摘要:

The use of liposomes has recently been the subject of considerable attention as a promising and versatile approach to drug delivery. Particularly intriguing is the possibility of targeting liposomes to specific areas of the body such as tumors or sites of inflammation or parasitic invasion for either local accumulation or release of associated drugs. This review focuses mainly on recentin vivowork having clinical potential. An extensive discussion of liposome preparation and entrapment of drugs for controlled releasein vivois also included. The stability of liposomes in biological fluids is a major problem. The mode of administration, either intraperitoneal, subcutaneous, local, oral, or respiratory, is closely related to the life of the liposomesin vivo. Followingin vivoadministration the lifetime of a liposome is critically dependent on its composition, size, and charge. Liposome toxicity appears to be minimal, but should be considered when administering liposomes to patients. Tissues such as the liver, spleen, and lungs, because of macrophage ingestion of liposomes, become potential sites of drug toxicity. The use of liposomes to deliver antiparasitic drugs in the treatment of malaria and leishmaniasis is promising; so is the use of surfactant‐carrying liposomes in the treatment of respiratory distress syndrome in premature babies. Recent cancer studies utilizing liposomes bothin vivoandin vitrohave shown promise. In tumor‐bearing animals a liposome drug delivery system has caused a regression, delayed tumor growth, and increased survival time. Although the clinical use of liposomes is only in its infancy, its potential in future therapy appears promising.

ISSN:0269-3127    

DOI:10.1118/1.595079

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper concerns a method of scaling photon fluence from one scattering material to another when the photon energies are such that the dominant mode of interaction is Compton scattering. The theorem establishes a one‐to‐one correspondence between points in the two scattering media where the spectra of primary and scattered photons have the same distribution in energy and angle, and where the fluence ratio equals the square of the electron density ratio. Experimental tests were made with cobalt‐60 gamma radiation using ionization‐chamber measurements in graphite, acrylic plastic, polystyrene, and water phantoms. The experimental results are consistent with the equality of photon spectral shapes and angular distributions at corresponding points. The fluence ratios may differ by a few percent from the predicted values, depending on distance from the source.

ISSN:0269-3127    

DOI:10.1118/1.595080

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Fermi‐Eyges theory of electron scattering overestimates the scattering of electron beams used in radiation therapy. The reason for this overestimate is the neglect of the loss of electrons which are scattered into highly oblique paths and removed from the beam at relatively shallow depths. A modification of Eyges’ solution to Fermi's equation is presented to take this loss of electrons into account. Equations for the calculation of isodose distributions for any medium using pencil beams are developed. Experimental confirmation is presented for electron beams of 13 and 18 MeV in homogeneous water, polystyrene, Lucite, and aluminum phantoms.

ISSN:0269-3127    

DOI:10.1118/1.595157

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An investigation has been undertaken of the effect of flow on NMR images. We term the new technique of flow analysis examined here, NMR rheotomography. Images were acquired through use of a spin–echo technique. They were of doped water passing through a glass U‐tube, simulating a small vein, which fitted into the magnetic field gradient and radiofrequency coils of an NMR imaging apparatus. For the case of the inlet arm of the U‐tube, the flow images displayed an intensity distribution which compared favorably to a simple physical model based on laminar flow. The intensity distribution observed for the outlet arm was explained through consideration of the perturbation to flow introduced by the U‐bend. Generally, the results of the feasibility study indicate that (a) the NMR spin–echo image is modified by flow in a predictable manner, (b) display of the velocity profile is possible, (c) through use of a U‐tube some separation can be demonstrated within a single image of the twin factors, proton incursion into the image “slice” and proton dephasing, which generally combine to modify the flow image intensity, and (d) discrimination between different modes of flow may be possible by NMR CT. The clinical potential suggested by the results is discussed; NMR rheotomography may prove to be particularly useful for the noninvasive diagnosis of structurally‐originating cardiovascular defects.

ISSN:0269-3127    

DOI:10.1118/1.595158

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A radioisotope excited XRF method based on the use of five 10‐mCi125I point sources arranged in a newly designed configuration is applied to the bromine determination in very small blood serum samples (⩽50 μL). The possibility of determination of 0.05 μg Br in blood serum during 100 s counting time with a standard deviation of ±10% is described. The simplicity and low cost of the present method are major advantages over the other nondestructive methods and render it suitable for medical laboratories.

ISSN:0269-3127    

DOI:10.1118/1.595159

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The hardening effects of hydrogenous and nonhydrogenous filters on ap(66)Be(49) neutron beam have been investigated. It was found that all materials studied, Teflon, aluminum, lead, steel, and polyethylene, harden the neutron beam, albeit polyethylene to a greater extent. Relationships were found to exist between the attenuation of a filter and its hardening effect, and also between the build‐up characteristics and the depth for half‐maximum dose of the hardened beams.

ISSN:0269-3127    

DOI:10.1118/1.595081

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Field shaping wedges made of Teflon, aluminum, steel, and lead were designed for ap(66)Be(49) therapeutic neutron beam. Design considerations, expectations, and actual measured changes in dose profiles are presented.

ISSN:0269-3127    

DOI:10.1118/1.595082

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A photon scattering method for measuring the trabecular bone mineral density (TBMD)in vitrois described. This method involves the measurement of the ratio of coherent to Compton 90° scattered photons from Am–241 by using a narrow beam geometry with an intrinsic germanium detector. The feasibility of using smaller scattering angles for better counting efficiency and the associated problems in their application forin‐vivomeasurements were investigated. Calibration of the system with fresh trabecular bone samples showed a linear relationship between the coherent to Compton ratioRof the detected counts and the TBMD (r= 0.94). The effect of the overlying soft tissue on theRratio was significant while the effect of self‐attenuation by the trabecular bone itself and the cortical layer was negligible. It was found that the marrow fat content could alter the value of theRratio. Our results show that for a 10% increase in the fat content in the interstices of the trabecular bone there is a 2.5% decrease in theRratio. This technique together with soft tissue corrections will enable us to measure the TBMD of the calcaneumin vivo, assuming a small variation in the trabecular fat content. The estimated absorbed dose to the bone marrow is about 139 mrad.

ISSN:0269-3127    

DOI:10.1118/1.595073

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two models for thick target emission spectra, one using constant depth of Bremsstrahlung and one using continuous slowing‐down of electrons and multiple thin targets, were used to predict filtered emission spectra at various angles with respect to the central axis. Integration of these spectra and comparison against measured heel effect gave excellent agreement for tungsten targets with take‐off angles of 7° and 10° at 60 and 100 kVp when the angular anisotropy of Bremsstrahlung was taken into account. Results for a 17.5° target at 60 kVp were poor, possibly indicating a need for a better model of Bremsstrahlung angular dependence.

ISSN:0269-3127    

DOI:10.1118/1.595074

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In the treatment of tumors using interstitial implants of radioactive seeds, the accuracy of computed dose distributions depends upon the accuracy with which the three‐dimensional source geometries are reconstructed from radiographs of the implants. The effect of geometric reconstruction errors in iridium–192 seed implants were studied, using tumor dose as the measure. Tumor dose was defined as the average dose around the periphery of the treatment volume. Three ideal mathematical implants and five actual patient implants were used. The implants were distorted by randomly moving a specified number of seeds a specified distance. Tumor doses were directly calculated for the ideal implants. For the actual implants, isodose distributions were plotted and were read by a radiotherapist. For both types of implants, percentage errors in the tumor doses were calculated for the distorted reconstructions relative to the correct reconstructions. It was found that the tumor dose was accurate to within 5% if all the seeds were reconstructed to within 0.5 cm of their actual positions. Furthermore, up to 5% of the seeds could be mismatched between films, or otherwise incorrectly reconstructed, with position errors as large as 20 cm, and not change the tumor dose by more than 5%.

ISSN:0269-3127    

DOI:10.1118/1.595075

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY