Wire Source Research Articles

Monte Carlo modeling of radiation dose distributions in intravascular radiation therapy

Radiation dose distributions are developed for balloon and wire sources of radioactivity within coronary arteries. The Monte Carlo codes MCNP 4B and EGS4 were used to calculate dose distributions for photons and electrons at discrete energies around such sources, with and without the presence of a high-density atherosclerotic plaque. An interactive computer program was developed which then calculates dose distributions for many radionuclides by applying the emission spectra to the discrete energy grids calculated by the Monte Carlo codes, weighting appropriately for electron energy and abundance. Results for Re-186 and Re-188 balloon sources are shown in comparison to an Ir-192 wire source. The program provides dose distributions as well as estimates of activity levels needed to deliver prescribed doses to the vessel wall at selected distances from the lumen in a selected time interval. In addition, dose calculations are presented in this paper for other organs in the body, from photon radiation as well as from possible loss of liquid activity into the bloodstream in the case of a balloon rupture. These results, especially the interactive computer program permitting easy comparison of various radionuclides and their physical characteristics, will greatly facilitate the comparison process and aid in the selection of the best candidate(s) for clinical use.

INVESTIGATION OF IGNITION OF THERMOPLASTICS THROUGH THE HOT WIRE IGNITION TEST

The purpose of this study was to investigate the ignition phenomena of selected polymeric materials—Plexiglas® (PMMA), polyethylene, Lexan® (polycarbonate), and nylon using the Hot Wire Ignition Test. Underwriters' Laboratories (UL) prescribe this test for polymeric materials used in enclosures or insulation systems of electrical equipment. The main objective of this study was to identify the effect of specimen thickness on the ignition time. In addition, temperature changes at the surface of selected polymeric materials during the different phases of the ignition process were also investigated. The apparatus used for this test was specified by ASTM D 3874–90a—Standard Test Method for Ignition of Materials by Hot Wire Sources. It consisted of a supply circuit capable of maintaining a continuous 60-Hz power density of 0.26 W/mm over 12 in. (304.8 mm) of No. 24 AWG, Nichrome V wire and a variable transformer for adjustment of the voltage to achieve the desired current. Temperature measurements were also performed using a Cole-Parmer Digi Source® 12-channel scanning thermometer and 30 AWG chromel–alumel (type K) thermocouples. A linear relationship between time to ignition and thickness was verified for the four polymers of thicknesses 1/16 in., 1/8 in., 3/16 in., and 1/4 in. The different polymers evaluated in terms of ease of ignition from most ignitable to least ignitable were in the order: Plexiglas, polyethylene, Lexan, and nylon. From the temperature measurements performed in tests with Plexiglas, it appears that the major factor affecting the time to ignition is the rate of temperature rise during the decomposition process. One important recommendation is that temperature measurements similar to those performed with four thicknesses of Plexiglas should be performed with Lexan (polycarbonate), nylon, and polyethylene. These data would confirm the principal role played by the rate of temperature rise during the decomposition process on the time to ignition of thermoplastic materials.

Transient thermal conductivity measurements: comparison of destructive and nondestructive techniques

Transient thermal conductivity methods have benefited from the rapid nature of the testing, but at the same time these methods are destructive because a sample must be prepared. Laser flash diffusivity is an example of a well-established transient method of measuring thermal conductivity, which requires samples to be prepared to a diameter of 12.4 mm. The newly introduced modified hot wire (MHW) and transient plane source (TPS) techniques are transient as well, but are nondestructive in nature. This is possible because both of these new techniques are reflectance methods that operate by applying and measuring heat at the same surface. Essentially any flat surface can be measured, with no maximum size constraints. These transient techniques have been used to evaluate materials of differing size, structure, and homogeneity. While results compare well for homogeneous materials from foams to ceramic, composite materials differ considerably. This is because the two methods penetrate the samples to different degrees, and the results are a reflection of the testing depth. As test times are varied in the reflectance techniques, information was gathered that has been related to the thermal conductivity as a function of position normal to the sample. Techniques are discussed and results on homogeneous materials from two different transient techniques are compared.

Determination of a calibration factor for the nondestructive assay of Guidant 32P brachytherapy sources

A calibration factor (`dial setting') for the nondestructive assay of Guidant TiNi-encapsulated 32P intravascular brachytherapy wire sources has been determined for measurements with the Capintec CRC-12 (sic. `dose calibrator') ionization chamber. The calibration factor was derived from ionization current measurements with the CRC-12 followed by very quantitative, destructive assays of the 32P content in two sources.

Endangered species: science writers in the Canadian daily press

The last comprehensive study of Canadian science journalists and science reporting was done over 20 years ago (Dubas and Martel 1973). A more recent content analysis of seven major Canadian dailies found most science stories are “hard” news in style and originate from wire sources (Einsiedel 1992). This 1994 survey of 105 of Canada's daily English-language newspapers and their commitment to science journalism revealed a general weakening and dilution of reporter allocation. Over 50 percent of Canadian dailies did not allocate a science reporter. Only 18 full-time science reporters were identified. A follow-up of 45 journalists covering science at least half-time revealed most covered science policy. The vast majority had no science training. Time problems and competing demands hindered professional development. Confirming suggestions of previous literature, this study also indicates the Canadian daily newspaper structure is not supportive of the style of reportage required for quality science writing, nor of the development of such writers.

The investigation of 32P wire for catheter-based endovascular irradiation.

The dose distribution from a 32P source has been measured and calculated in order to evaluate its application in endovascular irradiation. The source dimension was 27 mm in length and 0.3 mm in diameter and was embedded in the end of a Ni-Ti wire. Dose measurements were performed using radiochromic film in several specially designed tissue equivalent phantoms. Loevinger's point dose kernel was used for the calculation. The approximate dose rate at a radial distance of 1.5 mm from the center of the source was found to be 6.75 cGy/s per GBq (0.25 cGy/s per mCi), which allows the delivery of a therapeutic dose in a short time interval with a satisfactory homogeneity without stepping the source. However, the dose rate falls off almost exponentially along the radial distance. Therefore it may not be suitable for treating large diameter vessel from a centrally located source. The effect of a curved 32P wire source on the radial dose distribution was also investigated. The results showed that for a maximum bend of 180 degrees the dose rate was increased by as much as 20% along the inner radial distance but decreased by as much as 20% along the outer radial distance compared to the dose along a straight wire. However, for curvatures normally encountered in a clinical situation, the dose rate was changed less than 5%.

The electron-dose distribution surrounding an 192Ir wire brachytherapy source investigated using EGS4 simulations and GafChromic TM film

The steep dose gradient around 192Ir brachytherapy wire implants is predicted by the EGS4 (PRESTA version) Monte Carlo simulation. When considering radiation absorbion regions close to the wire source, the accurate dose distribution cannot be calculated by the GE Target II Sun Sparc treatment-planning system. Experiments using GafChromic TM film have been performed to prove the validity of the EGS4 user code when calculating the dose close to the wire source in a low energy range.

32 - Quality assurance for a 192Ir wire source dosimetry

32 - Quality assurance for a 192Ir wire source dosimetry

Pressure and cavitation measurements of an intra‐arterial angioplasty device

Variation of the acoustic field radiated from a low‐frequency (22.5 kHz) catheterlike angioplasty device in a relatively large (600 L) tank of water was presented earlier [I. R. S. Makin and E. C. Everbach, 2855(A) (1994)]. The normal volume of the human torso or the human limb into which the ultrasonic wire is inserted during therapy cannot be treated, however, as an infinite medium for the 66.6‐mm wavelength (calculated for water) due to the wire source. Therefore the sound field of the device was investigated in a liquid‐filled Plexiglasregistered cylinder (200‐mm diameter, 350‐mm height) to simulate anthropometric dimensions. Measurement of pressures along the axis of the cylinder for the wire source positioned axially at one end of the cylinder shows a marked standing wave pattern even up to an axial range of 250 mm. Such strong standing waves could potentially exist far away from the probe tip in vivo. In addition, the occurrence of cavitation at the tip of the wire was confirmed using a 20‐MHz passive cavitation detection technique.

Low-dose, beta-particle emission from 'stent' wire results in complete, localized inhibition of smooth muscle cell proliferation.

Restenosis after catheter-based revascularization has been demonstrated to be primarily caused by medial and/or intimal smooth muscle cell (SMC) proliferation. The objective of this study was investigate the ability of local emission of beta-particles from a 32P-impregnated titanium "stent" wire source to inhibit vascular SMC and endothelial cell proliferation in cell culture and to determine the dose-response characteristics of this inhibition. A series of experiments were performed using 0.20-mm-diameter titanium wires that were impregnated with varying low concentrations of 32P (activity range, 0.002 to 0.06 microCi/cm wire, n = 47) or 31P (nonradioactive control, n = 28) in cultures of rat and human aortic SMCs and in cultured bovine aortic endothelial cells. The zone of complete cell growth inhibition (in millimeters from stent wire) was measured using light microscopy in the cultures exposed to the radioactive (32P) or control (31P) wires at 6 and 12 days after plating. In both rat and human SMC cultures there was a distinct 5.5- to 10.6-mm zone of complete SMC inhibition at wire activity levels > or = 0.006 microCi/cm. In contrast, there was no zone of inhibition surrounding the control (31P impregnated) wires (P < .001 versus 32P wires at all wire activities > or = 0.006 microCi/cm for human and rat SMCs). Proliferating bovine endothelial cells were more radioresistant than SMCs, with no zone of inhibition observed at wire activity levels up to 0.019 microCi/cm (P < .001 versus SMCs at 0.006 microCi/cm and 0.019 microCi/cm). We conclude that very low doses of beta-particle emission from a 32P-impregnated stent wire (activity levels as low as 0.006 microCi/cm of wire) completely inhibit the growth and migration of both rat and human SMCs within a range of 5.5 to 10.6 mm from the wire. Endothelial cells appear to be much more radioresistant than SMCs. These data suggest that an intra-arterial stent impregnated with a low concentration of 32P may have a salutary effect on the restenosis process. Whether this approach can be used successfully and safely to inhibit restenosis in vivo and in the clinical setting is under investigation.

Dosimetric considerations of commercially fabricated esophageal applicators

Two commercially fabricated esophageal applicators for low dose rate intraluminal brachytherapy were compared using the dose non-uniformity ratio (DNR). One of the applicators has a single afterloading catheter while the other applicator has six afterloading catheters to accommodate low dose rate 192Ir ribbon or wire sources. Five sets of source configurations, each with a different ribbon length in the range of 5–17 cm were analysed using the DNR. The DNR shows the delivery of relatively higher total doses at higher dose rates to tissues closer to the applicators. The difference in the inhomogeneity of dose rates and the treatment volume exposed to higher dose rates than the prescribed dose rate between the two applicators are minimal. The dose homogeneity is better for the single-source applicator while the worst dose homogeneity for the six-source applicator is caused by the placement of each source closer to the surface of the applicator. The implications of higher dose rates were discussed using the α β model.

Radioactivity measurements of 192Ir wire sources with a microcalorimeter

Radioactivities of metallic 192Ir sources are measured with a microcalorimeter nondestructively. In order to convert the complete decay energy evolved from the source into thermal energy, a tungsten block is used as radiation absorber. The sample sources measured are four thin wires of Ir/Pt alloy with dimensions of 0.1 mm ø × 10 mm 1, each coated by 0.1 mm thick platinum, giving the final diameter of 0.3 mm ø. Overall uncertainty of the measurement for some 60 MBq activity is ±3.2%. Measurements of the wire sources with an ionisation chamber give smaller activities which are smaller by 10% than those from microcalorimetry mainly because of the autoattenuation effect.

Analytical calculations on radioactivity measurements of 192Ir metallic sources with a calibrated ionization chamber

The self-attenuation of photons was analytically calculated for an 192Ir metallic wire source and an aqueous standard source. The influence of the self-attenuation on the current of the calibrated ionization chamber was also evaluated. The self-attenuation did considerably affect the ionization current. It was concluded that the apparent radioactivity must be multiplied by a correction factor larger than unity (1.09±0.01 in the present case) to get the true radioactivity when the radioactivity of the 192Ir metallic wire source was measured with the ionization chamber calibrated with the aqueous standard source of 192Ir.

Transient electromagnetic modeling of shallow A‐type sections with 3-D inhomogeneities

A hard rock area underlain by granitic/gneissic or basaltic rocks often has an A‐type three‐layer geoelectric section in which resistivity increases with depth. The middle layer of moderate resistivity caused by fracturing/fissuring that lies between the surface‐weathered layer and the substratum of unfractured rock is not a good target for a direct current (DC) resistivity sounding since it is generally suppressed in the observations. Moreover, its definition requires expanding the electrode spacing to a length several times the depth of the target layer, and this may be a drawback if the target layer is either laterally variable or limited in its horizontal extent. We first studied the transient electric field of a horizontal electric dipole (HED) source excited by a step turn‐off current for a 1-D model of an A‐type geoelectric section. The results of this theoretical study are presented as graphs of normalized apparent resistivity versus a time‐related dimensionless parameter. Irrespective of the separation between the transmitter and receiver dipoles, these transient sounding curves become similar to the corresponding Schlumberger sounding curves at late time. Hence the transient electric field measurement offers the possibility of sounding at a fixed transmitter‐receiver spacing that may be shorter than the target depth. Also, at early times, for a certain ratio of the dipole separation to the target depth, there is a dramatic increase in the resolution of the response. Thus, it is possible to resolve suppressed layers of an A‐type section in this type of sounding. A study of the effects of transmitter ramp time and receiver bandwidth on the transient apparent resistivity curves shows that a very fast current shut‐off and wideband measurement are required to realize all the possibilities suggested by this modeling. Some 3-D transient electromagnetic (TEM) modeling was also done to simulate (1) a lateral variation in the resistivity of the middle layer of an A‐type section and (2) a weak zone of limited horizontal extent in the substratum of a two‐layer section. We observed that the 3-D inclusion has less effect at late time but is more pronounced at early time. In view of the above results, we conclude that the transient E‐field sounding with a grounded wire source can be used in place of a conventional DC resistivity sounding to overcome the problem of poor resolution due to the suppression of the intermediate layer in a geoelectric section where the resistivity increases with depth. As such, it has a potential application in groundwater as well as geotechnical surveys, because together with the overlying weathered layer, the fractured rock constitutes the aquifer in hard rocks.

192Ir brachytherapy sources: Calibration of the NPL secondary standard radionuclide calibrator

The increasing use of brachytherapy sources has highlighted the need to confirm the traceability to national standards of air kerma rate measurements made by suppliers and to provide a simple means for the user to check the activity of such sources. A programme of work has been initiated under the auspices of EUROMET to confirm the traceability to NPL of air kerma rate measurements made by Amersham International plc for wire sources of 192Ir. Intercomparison exercises are also under way between a number of national laboratories. The wires used in this work have been used to calibrate the NPL secondary standard radionuclide calibrator both in terms of the air kerma rate and the activity, and thus provide the hospital user with the basis of a simple and accurate quality control system for these particular types of source.

Observation of a possible surface-charge-density oscillation by the adsorption of an impurity.

Neon field-ion image spots of impurities deposited from a Ta wire source on the Ir(100) surface show a halo-ring structure. Using an atom-probe field-ion microscope, these impurities are tentatively identified as tantalum hydrides. This halo-ring image-spot structure is most probably produced by the perturbation of the electronic charge-density distribution of the substrate around the impurity by its adsorption.

Cultural Proximity in International News Coverage: 1988 U.S. Presidential Campaign in the Greek Press

Previous studies have shown that news selection is influenced by economic, social, political and geographic values, while this study tests the notion that cultural proximity also influences news choices and display. A content analysis of a liberal and a conservative Creek newspaper during the final three months of the 1988 U.S. Presidential campaign shows that Democrat Michael Dukakis, a Greek-American, was given more space — and that more favorable — than was Republican George Bush. The two newspapers used similar wire sources and differed little in overall U.S. campaign coverage, however, despite strong ideological differences.

Outside Over National News Agencies? A Study of Preferences in the French Regional Press

This survey of 27 French journalists responsible for choosing foreign news from the wire services to publish in regional newspapers outside Paris shows that Agence France Presse remains the major wire source. But the concise, unadorned style of the French language service offered by the Associated Press has gained many users. French journalists are specializing more, and about one-fourth of younger journalists have formal journalism training. There is less time for editing on some newspapers. The study suggests the unique style of Agence France Presse may have to give way to be more competitive, at a cost to a unique French emphasis on foreign news. International news may become like foreign news in any press.

Dimer and tetramer formation in an AsH3 cracker studied by calibrated quadrupole mass spectrometry

The reaction of AsH3 in a gas source with a tungsten filament (W-GS) was studied. The reaction products were detected by a quadrupole mass spectrometer and measured as a function of filament temperature and feed pressure. The mass dependence of the transmission of the quadrupole system was optimized and calibrated using a mixture of five noble gases. The results were corrected for ionization efficiency and cracking pattern. Background and noise effects were reduced by beam modulation. Conditions for buildup of an arsenic deposit within the gas source are discussed. Maximum As2 and As4 production was observed at 800 and 630 °C, respectively. The gas source which decomposes the hydride at a tungsten wire is compared with a second one which decomposes the hydride at a tantalum wire (Ta-GS). Operating conditions that yield predominantly arsenic dimers are achieved only with the tungsten wire source. Chemical reasons are discussed to explain the higher dimer yield of the tungsten source.

Intracavitary Irradiation of Carcinomas of the Uterus and Cervix: The Créteil Method

Abstract The Creteil method is a logical and simple system for intracavitary therapy of cervical and uterine cancer. The system is based on the use of a plastic cervico-vaginal moulage loaded with 0.5 mm diameter iridium 192 wire sources. The dimensions of both the moulage and the sources correspond to the size of the cervix being treated according to precisely defined relationships. The dose is specified on a reference isodose of a “fixed” value enclosing the pear-shaped target volume whose dimensions depend on the geometry of the sources. Source geometry in turn, depends on the size of the cervix. Thus, the dimensions of the target volume can be accurately predicted at the time of the application. Furthermore, treatment can be performed in a single application, with all sources having the same linear reference air kerma rate (or activity). Radioprotective measures are simple but effective and the patient is not subjected to the restrictions imposed by attachment to an after-loading apparatus. Our clinical results for early Stage Tl and T2a cervical tumors show excellent local control without major treatment complications or long term sequelae.