ISSN:0094-2405    

DOI:10.1002/j.2473-4209.1982.tb36175.x

出版商:Wiley    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1002/j.2473-4209.1982.tb36176.x

出版商:Wiley    

年代:1998

数据来源: WILEY

摘要:

A comprehensive theory of radiographic density fluctuations is developed. The theory incorporates film granularity, quantum mottle, the absorbed x‐ray photon energy distribution, fluctuations in phosphor light photon yield, and phosphor coating weight variations. The resulting expressions allow one to calculate the density fluctuations or, alternatively, the Wiener spectrum that will be obtained with a radiographic screen/film system from the physical parameters of the system. Reasonable agreement is obtained between experimental results and calculated values for three typical screen/film systems. An expression is also derived for the detective quantum efficiency (DQE) of screen/film systems, and the results of DQE calculations are presented for a commonly used screen/film combination as a function of density and of object size.

ISSN:0094-2405    

DOI:10.1118/1.595126

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A theoretical study of the application of pulsed nuclear magnetic resonance (NMR) to the measurement of fluid flow has been made. Calculations of detector signal line shapes, for a two‐coil NMR laboratory experiment, were performed using a computer to determine the appropriate choice of flow signature. Results of calculations are presented showing the sensitivity of the method to various experimental parameters. The position of the received peak signal, in time, was found to correlate directly with mean flow velocity. However, the shape of the received signal was observed to depend markedly upon the state of flow, i.e., laminar, turbulent (plug). Hence, accurate determinations of flow required a knowledge of flow type. The theoretical calculations provided a better understanding of the potential of NMR for measuring fluid or blood flow.

ISSN:0094-2405    

DOI:10.1118/1.595127

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

One type of system proposed for producing hyperthermia for cancer therapy of deep‐seated tumors consists of a cylindrical conducting coil (solenoid) in which the patient (or a limb of the patient) is placed. The current in the conducting coil creates an alternating magnetic field inside the patient, which induces eddy currents in the tissue. The eddy currents deposit power and hence produce heating. In this paper, steady‐state temperature profiles are calculated analytically, assuming a homogeneous one‐dimensional model which includes blood perfusion. Numerical methods are used to calculate the temperature distributions as a function of time for this homogeneous model and to calculate the temperature profiles when there are radial variations in tissue parameters including blood flow. Based on these computer simulations, it would appear that in most cases a solenoid‐type hyperthermia system will not be able to provide deep‐seated heating in the abdomen or thorax, but should be able to raise temperatures to therapeutic levels to about 6 cm depth in the abdomen and in the muscle layer in the thorax.

ISSN:0094-2405    

DOI:10.1118/1.595128

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A photon dosimeter based on degradation of polyacrylamide (PAA) in dilute aqueous solution has been found to be insensitive to neutrons from Cf‐252, independent of photon energy from 34 keVeffto 1.33 MeV, and useful over a dose range of 1 to 100 Gy. Because this dilute polymer solution is virtually 100% water, it is nearly tissue equivalent for photon absorption as well as neutron scatter and attenuation. Radiation response was determined by measuring decreases in viscosity three days after irradiation. The measured response is dose rate independent from 10 to over 300 Gy/h but increases rapidly at lower dose rates. Long‐term, radiation‐induced viscosity changes were also observed. The many advantages of this tissue equivalent system for photon/neutron dosimetry suggest the need for further studies of the observed over‐response at low dose rates.

ISSN:0094-2405    

DOI:10.1118/1.595129

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An optimization analysis of digital fluoroscopy has been carried out using (signal/noise)2/dose as a figure of merit. Three signal detection tasks have been considered: (i) small iodine‐containing blood vessels; (ii) regions of fat; and (iii) microcalcifications. The effects of x‐ray beam filtration and kilovoltage on the figure of merit have been parameterized and the associated x‐ray tube power loadings have been defined. Results are presented in a fashion that facilitates the choice of parameters (amount of aluminum filtration, kilovoltage, and x‐ray tube power loading) such that an acceptable balance can be achieved between signal detection sensitivity, patient dose, and x‐ray tube power loading.

ISSN:0094-2405    

DOI:10.1118/1.595130

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of transmission measurements with combinations of attenuating materials has been investigated as a method of determining the x‐ray energy spectrum from a 4‐MV linear accelerator. Simulation studies have been made with three 4‐MV x‐ray spectra of slightly different shapes. From each of these spectra, the narrow beam transmission curve was calculated, using lead to attenuate the beam for the first several points. Aluminum was then added to attenuate the beam to 0.001 of its initial intensity. A computer program using an iterative least‐squares technique has been developed to analyze the transmission data and determine the spectrum by applying some physically reasonable sensible constraints. The calculated transmission values agreed with the simulated data within the estimated experimental uncertainties, and it is concluded that the technique is reliable for the spectral reconstruction and is sensitive to small spectral changes.

ISSN:0094-2405    

DOI:10.1118/1.595131

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Tissue‐mimicking (TM) liver, previously reported by our laboratory, mimics the speed of sound, density, and attenuation coefficient (including frequency dependence) of liver; however, scatter properties are only qualitively simulated. Two new versions of TM liver are reported here which not only mimic liver with respect to speed of sound, density, and attenuation coefficient, but also with respect to backscatter coefficients, including the frequency dependence of the latter. Compositions, methods of production, and comparisons of ultrasonic properties with those found in the literature for human liver are presented.

ISSN:0094-2405    

DOI:10.1118/1.595134

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper discusses a principle for the determination of the blood perfusion rate. The method is based upon measurement of the phase‐shift between periodic heat flow and the corresponding temperature rise.

ISSN:0094-2405    

DOI:10.1118/1.595136

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY