ISSN:0269-3127    

DOI:10.1118/1.597544

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A water‐filled head phantom that is designed for use in boron neutron capture therapy is described. The shape of this ellipsoidal phantom, based on the Snyder head model, and its composition are designed to simulate the neutron slowing down properties of the human skull and brain. Small ion chambers or activation foils can be placed in many locations within the phantom volume. This permits accurate three‐dimensional mapping of all relevant dose components and use of these dose contours for beam development as well as for benchmarking of computer‐based patient treatment codes.

ISSN:0269-3127    

DOI:10.1118/1.597545

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Neutron penetration in tissue is a major limitation of thermal NCT, as such much work has centered upon the epithermal neutron beam in an effort to improve this situation. Further gains in neutron flux penetration, and thus therapeutic ratios, are possible if natural water is replaced with heavy water prior to therapy. Applying MCNP to a heterogeneous ellipsoidal skull/brain model, advantage depth and therapeutic depth parameters are studied as a function of heavy water replacement for a range of tumor to blood boron ratios. Both thermal (0.025 eV) and epithermal (2–7 keV) ideal neutron beams are analyzed. Using10B ratios in the range of documented human uptake, the thermal advantage depth improved by approximately 0.7 cm for 20% D2O replacement, however, the therapeutic depth increased by less than half this value. For the epithermal beam, both the advantage depth and the therapeutic depth increased by over 1 cm. Effects of heavy water replacement on10B requirements to therapeutically treat the midline of the brain are also evaluated.

ISSN:0269-3127    

DOI:10.1118/1.597585

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

As a follow‐on study to the feasibility of neutron capture therapy (NCT) with235U brachytherapy seeds, physical doses were calculated and measured for the radiation from a235U foil in a lucite phantom which was irradiated at the epithermal neutron irradiation port of the Brookhaven Medical Research Reactor. In addition, cell survival experiments were performed to obtain the relative biological effectiveness (RBE) for the neutron part of the radiation. The calculated absorbed doses agree with the measured ones. From cell survival experiments, it is deduced that the fission neutrons from the235U foil have a RBE of 3.0 while the fast neutrons in the beam have a RBE of 3.8. Also observed is that, with the cells 7 mm from the foil, a significant amount of absorbed dose comes from the β rays of235U fission events. This absorbed dose from β rays is a significant addition to the therapeutic dose. Due to the limited ranges of β rays in tissue, this absorbed dose is restricted to the vicinity of the foil. This is the first demonstration of β rays as part of NCT.

ISSN:0269-3127    

DOI:10.1118/1.597546

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597547

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In previous work it has been shown that a standard backpropagation neural network can be trained to reconstruct sections of single photon emission computed tomography (SPECT) images based on the planar image projections as inputs. In this study, it is demonstrated that an artificial neural network (ANN) trained on a series of simulated SPECT images or trained on a set of rudimentary geometric images can learn the planar data‐to‐tomographic image relationship for 64×64 tomograms. As a result, a properly trained ANN can produce accurate, novel image reconstructions but without the high computational cost inherent in some traditional reconstruction techniques. We also present a method of deriving activation functions for a backpropagation ANN that make it readily trainable for cardiac SPECT image reconstruction. The activation functions are derived from the estimated probability density functions (p.d.f.s) of the ANN training set data. The performance of the statistically tailored ANNs are compared with the performance of standard sigmoidal backpropagation ANNs, both in terms of their trainability and generalization ability. The results presented demonstrate that statistically tailored ANNs are significantly better than standard sigmoidal ANNs at reconstructing novel tomographic images based on a simulated SPECT image training set or a rudimentary geometric image training set. Neural network based image reconstruction has two potential advantages over conventional reconstruction methods. The first advantage is that ANNs can rapidly reconstruction tomograms. Secondly, the quality of the reconstructions produced are directly correlated to the quality of the images used to train the ANN.

ISSN:0269-3127    

DOI:10.1118/1.597586

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Bone mineral densitometry (BMD) is useful in predicting fracture risk, but, unfortunately, there is a significant degree of overlap in BMD measurements of patients who have a high risk of fracture and patients with a low risk of fracture. In this study, a method of characterizing trabecular bone structure in digitized radiographs of vertebrae is proposed and assessed. A significant correlation between bone “structure” and the compressive strength of vertebral bodies was found. The utility of the parameter for distinguishing between “weak” and “strong” bone samples was assessed using receiver operating characteristic (ROC) analysis. Using this analysis, the structural parameter produced an area under the ROC of 0.88±0.05, while a bone density measure produced an area of 0.79±0.07. The results suggest that the addition of a measure of bone structure to the conventional measures of bone density may prove useful in predicting the quality of bone when considering surgical or medical intervention for osteoporotic conditions.

ISSN:0269-3127    

DOI:10.1118/1.597548

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A computerized method for accurate determination of the ribcage boundary in chest images has been developed for use in computer‐aided diagnosis (CAD) schemes for automated detection of abnormalities such as the pulmonary lung nodules, pneumothorax, interstitial disease, cardiomegaly, and interval changes in clinical chest images. With our method, the average position of the top of the lung in the chest image is determined first. Top lung edges and ribcage edges are determined within search ROIs, which are selected over top lung cages and ribcages. Three polynomial functions are applied separately to yield smooth curves for top lung edges and right and left ribcage edges. The complete ribcage boundary is then obtained by smoothly connecting three curves. A total of 1000 radiographs were digitized to 1k×1k matrix size and a 10‐bit gray scale with a laser scanner and analyzed by our method. The subjective evaluation indicated that our method produced moderately to highly accurate results in approximately 96% of the 1000 cases examined.

ISSN:0269-3127    

DOI:10.1118/1.597549

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The principle of the kinestatic charge detector (KCD) for digital radiography depends on the synchronization of the scan velocity of a parallel plate drift chamber with the cation drift velocity. Compared with line‐beam scanners, this motion‐compensated imaging technique makes better use of the x‐ray tube output. A Frisch grid traditionally has been used within the KCD to minimize unwanted signal contributions from both cations and negative charge carriers during irradiation. In this work the charge induction process in a parallel plate geometry was investigated for the special case of the KCD. In the limit of infinite plates, the cathode charge density due to both cations and negative charge carriers increases quadratically in time for a kinestatically scanned narrow slit. In the KCD the cathode is segmented into an array of narrow electrodes, each aligned with the incident x‐ray beam. Our conformal mapping computation determined that the shape of the induced charge signal depends critically on Δx/w, the ratio of electrode width to drift gap. Our conclusion introduces the possibility of eliminating the Frisch grid from the KCD design because the value of Δx/wrequired for transverse sampling in the KCD is sufficiently low as to allow “self‐gridding” to take effect.

ISSN:0269-3127    

DOI:10.1118/1.597575

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A number of tests to evaluate the image transfer characteristics of laser film digitizers have been developed and these tests have been used to evaluate the performance of a Konica KFDR‐S laser film digitizer. These tests were designed to be simple in nature and to use materials readily available in clinical departments. The tests examined (i) the geometric accuracy of the laser film digitizer; (ii) the linearity of the digitizer's characteristic curve; (iii) the temporal/spatial response of the digitizer to abrupt changes in optical density; and (iv) the noise added by the digitizer to the digitized images. The tests were easy to use and could be performed in a short period of time. Results of the tests for the Konica laser film digitizer revealed several problems including minor geometric inaccuracies, structural noise added by a shading correction circuit in the digitizer, and spread of the laser beam before reaching the film plane, possibly caused by the f‐theta lens of the digitizer. The light spread made the optical density of small, high density structures measured by the digitizer dependent upon the optical density of the surrounding regions. Therefore, under some circumstances, film digitization with the Konica laser film digitizer could lead to erroneous results. In addition, our measurements suggest that these problems are not unique to the Konica digitizer. Thus, the testing of laser film digitizers is recommended, especially if the digitizer is to be used for quantitative measurements.

ISSN:0269-3127    

DOI:10.1118/1.597550

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY