Comparison of the quality of temporal subtraction images obtained with manual and automated methods of digital chest radiography

The authors have been developing a fully automated temporal subtraction scheme to assist radiologists in the detection of interval changes in digital chest radiographs. The temporal subtraction image is obtained by subtraction of a previous image from a current image. The authors' automated method includes not only image shift and rotation techniques but also a nonlinear geometric warping technique for reduction of misregistration artifacts in the subtraction image. However, a manual subtraction method that can be carried out only with image shift and rotation has been employed as a common clinical technique in angiography, and it might be clinically acceptable for detection of interval changes on chest radiographs as well. Therefore, the authors applied both the manual and automated temporal subtraction techniques to 181 digital chest radiographs, and compared the quality of the subtraction images obtained with the two methods. The numbers of clinically acceptable subtraction images were 147 (81.2%) and 176 (97.2%) for the manual and automated subtraction methods, respectively. The image quality of 148 (81.8%) subtraction images was improved by use of the automated method in comparison with the subtraction images obtained with the manual method. These results indicate that the automated method with the nonlinear warping technique can significantly reduce misregistration artifacts in comparison with the manual method. Therefore, the authors believe that the automated subtraction method is more useful for the detection of interval changes in digital chest radiographs.     

Comparison of three different techniques for dual-energy subtraction imaging in digital radiography: A signal-to-noise analysis

Dual-energy subtraction imaging techniques allow the tissue and bone structures in the patient to be visualized and studied in two separate images, thus removing the obscurity associated with overlapping of the two structures. In addition, they allow the subtraction image signals to be used for quantifying the tissue and bone thicknesses. Thus, capability for dual-energy subtraction imaging is often incorporated with new digital radiography systems. There are three different approaches to dual-energy image subtraction imaging techniques. Among them, the dual-kilovolt (peak) [kV(p)] and sandwich detector techniques have been two widely used approaches. A third approach is the single-kV(p) dual-filter technique, which allows some flexible control of the spectra while avoiding the technical complexity of kV(p) value switching in slit-scan imaging. In this report, the noise properties associated with these three techniques are studied and compared by computing the noise variances in the subtraction image signals as a function of the kV(p) values and filter thicknesses. It was found that the dual-kVp technique results in the least noisy subtraction images, whereas the dual-filter technique results in slightly less noisy subtraction images than the sandwich detector technique. Following optimization of the kV(p) value and filter thicknesses, the dual-filter and sandwich detector techniques result in a noise level of approximately three and four times higher than that resulted from the dual-kV(p) technique, respectively.     

Medical imaging strategies

Computed tomography (CT), intravenous digital subtraction angiography (IV-DSA), digital radiographic image processing and dual energy subtraction are four examples of medical imaging strategies that have met with various degrees of success as judged by diagnostic performance. The success of CT has been spectacular; IV-DSA has provided modest benefits; digital image processing of chest radiographs has been singularly disappointing; and the verdict on dual energy subtraction is undecided. The degree of success of each of these techniques can be understood by considering the degree to which each simplifies image interpretation or isolates a fingerprint of disease.     

Digital background subtraction for fluorescence imaging

A method of enhancing contrast in fluorescence imaging has been devised, based on real-time digital subtraction of a background video image from a signal-plus-background video image. Color filters are used to differentiate signal from background. The technique has been applied to detection of small tumors labeled with the tumor specific fluorescent drug hematoporphyrin derivative.     

Cross-correlation digital registration techniques for multispectral cell images.

Several variants of the cross-correlation technique of automatic digital image registration are tested and compared on a set of multispectral images of lymphoblastic bone marrow cells. Factors investigated are normalization, application of a gradient and subtraction of the average image density from each image point. Several of the variants are shown to have low error rates. A gradient method requiring a small degree of manual intervention is concluded to be the most sensitive approach.     

Computerized detection of masses in digital mammograms: analysis of bilateral subtraction images

A computerized scheme is being developed for the detection of masses in digital mammograms. Based on the deviation from the normal architectural symmetry of the right and left breasts, a bilateral subtraction technique is used to enhance the conspicuity of possible masses. The scheme employs two pairs of conventional screen-film mammograms (the right and left mediolateral oblique views and craniocaudal views), which are digitized by a TV camera/Gould digitizer. The right and left breast images in each pair are aligned manually during digitization. A nonlinear bilateral subtraction technique that involves linking multiple subtracted images has been investigated and compared to a simple linear subtraction method. Various feature-extraction techniques are used to reduce false-positive detections resulting from the bilateral subtraction. The scheme has been evaluated using 46 pairs of clinical mammograms and was found to yield a 95% true-positive rate at an average of three false-positive detections per image. This preliminary study indicates that the scheme is potentially useful as an aid to radiologists in the interpretation of screening mammograms.     

Improved contralateral subtraction images by use of elastic matching technique

A contralateral subtraction technique has been developed to assist radiologists in the detection of asymmetric abnormalities such as lung nodules on a single chest radiograph. With this technique, a contralateral subtraction image is obtained by subtracting a right/left reversed "mirror" image from the original one. The lesions in the subtraction image may be enhanced because most of the symmetric skeletal structures, such as peripheral ribs, are eliminated. Although the quality of the previous contralateral subtraction images is relatively good, severe misregistration artifacts, mainly due to serious asymmetry of the ribs in the two lungs of the original image, were observed in some cases, and minor misregistration artifacts were also observed in many cases. In this study, we employed three image warping techniques. An initial global warping technique was applied to reduce severe misregistration artifacts in the subtraction image caused by asymmetric rib structures. Additional two iterative warping techniques based on an elastic matching technique were used for accurate registration of the local structures of ribs, so that minor artifacts present in many subtraction images obtained with the previous technique were greatly reduced. With the new technique, the percentage of chest images, which were rated as being of adequate, good, or excellent quality of subtraction images by use of a subjective evaluation method, was improved from 91% to 97%. In particular, the number of cases with excellent quality was greatly increased from 15% to 42%. The contralateral subtraction technique can be used for detection of asymmetric abnormalities, such as lung nodules, pneumothorax, pneumonia, and emphysema, on peripheral lungs in single chest radiographs, and it therefore has potential utility in a large proportion of abnormal chest images.     

Contralateral subtraction: a novel technique for detection of asymmetric abnormalities on digital chest radiographs

A novel contralateral subtraction technique has been developed to assist radiologists in the detection of asymmetric abnormalities on a single chest radiograph. With this method, the lateral inclination is first corrected by rotating and shifting the original chest image so that the midline of the thorax is aligned with the vertical centerline of the original chest image. The rotated image is then flipped laterally to produce a reversed "mirror" image. Finally, the mirror image is warped and subtracted from the original image for derivation of the contralateral subtraction image. The three key techniques which are employed in this study are applied successively to the initial contralateral subtraction technique for acquisition of improved subtraction images. One hundred PA chest radiographs, including 50 normals and 50 abnormals, were used as the database for this study. The percentage of chest images, which were rated as being adequate, good, or excellent quality of subtraction images by employing a subjective evaluation method, was improved from 73% to 91% by use of the three key techniques. The contralateral subtraction technique can be used for detection of any asymmetric abnormalities, such as lung nodules, pneumothorax, pneumonia, and emphysema, on a single chest radiograph, and therefore has potential utility in a high proportion of abnormal cases.     

Dual-energy digital mammography utilizing stimulated phosphor computed radiography

Dual-energy subtraction is a radiographic technique for the acquisition of a material selective image by the weighted subtraction of low- and high-energy digital X-ray images. This is achieved by exploiting the energy dependence of the X-ray attenuation components in the image. This can allow the removal of background morphology to enhance the presentation of otherwise obscured details. The detection of microcalcifications in a mammogram by dual-energy techniques has previously been investigated. These investigations indicated that, using dual-energy techniques, small microcalcifications could be extracted from the background breast morphology with sufficient signal to noise ratio (SNR) to be full visualized. The authors present the extension of a theoretical dual-energy model to incorporate practical considerations and then compare the results with experimentally derived data using a commercially available computed radiography system. In particular the extended model now takes into account the energy dependent detective quantum efficiency of a system. This is thought to be a major factor in reducing the efficiency of dual-energy mammography. The theoretical model predicts that dual-exposure dual-energy mammography, utilizing HRIII image plates, could not provide a detail SNR of five for calcifications smaller than 470 mu m. The experimental results verify this and indicate that dual-energy subtraction mammography, utilizing computed radiography, is currently not a viable technique for the detection of clinically significant microcalcifications. Further advances in X-ray image detector efficiency will be required if the full potential of this technique is to be achieved.     

冠脉数字造影图像血管分割方法研究

为了从冠脉数字造影图中提取具有复杂形态结构的血管 ,以便于血管临床心血管疾病的定量分析与诊断 ,我们对造影图像设计了一种有效的血管分割算法 ,然而为了获得更加准确的血管形态 ,我们对造影图像和掩膜图像进行匹配减影 ,然后再从减影图像中分割血管 ,实验结果表明这样分割得到的血管较直接从造影图像分割得到的血管更加准确     

Automatic chart reader--an example of computer systems for patients' data interpretation

A new approach to the processing of circular medical charts based on "odd symmetry digital subtraction" is presented. The technique allows automatic digitisation of these charts using an image processing system based on a flat bed scanner. This approach is simple, fast, and reliable; and provides a system in which the digitised image is processed by a microcomputer to extract and quantify the plotted information. The main features and the potential applications of this approach in medical and nonmedical fields are discussed.     

Scatter-glare corrections in quantitative dual-energy fluoroscopy

Previous attempts to use time subtraction intravenous digital subtraction angiography for ventricular imaging have been hampered by artifacts due to cardiac and respiratory motion. We have previously reported a motion-immune dual-energy technique in which kVp is switched between 60 and 120, at 300-500 mA, 30 times/s. In order to quantitate parameters such as ejection fraction and left ventricular volume, it is necessary to correct for scatter and veiling glare (SVG), which are the major sources of nonlinearities in videodensitometric digital subtraction angiography (DSA). In this report, a convolution filtering method has been investigated to estimate SVG in DSA images. In the first step, a grey level transformation of the detected image is utilized to get an estimated SVG image. In the second step this image is convolved to produce an image with appropriate spatial frequency content. Estimates of SVG in several Humanoid chest phantom images were obtained using Gaussian convolution kernels with a full width at half-maximum (FWHM) of 51-125 pixels. The root-mean-square (rms) percentage error of these estimates was obtained by comparison with direct SVG measurement. A convolution kernel with a FWHM of 75 pixels in each dimension applied to 16 Humanoid phantom images with various projections, thicknesses, and beam energies resulted in an average rms percentage error of 9.7% in the SVG estimate, for the 16 cases studied. The SVG estimation consisting of grey scale-to-SVG fraction lookup table (LUT) is made based on previous measurements. The x-ray settings required for each patient are utilized to alter the LUT in order to account for patient thickness variations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Improvement of temporal and dynamic subtraction images on abdominal CT using 3D global image matching and nonlinear image warping techniques

Accurate registration of the corresponding non-enhanced and arterial-phase CT images is necessary to create temporal and dynamic subtraction images for the enhancement of subtle abnormalities. However, respiratory movement causes misregistration at the periphery of the liver. To reduce these misregistration errors, we developed a temporal and dynamic subtraction technique to enhance small HCC by 3D global matching and nonlinear image warping techniques. The study population consisted of 21 patients with HCC. Using the 3D global matching and nonlinear image warping technique, we registered current and previous arterial-phase CT images or current non-enhanced and arterial-phase CT images obtained in the same position. The temporal subtraction image was obtained by subtracting the previous arterial-phase CT image from the warped current arterial-phase CT image. The dynamic subtraction image was obtained by the subtraction of the current non-enhanced CT image from the warped current arterial-phase CT image. The percentage of fair or superior temporal subtraction images increased from 52.4% to 95.2% using the new technique, while on the dynamic subtraction images, the percentage increased from 66.6% to 95.2%. The new subtraction technique may facilitate the diagnosis of subtle HCC based on the superior ability of these subtraction images to show nodular and/or ring enhancement.     

实时动态数字减影血管造影系统

本文简要介绍最新研制的实时动态数字减影血管造影系统，该系统用普通微机及数字减影图象存储处理器实现实时动态数字减影。文中对系统硬件组成、电路结构、特别是对超大容量造影图象存储处理器、蒙片图象存储器及其电路逻辑框图作了较详细的论述，同时对系统的软件功能做一简要的介绍，最后给出左心室造影的减影图象     

最小二乘法的未知控制点检测

背景：数字减影血管造影系统使用X射线影像增强器作为图像检测器,获取的投影图像与真实图像之间存在几何失真。 目的：为实现脑血管的三维重建,对数字减影血管造影图像的几何失真进行校正。 方法：基于数学形态学检测可视控制点,在此基础上采用最小二乘法曲线拟合进行非可视控制点检测,利用获取的672个控制点对原始图像进行非线性局部校正。 结果与结论：实验结果表明在边缘校正的结果较好,圆弧区域基本不失真。使用最小二乘法曲线拟合得到的控制点坐标准确性高,图像校正结果好,可用于脑血管数字减影血管造影图像的校正,为后续的三维重建提供保证。     

Noise reduction methods for hybrid subtraction

In digital subtraction angiography, hybrid subtraction provides selective vessel images free of soft-tissue motion artifacts but with a lower signal-to-noise ratio (SNR) than temporal subtraction images. An image processing method called measurement-dependent filtering has been developed to enhance the SNR of hybrid images without losing resolution or selectivity. Linear combinations of four images consisting of a pre- and postcontrast dual-energy measurement pair form both the hybrid image and a lower noise but less selective vessel image. The noise-reduced image is derived by combining the low-frequency components of the hybrid image with the high-frequency components of the lower noise image in a variety of ways. The results of the filtering method, when tested on both phantom and clinical data, display images with about the same degree of conspicuity as the hybrid image and a SNR approaching that of the temporal image.     

一种基于查找表算法的数字减影血管造影系统

查找表算法在数字信号处理中已有广泛的应用，但在数字减影血管造影术中的应用却报道不多。本文讨论了不同查找表算法对数字减影血管造影图像的影响。结果表明：使用不同的输出查找表算法，会得到不同效果的数字减影血管造影图像。通过分析与比较，我们认为：在图像的后处理方面，应用查找表算法可以进一步改善数字减影血管造影图像的质量。     

Development of a Voxel-Matching Technique for Substantial Reduction of Subtraction Artifacts in Temporal Subtraction Images Obtained from Thoracic MDCT

A temporal subtraction image, which is obtained by subtraction of a previous image from a current one, can be used for enhancing interval changes (such as formation of new lesions and changes in existing abnormalities) on medical images by removing most of the normal structures. However, subtraction artifacts are commonly included in temporal subtraction images obtained from thoracic computed tomography and thus tend to reduce its effectiveness in the detection of pulmonary nodules. In this study, we developed a new method for substantially removing the artifacts on temporal subtraction images of lungs obtained from multiple-detector computed tomography (MDCT) by using a voxel-matching technique. Our new method was examined on 20 clinical cases with MDCT images. With this technique, the voxel value in a warped (or nonwarped) previous image is replaced by a voxel value within a kernel, such as a small cube centered at a given location, which would be closest (identical or nearly equal) to the voxel value in the corresponding location in the current image. With the voxel-matching technique, the correspondence not only between the structures but also between the voxel values in the current and the previous images is determined. To evaluate the usefulness of the voxel-matching technique for removal of subtraction artifacts, the magnitude of artifacts remaining in the temporal subtraction images was examined by use of the full width at half maximum and the sum of a histogram of voxel values, which may indicate the average contrast and the total amount, respectively, of subtraction artifacts. With our new method, subtraction artifacts due to normal structures such as blood vessels were substantially removed on temporal subtraction images. This computerized method can enhance lung nodules on chest MDCT images without disturbing misregistration artifacts.     

Digital image motion correction by spatial warp methods

A technique for correction of motion between images which are obtained in high-speed digital subtraction or cine angiographic acquisitions is being tested. The method is based on the application of quadratic polynomial equations which transform one image so that it matches a reference image. Images which have been processed in this manner can be summed to improve the signal-to-noise ratios over individual images. The technique for motion correction currently being tested uses operator interaction to establish the appropriate polynomial transformation. An operator selects fiducial (reference) points on an image which will be the reference. Then he selects the corresponding fiducial points on the image to be processed. The algorithm calculates the coefficients of a pair of quadratic polynomial equations and applies them to each pixel in the image. Results demonstrate the application of the technique in phantoms and in digitized cine angiograms.     

Application of temporal subtraction for detection of interval changes on chest radiographs: Improvement of subtraction images using automated initial image matching

The authors developed a temporal subtraction scheme based on a nonlinear geometric warping technique to assist radiologists in the detection of interval changes in chest radiographs obtained on different occasions. The performance of the current temporal subtraction scheme is reasonably good; however, severe misregistration can occur in some cases. The authors evaluated the quality of 100 chest temporal subtraction images selected from their clinical image database. Severe misregistration was mainly attributable to initial incorrect global matching. Therefore, they attempted to improve the quality of the subtraction images by applying a new initial image matching technique to determine the global shift value between the current and the previous chest images. A cross-correlation method was employed for the initial image matching by use of blurred low-resolution chest images. Nineteen cases (40.4%) among 47 poor registered subtraction images were improved. These results show that the new initial image matching technique is very effective for improving the quality of chest temporal subtraction images, which can greatly enhance subtle changes in chest radiographs.