Patient dose management in digital radiography

Purpose:

To present the experience in patient dose management and the development of an online audit tool for digital radiography.

Materials and methods:

Several tools have been developed to extract the information contained in the DICOM header of digital images, collect radiographic parameters, calculate patient entrance doses and other related parameters, and audit image quality.

Results:

The tool has been used for mammography, and includes images from over 25,000 patients, over 75,000 chest images, 100,000 computed radiography procedures and more than 1,000 interventional radiology procedures. Examples of calculation of skin dose distribution in interventional cardiology based upon information of DICOM header and the results of dosimetric parameters for cardiology procedures in 2006 are presented.

Conclusion:

Digital radiology has great advantages for imaging and patient dose management. Dose reports, QCONLINE systems and the MPPS DICOM service are good tools to optimise procedures and to manage patient dosimetry data. The implementation of the ongoing IEC-DICOM standard for patient dose structured reports will improve dose management in digital radiology.     

Dose efficiency and low-contrast detectability of an amorphous silicon x-ray detector for digital radiography

The effect of dose reduction on low-contrast detectability is investigated theoretically and experimentally for a production grade amorphous silicon (a-Si) x-ray detector and compared with a standard thoracic screen-film combination. A non-prewhitening matched filter observer model modified to include a spatial response function and internal noise for the human visual system (HVS) is used to calculate a signal-to-noise ratio (SNR) related to object detectability. Other inputs to the SNR calculation are the detective quantum efficiency (DQE) and the modulation transfer function (MTF) of the imaging system. Besides threshold detectability, the model predicts the equivalent perception dose ratio (EPDR), which is the fraction of the screen film exposure for which the digital detector provides equal detectability. Images of a contrast-detail phantom are obtained with the digital detector at dose levels corresponding to 27%, 41%, 63% and 100% of the dose used for screen-film. The images are used in a four-alternative forced choice (4-AFC) observer perception study in order to measure threshold detectability. A statistically significant improvement in contrast detectability is measured with the digital detector at 100% and 63% of the screen-film dose. There is no statistical difference between screen-film and digital at 41% of the dose. On average, the experimental EPDR is 44%, which agrees well with the model prediction of 40%.     

数字X射线摄影机床边胸部摄影时周围散射线量测量

目的测量移动式数字X射线摄影（DR）机床边胸部摄影时照射野外的散射线辐射剂量。方法采用移动式DR机,用80kV、240mA、10ms和110cm焦-片距,垂直照射胸部体模,将测量仪分别放在360°均等分的8个角度、距照射野外1m和2m处,共测16个点。结果在0°和180°2m远距离处,测量仪上散射线量指示值最大,均为0.014μSv。在90°和270°2m远距离处最小,分别为0.011μSv和0.010μSv。结论一次移动式DR床边机胸部摄影,如患者和工作人员距照射野外2m远,其散射线辐射剂量低于人体7min所接受的自然本底照射剂量,不会产生危害后果。     

影响DR图像质量因素分析

目的研究影响DR图像质量因素和规避。方法通过DR成像系统成像后,经工作站后处理,调整至最佳传至激光相机打片。选取600幅照片,其中男性326例,女性274例;年龄2天～78岁,平均年龄45岁。照片所摄部位以胸片、脊柱片、四肢骨和关节片为主,通过X射线评片标准,对其质量进行评价,探讨影响成像质量因素。结果甲级片498张,非甲级片102张。通过着重对非甲级片分析,影响DR图像质量因素如下。①技术人员的操作技能和业务技术水平。此原因所致68张,占66%。②摄片技术参数。此原因所致23张,占23%。③机器性能。此原因所致3张,占3%。④图像后处理技术。此原因所致6张,占6%。⑤激光相机。此原因所致2张,占2%。结论影响DR照片质量因素很多,技术人员的操作技能和水平是其主要因素。应用DR设备过程中,只有不断提高技术人员的综合素质水平,规范操作和不断总结使用经验,增强对设备原理的学习,探索更多临床经验,才能得到质量更佳图像照片。     

Screen optics effects on detective quantum efficiency in digital radiography: zero-frequency effects

Indirect flat panel imagers have been developed for digital radiography, fluoroscopy and mammography, and are now in clinical use. Screens made from columnar structured cesium iodide (CsI) scintillators doped with thallium have been used extensively in these detectors. The purpose of this article is to investigate the effect of screen optics, e.g., light escape efficiency versus depth, on gain fluctuation noise, expressed as the Swank factor. Our goal is to obtain results useful in optimizing screens for digital radiography systems. Experimental measurements from structured CsI samples were used to derive their screen optics properties, and the same methods can also be applied to powder screens. CsI screens, all of the same thickness but with different optical designs and manufacturing techniques, were obtained from Hamamatsu Photonics Corporation. The pulse height spectra (PHS) of the screens were measured at different x-ray energies. A theoretical model was developed for the light escape efficiency and a method for deriving light escape efficiency versus depth from experimental PHS measurements was implemented and applied to the CsI screens. The results showed that the light escape efficiency varies essentially linearly as a function of depth in the CsI samples, and that the magnitude of variation is relatively small, leading to a high Swank factor.     

Evaluating radiographic parameters for mobile chest computed radiography: phantoms, image quality and effective dose

Conventional chest radiography is technically difficult because of wide variations in tissue attenuations in the chest and limitations of screen-film systems. Mobile chest radiography, performed bedside on hospital inpatients, presents additional difficulties due to geometric and equipment limitations inherent in mobile x-ray procedures and the severity of illness in the patients. Computed radiography (CR) offers a different approach for mobile chest radiography by utilizing a photostimulable phosphor. Photostimulable phosphors overcome some image quality limitations of mobile chest imaging, particularly because of the inherent latitude. Because they are more efficient in absorbing lower-energy x-rays than rare-earth intensifying screens, this study evaluated changes in kVp for improving mobile chest CR. Three commercially available systems were tested, with the goal of implementing the findings clinically. Exposure conditions (kVp and grid use) were assessed with two acrylic-and-aluminum chest phantoms which simulated x-ray attenuation for average-sized and large-sized adult chests. These phantoms contained regions representing the lungs, heart and subdiaphragm to allow proper CR processing. Signal-to-noise ratio (SNR) measurements using different techniques were obtained for acrylic and aluminum disks (1.9 cm diameter) superimposed in the lung and heart regions of the phantoms, where the disk thicknesses (contrast) were determined from disk visibility. Effective doses to the phantoms were also measured for these techniques. The results indicated that using an 8:1, 33 lines/cm antiscatter grid improved the SNR by 60-300 % compared with nongrid images, depending on phantom and region; however, the dose to the phantom also increased by 400-600%. Lowering x-ray tube potential from 80 to 60 kVp improved the SNR by 30-40%, with a corresponding increase in phantom dose of 40-50%. Increasing the potential from 80 to 100 kVp reduced both the SNR and the phantom dose by approximately 10%. The most promising changes in technique for trial in clinical implementation include using an antiscatter grid, especially for large patients, and potentially increasing kVp.     

婴幼儿胸部DR摄影源像距与辐射剂量和图像质量的相关性研究

目的 探讨婴幼儿胸部数字X射线摄影(DR)的源像距(SID)与图像质量及辐射剂量的相关性.方法 先进行仿真胸部体模(成都剂量体模)实验性曝光,采用同一管电压(65 kVp)和不同SID进行曝光组合,记录每次曝光的毫安秒(mAs)、单位面积剂量:在实验的基础上进行婴幼儿胸部DR,收集2008年5月～2010年5月初次人院的50例婴幼儿行胸部DR(其中男性28例,女性22例;年龄5个月～2岁,平均年龄10个月),住院期间复查1次作为对照,初检组均使用110cmSID,复检组均使用90cmSID进行DR胸部摄影.由3名资深影像学专家对获取的100幅图像质最进行评判,评判结果运用统计学软件SPSS 13.0进行分析,使用接受者操作特征曲线(ROC)分析和t检验.结果 100例初检及复查婴幼儿的图像质量均符合诊断要求(P>0.05);而不同的SID曝光的面积剂量差异有统计学意义(P<0.05).结论 婴幼儿胸部DR的不同SID,其辐射剂量差异具有统计学意义,且图像质量均能满足诊断要求,90cmSID在婴幼儿胸部DR中值得提倡.     

Application of contrast-to-noise ratio in optimizing beam quality for digital chest radiography: comparison of experimental measurements and theoretical simulations

The contrast-to-noise ratio (CNR) has been employed in optimizing beam quality for imaging a chest phantom using digital radiography. The relationship between CNR and tube potential has been studied for regions of different attenuations representing the lung, heart and abdomen, and a figure of merit (FOM) incorporating effective dose has been calculated to enable dose performance to be included. Direct measurements of imaging performance have been compared with simulations based on a model representing object attenuations. The study has shown reasonable agreement between measurements of CNR and calculated values. The CNR values in the lung and heart regions are higher at 60-80 kV, while those for the abdomen are higher at 90-110 kV. Incorporating a 0.2 mm copper filter has minimal effect on image quality and the FOM is higher because of the reduction in dose. For imaging the heart and abdomen, performance was improved through use of a technique to remove scatter, with an air-gap technique giving a higher FOM because of the lower dose. The CNR and FOM can provide useful quantities for evaluating imaging performance to optimize beam quality for different imaging tasks.     

Filter materials for dose reduction in screen-film radiography

A computer program was developed to calculate both integral absorbed dose in a water phantom and entrance exposure, for the imaging of iodine contrast with x-ray intensifying screens. The effect of filtration of the x-ray beam on integral absorbed dose and entrance exposure was studied for 27 different filter materials and four types of intensifying screens. The dose and exposure were calculated, keeping the image contrast and energy absorption in the screens constant. To check the validity of the calculations, a number of measurements were performed, the results of which agreed well with our calculations. A remarkable result is that dose and exposure reduction can be achieved almost equally well with conventional filters (aluminum and copper) as with a number of K-edge filters. This was found for situations commonly encountered in diagnostic radiology (60-80 kV, 20 cm water). This finding is in contrast to a number of earlier studies, in which K-edge filters were found to be superior to conventional filters.     

A new photoconductor imaging system for digital radiography

Amorphous selenium is a material often used in the x-ray imaging system. The main application is in xeroradiography where the structure of the sensor is a layer of selenium on a conductive substrate. The signal is a charge density on the surface which is revealed by a toner or by electrostatic probe for digitalization. In the system described here, the sensor structure is different for the sensor is covered by an electrode, a thin layer of metal, which gives another interface. The reading system needs the scanning of a light beam and the resolution power depends on the size of the beam. It is easier to scan a light beam than electrostatic probes so a more compact system can be realized. In the process, there are two phases: the storage and the reading. The time spent between the two phases reduces the quality of the image, and an in situ reading system, integrated to the radiographic machine will be, for this reason, more efficient. Also, the sensor needs good memory effect. One has investigated different sensors based on a structure of a thin photoconductive layer between two electrodes to find a memory effect. We have already seen this phenomena in the Bi12 SiO20 (B. Richard, "Contribution à l'étude d'un procédé d'imagerie radiologique utilisant le photoconducteur BO12 SiO20," Ph.D. thesis, Paris, 1987). In amorphous selenium with some dopants and some type of metallic contact, the memory effect is important enough to realize a system. With 2 X 2 cm samples, a complete x-ray digital imaging system has been built.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calibrating automatic exposure control devices for digital radiography

The energy responses of digital radiography detectors differ from those of screen-film systems. To provide a consistent level of image quality at different tube potentials automatic exposure control (AEC) devices must be calibrated to suit the energy response of the image receptor with which they are intended for use. AEC calibration for digital radiography systems requires an alternative parameter to optical density, ideally one related to the quality of a digital image. Energy responses of computed radiography (CR) and indirect digital radiography (IDR) image receptors have been calculated, and compared with those for screen-film systems. Practical assessments of the relative sensitivities of a CR detector made using the detector dose indicator (DDI), pixel value and signal-to-noise ratio showed similar variations with tube potential. The DDI has been used to determine the correct kV compensation curve required to calibrate the AECs for the loss in detector sensitivity with tube potential. AECs are set up relative to a predetermined air kerma incident on the detector at 80 kV for CR and IDR systems using this curve and the method used is described. Factors influencing the calibration of AECs for digital radiography including techniques, types of phantom and contributions from scatter are reviewed, and practical methods recommended for use.     

Assessment of the effects of pixel loss on image quality in direct digital radiography

Modern digital radiographic 'flat panel' detectors can exhibit a progressive form of image degradation arising from non-functioning pixels. The effect of these 'dead pixels' on the quantitive image quality measures of modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) is investigated by a simulated degradation of images obtained from an Hologic EPEX system. The effects on the semi-quantitive measures obtained from contrast threshold test objects and resolution gratings are also investigated. Results suggest that the contrast-detail tests often employed in quality assurance measures are not sufficient to reveal the presence of dead pixels until well beyond the recommended replacement point for the flat panel detector. However, measurements of spatial resolution using a line pairs phantom were found to be more sensitive to pixel loss. Measurement of the MTF, NPS and DQE can reveal small changes in image quality with increasing pixel loss, with a distinctive pattern in the trend of the NPS.     

Imaging properties of digital magnification radiography

Flat panel detectors exhibit improved signal-to-noise ratio (SNR) and display capabilities compared to film. This improvement necessitates a new evaluation of optimal geometry for conventional projection imaging applications such as digital projection mammography as well as for advanced x-ray imaging applications including cone-beam computed tomography (CT), tomosynthesis, and mammotomography. Such an evaluation was undertaken in this study to examine the effects of x-ray source distribution, inherent detector resolution, magnification, scatter rejection, and noise characteristics including noise aliasing. A model for x-ray image acquisition was used to develop generic results applicable to flat panel detectors with similar x-ray absorption characteristics. The model assumed a Gaussian distribution for the focal spot and a rectangular distribution for a pixel. A generic model for the modulated transfer function (MTF) of indirect flat panel detectors was derived by a nonlinear fit of empirical receptor data to the Burgess model for phosphor MTFs. Noise characteristics were investigated using a generic noise power spectrum (NPS) model for indirect phosphor-based detectors. The detective quantum efficiency (DQE) was then calculated from the MTF and NPS models. The results were examined as a function of focal spot size (0.1, 0.3, and 0.6 mm) and pixel size (50, 100, 150, and 200 microm) for magnification ranges 1 to 3. Mammography, general radiography (also applicable to mammotomography), and chest radiography applications were explored using x-ray energies of 28, 74, and 120 kVp, respectively. Nodule detection was examined using the effective point source scatter model, effective DQE, and the Hotelling SNR2 efficiency. Results indicate that magnification can potentially improve the signal and noise performance of digital images. Results also show that a cross over point occurs in the spatial frequency above and below which the effects of magnification differ indicating that there are task dependent tradeoffs associated with magnification. The cross over point varies depending upon focal spot size, pixel size, x-ray energy, and source-to-image-distance (SID). For mammography, the cross over point occurs for a 0.3 mm focal spot while a 0.6 mm focal spot indicates that magnification does not improve image quality due to focal spot blurring. Thus, the benefit of magnification may be limited. For general radiography (as well as mammotomography), and chest radiography, the cross over point changes with SID. For a system with a 0.3 mm focal spot, 100 microm pixel size, a 2 m SID, and the applicable tissue thickness and scatter components, optimal magnification improved SNR2 by approximately 1.2 times for mammography and 1.5 times for general radiography (and mammotomography). These results indicate that the optimal geometry can improve image quality without changing patient dose or otherwise reduce dose without compromising image quality.     

Image processing in digital radiography: Basic concepts and applications

Digital x-ray images are routinely processed to enhance diagnostic information and to suppress irrelevant detail, and also to extract quantitative information. The basic concepts and terminology of image processing as it applies to x-ray projection radiography are discussed and defined. In general, the processing of an image involves one or more point, local, and/or global operations. Clinical examples of linear and nonlinear gray-scale and algebraic point operations are presented. Examples are also given of local operations. Included in the latter group are distortion corrections, misregistration corrections, linear filtering, and nonlinear filtering.