数字X射线摄影中散射线滤除模板成像质量与辐射剂量学评价

目的比较散射线滤除模板及常规滤线栅对数字X射线影像进行散射线校正所成影像质量与辐射剂量差异。方法 以对比度-细节体模(CDRAD 2.0)及ROC统计学体模(ALVIM TRS)为成像对象,分别使用散射线滤除模板技术和常规滤线栅技术,获取经散射线校正的体模数字X射线影像,比较两种散射线滤除方法所得的体模影像图像质量因子(IQF)和信号检出概率,分析两种方法图像质量及体模表面入射剂量差别。结果 在不同体模表面照射剂量条件下,应用散射线滤除模板技术和常规滤线栅技术,消除散射线后的数字影像其图像质量因子(IQF)和信号检出概率(P_det)差异均有统计学意义(P<0.05)。结论 数字X射线摄影时,应用散射线滤除模板技术(SFTT)可以有效滤除散射线。与滤线栅相比,相同照射条件下SFTT能够明显改善图像质量;在获得相同图像质量前提下,应用SFTT,体模表面入射剂量比使用滤线栅降低30%;SFTT为数字X射线摄影中有效滤除散射线、提高X射线图像质量、降低患者剂量的可选择方法。     

Vessel diameter measurement using digital subtraction radiography

A method for obtaining absolute diameter and cross-sectional area measurements on subtraction digital images is described and tested in phantom vessels from 1.5 to 5.5 mm in diameter filled with iodine contrast at concentrations from 23 to 185 mg I/ml. A highly linear correlation of true vs. calculated diameter is demonstrated, with accuracy and reproducibility of the method varying from +/- 1% to 2% at the highest iodine concentration to +/- 30% in the smallest tube at the lowest concentration. A method is described for correction of the observed video density values to allow for nonlinearity of response of the imaging system to iodine density, and its effect on the measured diameters is demonstrated.     

State-of-the-art digital radiography.

Technologic advances in digital radiography have improved the ways in which radiographic images are acquired, displayed, transmitted, recorded, and archived. With computed radiography, performed with storage phosphor plates and interactive high-resolution workstations, radiation dose is reduced and repeat exposures necessitated due to technical errors are eliminated. Digital fluorography allows reductions in dose, procedure time, and film costs. These digital imaging modalities have been well accepted clinically and are equal in diagnostic accuracy to conventional methods. Teleradiology has advanced with the development of laser film digitization, fiberoptic networks, and dial-up circuit switching technology. Laser film printers yield improved hard copies of transmitted images, but further work is needed to faithfully reproduce the images displayed on high-resolution work-stations. Although the capacity for archiving digital image data has increased (260,000 examinations or 23,500 Gbytes can be stored in a six-unit optical disc library), higher capacity storage media are needed. Further technologic advances in the speed of image transmission and storage capacity are anticipated.     

Comparative scatter and dose performance of slot-scan and full-field digital chest radiography systems

PURPOSE: To evaluate the scatter, dose, and effective detective quantum efficiency (DQE) performance of a slot-scan digital chest radiography system compared with that of a full-field digital radiography system. MATERIALS AND METHODS: Scatter fraction of a slot-scan system was measured for an anthropomorphic and a geometric phantom by using a posterior beam-stop technique at 117 and 140 kVp. Measurements were repeated with a full-field digital radiography system with and without a 13:1 antiscatter grid at 120 and 140 kVp. For both systems, the effective dose was measured on posteroanterior and lateral views for standard clinical techniques by using dosimeters embedded in a female phantom. The effective DQEs of the two systems were assessed by taking into account the scatter performance and the DQE of each system. The statistical significance of all the comparative differences was ascertained by means of t test analysis. RESULTS: The slot-scan system and the full-field system with grid yielded scatter fractions of 0.13-0.14 and 0.42-0.48 in the lungs and 0.30-0.43 and 0.69-0.78 in the mediastinum, respectively. The sum of the effective doses for posteroanterior and lateral views for the slot-scan system (0.057 mSv +/- 0.003 [+/- standard deviation]) was 34% lower than that for the full-field system (0.086 mSv +/- 0.001, P < .05) at their respective clinical peak voltages (140 and 120 kVp, respectively). The effective DQE of the slot-scan system was equivalent to that of the full-field system in the lung region but was 37% higher in the dense regions (P < .05). CONCLUSION: The slot-scan design leads to marked scatter reduction compared with the more conventional full-field geometries with a grid. The improved scatter performance of a slot-scan geometry can effectively compensate for low DQE and lead to improved image quality.     

Measurement of scatter fractions in clinical bedside radiography.

The authors present measurements of scatter fraction (SF), the ratio of scattered to total imaged photons, from clinical bedside radiographs of 102 patients. These measurements were obtained by using a new posterior beam-stop technique that does not alter the diagnostic image but that simultaneously provides SF measurements at 224 locations in the image. The SF values in the lung were found to be consistent with previous measurements, while the SF values in the mediastinal and retrocardiac areas were larger than previously reported. SFs in diseased lung were significantly larger than SFs in normal lung. The range of SF values was large for all anatomic locations. For applications in which accurate scatter estimation is required, this wide range of values suggests that SFs should be measured in each individual image.     

A simplified method to obtain perceptibility curves for direct dental digital radiography.

OBJECTIVES: To derive and test a simplified method to construct Perceptibility Curves (PCs) for dental digital detectors. METHODS: Mathematical expressions were derived to make it possible to construct PCs from viewer data obtained at two exposures, one low and one high. PCs were constructed applying these expressions and compared with data previously obtained employing the conventional method. RESULTS: PCs constructed according to the simplified method agree extremely well with conventionally obtained data. CONCLUSIONS: Reliable PCs may be constructed according to the simplified method.     

A method of estimating doses in digital luminescent radiography

Digital storage phosphor radiography can produce images of a constant optical density over a wide range of exposure dose by adjusting reading sensitivity. Since overexposed images are not as readily recognized as with the conventional film-screen technique, a formula was designed, which calculates the radiation dose in the film plane from image sensitivity (S factor), latitude (L factor) and average grey value of the region of interest. To verify the formula, 168 measurements with variation of dose, kVp, L factor, S factor and the readout algorithm were performed using the DIGISCAN storage phosphor system (Siemens). The experiments confirmed the validity of the formula for an S factor of below 5000 (values during routine use 50-400). Correlation between dosimetrically measured radiation dose and the calculated dose was 0.997.     

数字X线摄影的MTF检测方法的进展

随着计算机技术和数字化技术的发展，数字摄影，特别是CR(computed radiography)、DR(direct radiography)正逐步代替传统屏一片X线摄影系统。成像质量评价也上了一个新台阶，即对数字影像的评价。调制传递函数是评价模拟X线摄影影像质量的一个重要参量，其对于数字X线摄影仍很重要。但是因混叠误差的存在，其检测方法与模拟X线摄影不同。本文就MTF检测方法在数字X线摄影中的应用历史和进展作以回顾。     

Detector for dual-energy digital radiography

A detection scheme is described that allows one to accomplish dual-energy scanned projection digital radiography without switching the x-ray tube voltage. The method employs a high/low atomic number detector sandwich that simultaneously separates the x-ray beam transmitted by the patient into low and high energy components. To test the method, the response of a scanning linear array of energy-sensitive detectors was simulated, and bone and soft tissue images of an anthropomorphic chest phantom were obtained at 140 kVp. These were compared with similar images obtained by switching the x-ray tube voltage from 80 kVp to a heavily filtered 140 kVp. For comparable entrance skin exposures, the dual-energy detector images required a lower tube load and resulted in higher noise levels. The latter is attributable to the fact that the separation in energy between the high and low energy components is smaller with the dual-energy detector than with the voltage switching technique, and to misregistration problems associated with the simulation methodology. A detector design is also discussed that would result in improved energy separation and lower noise levels. In view of this possibility and the tube loading advantage, the method looks promising for digital scanned projection radiography.     

New computed radiography technologies in digital radiography

Digital radiography (DR) has become integral to modern diagnostic radiology. One of the earliest forms of DR, computed radiography (CR) using storage phosphors, has established itself as the mainstay of DR-based diagnostic imaging over the past 20 years. More recently, flat-panel DR systems based on solid state X-ray detectors with integrated, large-area, active-matrix readout electronics are promising further improvements in clinical workflow and image quality. Despite CR's longevity, innovations continue to be made. New developments in CR screen technologies, like structured (needle) screens, and new scanner concepts based on line-at-a-time reading promise major improvements in image quality (comparable to that of flat-panel systems), system through-put and physical size, at a cost comparable to that of today's systems. Thus, despite the advent of flat-panel acquisition systems, there will still be an important role for CR in the foreseeable future. After a brief review of the current state of CR technology, this paper will explore several of these new CR developments and present some examples of their potential clinical impact.     

A method of stereotaxic localization adopted for conventional and digital radiography

Summary A method for the determination of stereotaxic coordinates in radiography, e.g. angiography, pneumoencephalography or digital vascular radiography, is described. A special localization frame containing radiopaque structures and scales defines a diagnostic coordinate system. This frame is fixed to the X-ray-table prior to the radiographic procedure and two projections are obtained at arbitrary angles to each other. The focus-film distances do not how to be fixed. The target coordinates are then determined either by a simple graphical procedure or with the use of a digitizing x-y-table, by a computer. With the computer method the films are placed on the digitizing table and the target and a few reference points are marked using a cursor. From the relative positions the computer calculates the coordinates. With the special head fixation system, coordinates of structures visualized in radiographic examinations can be transferred to various therapeutic or diagnostic stereotaxic devices.     

Gastrointestinal examinations with digital radiography.

Basic imaging properties and clinical usefulness of an upgraded digital radiography system were evaluated. The system, which has 1,024 x 1,024 and 2,048 x 2,048 matrices, was upgraded with smaller focal spots (0.3 and 0.8 mm) and reduced thickness of the photoconductive layer of the video camera. Screen-film and digital images (with and without postprocessing) of the upper and lower gastrointestinal (GI) tract were used in the clinical evaluation. Overall modulation transfer functions of the upgraded digital system were comparable to those of the screen-film system, especially at the lower spatial frequency. Threshold contrasts of the two systems were similar despite a 50% reduction in incident exposure for the digital system. For the upper GI tract, digital images processed with unsharp masking techniques were comparable in quality to screen-film images before and after upgrade of the system. For the lower GI tract, screen-film images were better than digital images, except for those produced with a 2,048 x 2,048 matrix with unsharp masking. Further evaluation of the system for examination of other parts of the body seems warranted.     

Artifacts in digital radiography

OBJECTIVE: The purpose of this article is to discuss flat-panel digital radiography (DR) artifacts to help physicists, radiologists, and radiologic technologists visually familiarize themselves with an expanded range of artifact appearance. CONCLUSION: Flat-panel DR is a growing area of general radiography. As a radiology community, we are still becoming familiar with these systems and learning about clinically relevant artifacts and how to avoid them. These artifacts highlight important limitations or potential complications in using flat-panel DR systems.     

A dual-photopeak window method for scatter correction.

The imaging of scattered photons degrades contrast and is a major source of error in the quantitation of activity. It was hypothesized that, if the photopeak was divided into two nonoverlapping energy windows, a regression relation could be obtained between the ratio of counts within these windows and the scatter fraction for counts within the total region. This idea was tested by acquiring dual photopeak window acquisitions of a 99mTc point source in an elliptical attenuator, and at the same locations in air. From these, a regression between the scatter fraction and window ratio was determined. When this regression was applied to estimate the scatter distribution for acquisitions in both uniform and nonuniform elliptical attenuators, the residual scatter fraction was reduced approximately ten-fold and the estimated scatter line spread functions matched very closely the tails of the total line spread functions. In SPECT acquisitions, dual-photopeak window scatter correction was observed to significantly increase the contrast of "cold" spheres, improve the accuracy of estimating activity at the center of "hot" spheres, and return the three-dimensional modulation transfer function for point sources in an elliptical attenuator to near their in-air shape.     

The effect of scatter reduction on the signal-to-noise ratio in computed radiography.

Signal-to-noise ratio (SNR) in computed radiography (CR) was assessed by using the computerized image data from storage phosphor radiographs in a modification of the Rose model. A multiple pencil-beam (MPB) imaging device, a conventional 1:12 grid, and an air gap of 90 cm were compared in terms of improvement of the signal-to-noise ratio caused by the reduction of scatter. The MPB device showed better SNRs by a factor of 1.25 compared to the grid and air gap which were approximately equal to each other. This is related to its superiority in scatter control, which has also been shown previously. Air gap screening has not been very popular because of geometrical problems, but in scatter reduction it is still comparable to today's grid technology. The optimization of image information content in CR is briefly discussed.     

Mineral content of bone: measurement by energy subtraction digital chest radiography

Several methods are presently available for measuring the mineral content of bone. Those in widespread use include dual-photon absorptiometry and quantitative CT. The feasibility of using dual-energy digital chest radiography for determination of the mineral content of posterior ribs on digital chest images was studied by using a prototype unit. The results showed a significant difference in the mineral density of the posterior ribs of control subjects and those of patients who had osteoporosis (251.1 +/- 36 mg Ca2+/cm2 of rib vs 158.8 +/- 48 mg Ca2+/cm2, p less than or equal to .01) and a close correlation with values obtained by dual-photon absorptiometry of the lumbar spine (r = .77). The results suggest that this technique can provide an accurate assessment of the presence or absence of osteoporosis.     

A new method for the automated alignment of dental radiographs for digital subtraction radiography

OBJECTIVES: To provide a robust and convenient method by which radiographic images can be spatially aligned for digital radiographic subtraction without the use of manually selected reference points. METHODS: An automated method for image alignment is described which begins with the extraction of a large number of edge features (1500 or more pixels) from each of two radiographic images taken of the same anatomical region of a given patient. The features in the first radiograph are paired, pixel by pixel, with those in the second radiograph using a nearest neighbor criterion. The edge features in the first radiograph are aligned with those in the second radiograph by performing an affine transformation that is consistent with the projection geometry for a plantar parallel X-ray beam. Transformation parameters are determined which provide the smallest spatial alignment error between the two sets of equivalent features. These parameters are found by a closed-form analytic solution, thus enabling a computationally efficient implementation. The final transformation is then applied to the entire first image resulting in a close spatial match to the second image. The performance of three dentists using the automatic method was compared with their performance using a manual method of alignment for eight pairs of images. RESULTS: The root mean squared error in image alignment for the automatic method was 14% lower than that with manual alignment. The variability for the automatic method was half that of the maximal method as measured by the residual error. The automatic method was also three times faster than the manual method. CONCLUSIONS: This method could make digital subtraction more accessible to researchers and practising dentists. Batch mode implementation could enable the processing of large volumes of data. Restriction to a region of interest and improved feature extraction could further improve performance.     

Image processing in digital radiography

Image processing is a critical part of obtaining high-quality digital radiographs. Fortunately, the user of these systems does not need to understand image processing in detail, because the manufacturers provide good starting values. Because radiologists may have different preferences in image appearance, it is helpful to know that many aspects of image appearance can be changed by image processing, and a new preferred setting can be loaded into the computer and saved so that it can become the new standard processing method.Image processing allows one to change the overall optical density of an image and to change its contrast. Spatial frequency processing allows an image to be sharpened, improving its appearance. It also allows noise to be blurred so that it is less visible. Care is necessary to avoid the introduction of artifacts or the hiding of mediastinal tubes.     

Validation of a digital videodensitometric program analysis for measurement of left ventricular ejection fraction

Left ventricular (LV) function was studied in 30 patients using digital subtraction angiography by the intravenous approach. Each ventriculogram was processed with a specific videodensitometric analysis to determine LV ejection fraction. The program was verified in an experimental set-up consisting of nine latex balloons filled with contrast medium. Its validation has been established by comparing videodensitometric results with classical results supplied by geometric methods. A good correlation was obtained (r = 0.9449) and, furthermore, with experimental models, videodensitometric analysis seemed to be more accurate than geometric analysis. Digital videodensitometry appears to be a valuable and accurate method for quantifying LV function, and a promising technique for determination of the real volumes.