摘要:

Single kernel scatter correction algorithms are based on the model that the scatter field can be predicted by convolution of the primary intensity (Iprim) with a spatially invariant scatter point‐spread function (PSF). Practical limitations (Iprimunknown) suggest the substitution of the total detected intensity (Idet) forIprimas the source image in the convolution. In regions of high scatter fraction (SF),Idetis a poor approximation ofIprim, thereby causing an overestimation of scatter originating in the region. This contributes to errors in estimating detected scatter in the mediastinum and neighboring regions. A technique using a regionally variable point‐spread function that significantly reduces RMS error in estimation of the primary image as compared to the single PSF method is investigated. The regionally variable convolution method employs a larger PSF in the mediastinum and a smaller PSF in the lungs to reduce the error in estimating the scatter throughout the image. The method to allow for patient differences has also been expanded and various implementations of these methods have been compared. Results show that the dual‐kernel algorithm is always more effective than an equivalent single‐kernel algorithm. The dual‐kernel algorithm using a predicted scatter fraction curve gives an overall RMS error in the primary of as low as 20.8% which is equivalent to 8.7% RMS error in the scatter. The dual‐kernel method using a predicted scatter fraction curve approaches the accuracy of the single‐kernel method using patient specific scatter measurements. Because using individual scatter measurements is a less desirable method for clinical use, we feel that the dual‐kernel algorithm which uses two region specific convolution kernels and a variable scatter fraction curve is the preferable method.

ISSN:0094-2405    

DOI:10.1118/1.597297

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Random walk theory is used to calculate the line spread function (LSF) of photons as they cross the midplane of a slab of finite thickness. The relationship between the LSF and the photon transit time in transillumination time‐resolved experiments is investigated. It is found that the LSF is approximately Gaussian distributed, with a standard deviation, σ, which can be used as a criterion of the spatial resolution of the imaging system. Results are substantiated by comparison with actual data in the literature. Any given resolution can be improved by reducing the excess transit time Δt, but heterogeneity of the scattering medium and low levels of detected light enormously complicate the achievement of subcentimeter spatial resolution. The latter point is discussed by using optical parameters of breast tissues for visible and near‐infrared radiation (NIR) light.

ISSN:0094-2405    

DOI:10.1118/1.597298

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The principal component (PC) approach offers compressions of an image sequence into fewer images and noise suppressing filters. Multiple MR images of the same tomographic slice obtained with different acquisition parameters (i.e., with differentTR,TE, and flip angles), time sequences of images in nuclear medicine, and cardiac ultrasound image sequences are examples of such input image sets. In this paper noise relationships of original and linearly transformed image sequences in general, and specifically of original, PC, and PC‐filtered images are discussed. As the spinoff, it introduces locally weighted PC transforms and filters, nonlinear PC's, and a single‐image based filter for suppression of noise. Examples illustrate increased perceptibility of anatomical/functional structures in PC images and PC‐filtered images, including extraction of physiological functional information by PC loading curves. Generally, the more correlated the original images are, the more effective is the PC approach.

ISSN:0094-2405    

DOI:10.1118/1.597374

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of x‐ray tube potential and prepatient and interdetector filtration in single exposure dual energy chest imaging has been studied employing a carefully benchmarked model. The analysis utilized published methodology. Noise in simulated lung and mediastinum fields of the aluminum (bone) and Lucite (soft tissue) images were studied at fixed entrance skin exposure (ESE) for commonly employed sandwich detector and sandwich imaging plate configurations. Our results indicate noise in the lung increases slowly with tube potential above 120 kVp, while noise in the mediastinum decreases rapidly. Also, at high tube potential (≥120 kVp) adding moderate amounts of prepatientK‐edge filtration (≅100 mg/cm2) while optimizing imaging conditions for the lung tends to decrease noise in the lungs by ≅30% while increasing noise in the mediastinum by a similar amount. WithoutK‐edge prepatient filtration, noise in the lung is minimized with Cu interdetector filter weights near 400 mg/cm2. In the mediastinum noise is minimized with heavier interdetector filter and prepatientK‐edge filter weights. PrepatientK‐edge filter weights that minimize image noise in either field can increase the tube loading by factors ranging from 10 to 1010. Systems designed with sandwich detectors using commercially available phosphors and coating weights can produce contrast‐to‐noise ratios (CNRs) as high as 50% of the theoretical limit (defined as an optimized system with a totally absorbing rear detector).

ISSN:0094-2405    

DOI:10.1118/1.597373

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dual‐energy digital subtraction angiography (DSA) is immune to the misregistration artifacts which plague conventional temporal subtraction DSA. However, since the signal‐to‐noise ratio (SNR) of dual‐energy DSA is lower than temporal subtraction it is important to optimize the dual‐energy technique with respect to the SNR. This study investigates the optimization of x‐ray exposure parameters for the low‐ and high‐energy x‐ray beams, drawing upon our experience with dual‐energy DSA studies in 20 cardiac patients. It is shown that the SNR is optimized, with respect to patient exposure, when the image intensifier (II) exposure for the high‐energy beam is approximately 5 times higher than the low‐energy beam. The unequal distribution of exposure between the low‐ and high‐energy beams is achieved by selecting different video apertures for the two beams, and is therefore referred to as the “dual‐aperture” technique. The dual‐aperture technique achieves the same SNR as the single‐aperture technique (which uses equal II exposures for the two beams) with 45% less patient exposure. In addition, the dual‐aperture technique enhances the ability to maintain an optimum low‐energy kVp, which increases the dual‐energy signal amplitude. The optimum high‐beam filter thickness for use with the dual‐aperture approach is determined. The optimum filter thickness, which is dependent upon the desired tube loading, is found to range from 1.3 to 2.0 mm Cu for tube loads ranging from 188 to 688 J/beam pair.

ISSN:0094-2405    

DOI:10.1118/1.597376

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Studies were conducted to determine the optimal metal/phosphor screen for on‐line video verification of radiation treatment portals. Screens were evaluated for luminance and spatial resolution as a function of composition and thickness at 6‐ and 23‐MV x‐ray energies. A new video technique was used to determine modulation transfer functions. Gd2O2S was found to be the most efficient (brightest) phosphor for this application. Luminance was found to vary linearly with phosphor thickness up to a thickness of 500 mg/cm2. Metal plates made of iron, brass, copper, lead, and sintered tungsten of various thicknesses were also tested for luminance and resolution with Gd2O2S phosphor. Brightness peaked at about 2‐mm thickness for most metals. Significant contributions to the brightness were found to come from x rays interacting with the phosphor itself.

ISSN:0094-2405    

DOI:10.1118/1.597299

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Accurate extraction of the radiation field boundary from portal images is required in selective histogram equalization techniques for contrast enhancement of portal images, and is required in portal‐simulator image correlation. Local edge detectors are so noise sensitive that delineation of the field from anatomy edges and noise is very difficult. Optimization in edge localization and noise suppression can be achieved with optimal edge detectors, but the required calculation is very time consuming. A portal‐image segmentation algorithm based on morphological edge detection and morphological segmentation techniques is presented. Relying on two predefined criteria that are nonsensitive to variation of portal‐image types, the algorithm can automatically search the optimal threshold value, which is sensitive to the variation of the type and quality of portal images. With two stages of the searching procedure, the algorithm can accommodate a large variation of single and double exposure portal images obtained from different therapy machines. Results of our morphological edge detector are also compared to that of an optimal edge detector. Portal‐simulator image correlation using the inertia moments of the radiation field mask is investigated, and compared to correlation obtained using inertia moments of the radiation field contour. It is shown that the mask method is less sensitive to small variations and distortions in the field shape, resulting in a more accurate correlation. This would substantially simplify the task of treatment verifications.

ISSN:0094-2405    

DOI:10.1118/1.597300

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Pulsed fluoroscopy (hereafter called pulsed) at reduced acquisition rates, typically 15 acq/s (pulsed‐15), is proposed to reduce x‐ray dose in interventional procedures. However, since the human visual system (HVS) acts as a temporal low‐pass filter that interacts with such acquisitions, the proper dose for pulsed must be obtained in perception experiments. We determine the dose for low‐frame‐rate pulsed that gives visualization equivalent to that of conventional 30 acq/s fluoroscopy, hereafter called continuous. Computer‐generated phantoms are used. They consist of stationary, low‐contrast disks on a flat background containing Poisson noise that mimics quantum noise in fluoroscopy. Image sequences are displayed on the video tachistoscope, a device with considerable display flexibility. Three experimental paradigms are used. (1) In a paired‐comparison study, pulsed and continuous are displayed side‐by‐side on the same monitor, and the visibility of a contrast detail phantom is compared. (2) Using this same display, subjects record the minimally detectable disk contrast (the min‐contrast measurement). (3) In a four‐alternative forced‐choice experiment, a disk is placed in one of four positions, and the subject determines the position of the disk. The methods are complementary—the forced‐choice experiment properly eliminates the subjectivity of the observer threshold while the paired‐comparison study is much more time efficient. With regard to pulsed and continuous comparisons, remarkable similarity is found between the supra‐threshold experiments (1 and 2) and the detectability experiment (3); i.e., the average absolute differences in the equivalent‐perception dose as determined by the three measures is approximately 3%. No difference is found between interlaced and noninterlaced display. A relatively small dependence of dose savings on disk size is found with larger disks giving increased dose savings. Average dose savings of 22%, 38%, and 49% are found for pulsed‐15, pulsed‐10, and pulsed‐7.5, respectively.

ISSN:0094-2405    

DOI:10.1118/1.597285

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technique for the measurement of the modulation transfer function (MTF) of image intensifier systems by means of video signal analysis is presented. The MTF can be measured in both horizontal and vertical directions. The method is applicable to routine quality assurance testing in the field. The limitations on accuracy of the technique are investigated by computer simulation. Experimental results reported for several image intensifier systems correlate well with subjective assessments of limiting resolution and other parameters.

ISSN:0094-2405    

DOI:10.1118/1.597286

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The choice of clinical cases used to train and test a computer‐aided diagnosis (CAD) scheme can affect the test results (i.e., error rate). In this study, we deliberately modified the components of our testing database to study the effects of this modification on measured performance. Using a computerized scheme for the automated detection of breast masses from mammograms, it was found that the sensitivity of the scheme ranged between 26% and 100% (at a false positive rate of 1.0 per image) depending on the cases used to test the scheme. Even a 20% change in the cases comprising the database can reduce the measured sensitivity by 15%–25%. Because of the strong dependence of measured performance on the testing database, it is difficult to estimate reliably the accuracy of a CAD scheme. Furthermore, it is questionable to compare different CAD schemes when different cases are used for testing. Sharing databases, creating a common database, or using a quantitative measure to characterize databases are possible solutions to this problem. However, none of these solutions exists or is practiced at present. Therefore, as a short‐term solution, it is recommended that the method used for selecting cases, and histograms or mean and standard deviations of relevant image features be reported whenever performance data are presented.

ISSN:0094-2405    

DOI:10.1118/1.597287

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY