摘要:

The microwave‐powered electron linear accelerator, or linac, is becoming the dominant radiotherapy treatment unit. Several technical advances, combined with attention to how patients are most effectively set up and treated, have led to continuing improvements in linac radiotherapy. This review describes: improvements in accelerator structures, widely variable energy linacs, microtrons, beam transport systems, and treatment head design.

ISSN:0269-3127    

DOI:10.1118/1.595617

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Videofluorography is the technique of obtaining radiographic images by photographing with a multiformat camera the television images produced during x‐ray image intensified fluoroscopy. This technique of producing images has received considerable clinical use, but the radiation exposure used has been in doubt. Experiments were performed to evaluate this radiation exposure. It was found that the imaging chain was reducing mottle by temporal integration. These results were tested by comparing the image quality of videofluorographs with that of photofluorographs. The blurring effect of motion on images was also evaluated as this is a factor which must be balanced against the improvement of mottle due to temporal integration. It was found that exposures used for videofluorography were of the order of 10 μR(2.58×10−9C kg−1) to the input of the x‐ray image intensifier.

ISSN:0269-3127    

DOI:10.1118/1.595488

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The charging characteristics of ionographic latent images are discussed in terms of the saturation characteristics of ionographic chambers. It is shown that latent images are essentially foil electrets with surface charge densities modulated by the x‐ray transmission through the object. Empirical methods for the calculation of saturation current densities and extrapolated electric fields are given. Both the saturation current density and the extrapolated field consist of two components: one, which is linear with the air gap thickness, is produced by primary photon interactions in the chamber‐sensitive volume; the other, exhibiting an exponential saturation, is attributed to photoelectrons backscattered from the polarizing electrode into the chamber‐sensitive volume. The minimum applied electric field needed for an optimized charge collection in the ionographic chamber is presented in terms of both the characteristic polarization time and the electret relaxation time. The maximum possible surface charge density in the latent image is calculated, first exhibiting direct proportionality with the polarizing voltageV0, and then following aV1/20dependence for voltages larger than the breakpoint voltage. The optimum exposure for latent image production is calculated. Exposures below this optimum give insufficient charge densities for subsequent development, while exposures above it degrade the image and eventually result in a uniform foil electret charged to its maximum theoretical value.

ISSN:0269-3127    

DOI:10.1118/1.595489

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Needle‐shaped thermoseeds have been manufactured from an alloy consisting of 70.4% nickel and 29.6% copper. The magnetic properties of the alloy were measured at various temperatures and from this the heating power produced by a thermoseed exposed to an electromagnetic induction field was computed as a function of the seed temperature. Calorimetric measurements were also performed. From these data, temperature distributions in simple tumor models assumed to be heated by an array of nickel–copper implants were computed. It was found that the nickel–copper implants produce substantially better temperature homogeneity than readily available constant power seeds, especially in tumors with unpredictable rates of blood perfusion or when the implant arrangement is not perfectly regular. Since such conditions are likely to be present in actual patients, the nickel–copper implants should be very useful in clinical hyperthermia.

ISSN:0269-3127    

DOI:10.1118/1.595490

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method of blood flow measurement is described that determines flow through a particular artery in absolute units or as a fraction of the cardiac output. For measuring flow as a percentage of cardiac output, no assumption about the vessel's cross‐sectional shape is necessary. If one assumes that the vessel's cross section is circular, then absolute flow determination is possible. All methods would require only a single intravenous injection of contrast material. Details of the theory of these methods are presented and alternative data analysis options are developed and discussed.

ISSN:0269-3127    

DOI:10.1118/1.595491

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The image artifacts characteristic of a scanning chest radiographic system are reviewed. The technique employs a pulsed beam of radiation swept in an overlapping raster pattern that can result in severe ripple and scan line artifacts with improper scanning parameters. A one‐dimensional treatment of the scanner geometry shows that the artifacts can be eliminated when the beam width is an integral multiple of interpulse spacing. An extension to a two‐dimensional analysis indicates that with the collimator geometries employed, artifact‐free images are not possible with a fixed x‐ray frequency but can be achieved when a variable frequency source is used. A treatment of the sensitivity for artifact formation shows that with proper choice of scanning parameters sizable errors in beam width can be tolerated without significant artifact formation.

ISSN:0269-3127    

DOI:10.1118/1.595492

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method has been devised to accurately measure the modulation transfer function (MTF) of digital x‐ray systems up to and, for undersampled systems, beyond the pixel Nyquist frequency ( fN). A phantom consisting of an array of parallel tungsten or similar wires is imaged, and discrete Fourier transforms of rows of pixel values are computed. Under suitable conditions of phantom orientation, wire diameter, wire spacing, and image magnification, the envelope of the modulus of the mean Fourier transform represents the system MTF. Experimental results extending beyond  fNare presented for an undersampled prototype digital chest x‐ray system and shown to be in reasonable agreement with predicted values. Employment of the method with other digital imaging modalities [i.e., computerized tomography (CT) scanners and nuclear magnetic resonance (NMR) units] is also discussed as well as error considerations and practical problems in implementing the method.

ISSN:0269-3127    

DOI:10.1118/1.595493

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A theoretical derivation for the point spread function (PSF) which describes the veiling glare in x‐ray image intensifiers (II) is presented. The PSF is dependent on two parameters which can be determined experimentally for a given II. An experimental investigation of the linearity of veiling glare phenomenon is undertaken. The experimental results indicate that veiling glare could be described as a linear process to a high degree of accuracy.

ISSN:0269-3127    

DOI:10.1118/1.595494

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A nuclear magnetic resonance (NMR) imaging system signal‐to‐noise calibration technique based on an NMR projection of distilled water in a cylindrical bottle is proposed. This measurement can characterize any arrangement of rf coils in any magnetic field as signal to noise per ml times root Hz. Inductive losses in a typical patient must be included in the calibration, and such losses can be simulated in a particular system by an externally attached resistor(s) appropriate to that system. Alternatively, an rf inductive damping phantom consisting of a conducting loop of wire containing an appropriate resistor is suggested that can be inserted into any NMR imaging coil to simulate subjectQdamping. The same resistor can be used, independent of the details of the coil construction. Furthermore, if the loop inductance is tuned out at each frequency with a series capacitor, then the same loop resistance will serve for all frequencies as a good approximation to human subject damping. This “projection method” signal‐to‐noise ratio is related to the conventional signal‐to‐noise ratio measured from a Lorentzian‐shaped spectral line as ψP=ψL[2/T2]1/2, where ψ stands for signal‐to‐noise ratio, subscriptsPandLstand, respectively, for the projection and “Lorentzian” methods, andT2is the transverse relaxation time of the spectral line used in the Lorentzian method.

ISSN:0269-3127    

DOI:10.1118/1.595484

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two sets of newly designed large wedge filters for field sizes up to 20×20 cm2have become commercially available for use with 4‐ and 6‐MV linear accelerators. Such field sizes are sometimes required to ensure adequate coverage in certain treatment techniques. This work reports base line data resulting from an investigation of the dosimetric properties of these wedges. Measurements of wedge angles, transmission factors, and beam hardening effects are described, and comparisons are made with the smaller standard wedges.

ISSN:0269-3127    

DOI:10.1118/1.595485

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY