摘要:

A chest radiographic equalization system using lung‐shaped templates mounted on filter wheels is under development. Using this technique, 25 lung templates for each lung are available on two computer controlled wheels which are located in close proximity to the x‐ray tube. The large magnification factor (≳10×) of the templates assures low‐frequency equalization due to the blurring of the focal spot. A low‐dose image is acquired without templates using a (generic) digital receptor, the image is analyzed, and the left and right lung fields are automatically identified using software developed for this purpose. The most appropriate left and right lung templates are independently selected and are positioned into the field of view at the proper location under computer control. Once the templates are positioned, acquisition of the equalized radiographic image onto film commences at clinical exposure levels. The templates reduce the exposure to the lung fields by attenuating a fraction of the incident x‐ray fluence so that the exposure to the mediastinum and diaphragm areas can be increased without overexposing the lungs. A data base of 824 digitized chest radiographs was used to determine the shape of the specific lung templates, for both left and right lung fields. A second independent data base of 208 images was used to test the performance of the templates using computer simulations. The template shape characteristics derived from the clinical image data base are demonstrated. The detected exposure in the lung fields on conventional chest radiographs was found to be, on average, three times the detected exposure behind the diaphragm and mediastinum. The simulated filter wheel equalization technique yielded detected exposure levels that were approximately equal in both the lung fields and the rest of the image. In addition to illustrating that the filter‐wheel equalization technique may be feasible for chest radiography, the simulations also provided important information for the mechanical construction of a filter wheel equalization system.

ISSN:0094-2405    

DOI:10.1118/1.597510

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A computer simulation study was performed to assess the errors due to x‐ray beam hardening in the fat and bone estimates of a post‐processing dual‐energy quantitative computed tomography technique. The “central” calibration method was employed in which calibration standards are inserted within a torso phantom of a size similar to that of the “patient.” Although beam hardening errors are reduced with this method, they still occur as a result of mismatches between thetorsophantom andpatientbody sizes. Two mismatch situations were investigated. In one, a singletorsophantom was used for all subject sizes (i.e.,one‐size‐fits‐all). In the other, closest matches were made from a set of three different sized torso phantoms (small, medium, and large). In all cases, the compositions of the calibration standards that were inserted into the torso phantoms consisted of bone, fat (glycerol trioleate), and an average fat‐free red marrow. Fifteen patient sizes were simulated ranging from 20 to 34 cm in diameter. There were 21 patients of each size. The vertebrae in these subjects contained known amounts of bone mixed in marrows of composition determined from chemical analyses of cadaver marrow samples. Vertebrae consisting of mixtures of the calibration standard materials were also studied. The computed effective x‐ray beam energies at the vertebra location for the various subject sizes ranged from 54.3 to 56.4 keV at 80 kVp and from 74.4 to 78.8 keV at 140 kVp. Maximum root mean square errors due to beam hardening were 1.2 mg/ml bone and 0.007 mass fraction fat forone‐sizefits‐allcentral calibration, and 0.31 mg/ml bone and 0.002 mass fraction fat forclosestmatchcentral calibration. These errors are two to four times smaller than themaximumbeam hardening errors obtained when the calibration standards and vertebrae are made of identical materials. This study indicates that for a realistic situation in which the compositions of the calibration standards and patient's marrow differ to some extent, the contribution of beam hardening to the overall error in fat and bone estimates is small. Furthermore, these errors can be minimized through the use of a central calibration technique in which a closest match to the patient's body size is made from a set of three different sized torso phantoms.

ISSN:0094-2405    

DOI:10.1118/1.597590

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper presents a new reference data set and associated quantification methodology to asses the accuracy of registration of computerized tomography (CT) and magnetic‐resonance (MR) images. Also described is a new semiautomatic surface‐based system for registering and visualizing CT and MR images. The registration error of the system was determined using a reference data set that was obtained from a cadaver in which rigid fiducial tubes were inserted prior to imaging. Registration error was measured as the distance between an analytic expression for each fiducial tube in one image set and transformed samples of the corresponding tube obtained from the other. Registration was accomplished by first identifying surfaces of similar anatomic structures in each image set. A transformation that best registered these structures was determined using a nonlinear optimization procedure. Even though the root‐mean‐square (rms) distance at the registered surfaces was similar to that reported by other groups, it was found that rms distances for the tubes were significantly larger than the final rms distances between the registered surfaces. It was also found that minimizing rms distance at the surface did not minimize rms distance for the tubes.

ISSN:0094-2405    

DOI:10.1118/1.597591

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Consensus has been developing over the past few decades on a number of measurements required for the laboratory assessment of medical imaging modalities. Nevertheless, understanding of the connection between these measurements and human observer performance in a broad range of tasks remains far from complete. Focusing primarily on projection radiography to provide concrete examples, this overview indicates areas in which consensus on methodology for physical image‐quality measurement has been established. Concepts such as “noise equivalent quanta” (NEQ) and “detective quantum efficiency” (DQE) have been found useful for normalizing physical measurements on an absolute scale and for relating those measurements to the decision performance of a hypothetical “ideal observer” that effectively performs decision tasks from the image data. The connection between ideal observer performance and human performance, as determined by receiver operating characteristic (ROC) analysis, remains to be understood for many clinically relevant tasks.

ISSN:0094-2405    

DOI:10.1118/1.597511

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Detection of radiotherapy field misalignments with electronic portal imaging devices requires the precise initial registration of the digital portal image with a reference image indicating the prescribed field alignment. Moreover, for real‐time continuous detection this registration must be performed rapidly—arguably within 250 ms. The quality of this registration is sensitive to the ability of the user to accurately identify corresponding anatomical landmarks in the image pair. To improve the accuracy of the registration and, ultimately, that of the field misalignment measurement, we have developed a sequential digital portal image registration method using both user‐identified anatomical matchpoints and image information. A first pass generates registration parameters from user‐provided matchpoint coordinates and explicitly accounts for the uncertainty in matchpoint identification. The second pass uses both the initial registration parameters and image information to further improve the registration quality by maximizing cross correlations between segments of the image pair. As this registration method does not use massive matrix/vector computations common to other algorithms, it is inherently faster and well‐suited for real‐time field placement error detection. On a platform representative of those controlling many commercial electronic portal imaging devices (486 CPU), this algorithm registers portal images in times of less than 6 ms per matchpoint with errors of less than 2% in magnification, 0.5° in in‐plane rotation, and less than 1 pixel dimension in in‐plane translation. As the algorithm assumes a rigid‐body geometry, it is sensitive to out‐of‐plane rotations. A quantitative analysis of this algorithm is presented, indicates its accuracy, and describes its sensitivity to out‐of‐plane rotations.

ISSN:0094-2405    

DOI:10.1118/1.597592

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The subject contrast of bony anatomy in megavoltage medical radiographs is very low, making detection of bony landmarks difficult if additional noise sources are introduced into the images. One source of noise, which is inherent to the x‐ray detection process, isx‐rayenergyabsorption noise. X‐ray energy absorption noise results from variations in the amount of energy deposited in the imaging detector per interacting x ray. These variations increase the noise content of the image. In this study, EGS4 Monte Carlo simulations of x‐ray interactions in metal plate phosphor screen detectors have been performed to determine the distribution of energy absorption events within the phosphor screen. From these “absorbed energy distributions (AEDs)”, the x‐ray energy absorption noise and the quantum absorption efficiency of the detector are determined. These calculations are performed for a range of detector thicknesses (0.1–4 mm) and x‐ray energies (0.1–10 MeV). A number of conclusions can be drawn from these investigations. (i) The x‐ray absorption noise reduces the detective quantum efficiency (DQE) of metal plate/phosphor screen detectors by as much as 50% at energies used in megavoltage imaging (1–10 MeV). (ii) It is important to include secondary particle (electron) transport in estimating the quantum absorption efficiency of these detectors. For instance, the quantum efficiency of a typical portal detector is approximately 2%, even though 4%–5% of the incident photons are attenuated. (iii) The metal “conversion” plate commonly used in megavoltage imaging enhances the DQE of the phosphor screen by increasing the quantum absorption efficiency and reducing the magnitude of the x‐ray absorption noise.

ISSN:0094-2405    

DOI:10.1118/1.597593

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The purpose of this study is to quantify and optimize the performance of an automatic portal image analysis procedure under clinical conditions and to compare the performance with that of human operators. A new method, based on analysis of variance, is introduced to quantify the clincial performance of portal image analysis tool in terms of systematic and random variations. The automatic portal image analysis procedure is based on chamfer matching. Two image enhancement techniques have been investigated in the automatic procedure: morphological top‐hat (MTH) transformation and multiscale medial axis (MMA) transformation. The performance of these enhancements was quantified and optimized as a function of filter size using images obtained from clinical treatment. All images used for this study were obtained from pelvic treatment fields by means of an electronic portal imaging device. The random variations in the alignment of AP fields are typically 0.5 mm and 0.5° (1 SD) for both the human operators and the optimized automatic analysis procedure. Random variations in the alignment of lateral pelvic fields are typically twice as large for all operators. MMA enhancement yields smaller random variations than MTH enhancement for lateral fields, but the differences are marginal for AP fields. The optimized automatic analysis procedure has a success rate ranging from 99% for AP large fields to 96% for lateral fields and 85% for AP boost fields. The accuracy of the method is comparable with the accuracy of the human operators for most investigated fields. For lateral boost fields and simultaneous boost fields, the random variations of the automatic analysis are typically two times larger than the variations of the human operators. Automatic analysis is 4 to 20 times faster than human operators yielding a large reduction in work load.

ISSN:0094-2405    

DOI:10.1118/1.597610

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Current intensive investigations of electronic portal imaging devices (EPIDs) have prompted their potential application to portal dosimetry. In this paper, the progress made in using a commercial liquid ion chamber matrix EPID for portal dosimetry is discussed. The pixel value of the liquid ion chamber element was calibrated against dose by exposing the imager to 6‐MV x‐ray beams of various intensities obtained with various thicknesses of lead attenuators and a range of source to detector distances. Absolute dose values were determined using an ion chamber on the central axis at the depth of maximum dose in a solid water phantom. The pixel values of the matrix were determined for various field sizes in order to evaluate the dependence of pixel value on dose at those field sizes. It was confirmed that the pixel value was proportional to the square root of the dose rate and was nearly independent of the field size. The 2D pixel values were converted to 2D dose maps in the water phantom after applying a correction for the effect of horns in the flood calibration field. The flood calibration field was used to obtain the relative sensitivity of each pixel. Good agreement was observed (normally better than 1% in relative standard deviation) between the converted dose distribution obtained from the pixel matrix and the direct dose measurement using an ion chamber scanned in a water phantom in regions of shallow dose gradient. For application to on‐line portal dosimetry, both the short‐ and long‐term stability of this EPID system were found to be within 1% relative standard deviation. This system, together with an accurate portal dose calculation procedure, can be used for on‐line radiotherapy dose verification.

ISSN:0094-2405    

DOI:10.1118/1.597502

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A mathematical model of the average edge slope across the radiation field border in a portal image was investigated in order to improve the precision of edge localization in a previously reported algorithm for automatic extraction of the radiation field from double‐ and single‐exposure portals. The model involves a global rather than a local approach to edge localization, and employs a hyperbolic function with four parameters to characterize the behavior of the radiation field penumbra. The location of the radiation field edge is determined from one of these parameters. This model was tested on a group of portal images acquired with different conditions in our clinic. Evaluation results of this model and improvements in the performance of our portal image segmentation algorithm will be presented.

ISSN:0094-2405    

DOI:10.1118/1.597503

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An MR compatible flow simulator is described which generates physiologically realistic pressure and flow waveforms. The simulator is based on a servomotor‐driven gear pump which produces pulsatile flow by modulation of the servomotor rotation rate. Operation of the simulator is under the control of a personal computer, which executes an iterative feedback loop to minimize errors between measured and desired pressure waveforms. The simulator is totally automatic, requiring only a few minutes of iteration to generate the desired pressure waveform. Accurate sinusoidal waveforms with frequencies up to 10 Hz have been generated using the simulator, with high‐frequency contamination of the measured waveform at least 80 dB below the fundamental frequency. Aortic waveforms have been produced with realistic flow rates and pressure variations. The pump assembly is mechanically straightforward and can operate at an 8‐m distance from the flow phantom to allow the device to be isolated from the MR magnet room.

ISSN:0094-2405    

DOI:10.1118/1.597504

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY