摘要:

In 1912 many physicians considered radium a cure for cancer but few could afford it because radium cost a fortune. William Duane, Marie Curie's associate, discovered that “radium milk” (later officially named radon) was easier for physicians to use. In 1915 he built Boston's first radium “cow” and thousands of patients were treated with its “milk.” But because radon decayed with high‐energy alpha emissions, it also became the first “atom smasher.” Making radon available to nuclear scientists was one of America's major contributions to an evolving nuclear age.

ISSN:0094-2405    

DOI:10.1118/1.596947

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597159

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is well recognized that both trabecular bone density and structure affect the overall bone quality and strength. In this study the aim is to quantify variations in the structural network of trabeculae using the concepts of fractal geometry. Fractal objects are objects that appear statistically similar over a range of scales. Typically fractals do not have smooth surfaces, but instead have rather complex structures with highly convoluted surfaces. Associated with every fractal is a characteristic dimension, called the fractal dimension. In this study, techniques of fractal analysis were used to characterize the trabecular bone matrix on digital images acquired by quantitative computed tomography (QCT) of dried excised human vertebral bodies (density ranging from 76–220 mg/cc) and photomicrography of transiliac crest biopsies. An automatic boundary tracking algorithm was used to identify the trabecular‐bone and bone marrow interface, and a box‐counting algorithm was used to estimate the fractal dimension of the trabecular boundary. Using this technique for fractal objects, if the boundary being analyzed is covered with boxes of differing sizes, ε, then the number of boxesNrequired to cover the surface increases indefinitely according to the relationN=ε−D, whereDis the fractal dimension. Using this relationship in a preliminary study on five specimens we have found that the trabecular‐bone boundary is fractal in nature. Using photomicrographs of iliac crest biopsies, it is found that the fractal dimension changes with the fractional trabecular bone content. These results suggest that fractal analysis may be useful in distinguishing osteoporotic bone structure from normal. Analysis and technique‐dependent factors that affect the estimated fractal dimension are also discussed.

ISSN:0094-2405    

DOI:10.1118/1.596948

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dynamic range and the number of gray levels, Γs, required for digital mammography has been evaluated using an energy transport model. The effects of molybdenum (Mo) and tungsten (W) target spectra and the energy‐dependent attenuation by elemental filters, breast tissue, and a phosphor screen were included in the model. For detectors with ideal optical coupling and no inherent detector noise, 3100 gray levels are discernable (requiring 12 bits per pixel), assuming a 40 kVp, W target spectrum (1.0 mm Al filtration), a mean glandular dose to a 5 cm thick breast of 0.6 mGy, and an ideal observer with a 5 mm diam viewing aperture. The effects of inherent detector noise and realistic coupling efficiencies on Γswere also examined. For the 40 kVp, W spectrum, a detector with total coupling efficiency of 16 electrons (e−) per x‐ray interaction and a dynamic range of 3000 (maximum carrier signal of 1.93×105e−/pixel and inherent detector noise of 64e−pixel) would decrease the number of gray levels that could be resolved by only 2% compared to a detector with ideal coupling and no inherent noise. A detector with a total coupling efficiency of 2.0 electrons per x‐ray interaction and a dynamic range of 240 (maximum carrier signal 2.41×104e−/pixel and inherent detector noise of 100e−/pixel) would reduce the number of gray levels by 26% for the 40 kVp spectrum. On the basis of dynamic range, W spectra are preferable for digital mammography, since Mo spectra yielding the same signal‐to‐noise ratio require a detector with dynamic range twice as large, and with a 30% greater saturation signal. When no scatter rejection method is used, scattered radiation over a 254 cm2imaging field reduces the number of discernable gray levels by 23% for a 5 cm thick breast and 34% for an 8 cm thick breast.

ISSN:0094-2405    

DOI:10.1118/1.596949

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Helical CT is an important recent development in x‐ray CT. In helical CT, planar projection sets are synthesized from raw projection data via interpolation. Among various interpolation schemes, linear interpolation is usually preferred due to its efficiency and performance. In this paper, image noise variance is derived for typical helical CT linear interpolation techniques, including the full scan (FS), under‐scan (US), full scan with interpolation (FI), half‐scan (HS), half‐scan with interpolation (HI) and half‐scan with extrapolation (HE) methods. Image noise deviation ratios of helical CT to conventional 360° reconstruction (CR) are tabulated. These are consistent with previously reported simulation results. The theoretical results provide further understanding of helical CT noise performance. It is shown that helical CT image noise deviation is independent of transaxial position, proportional to the raw projection noise deviation, and not affected by the fan angle (approximately for the HE method). Also, helical CT image noise variance is proportional to the area under the square of the reconstruction filter.

ISSN:0094-2405    

DOI:10.1118/1.596950

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Video‐based systems for on‐line portal imaging utilize a metal plate coated with Gd2O2S phosphor at a typical thickness of 500 mg/cm2. A new screen design is proposed wherein the conventional flat phosphor coating is replaced by a much thicker phosphor layer (1000–2000 mg/cm2) penetrated by either lineal grooves or pyramidal holes comparable to the system pixel size. By increasing the surface area of the phosphor, the grooves or holes allow light from deep layers of the phosphor to escape by a process of internal reflection. In addition, the escaping light is strongly forward peaked, improving optical coupling to the video camera. The processes by which grooved screens intensify light output have been modeled in a simple computer program that gives approximate agreement with experiment. Prototype screens have been constructed that provide several times the forward light output of flat screens, and that improve DQE(f) in light photon limited systems for spatial frequencies below 0.4 mm−1.

