ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1981.tb36158.x

出版商:Wiley    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1981.tb36159.x

出版商:Wiley    

年代:1998

数据来源: WILEY

摘要:

Living cells may be modified in diverse ways by the combined action of visible light and photosensitizing molecules. The effects appear most frequently as disruptions of subcellular structure, changes in surface membrane function or inhibition of mitotic ability. This review concentrates on the four most thoroughly studied cell types—yeast cells, nerve cells, erythrocytes, and cultured tumor cells. Research on these cells indicates that potency of sensitization depends at least as much on the factors affecting an association between sensitizer and cell prior to illumination as on photochemical properties. While sensitizers which permeate may lead to altered DNA, it appears that surface membrane modification occurs simultaneously and may be critical in the inactivation mechanism. There is much circumstantial evidence suggesting that excited singlet molecular oxygen acts as an intermediate between photoexcited sensitizer and target alteration. Proteins, lipids, and nucleic acids are all susceptible to photosensitized attack, but the correlation between cellular and molecular modification remains ill‐defined. The use of the photodynamic process as a therapeutic technique, particularly in the treatment of malignant tumors, holds great promise, but awaits further research to develop greater selectivity of action.

ISSN:0269-3127    

DOI:10.1118/1.595020

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Theoretical and experimental techniques have been used to study optimal x‐ray spectra for screen‐film mammography. A simple model of mammographic imaging predicts optimum x‐ray energies which are significantly higher than theK‐characteristic energies of Mo. A subjective comparison of x‐ray spectra from Mo‐anode and W‐anode tubes indicates that spectra produced by a W‐anode tube filtered with materials of atomic number just above that of Mo are more suitable for screen‐film mammography than spectra produced by the Mo‐anode/Mo‐filter system. The imaging performance ofK‐edge filtered, W‐anode tube spectra was compared to the performance of Mo‐anode spectra using phantom measurements and mastectomy specimen radiography. It was shown that optimal W‐anode spectra can produce equal contrast with an exposure reduction of a factor of two to three, a dose reduction of a factor of two, and equal or reduced tube loading, compared to Mo‐anode spectra. A computer simulation was carried out to extend the initial, monoenergetic theory to the case of real, polychromatic sources. The effects of varying filter material and thickness, tube operating potential, and breast thickness were all studied. Since W‐anode x‐ray tubes are considered to be better for Xerox mammography than Mo‐anode tubes, this study has shown that both Xerox and screen‐film techniques can be performed optimally with a single, properly designed, W‐anode x‐ray tube.

ISSN:0269-3127    

DOI:10.1118/1.595021

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A multisegmented, ionization chamber capable of determining dosimetric profiles from a CT scanner has been developed and tested. The chamber consists of a number of 2 mm wide electrically isolated segments from which ionization currents may be measured. Presented here are the performance characteristics of the chamber including energy response, dose linearity, and corrections for “cross talk” between segments. Sample dosimetric profiles are depicted for 3 and 6 mm nominal beam widths at two locations in a dosimetric phantom positioned in the x‐ray beam of a fourth generation CT scanner. The results agree well with the conventional method of obtaining dosimetry measurements with TLD chips.

ISSN:0269-3127    

DOI:10.1118/1.595022

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We report results of the variation of signal detectability with radiographic film density for Lanex screens and four different x‐ray films. We found that maximum signal detectability occurs near (but not precisely at) the maximum slope of the characteristic curve. We interpret our results using a model that includes film gamma quantum noise, film granularity, and an intrinsic observer contrast threshold.

ISSN:0269-3127    

DOI:10.1118/1.595023

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A variety of digital radiographic and fluoroscopic devices have been developed which can isolate small concentrations of iodine within the cardiovascular system. Using these devices, time dependent subtraction images have been formed which only display opacified vasculature. Theory is presented and simple methods have been developed for determining vessel diameters and iodine concentration from such subtraction images. The methods have been verified using plexiglass and aluminum vessel phantoms imaged with a computerized radiography device. Using this device the diameter of a 5 mm diameter vessel could be determined to within 6% (0.28 mm) even though the pixel width in the digitized image corresponded to 1.34 mm. In the same vessel, it is estimated that an iodine concentration of 46 mg/cm3could be determined with 10% accuracy.

ISSN:0269-3127    

DOI:10.1118/1.595024

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dual energy basis decomposition techniques apply to single projection radiographic imaging. The high and low energy images are non‐linearly transformed to generate two energy‐independent images characterizing the integrated Compton/photoelectric attenuation components. Characteristic linear combinations of these two basis images identify unknown materials, cancel known materials, and generate synthesized monoenergetic images. The problems of intervening materials and material displacement are solved in general for a wide class of clinical imaging tasks. The basis projection angle indentifies one from a family of energy selective imaging tasks, and such performance measures as the contrast enhancement factor (CEF) and signal to noise ratio (SNR) are expressed as functions of this angle. Algorithms for the decomposition of high and low energy measurements are compared and experimental images are included.

ISSN:0269-3127    

DOI:10.1118/1.595025

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have identified the sources of neutron production in the beam transport system of the 720‐MeV helium beam used for radiation therapy at the 184‐in synchrocyclotron of the Lawrence Berkeley Laboratory, and determined their magnitude. Measurements with activation detectors of differing energy response were used to unfold secondary particle spectra at various locations on the patient table. The effect of charged particles was estimated using a calculation of neutron‐flux densities derived from published cross sections. The absorbed dose, as a function of distance from the beam axis, was calculated using the unfolded spectra and evaluated fluence‐to‐dose conversion factors. The values of absorbed dose obtained from the unfolding of experimental data agree with the values obtained from the calculated spectra within the estimated uncertainty of ±25%. These values are ∼5×10−3rad on the beam axis and ∼1×10−3rad at distances greater than 20 cm, perpendicular to the beam axis, per rad deposited by the incident alpha‐particle beam in the plateau. Estimates of upper limits of dose to two critical organs, the lens of the eye and red bone narrow, are ∼25 rad and ∼5 rad, respectively, for a typical treatment plan.

ISSN:0269-3127    

DOI:10.1118/1.595026

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Luminance scatter fractions were measured for patients and phantoms in various regions of the chest film. Scatter fractions were found to be highly variable, being quite high in the regions of the chest with a large equivalent tissue thickness (e.g. mediastinum, chest wall, and subdiaphramatic areas). For example, approximately 90% of the radiation reaching the mediastinum region of the chest film was found to be scattered radiation when a grid was not used. Even with a 12:1 grid, less than 50% of the available primary beam contrast was being displayed in the mediastinum. In regions of the chest film with high scatter fractions, film latitude was also a problem, further reducing image contrast in these regions. Image contrast and scatter fractions in chest radiography were investigated for different scatter rejection techniques including air gaps, antiscatter grids, and slit radiography.

ISSN:0269-3127    

DOI:10.1118/1.595027

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY