摘要:

Most radiologists do not use texture information contained in the trabecular patterns of hand radiographs to diagnose erosive changes and demineralization due to systemic inflammatory diseases that affect the skeletal system. However, high‐resolution digitization achievable by a laser digitizer now makes it possible to access texture information that may not be perceived visually. We are studying the feasibility of computer‐assisted early detection of these processes with particular attention to patients with hyperparathyroidism. In this paper the methods used to extract a region of interest (ROI) for texture analysis are discussed. The techniques include multiresolution sensing, automatic adaptive thresholding, detection of orientation angle, and projection taken perpendicular to the line of least second moment. The methods were tested on a database of 50 pairs of hand radiographs. We segmented the middle and the index fingers with an average success rate of 83% per hand. For the segmented finger strips, we located ROIs on both the middle and the proximal phalanges correctly over 84% of the times. Texture information was collected in the form of a concurrence matrix within the ROI. This study is a prelude to evaluating the correlation between classification based on texture analysis and diagnosis made by experienced radiologists.

ISSN:0269-3127    

DOI:10.1118/1.597402

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The problem of accurate stereotactic localization and registration of targets in computed tomography (CT) data sets is addressed, in particular the effect of using a single transformation matrix to map voxel coordinates onto stereotactic coordinates. An algebraic approach to the calculation of stereotactic target coordinates in tomographic data acquired with conventional stereotactic localizers is presented. The volume transformation matrix (VTM) is discussed, which is useful for the registration of volumetric data sets, and also corresponds to the rigid body transformation matrix used in many so‐called frameless registration methods. The VTM can lead to accuracy degradation, in particular due to patient movement during scanning. Simulations were performed and CT data sets acquired with patients fitted with the CRW or the GTC stereotactic localizer were analyzed. Comparison of STM‐ and VTM‐derived stereotactic coordinates shows an average overall registration error of 0.1 mm for anesthetized patients and in the range 0.6–1.4 mm for nonanesthetized patient. Accuracy maps are described that enable the user to visualize the registration error in relation to the data. It is shown that the effect of fiducial point localization error and patient movement for VTM‐based localization is minimized when all available fiducials in the region of interest are used. The significance of these results is discussed, and methods are proposed to minimize these effects for frame‐based and frameless registration methods.

ISSN:0269-3127    

DOI:10.1118/1.597403

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An analytical theory of photon propagation and detection in single‐photon emission computed tomography (SPECT) for collimated detectors is developed from first principles. The total photon detection kernel is expressed as a sum of terms due to the primary and the Compton scattered photons. The primary as well as contributions due to every order of Compton scattering are calculated separately. The model accounts for the three‐dimensional depth dependence of the collimator holes as well as for nonhomogeneous attenuation. No specific assumptions about the boundary or the homogeneity of the attenuating medium are made. The energy response of the detector is also modeled by the theory. Analytical expressions are obtained for various contributions to the photon detection kernel, and the multidimensional integrals involved are calculated using standard numerical integration methods. Theoretically calculated projections and scatter fractions for the primary and the first through second scattering orders are compared with our own experimental results for a small cylindrical primary radiation source immersed at various positions in a uniform cylindrical phantom. Also, theoretically calculated scatter fractions for a small spherical (pointlike) source in a uniform elliptic phantom are compared with experimental and Monte Carlo simulation results taken from the recent literature. The results from the analytical method are essentially exact and are free from the inaccuracies inherent in the numerical simulation methods used to deal with the photon propagation and detection problem in SPECT so far. The method developed here is unique in the sense that it provides accurate theoretical predictions of results averaged over an infinite number of simulations or experiments. We believe that our theory enhances an intuitive understanding of the complex image formation process in SPECT and is an important step toward solving the inverse problem, that of reconstructing the primary radiation source distribution from the measured gamma camera projections.

ISSN:0269-3127    

DOI:10.1118/1.597201

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Under certain conditions, laser light incident on a target material can induce an explosive removal of some material, a process called laser ablation. The photomechanical model of laser ablation asserts that this process is initiated when the laser‐induced stresses exceed the strength of the material in question. Although one‐dimensional calculations have shown that short pulsed lasers can create significant transient tensile stresses in target materials, the stresses last for only a few nanoseconds and the spatial location of the peak stresses is not consistent with experimental observations of material failure in biological tissues. Using the theory of elasticity, analytical expressions have been derived for the thermoelastic stresses and deformations in an axially symmetric three‐dimensional solid body caused by the absorption of laser light. The full three‐dimensional solution includes three stresses, radial, circumferential and shear, which are necessarily absent in the simple one‐dimensional solution. These stresses have long‐lived components that exist for eight orders of magnitude longer in time than the acoustic transients, an important point when the details of dynamic fracture are considered. Many important qualitative features are revealed including the spatial location of the peak stresses, which is more consistent with experimental observations of failure.

ISSN:0269-3127    

DOI:10.1118/1.597202

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A fully automated two‐dimensional image registration technique based on cross correlation is presented. This technique is evaluated using magnetic resonance images of human brain. Results indicate that this algorithm is capable of accurately estimating both linear and angular offsets.

ISSN:0269-3127    

DOI:10.1118/1.597405

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597203

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597204

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1994.tb00963.x

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY