modeling and analyzing the scatter, attenuation, and image formation processes in quantitative imaging of bremsstrahlung for RIT dosimetry.

Humans↗
PubMed1978

Mapping the ATP-binding site in the catalytic subunit of adenosine-3':5'-monophosphate-dependent protein kinase. Spatial relationship with the ATP site of the undissociated enzyme.

A set of 24 ATP analogs modified at various positions of the ATP molecule was used for mapping the ATP-binding site in the free catalytic subunit (C) of cAMP-dependent protein kinase (type I). Ki values for these analogs (of which 23 were shown to be competitive with ATP) were measured and compared with Ki values previously obtained for the same set of analogs upon binding to the undissociated form of the enzyme (R2C2). It was found that modifications at the adenine part of ATP bring about a considerable reduction in affinity between C and the resulting analog. The other parts of the ATP molecule play a less important, though definite, role in the binding of this nucleotide to C. By measuring the effect of each given modification in ATP on its binding to C, and comparing the effect of this modification on the binding of the same analog to R2C2, it was possible to obtain 'specificity profiles' for both forms of the kinase. Using such profiles it is shown that the adenine-binding subsite in C may well coincide with the adenine-binding subsite in R2C2. Two plausible models describing the spatial relationship between the ATP sites in C and R2C2 are proposed.

Adenosine Triphosphate↗
PubMedDate not supplied

[The dependence of spatial-motor asymmetry on the nature of the reinforcement].

Spatial-motor asymmetry (SMA) of rats goal-directed running was studied in condition of numerous alternative choices in complex T-maze. In condition of free choices SMA in trained rats depended on reinforcement biological significance, volume and disposition scheme. In condition of non-free choices a change was possible of asymmetry of sequence of goal-directed motor acts adequate to situation, i.e. SMA regulation. Examples of the presence of properties of ego- and allocentric systems in SMA are given. The results of experiments testify to SMA dependence on the motivation of running, to rightfulness of SMA interpretation as behavioural tactics and to non-coming of SMA of goal-directed running to a simple rotation of animals.

Animals↗
PubMed1992

Visual hemispheric asymmetries depend on which spatial frequencies are task relevant.

Observers classified sine-wave and square-wave gratings on the basis of fundamental frequency (Are the bars wide or narrow?) or on the basis of higher harmonic frequencies (Are the bars sharp or fuzzy?). Stimuli were presented in either the left (LVF) or right (RVF) visual field. When the classification was made on the basis of the fundamental frequencies (1 or 3 c/deg), there was a LVF/right hemisphere advantage. However, when the classification was on the basis of a sharp/fuzzy distinction which involved searching for the higher harmonic frequencies, then a RVF/left hemisphere advantage was found.

Adult↗
PubMed1987

Some task and signal dependent rules for spatial vision.

In this paper we consider the types of computational processes which may be involved in solving a variety of perceptual problems from the detection of signals in the presence of others, to texture discrimination, and some aspects of pattern recognition. These processes centre around the involvement of correlational computations, the transduction of their input/output values, and the apparent involvement of selective filtering mechanisms. Our results suggest that even if fixed detectors (in tuning characteristics) are involved in low-level vision, the human observer apparently employs much more adaptive (variable tuning characteristic) filters and nonlinear mechanisms in more complex spatial tasks.

Humans↗
PubMed1987

Nonlinear directionally selective subunits in complex cells of cat striate cortex.

1. We have analyzed receptive fields (RFs) of directionally selective (DS) complex cells in the striate cortex of the cat. We determined the extent to which the DS of a complex cell depends on spatially identifiable subunits within the RF by studying responses to an optimally oriented, three-luminance-valued, gratinglike stimulus that was spatiotemporally randomized. 2. We identified subunits by testing for nonlinear spatial RF interactions. To do this, we calculated Wiener-like kernels in a spatial superposition test that depended on two RF positions at a time. The spatial and temporal separation of light and dark bars at these two positions varied over a spatial range of 8 degrees and a temporal range of +/- 112 ms in increments of 0.5 degree and 16 ms, respectively. 3. DS responses in complex cells cannot be explained by their responses to single light or dark bars because any linear superposition of responses whose time course is uniform across space shows no directional preference. 4. Nonlinear interactions between a flashed reference bar that is fixed in position and a second bar that is flashed at surrounding positions help explain DS by showing multiplicative-type facilitation for bar pairs that mimic motion in the preferred direction and suppression for bar pairs that mimic motion in the null direction. Interactions in the preferred direction have an optimal space/time ratio (velocity), exhibited by elongated, obliquely oriented positive domains in a space-time coordinate frame. This relationship is inseparable in space-time. The slope of the long axis specifies the preferred speed, and its negative agrees with the most strongly suppressed speed in the opposite direction. 5. When the reference bar position is moved across the RF, the spatiotemporal interaction moves with it. This suggests the existence of a family of nearly uniform subunits distributed across the RF. We call the subunit interaction, as averaged across the RF, the "motion kernel" because its spatial and temporal variables are those necessary to specify the velocity, the only parameter that distinguishes a moving image from a temporally modulated stationary image. The nonlinear interaction shows a spatial periodicity, which suggests a mechanism of velocity selectivity for moving extended images.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Large time behavior in a nonlinear age-dependent population dynamics problem with spatial diffusion.

In this work we analyze the large time behavior in a nonlinear model of population dynamics with age-dependence and spatial diffusion. We show that when t----+ infinity either the solution of our problem goes to 0 or it stabilizes to a nontrivial stationary solution. We give two typical examples where the stationary solutions can be evaluated upon solving very simple partial differential equations. As a by-product of the extinction case we find a necessary condition for a nontrivial periodic solution to exist. Numerical computations not described below show a rapid stabilization.

Aging↗