摘要:

Increasingly, hemibody and total body irradiation are used to treat systemic and disseminated diseases. Certain dosimetry considerations are common to conventional treatments at short source‐to‐axis distances and large field treatments at extended treatment distances. These dosimetry considerations include: irradiation geometry, dose rates or outputs in the treatment plane, radiation intensity uniformity, confirmation of the dose ratio parameters such as percentage depth doses, tissue air ratios, tissue maximum ratios, and tissue phantom ratios, the surface doses and the doses in the region of build‐up to maximum ionization, the exit doses, the dose prescription point or points, dose uniformity in the patient, and dose confirmation and verification. Some radiotherapists are reluctant to treat patients using large fields at extended treatment distances because they are unfamiliar with the dosimetry of such treatments. This article reviews the dosimetry and radiobiology of large field treatments at the extended treatment distances required by hemibody or total body irradiation, as performed with conventional single source teletherapy units.

ISSN:0269-3127    

DOI:10.1118/1.595091

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The statistical quality of conventional nuclear medical imagery is limited by the small signal collect through low‐efficiency conventional apertures. Coded‐aperture imaging overcomes this by employing a two‐step process in which the object is first efficiently detected as an “encoded” form which does not resemble the object, and then filtered (or “decoded”) to form an image. We present here the imaging properties of a class of time‐modulated coded apertures which, unlike most coded apertures, encodeprojectionsof the object rather than the object itself. These coded apertures can reconstruct a volume object nontomographically, tomographically (one plane focused), or three‐dimensionally. We describe a new decoding algorithm that reconstructs the object from its planar projections. Results of noise calculations are given, and the noise performance of these coded‐aperture systems is compared to that of conventional counterparts. A hybrid slit‐pinhole system which combines the imaging advantages of a rotating slit and a pinhole is described. A new scintillation detector which accurately measures the position of an event in one dimension only is presented, and its use in our coded‐aperture system is outlined. Finally, results of imaging test objects and animals are given.

ISSN:0269-3127    

DOI:10.1118/1.595171

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The 40‐MeV direct beam bremsstrahlung irradiation facility of the Sagittaire clinical electron accelerator was used to produce197Ir via the reaction198Pt(γ,p)197Ir. Gamma emitted in the Ir decay were detected with an intrinsic germanium system and multichannel analyzer. 106 of the spectrum peaks could be analyzed for energy, relative intensity, and half‐life. Half‐life results indicate that197Ir decays from two isomers, one with half‐life 6.1±0.1 min and another with half‐life 8.9±0.1 min.

ISSN:0269-3127    

DOI:10.1118/1.595172

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The interpretation of two‐dimensional radiographs or computerized tomography (CT) or ultrasound (US) sections with inferences made about three‐dimensional anatomy is the foundation of diagnostic radiology. A large variety of estimation methods which can be applied to two‐dimensional CT/US sections to estimate their three‐dimensional properties is presented. The techniques are easily utilized in a research or clinical setting, do not require computerization, and can provide statistically accurate three‐dimensional information based on two‐dimensional sampling with a minimum of effort. Although the examples are directed at diagnostic radiology, the estimation procedures may be employed in analogous fields such as microscopy or section anatomy.

ISSN:0269-3127    

DOI:10.1118/1.595173

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper extends the results of Part I to estimates of three‐dimensional anatomy and morphology obtained form measurements on a projection plane. Ambiguities and overlap obscuration in projections are discussed. Estimates of vascular (angiography) network geometry are easily obtained and applications to particle volume, surface area, spacing, and number are possible.

ISSN:0269-3127    

DOI:10.1118/1.595095

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In dose calculations for external beams, Batho's correction factor applies to internal heterogeneities that cover the entire cross section of the beam. This study examines the possibility of combining Batho's correction factor with a scatter summation method, akin to that of Day, for heterogeneities that cover only part of the beam and for arbitrarily located points. The results of such calculations for beams of60Coγ photons are compared to Monte Carlo calculations by Webb and Fox and experimental data by Sontag. It is concluded that the technique, with some exceptions, yields results that are superior to the simple Batho correction factor.

ISSN:0269-3127    

DOI:10.1118/1.595174

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An ion‐chamber gas mixture having an atomic composition similar to that of A150 plastic has been tested experimentally. This “A150 gas”, air, and methane‐based TE gas were each flowed through A150 plastic‐walled ion chambers of various sizes, and irradiated with60CO γ rays and 14.8‐MeV neutrons, separately administered. Observed ionization was measured and corrected for recombination losses. Best values forW̄A150, averaged over all chamber sizes, were 26.2 eV/ion pair for60CO γ rays and 28.2 eV/ion pair for 14.8‐MeV neutrons.

ISSN:0269-3127    

DOI:10.1118/1.595211

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new multiple‐scanning‐slit or scanning‐grid scatter‐reduction geometry consisting of coaxial rotating aperture (RA) cones is described and compared with other RA assemblies such as the rotating aperture wheel (RAW) device. A unique feature of the new design is that the geometric and rotational axes of the conical RA surfaces coincide and are collinear with the x‐ray focal spot. This arrangement of axes should provide the potential for greatly improved mechanical rigidity, higher rotational velocities, and the capability for static slit‐pattern alignment. The common rotational axis of the cone assembly is angled obliquely to the central x‐ray beam such that the intersections of the irradiated portion of the RA surfaces with the plane formed by the x‐ray central beam and cone axes are straight lines parallel to the film plane. This geometry is compatible with the standard source‐patient‐image‐receptor radiographic relationship and allows for variable source‐to‐image‐receptor distance.

ISSN:0269-3127    

DOI:10.1118/1.595224

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A limitation of the use of multiple scanning beams for scatter reduction in radiography is the possible creation of artifactual aperture patterns which can be caused: (1) by a synchronization between the pattern scanning frequency and the radiation waveform ripple frequency, or (2) by the fractional overlap of the scanning beams during the final beam pass. An analysis of the stages of artifact formation and the implications for various scanning beam approaches is presented.

ISSN:0269-3127    

DOI:10.1118/1.595060

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper addresses the physical probability of finding, by random chance of noise fluctuations, false‐positive events in the background field of view of a noisy image. The signal levels of these random events are characterized by a general noise power spectrum; if they are comparable in magnitude with that of a true target signal, averaged over an area equal to that of the true target signal, then the false‐positive events obscure the confidence of the true signal identification. The theory shows that the statistics of this phenomenon depend very strongly, and with distinct threshold behavior, upon the generalized power‐signal‐to‐noise ratio of the true target signal; the behavior depends only weakly upon other factors, e.g., the background field of view area examined with respect to the true target area. Evaluation of even the simplest model of detection confidence, based upon this theory, yields several immediate results. The predicted SNR at threshold is approximately 3.5, with a variation of ±0.5 from 10%–95% confidence‐of‐detection levels. The theory is applied also to systems where display or intrinsic signal detection properties, rather than system inputs, limit the statistics. In this case, the theory predicts, in agreement with experiments, contrast‐limiting effects of 1% contrast for the human eye vision system and 5% contrast for typical TV displays of scintigraphic images. When experimental detectability studies of numerous investigators all are converted to a uniform specification of the output power‐average‐signal‐to‐noise ratio, then the theoretical predictions here give an excellent description of all major aspects of the empirical results.

ISSN:0269-3127    

DOI:10.1118/1.595061

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY