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At least 487 records · Page 27Linked to original sources
PubMed2002

Effects of particle size, helium gas pressure and microparticle dose on the plasma concentration of indomethacin after bombardment of indomethacin-loaded poly-L-lactic acid microspheres using a Helios gun system.

We investigated the effects of the particle size of indomethacin-loaded poly-L-lactic acid microspheres (IDM-loaded PLA MS), the helium pressure used to accelerate the particles, and the bombardment dose of PLA MS on the plasma concentration of IDM after bombarding with IDM-loaded PLA MS of different particle size ranges, 20-38, 44-53 and 75-100 microm, the abdomen of hairless rats using the Helios gene gun system (Helios gun system). Using larger particles and a higher helium pressure, produced an increase in the plasma IDM concentration and the area under the plasma concentration-time curve (AUC) and resultant F (relative bioavailability with respect to intracutaneous injection) of IDM increased by an amount depending on the particle size and helium pressure. Although a reduction in the bombardment dose led to a decrease in C(max) and AUC, F increased on decreasing the bombardment dose. In addition, a more efficient F was obtained after bombarding with IDM-loaded PLA MS of 75-100 microm in diameter at each low dose in different sites of the abdomen compared with that after bolus bombardment with a high dose (dose equivalent). These results suggest that the bombardment injection of drug-loaded microspheres by the Helios gun system is a very useful tool for delivering a variety of drugs in powder form into the skin and systemic circulation.

Air Pressure↗
PubMed2006

Radiation sources based on laser-plasma interactions.

Plasma waves excited by intense laser beams can be harnessed to produce femtosecond duration bunches of electrons with relativistic energies. The very large electrostatic forces of plasma density wakes trailing behind an intense laser pulse provide field potentials capable of accelerating charged particles to high energies over very short distances, as high as 1GeV in a few millimetres. The short length scale of plasma waves provides a means of developing very compact high-energy accelerators, which could form the basis of compact next-generation light sources with unique properties. Tuneable X-ray radiation and particle pulses with durations of the order of or less than 5fs should be possible and would be useful for probing matter on unprecedented time and spatial scales. If developed to fruition this revolutionary technology could reduce the size and cost of light sources by three orders of magnitude and, therefore, provide powerful new tools to a large scientific community. We will discuss how a laser-driven plasma wakefield accelerator can be used to produce radiation with unique characteristics over a very large spectral range.

Journal Article↗
PubMed1991

Prediction of Saturne II+ 10 MV and 23 MV photon beam output factors.

A two-variable, three-parameter formula for modified, virtual equivalent square sides has been worked out which, when incorporated into empirical output-square field size function, generates output values of rectangular fields taking into account the asymmetry of the collimating system of the Saturne II+ accelerator generating 10 MV and 23 MV photon beams.

Particle Accelerators↗
PubMed1999

Intensity-modulated radiation therapy with dynamic multileaf collimators.

Intensity-modulated radiotherapy (IMRT) has been considered as a means of providing dose distributions that conform to concave target volumes. For computer-controlled multileaf collimators (MLCs) to be used to modulate x-ray beams, some procedure must be used to determine the sequence of leaf positions used to produce the desired modulation. This article derives and compares four leaf-sequencing algorithms. MLC leaf sequencing can be accomplished by representing the areal intensity modulation of a beam with a series of beam profiles. A velocity-modulation equation for computing the modulation required for a one-dimensional profile, described originally using more extensive algebra, is derived using a graphic approach. The velocity-modulation approach is compared with an equal incremental step-and-shoot approach derived by Bortfeld and Boyer. An areal step-and-shoot technique derived by Xia and Verhey is introduced and compared with the profile-by-profile methods. Finally, an approach is considered using multiple repeated arcs developed by Yu. This wide variety of methods can yield an approach to IMRT that conforms to the engineering constraints imposed by the design of a particular linear accelerator.

Algorithms↗
PubMed1999

Monitor unit calculation on the beam axis of open and wedged asymmetric high-energy photon beams.

An ESTRO booklet and a report of the Netherlands Commission on Radiation Dosimetry have been published recently describing empirical methods for monitor unit (MU) calculations in symmetrical high-energy photon beams. Both documents support the same basic ideas; firstly the separation of head scatter and volume scatter components and secondly the determination of head scatter quantities in a mini-phantom. Based on these ideas the methods previously described for MU calculations in symmetrical beams are extended to asymmetrical open and wedged beams in isocentric treatment conditions. All required dosimetric parameters (normalized head scatter factors, phantom scatter correction factors, wedge factors, off-axis ratios, quality index, and depth dose parameters) are determined as a function of beam axis position in order to study their off-axis dependence. Measurements are performed for 6 MV and 18 MV photon beams provided by two different dual-energy linear accelerators, a GE Saturne 42 and a Varian 2100 CD linac.

Models, Statistical↗
PubMed2004

A generalized extension to the Hounsell-Wilkinson head scatter model.

Hounsell and Wilkinson (1997 Phys. Med. Biol. 42 1737-49) have demonstrated that the concept of a scatter-plane source can accurately describe head scatter in irregularly shaped fields produced by the MLC of an Elekta linear accelerator. In these linacs, the solid jaw is below the MLC set and plays no part on defining the view of the main source of head scatter, the flattening filter. A more complicated situation arises for linear accelerator head designs which incorporate a solid upper jaw pair with the MLC jaws further from the source. Application of the Hounsell and Wilkinson technique to these accelerator designs does not achieve the accuracy in the Elekta application. We present a generalized extension to Hounsell and Wilkinson's model and present results for typical treatment field shapes on a Siemens Primus linear accelerator.

Biophysical Phenomena↗
PubMed2001

Radiation safety systems for accelerator facilities.

The radiation safety system RSS) of an accelerator facility is used to protect people from prompt radiation hazards associated with accelerator operation. The RSS is a fully interlocked, engineered system with a combination of passive and active elements that are reliable, redundant and fail-safe. The RSS consists of the access control system (ACS) and the radiation containment system (RCS). The ACS is to keep people away from the dangerous radiation inside the shielding enclosure. The RCS limits and contains the beam/radiation conditions to protect people from the prompt radiation hazards outside the shielding enclosure in both normal and abnormal operations. The complexity of an RSS depends on the accelerator and its operation. as well as associated hazard conditions. The approaches of RSS among different facilities can be different. This report gives a review of the RSS for accelerator facilities.

Humans↗
PubMedDate not supplied

Relative measurements of fast neutron contamination in 18-MV photon beams from two linear accelerators and a betatron.

Fast neutron contamination in photon beams in the 20 MV range have been reported in recent years. In order to determine if the variations were due mainly to differences in measurement procedures, or inherent in the design of the accelerators, three different 18-MV (BJR) photon beams were compared using identical analytical techniques. The units studied were a Philips SL/75-20 and a Siemens Mevatron-20 linear accelerators and a Schimadzu betatron. Gamma spectroscopy of an activated aluminum foil was the method used. By comparing the relative amounts of neutron contamination, errors associated with absolute measurements such as detector efficiency and differences in activation foils were eliminated. Fast neutron contaminations per rad of x rays in a ratio of 6.7:3.7:1 were found for the Philips, Schimadzu and Siemens accelerators, respectively.

Fast Neutrons↗
PubMedDate not supplied

On the selection of stopping-power and mass energy-absorption coefficient ratios for high-energy x-ray dosimetry.

A method for the selection of average stopping-power (L/rho)medair and energy-absorption coefficient (mu en/rho)medair ratios has been developed. The quality of the x-ray beam is characterized by the ratio of ionization chamber readings at depths of 20 and 10 cm in water (TMR)2010. For convenience, a relationship is established between experimental (TMR)2010 and the nominal accelerating potential (MV) of the accelerator. Experimental (TMR)2010 are related to (L/rho)medair and (mu en/rho)medair in a three-step process. First, using experimental and theoretical spectra in the range 60Co to 45 MV, (TMR)2010 were calculated for primary and first-scatter photons, and a graph of experimental versus calculated (TMR)2010 for these same spectra was constructed. Second, (L/rho)medair and (mu en/rho)medair were calculated for a large number of primary spectra [for most of which experimental (TMR)2010 were not available] and a graph constructed that related these quantities and (TMR)2010 calculated as above for this group of spectra. Third, using the graphs from the preceding steps, graphs relating the calculated (L/rho)medair and (mu en/rho)medair with experimental (TMR)2010 were constructed. Data are presented for water, polystyrene, acrylic, graphite, A-150, C-552, Bakelite, and nylon for beams with nominal accelerating potentials in the range 2-45 MV.

Humans↗
PubMed1982

[Measurement of the air activation caused by the use of medical accelerators].

The air is activated when a medical accelerator is operated in order to produce a continuous radiation the maximum energy of which is higher than 10.55 MeV. The induced activity can be measured by a simple method. The consequences regarding radiological protection and the organization of continuous radiation therapy with the examined accelerators (Clinac 20, Betatron 42 MeV) are discussed.

Activation Analysis↗