ISSN:0094-2405    

DOI:10.1118/1.597160

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A conventional x‐ray image intensifier (XRII) has been modified to enable the field of view (FOV) to be varied continuously, by adjusting the potentials at the focusing electrodes. The benefit, to system resolution, from decreasing the FOV has been characterized by measuring the modulation transfer function (MTF) of the XRII coupled to a high‐resolution photo‐diode array (PDA), at a number of different FOVs achieved either by electronic or optical zooming. Electronic zooming of the XRII from FOV=24 cm to FOV=10 cm led to an increase inf0.1(the frequency at which MTF=0.1) from 1.41 to 3.05 mm−1, while optical zooming increasedf0.1from 1.41 mm−1only to 1.88 mm−1. It is proposed that the advantage, with respect to resolution gain, of electronic zooming over optical zooming was realized only when the XRII limits system resolution. The MTF of the XRII coupled to a video camera, with lower resolving power than the PDA, was measured at different FOVs to show that using electronic zooming is only marginally beneficial when the optical detector and the XRII contribute equally to the resolution degradation. However, when a higher‐resolution optical detector is used, electronic zooming always yields a greater gain in resolution.

ISSN:0094-2405    

DOI:10.1118/1.596951

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A computerized scheme for the automated detection of clustered microcalcifications from digital mammograms is being developed. This scheme is one part of an overall package for computer‐aided diagnosis (CAD), the purpose of which is to assist radiologists in detecting and diagnosing breast cancer. One important step in the computer detection scheme is to group or cluster microcalcifications, since clustered microcalcifications are more clinically significant than are isolated microcalcifications. Previously a “growing” technique in which signals (possible microcalcifications) were clustered by grouping those that were within some predefined distance from the center of the growing cluster was used. In this paper, a new technique for grouping signals, which consists of two steps, is introduced. First, signals that may be several pixels in area are reduced to single pixels by means of a recursive transformation. Second, the number of signals (nonzero pixels) within a small region, typically 3.2×3.2 mm, are counted. Only if three or more signals are present within such a region are they preserved in the output image. In this way, isolated signals are eliminated. Furthermore, this method can eliminate falsely detected clusters, which were identified by a previous detection scheme, based on the spatial distribution of signals within the cluster. The differences in performance of the CAD scheme for detecting clustered microcalcifications using the old and new clustering techniques was measured using 78 mammograms, containing 41 clusters. The new clustering technique improved the detection scheme by reducing the false‐positive detection rate from 4.2 to 2.5 per image, while maintaining a sensitivity of approximately 85%.

ISSN:0094-2405    

DOI:10.1118/1.596952

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The contribution of penetrating photon energy to tumor dose is usually ignored because of the difficulty in calculating absorbed fractions and because it is frequently assumed to represent a small proportion of the total energy. The MABDOSE software—written explicitly to simulate photon transport for the calculation of penetrating radiation absorbed fractions—was used to simulate the131I photon spectrum originating from the liver, spleen, and whole body source organs. Specific absorbed fractions were calculated for tumors of radius 1.0, 1.5, and 2.0 cm placed near the liver and spleen in the Reference Man geometry. Cumulated activities were estimated using values reported from the literature. Dosimetry estimates from the combined cumulated activity and specific absorbed fractions indicate that neglecting the photon contribution underestimates the tumor dose by 10%–25%.

ISSN:0094-2405    

DOI:10.1118/1.596953

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this paper, a unified framework of iterative algebraic reconstruction for emission computed tomography (ECT) and its application to positron emission tomography (PET) is presented. The unified framework is based on an algebraic image restoration model and contains conventional iterative algebraic reconstruction algorithms: ART, SIRT, Landweber iteration (LWB), the generalized Landweber iteration (GLWB), the steepest descent method (STP), as well as iterative filtered backprojection (IFBP) reconstruction algorithms: Chang's method, Walters' method, and a modified iterative MAP. The framework provides an effective tool to systematically study conventional iterative algebraic algorithms and IFBP algorithms. Based on this framework, conventional iterative algebraic algorithms and IFBP algorithms are generalized. It is shown from the algebraic point of view that IFBP algorithms are not only excellent methods for correction of attenuation (either uniform or nonuniform) but are also good general iterative reconstruction algorithms (they can be applied to either attenuated or attenuation‐free projections and converge very fast). The convergence behavior of iterative algebraic algorithms is discussed and insight is drawn into the fast convergence property of IFBP algorithms. A simulated PET system is used to evaluate IFBP algorithms and LWB in comparison with the maximum likelihood estimation via expectation maximization algorithm (MLE‐EM) and the filtered backprojection (FBP) algorithm. The simulation results indicate that for both attenuation‐free projection and attenuated projection cases IFBP algorithms have a significant computational advantage over LWB and MLE‐EM, and have performance advantages over FBP in terms of contrast recovery and/or noise‐to‐signal ratios (NSRs) in regions of interest.

ISSN:0094-2405    

DOI:10.1118/1.596954

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY