摘要:

Bremsstrahlung spectra and on‐axis depth‐dose distributions due to 25.5‐MeV electrons incident on seven different Ta and one Al radiator were studied, with and without beam‐hardening filters of Al and Pb. The photon spectra were measured with a gamma‐ray spectrometer consisting of a liquid deuterium target viewed by a time‐of‐flight detector of photoneutrons. A remote‐controlled, miniature ion chamber was used to measure depth‐dose profiles in a water phantom. The Ta radiators ranged in thickness from 0.012 radiation length up to 1.0X0. The thinner the radiator, the harder was the measured spectrum. The maximum forward yield was obtained for a thickness of about 0.3X0. The forward yield and spectral shape of the photon beam from 0.07X0of Al as radiator were very similar to those from 0.06X0of Ta. Beams produced in Ta radiators and filtered by 27.28 g/cm2of Al gave a harder spectrum than those filtered by 15.06 g/cm2of Pb. Total absorption cross sections of Al and Pb for photon energies between 4 and 24 MeV were obtained which agree with theoretical calculations, within measurement errors. The depths in water at which the dose fell to 0.5 of peak dose was greater when the Al filter was used than when the Pb filter was used. For end point energies greater than 10 MeV, beam‐hardening and field‐flattening filters should be made, if space permits, of low‐atomic‐number materials.

ISSN:0269-3127    

DOI:10.1118/1.1637293

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

The aim of this study is to present a comparison between different mathematical models that can be used to represent renal function, as characterized by the I–hippuran method. Based on physiological considerations, a general model for renal function is established, and three special and less complex models are deriveda prioriby choosing special values for some of the parameters of the general model. The three special models have previously been investigated by other authors. A general‐purpose computer program has been developed to fit any of the four models to the kinetic data. Parameters inherently dependent on the detection technique are calculated by a special method independent of the estimation of the physiologically relevant model parameters. Output from the computer program is used to determine which model gives the most relevant interpretation of I–hippuran renographic curves. In accordance with recent examinations performed by other workers, the results seem to indicate that each kidney can be satisfactorily represented by an ordinary compartment in series with a “delay compartment.”

ISSN:0269-3127    

DOI:10.1118/1.1637294

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

Fast quantitative dynamic studies with scintillation camera systems require deadtime correction. Although such systems are semiparalyzable, they are most conveniently treated either as paralyzable or nonparalyzable. Paralyzing deadtime (τ) is that deadtime during which a system is unable to provide a second output pulse unless there is a time interval of at least τ between two successive events. Paralyzable systems are characterized by Poisson statistics, so that the “true” counting rateN = R eN τ, whereRis the observed counting rate. Nonparalyzing deadtime (T) is that deadtime during which a system is insensitive after eachobservedevent. The period of insensitivity is not affected by any additional “true” events before full recovery occurs. For nonparalyzable systems the corrected counting rateN = R / (1 − RT). A two‐source method protocol is presented for deadtime measurement. The paralyzing deadtime is calculated by a 5‐dimension Newton–Raphson iteration. The nonparalyzing deadtime is calculated by a quadratic equation. Approximation equations are also presented not requiring a computer. Deadtimes are fitted to polynomial equations as dependent variables of measured counting rate. Algorithms incorporating the polynomials are presented for the deadtime correction of histogram curves. Using either the paralyzing or the nonparalyzing approach, precise deadtime corrections are demonstrated.

ISSN:0269-3127    

DOI:10.1118/1.1637295

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

In field emission x‐ray tubes, the target is conical in shape and the projection of the focal spot along the central axis a circular disk. To determine the resolution capability of such tubes, a theoretical expression has been derived for the MTF of a circular focal spot. It has been shown that the MTF is given by the equationMTF = 2J1[πf0δ(m − 1) / m] / [πf0δ(m − 1) / m]wheremis the magnification factor,f0the spatial frequency in the object plane, and δ the diameter of the focal spot.J1stands for the Bessel function of the first order. The above expression has been tested experimentally and used to compare circular focal spots with square focal spots.

ISSN:0269-3127    

DOI:10.1118/1.1637296

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

Until recently, the photon spectrum of226Ra has not been known with sufficient accuracy to permit calculation of the exposure rate constant. Workers concerned with calculations involving this constant have relied upon measured values. The authors have taken compiled photon spectra authored by M. J. Martin and P. H. Blichert‐Toft inNuclear Data Tablesalong with photon cross sections authored by E. Storm and H. Israel inNuclear Data Tablesand calculated theoretical values for the exposure rate constant for226Ra. Using the most recent 72‐line spectrum for226Ra and its daughter products, an exposure rate constant of 12.44 R‐cm2/mg‐h was obtained for unfiltered radium. A large portion of this value was contributed by some low energy x‐ray lines which would not be seen in even a very lightly filtered source. Using a 3‐mm‐o.d. sphere (90%‐Pt, 10%‐Ir, 0.5‐mm filter), a value of 8.12 R‐cm2/mg‐h was obtained for the filtered source. Primary, scattered, and bremsstrahlung contributions to this value were 7.610, 0.363, and 0.149, respectively. This value is within 1.6% of the presently accepted value for226Ra, 8.25.

ISSN:0269-3127    

DOI:10.1118/1.1637297

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

A 52.5‐cm‐long steel collimator for 15‐MeV neutron therapy beams has been evaluated in terms of the distribution of the collimator‐scattered neutrons in the field and penumbra, the distribution in the collimator of the neutron scattering sites, and the energy of the neutrons scattered to one position in the penumbra, in order to assess the need for multiple tapers on such collimators. The associated particle technique was used to eliminate background neutrons from the measurement. We conclude that because collimator‐scattered neutrons constitute only a small part of the neutron flux in the field and most of the scattered neutrons have energies greater than 12 MeV, double‐taper collimators are not necessary.

ISSN:0269-3127    

DOI:10.1118/1.1637298

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

The influence of lung tissue on depth dose distributions, produced by beams of fast neutrons (generated at the Naval Research Laboratory cyclotron by bombarding a thick Be target with 35‐MeV deuterons) was determined. Muscle‐equivalent ionization chambers were utilized in conjunction with thermoluminescent dosimeters to establish the neutron and gamma ray‐dose deposited in a phantom of the thoracic region of the body containing lung components. All irradiations were carried out at a target–skin distance of 125 cm with neutron beams of 10 × 10‐cm field size at the surface of the phantom. From these measurements, the authors conclude that the presence of lung tissue results in higher dose levels for points at depths greater than the midpoint of the lungs as a result of the increased transmission of neutrons by low‐density lung tissue. An examination of the dose measurements revealed that the portion of the total dose due to gamma rays is of the order of 4–10% for points in lung and muscle‐equivalent fluid.

ISSN:0269-3127    

DOI:10.1118/1.1637299

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

An approach to summarizing interrelations between functions used in radiotherapy dose calculations is suggested. A mnemonic diagram is proposed which should be useful for radiation physicists in radiotherapy to obtain at a quick glance the definitions and interrelations between the commonly used functions like back‐scatter factor, percent depth dose, tissue–air ratio, scatter–air ratio, and scatter function. The conditions under which the measurements must be made for these functions and interrelations to be valid are described. Rules for application of the diagram and some practical examples are included. The diagram also serves as a very effective teaching aid.

ISSN:0269-3127    

DOI:10.1118/1.1637300

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.1637301

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1974.tb36050.x

出版商:Wiley    

年代:2003

数据来源: WILEY