摘要:

We propose the use of conic and cubic surface equations (surfaces of second and third order) to directly approximate the dual‐energy equations (the integral equations for the dual‐energy log‐signal functions, i.e., the negative logarithms of the relative detector signals, considered as functions of the basis‐material component thicknesses of the object) and especially their inverses. These types of surface equations require a minimum number of calibration points, and their solutions are smooth, monotonic functions with the correct linear asymptotic behavior. The accuracy of this method is investigated and compared to that of conventional polynomial approximations, both for simulated and real calibration data, taken from two split‐detector systems. These systems provide a more stringent test of our method than comparable dual‐kVp systems, due to the greater nonlinearity of their log‐signal and inverse functions. For these systems, we show that direct approximation of the inverse dual‐energy equations using the simple eight‐term rational form of the conic surface equation provides an extremely fast decomposition algorithm, which is accurate, robust in the presence of noise, and which can be calibrated with as few as 9 calibration points, or robustly calibrated, with a built‐in accuracy check, using only 16 calibration points. Also, we show that extreme accuracy of approximation (to within less than 10−6in log‐signal and 1 μm in material thickness) is theoretically attainable using the eighteen‐term form of the cubic surface equation, which has a closed‐form analytic solution. Finally, we consider the effects of noise on calibration accuracy, and derive simple formulas which relate the true and apparent root‐mean‐square (rms) accuracies. These formulas then allow the comparison of the true rms calibration accuracies of various surface approximations, considered as functions of the total calibration heat loading of the x‐ray tube.

ISSN:0269-3127    

DOI:10.1118/1.596512

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We are developing an automated method for determining a number of parameters related to the size and shape of the heart and of the lungs in chest radiographs. In order to obtain standard patterns of the cardiac shadow as “gold standards,” four radiologists traced their best estimates of the entire contour of the heart, including the largely invisible inferior margin, on 11 radiographs. These contours were analyzed by Fourier transform, and the results were used as a guide to obtain a shift‐variant cosine function which was applied to the prediction of the cardiac contour by fitting a limited number of detected heart boundary points. These points were obtained from analysis of edge gradients in two orthogonal directions. A simple observer study indicated that the contours of the heart shadows computed for 60 chest radiographs were generally acceptable to radiologists for estimation of the size and area of the projected heart. We also detected the rib cage and the edges of the diaphragm, which enabled us to determine the projected thoracic area. From these results, we calculated the cardiothoracic ratio and other parameters, such as the ratio of the projected heart area to the projected thoracic area.

ISSN:0269-3127    

DOI:10.1118/1.596513

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A series of Monte Carlo simulations have been performed in order to investigate the feasibility of constructing a time‐of‐flight transillumination imaging system that could be used as an effective tool for screening for breast disease. The conceptual design of such a system is described, and simulated images are presented that demonstrate its likely performance. It is found that, whereas the spatial resolution achievable with such a system is only dependent upon its temporal resolution, the scattering characteristics of the tissue being imaged will strongly affect the ultimate imaging performance of such a system.

ISSN:0269-3127    

DOI:10.1118/1.596514

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The results of a numerical simulation indicate that scanning equalization radiography (SER) provides the opportunity to use film/screen combinations of significantly higher contrast than currently practical in conventional chest radiography. These calculations which attempt to maximize overall image contrast while minimizing areas of over and under exposure demonstrate that optimum film gradient varies inversely with scanning beam area. Furthermore, we show that in an SER system which we have optimized for chest imaging that the peak film gamma gradient can be increased from that typical of conventional films (3.0–3.5) to approximately 7 with advantage.

ISSN:0269-3127    

DOI:10.1118/1.596515

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We present a solution to Maxwell's equations which describe the inductive interaction between an irregularly shaped sample in a magnetic resonance imaging rf coil. It is demonstrated that the inductive interaction between the coil and sample can induce charge densities on the sample surface and that these charge densities lead to observable changes in coilQ, resonant frequency offset and sample power dissipation.

ISSN:0269-3127    

DOI:10.1118/1.596516

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In addition to formidable theoretical obstacles that a proposed solution to the inverse electrocardiology problem must overcome, there are great practical difficulties in establishing its accuracy in actual clinical application. However, the recent appearance of two fundamentally independent treatments of the inverse problem raises the possibility that they may be used in tandem to help establish their individual accuracy. Thus, if the two methods give incompatible results in application then one of the methods must be inaccurate. Conversely, if the two methods give compatible results then the accuracy of both methods is supported (for the particular quantities measured) subject only to the validity of the assumptions common to both methods. We have compared results from the application of a quantitative “integral equations based” method with that of a qualitative “differential topology inspired” approach in three healthy volunteers. The output examined consists of measurements of the times of appearance of epicardial sources (depolarization wavefront breakthroughs) and sinks of the ventricular surface activation map. The extent of agreement on source/sink times between the methods was consistent with the resolution limits imposed by noise and discrete sampling on derivatives of the electrocardiogram. When events defined by the integral method occurring within 2 ms of each other are grouped together (and their times averaged), the two methods agreed on source/sink times to within 3 ms except in two instances where they differed by 5 ms. The measurements made by the two methods were found to be highly correlated (R=0.95). While the quantitative method alone rests on a variety of modeling and procedural assumptions, the only assumption common to both methods is the uniform dipole layer hypothesis. Thus, subject to this single assumption, one may infer the accuracy of the quantitative method in healthy individuals for epicardial source/sink times. On the other hand, coupling with the far more detailed quantitative method allows further useful characterization of the output of the qualitative method. In particular, this study provides convincing evidence that the major deflections of the spatial velocity electrocardiogram are coupled to particular epicardial sources and sinks, as has been previously conjectured on theoretical grounds. This raises the possibility of bedside evaluation of these epicardial events.

ISSN:0269-3127    

DOI:10.1118/1.596568

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new ultrasonically and thermodynamically tissue‐mimicking material is reported. The material is well suited for use in phantoms for testing ultrasound hyperthermia systems or related predictive models. Controlled convective heat transfer effects, mimicking to some extent perfusive heat transfer in tissues, can be instituted in the material with appropriate fluid sources and sinks. The material consists of closely packed agar spheres varying in diameters from 0.3–3.6 mm. The interstitial space between spheres is filled with 10%n‐propanol solution. The material has two practical advantages over the solid‐gel‐type tissue‐mimicking materials. The first advantage is that it allows rapid return of a hyperthermia phantom to thermal equilibrium following a heating test by rapid circulation of the perfusion fluid. The second advantage is that the material is in a “liquid” form. It can be easily siphoned in and out of phantom containers of any geometric shape for different purposes without change in its physical properties. Methods for measuring ultrasonic and thermodynamic properties of the material and the results of the measurements are reported. The physical parameters measured are the intensity attenuation and absorption coefficients, the ultrasonic speed, the thermal conductivity, specific‐heat capacity and the mass density. Temperature measurements in a hyperthermia phantom made of the material are also reported.

ISSN:0269-3127    

DOI:10.1118/1.596517

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The fabrication of retracted missing tissue compensators which account only for missing primary attenuation produces compensation which is less than optimum. Experimental results can be accurately reproduced with theoretical calculations based on primary and first‐order scattered radiation. This method of analysis may be extended to predict the shape of a compensator which will produce the desired dose distribution at a given depth in phantom. An optimized compensator is constructed based on these theoretical considerations and the analysis of its performance is presented.

ISSN:0269-3127    

DOI:10.1118/1.596518

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method to achieve the optimal design of radiation therapy compensating filters in multibeam situations is proposed. Dose distribution is optimized such that it approaches the prescribed value in the target zone, is minimized everywhere else, and does not exceed designated limits at vulnerable regions. Field shapes, sizes, and weights, together with filter thicknesses that take into account missing tissues and body heterogeneities, are calculated as a result of this process. The dose distribution, after the application of the calculated filters and beams, is also predicted. The method is rigorous, and its accuracy depends only upon the accuracy of the applied dose calculation method. Examples are discussed and verified using data obtained from Theraplan 300L treatment planning system.

ISSN:0269-3127    

DOI:10.1118/1.596519

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In a series ofinvivoexperiments on five adult canines, a small cylindrical permanent magnet (∼5‐mm diameter×5 mm long) was magnetically moved under fluoroscopic guidance from an occipital‐lobe burr hole to a predetermined destination within the brain and then removed. On three of the animals, dorsal and temporal skull markers were used to establish a coordinate system against which the motions of the seed were referenced. These procedures were sufficiently accurate to permit the guided motion of the seed along nonlinear paths within the brain, including traversal of the midline through the corpus callosum. For removal, the seed could be steered either to a frontal lobe location for extraction through an auxiliary burr hole, or back to the same burr hole through which it had been inserted. This article discusses the way in which stereotactic motions were obtained, the performance limits of the instrumentation and the precision of motion achieved.

ISSN:0269-3127    

DOI:10.1118/1.596520

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY