ISSN:0269-3127    

DOI:10.1118/1.597220

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper describes a method to perform reconstruction of vascular cross‐sectional images from a limited number of x‐ray angiographic cone‐beam projections. It is assumed that the projection data can be simplified by identifying blood vessels in each angiogram and removing signals due to other structures. Under these conditions, the x‐ray attenuation coefficient, μ, can be modeled as a binary variable having a value μ0within the vessel and “0” outside. The reconstruction is performed by minimizing a cost function using the method of simulated annealing. In this paper, we demonstrate that the introduction ofaprioriinformation allows one to reconstruct a sphere and a simulated branched vessel from three views with, respectively, 97% and 93% of voxels having correct values. The addition of a continuity constraint for the reconstruction of the branched vessel resulted in further reduction in the percentage of misplaced voxels. Calculations require from one to six hours of CPU time on a Sun SparcStation 2 computer for the cases investigated here. The effect of noise, “cooling” schedule, and number of views on the reconstruction are examined using simulated vessel projections. Modifications to our approach to accelerate the reconstruction are also discussed.

ISSN:0269-3127    

DOI:10.1118/1.597223

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The purpose of this investigation was to evaluate the potential of monoenergetic x‐ray sources to improve image quality and reduce patient dose compared to conventional tungsten anode x‐ray spectra. This was purely a computer simulation study. X‐ray spectra were generated using the Birch and Marshal spectral model, patient x‐ray transmission was calculated using Monte Carlo techniques, and a numerical method was developed for determining antiscatter grid performance. A 120 mg/cm2Gd2O2S intensifying screen was simulated for radiography and a 144 mg/cm2CsI image intensifier was simulated for fluoroscopy. The source of subject contrast that was simulated included tissue, calcium, and iodine targets which varied in mass thicknesses from 10 to 1000 mg/cm2. The figure of merit of the [contrast to noise ratio] /[integral dose]was used as a relative measure of dose utilization. Depending on the object thickness, monoenergetic x‐ray sources with a screen‐film detector exhibited a 1.4 to 2.4 improvement over tungsten anode spectra for iodine contrast, a 1.5 to 2.0 improvement for calcium imaging, and about a 1.4 to 1.6 improvement for tissue contrast. The thicker patients (30 cm) benefitted more than thinner (10 cm) ones. For the image intensifier as a detector, a 1.4–2.3 improvement factor was found for monoenergetic sources and an iodine signal object. For the practical range of radiographic imaging scenarios using present‐day detector technologies, monoenergetic sources may provide an improvement in dose utilization that is comparable to the improvements that can be expected with scanning slit devices over conventional antiscatter grids.

ISSN:0269-3127    

DOI:10.1118/1.597185

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simulation model of clustered microcalcifications superimposed on normal mammographic backgrounds has been developed and evaluated. A cluster is described according to its size, the number of microcalcifications it contains, and their density. A microcalcification is defined on the basis of its size, shape, location within the cluster, its contrast, and the distribution of the gray‐level values between pixels that belong to it. Gray‐level parameters are dependent on the surrounding tissue, which is also slightly modified. The main dependencies between the parameters are taken into account. All the parameters are randomly sampled using distribution laws determined from the statistical analysis of 408 real clusters containing a total of 8611 microcalcifications. Some statistical characteristics depend on the structural mammographic pattern on which the cluster is superimposed. The evaluation of the simulation model, performed by two radiologists on a test set of 100 real and simulated clusters, did not reveal any statistically significant difference between the simulated clusters and the real ones. This method provides clusters of microcalcifications with well‐defined characteristics and can be used to evaluate the accuracy of computerized detection methods.

ISSN:0269-3127    

DOI:10.1118/1.597186

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Last‐image hold (LIH) is used in x‐ray fluoroscopy systems as a convenience and dose savings feature. In the case of an image sequence, temporal filtering in the human visual system (HVS) reduces perceived noise. In the case of a constant, single image frame, this phenomenon is not present: the image looks noisier, and low‐contrast objects disappear. Using low‐contrast, stationary cylinder and disk phantoms in noise, perception of single frames are compared with that of conventional 30 acq/s continuous fluoroscopy (continuous). The dose of continuous is fixed at Q/acq, and the dose ofsingle‐framepresentations is varied in order to determine an “equivalent‐perception dose” for a paired‐comparison task. The equivalent‐perception dose depends upon the shape and size of an object. As cylinder diameter increases from 1 to 21 pixels, the equivalent‐perception dose decreases from 4.6 to 2.8 Q/acq. At equal equivalent‐perception dose values, the relationship between cylinder and disk diameters are determined; a cylinder diameter of 10 pixels is roughly equivalent to a disk diameter of 20 pixels. For interventional angiography, an average equivalent‐perception dose of ≊3.5 Q/acq for a single‐frame presentation is predicted. Thus processing by the HVS effectively reduces noise variance by a factor of 3.5, corresponding to aneffectiveaveraging time of 3.5 frames or 120 ms. Several variance reduction techniques are suggested to create an LIH frame having perception equal to the fluoroscopy sequence.

ISSN:0269-3127    

DOI:10.1118/1.597225

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597224

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597226

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A spatially variant technique for quantitative single photon emission computed tomographic (SPECT) image reconstruction using an artificial neural network (ANN) is presented. This network was developed to simultaneously compensate for collimator, attenuation, and scatter effects during the reconstruction process. The network was trained using a supervised scheme which implemented the generalized delta rule. Training ended once the mean‐squared error (MSE) between the ideal and reconstructed images converged to a minimum. After training, the ANN weights were held constant and could be used to reconstruct source distributions other than those used while training. In the absence of noise when only collimator effects were present, reconstruction of a Hoffman brain phantom had a 89% reduction in MSE compared to standard filtered backprojection. When collimator‐and‐attenuation and collimator‐attenuation‐and‐scatter trials were tested against filtered backprojection with Chang attenuation compensation, the corresponding ANN reconstructions demonstrated 85% and 86% decreases in MSE, respectively. With noise present, and with standard noise reduction filters implemented prior to reconstruction, the ANN reconstructions displayed up to a 50% decrease in MSE compared to filtered backprojection reconstructions for 200 000 count data. These results demonstrate that an ANN can be used to reconstruct SPECT images with improved quantitative accuracy.

ISSN:0269-3127    

DOI:10.1118/1.597167

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The biological effects of Auger‐electron‐emitting radionuclides can be as severe as those of alpha particles of high linear energy transfer. A great deal of effort has been expended in exploring the biological effects of Auger electron emitters. Much of this effort has been devoted to improving theoretical and experimental techniques required to calculate absorbed doses and correlate them with the observed biological effects. Given that the main purpose of dosimetry is to obtain a physical descriptor with which to correlate radiation toxicity, then nowhere is this challenge greater than when biological specimens are subject to Auger electron cascades. The dense shower of short‐range Auger electrons released by radionuclides, which decay by electron capture or internal conversion, results in biological damage that is highly dependent on the location of the decay site within the cell. In this report, different approaches to Auger electron dosimetry are described and compared. Methods to calculate the absorbed dose from Auger electron emitters at the DNA, cellular, multicellular, and organ levels are described as they relate to the biological effects. The concept of a radiation weighting factor for Auger electrons to be used in the calculation of equivalent dose is reviewed. The importance of subcellular distribution of Auger emitters in determining the biological effects of these radionuclides is emphasized and incorporated into the equivalent dose formalism. The Task Group recommends that a preliminary radiation weighting factor of 10 be used for deterministic effects of Auger electrons, and a value of 20 for stochastic effects.

ISSN:0269-3127    

DOI:10.1118/1.597227

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Detector geometry, spatial sampling, and more fundamentally, positron range and noncollinearity of annihilation photon emission define Positron Emission Tomography (PET) spatial resolution. In this paper, a strong magnetic field is used to constrain positron travel transverse to the field. Measurement of the spread function from a 500 μm diameter68Ga impregnated resin bead shows a squeezing of the full width at half maximum (FWHM) by a factor of 1.0, 1.22, 1.42, and 2.05, at 0, 4.0, 5.0, and 9.4 Tesla, respectively. The full width at tenth maximum (FWTM) decreases by a factor of 1.0, 1.73, 2.09, and 3.20, at 0, 4.0, 5.0, and 9.0 Tesla, respectively. Acquiring a PET image in a magnetic field should significantly reduce resolution loss due to positron range.

ISSN:0269-3127    

DOI:10.1118/1.597178

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY