数字化平板血管造影系统辐射剂量优化研究进展

目的:调查研究平板型数字减影血管造影系统剂量优化进展。方法:通过校园图书馆平台,查阅近10年间(从2001年到2011年)收录在中国知网、PubMed、Springerlink等文库的国内外科研文献,以及介绍DSA发展的相关书籍,并做回顾性的分析。结果:从数字化血管造影系统问世以来,在剂量优化上国内外学者作了大量工作。随着平板探测器技术在DSA中的应用,平板型血管造影系统的剂量优化国外学者作了大量研究,而国内做的研究相对较少。平板探测器具有高DQE的特点,决定了平板探测器具有较高的组织密度分辨能力。平板型数字减影血管造影系统具有高空间分辨率,宽动态范围,方形视野,几何失真小等特点,这些特点使得平板型数字减影血管造影系统在临床应用中有着传统影像增强器系统无法企及的优势,辐射剂量的降低也成为可能。而后旋转3D技术等后处理程序在DSA中的应用,也使得平板探测器的应用达到一个全新的高度。结论:平板型数字减影血管造影系统能够在保证临床需要的情况下更合理的优化辐射剂量。     

甲状腺-乳腺防护衣在乳腺X线摄影中降低辐射剂量的研究

目的:采用屏蔽防护法屏蔽和降低乳腺X射线摄影中散射辐射对受检者甲状腺及乳腺所致辐射剂量的方法,并对其采用防护措施的防护效果进行试验研究。方法:在数字化乳腺X射线摄影(DR)中,随机选取30例乳腺X射线受检者,采用本中心新研制的0.5 mm Pb铅橡胶甲状腺-乳腺防护衣对受检者进行屏蔽。采集数据方法:在防护衣内侧和外侧分别各放置一组LiF(Mg·Cu·P)热释光剂量探测器,在同一检测条件下进行照射后,分别测量甲状腺-乳腺防护衣内侧和外侧两组入射空气比释动能数值。结果:防护衣内外入射空气比释动能的差异具有统计学意义(P0.05)。防护衣对甲状腺的防护效率为(99.25±0.99)%,对侧乳腺的防护效率为(99.17±1.03)%。结论:甲状腺-乳腺防护衣可以降低甲状腺和乳腺在X射线摄影时的散射辐射,并且屏蔽防护效果显著。     

量子检测效率在平板探测器评估中的研究进展

平板探测器(flat panel detector,FPD)是X射线摄影系统中至关重要的部分,其性能直接影响所采集图像的质量。量子检测效率(detective quantum efficiency,DQE)涉及探测器的噪声、分辨率、剂量、调制传递函数、噪声功率谱等多项参数,被公认是X射线成像性能最准确的评估指标,DQE越高,说明影像系统在低X射线入射剂量的情况下,获得高质量影像的能力越强。我们简单介绍了平板探测器的种类,对其DQE的检测技术的研究现状、进展及应用作了综述,归纳了在DQE检测过程中的有关影响因素,并进行了总结展望。     

数字X线探测器的性能参数

在数字Ｘ线成像中，探测器的性能占有重要的地位，它直接决定了Ｘ线成像的质量。重要的特性有：视野范围、几何学特性、量子效率、灵敏度、空间分辨率、噪声特性、动态范围、均匀性、探测速度、帧频及费用，大多数情况下，不同探测器技术都有必要综合考虑这些因素。本文主要对以上参数进行了讨论。     

数字X线探测器的性能参数

在数字Ｘ线成像中，探测器的性能占有重要的地位，它直接决定了Ｘ线成像的质量。重要的特性有：视野范围、几何学特性、量子效率、灵敏度、空间分辨率、噪声特性、动态范围、均匀性、探测速度、帧频及费用，大多数情况下，不同探测器技术都有必要综合考虑这些因素。本文主要对以上参数进行了讨论。     

核磁共振序列及在体检测γ-氨基丁酸的双量子滤波序列设计

目的：近年来,核磁共振技术在医学领域得到了广泛的应用及发展,为使其应用价值更大,需要不断探寻新的或改进原有的技术方法。以基本原理为出发点结合目前研究进展,对核磁共振技术的发展前景进行展望。本文旨在探索、改进和开发多（双）量子滤波序列以及其在检测γ-氨基丁酸（GABA）中的作用和检测效率。方法：针对γ-氨基丁酸的检测,分析总结自旋回波（SE）序列、点分辨自旋回波序列（PRESS）并在此基础上研究双量子点分辨自旋回波序列;利用量子滤波技术可以在单次激发中抑制其它信号,通过优化序列设计实现选择性的检测γ-氨基丁酸。结果：通过在点分辨自旋回波序列上编辑新的双量子滤波序列,实现了在体检测γ-氨基丁酸。结论：因γ-氨基丁酸是维持正常脑功能的一种重要的抑制性神经递质,且由于γ-氨基丁酸的相对浓度很低以及其微弱的信号与其它物质信号的重叠,要有效地利用量子滤波技术能够很好地检测γ-氨基丁酸,对定位序列、读出脉冲、滤波梯度的应用仍需进一步改进。基于在MRS中量子滤波的技术特性,解决或改进其在序列设计中存在的问题将大大提高该技术在临床中的应用价值。     

CR系统在乳腺钼靶摄影中的应用

目的 探讨计算机X线成像(computedradiography,CR)在乳腺钼靶X检查中的优越性.方法 抽取2500例来我科作乳腺检查者,其中行普通钼靶检查者2000例,另外进行经CR处理的数字化乳腺摄影者500例,对其照片进行分析、评价.结果 普通钼靶摄影组的满意率、重摄率、阳性率分别为84%、11%和82%;数字化乳腺摄影组的满意率、重摄率、阳性率分别为100%、0和89%.结论 现有普通钼靶X线机及CR系统的结合实现了乳腺检查的半数字化,可提高其诊断效果.     

基于DPDW的乳腺吸收系数重建算法研究

用光子密度波（diffuse photon density waves,DPDW）检测乳腺癌是一种新的无创测量方法。由于乳腺腺体与癌变组织对光的吸收系数不同,通过光子密度波的变化重建出异物的吸收系数,可以应用于乳腺癌检测。但在实际应用中采用有限的光源-探测器对的探测数据对乳腺组织进行重建,离散化过程中引入的位置信息误差会严重影响最后的检测结果。本文提出了权值矩阵均值法,克服了离散化过程误差。仿真验证了该方法的有效性。     

基于定量影像组学的乳腺肿瘤良恶性诊断

乳腺癌是女性致死率最高的恶性肿瘤之一。为提高诊断效率,提供给医生更加客观和准确的诊断结果。借助影像组学的方法,利用公开数据集BreaKHis中82例患者的乳腺肿瘤病理图像,提取乳腺肿瘤病理图像的灰度特征、Haralick纹理特征、局部二值模式(LBP)特征和Gabor特征共139维影像组学特征,并用主成分分析(PCA)对影像组学特征进行降维,然后利用随机森林(RF)、极限学习机(ELM)、支持向量机(SVM)、k最近邻(kNN)等4种不同的分类器构建乳腺肿瘤良恶性的诊断模型,并对上述不同的特征集进行评估。结果表明,基于支持向量机的影像组学特征的分类效果最好,准确率能达到88.2%,灵敏性达到86.62%,特异性达到89.82%。影像组学方法可为乳腺肿瘤良恶性预测提供一种新型的检测手段,使乳腺肿瘤良恶性临床诊断的准确率得到很大提升。     

乳腺非介入管电压影响因素研究

目的:参照IEC 61676标准,研究随着管电压的增加,距离、过滤条件和阳极角等影响因素在两种乳腺辐射场（Mo/Mo和W/Rh）中的改变引起Fluke X2和RTI piranha 657两类非接入测量仪器测量结果的变化及差异。方法:利用介入法测量确定管电压稳定性,在两种乳腺辐射场情况下改变距离、过滤条件和阳极角进行非介入管电压测量,根据测量结果计算两种辐射场和两类仪器测量结果的变化趋势,并用蒙特卡罗模拟获得两种辐射场条件下因为过滤变化达到仪器的计数比值差异（R值变化率）并分析原因。结果:在W/Rh辐射场下,即使距离和阳极角差异较大、过滤条件发生微小变化,对两类仪器测量结果的影响符合IEC 61676的要求,都在0.5 kV以内;但是在Mo/Mo辐射场下,同样过滤条件变化引起的两类仪器测量结果差异较大,其中Fluke X2测量结果最大变化甚至达到了1.6 kV。结论:当对乳腺非介入管电压测量仪器进行校准时,各个实验室应该考虑确立统一的校准距离、球管阳极角和过滤要求作为重要的参考条件,特别是在Mo/Mo辐射场下对过滤的限制异常重要,以减少对非介入管电压平均峰值电压测量结果的影响,保证各个校准实验室间结果的一致性,提高可比性;建议进行乳腺非介入管电压测量的时候尽量不使用平均峰值电压来反应测量结果,而应使用实用峰值电压。     

Comparison of edge analysis techniques for the determination of the MTF of digital radiographic systems

The modulation transfer function (MTF) is well established as a metric to characterize the resolution performance of a digital radiographic system. Implemented by various laboratories, the edge technique is currently the most widespread approach to measure the MTF. However, there can be differences in the results attributed to differences in the analysis technique employed. The objective of this study was to determine whether comparable results can be obtained from different algorithms processing identical images representative of those of current digital radiographic systems. Five laboratories participated in a round-robin evaluation of six different algorithms including one prescribed in the International Electrotechnical Commission (IEC) 62220-1 standard. The algorithms were applied to two synthetic and 12 real edge images from different digital radiographic systems including CR, and direct- and indirect-conversion detector systems. The results were analysed in terms of variability as well as accuracy of the resulting presampled MTFs. The results indicated that differences between the individual MTFs and the mean MTF were largely below 0.02. In the case of the two simulated edge images, all algorithms yielded similar results within 0.01 of the expected true MTF. The findings indicated that all algorithms tested in this round-robin evaluation, including the IEC-prescribed algorithm, were suitable for accurate MTF determination from edge images, provided the images are not excessively noisy. The agreement of the MTF results was judged sufficient for the measurement of the MTF necessary for the determination of the DQE.     

Imaging performance of amorphous selenium based flat-panel detectors for digital mammography: characterization of a small area prototype detector

Our work is to investigate and understand the factors affecting the imaging performance of amorphous selenium (a-Se) flat-panel detectors for digital mammography. Both theoretical and experimental methods were developed to investigate the spatial frequency dependent detective quantum efficiency [DQE(f)] of a-Se flat-panel detectors for digital mammography. Since the K edge of a-Se is 12.66 keV and within the energy range of a mammographic spectrum, a theoretical model was developed based on cascaded linear system analysis with parallel processes to take into account the effect of K fluorescence on the modulation transfer function (MTF), noise power spectrum (NPS), and DQE(f) of the detector. This model was used to understand the performance of a small-area prototype detector with 85 microm pixel size. The presampling MTF, NPS, and DQE(f) of the prototype were measured, and compared to the theoretical calculation of the model. The calculation showed that K fluorescence accounted for a 15% reduction in the MTF at the Nyquist frequency (fNy) of the prototype detector, and the NPS at fNy was reduced to 89% of that at zero spatial frequency. The measurement of presampling MTF of the prototype detector revealed an additional source of blurring, which was attributed to charge trapping in the blocking layer at the interface between a-Se and the active matrix. This introduced a drop in both presampling MTF and NPS at high spatial frequency, and reduced aliasing in the NPS. As a result, the DQE(f) of the prototype detector at fNy approached 40% of that at zero spatial frequency. The measured and calculated DQE(f) using the linear system model have reasonable agreement, indicating that the factors controlling image quality in a-Se based mammographic detectors are fully understood, and the model can be used to further optimize detector imaging performance.     

Intercomparison of methods for image quality characterization. II. Noise power spectrum

Second in a two-part series comparing measurement techniques for the assessment of basic image quality metrics in digital radiography, in this paper we focus on the measurement of the image noise power spectrum (NPS). Three methods were considered: (1) a method published by Dobbins et al. [Med. Phys. 22, 1581-1593 (1995)], (2) a method published by Samei et al. [Med. Phys. 30, 608-622 (2003)], and (3) a new method sanctioned by the International Electrotechnical Commission (IEC 62220-1, 2003), developed as part of an international standard for the measurement of detective quantum efficiency. In addition to an overall comparison of the estimated NPS between the three techniques, the following factors were also evaluated for their effect on the measured NPS: horizontal versus vertical directional dependence, the use of beam-limiting apertures, beam spectrum, and computational methods of NPS analysis, including the region-of-interest (ROI) size and the method of ROI normalization. Of these factors, none was found to demonstrate a substantial impact on the amplitude of the NPS estimates (< or = 3.1% relative difference in NPS averaged over frequency, for each factor considered separately). Overall, the three methods agreed to within 1.6% +/- 0.8% when averaged over frequencies > 0.15 mm(-1).     

Performance of a 41 x 41 cm2 amorphous silicon flat panel x-ray detector designed for angiographic and R&F imaging applications

We measured the physical imaging performance of a 41 x 41 cm2 amorphous silicon flat panel detector designed for angiographic and R&F imaging applications using methods from the emerging IEC standard for the measurement of detective quantum efficiency (DQE) in digital radiographic detectors. Measurements on 12 production detectors demonstrate consistent performance. The mean DQE at the detector center is about 0.77 at zero frequency and 0.27 at the Nyquist frequency (2.5 cycles/mm) when measured with a 7 mm of Al HVL spectrum at about 3.6 microGy. The mean MTF at the center of the detector for this spectrum is 0.24 at the Nyquist frequency. For radiographic operation all 2048 x 2048 detector elements are read out individually. For fluoroscopy, the detector operates in two 30 frame per second modes: either the center 1024 x 1024 detector elements are read out or the entire detector is read out with 2 x 2 pixel binning. A model was developed to predict differences in performance between the modes, and measurements demonstrate agreement with the model. Lag was measured using a quasi-equilibrium exposure method and was found to be 0.044 in the first frame and less than 0.007 after 1 s. We demonstrated that it is possible to use the lag data to correct for temporal correlation in images when measuring DQE with a fluoroscopic imaging technique. Measurements as a function of position on the detector demonstrate a high degree of uniformity. We also characterized dependences on spectrum, exposure level, and direction. Finally, we measured the DQE of a current state of the art image intensifier/CCD system using the same method as for the flat panel. We found the image intensifier system to have lower DQE than the flat panel at high exposure levels and approximately equivalent DQE at fluoroscopic levels.     

Measurement of the detective quantum efficiency in digital detectors consistent with the IEC 62220-1 standard: practical considerations regarding the choice of filter material

As part of a larger evaluation we attempted to measure the detective quantum efficiency (DQE) of an amorphous silicon flat-panel detector using the method described in the International Electrotechnical Commission standard 62220-1 published in October 2003. To achieve the radiographic beam conditions specified in the standard, we purchased scientific-grade ultrahigh purity aluminum (99.999% purity, type-11999 alloy) filters in thicknesses ranging from 0.1 through 10.0 mm from a well-known, specialty metals supplier. Qualitative evaluation of flat field images acquired at 71 kV (RQA5 beam quality) with 21 mm of ultrahigh purity aluminum filtration demonstrated a low frequency mottle that was reproducible and was not observed when the measurement was repeated at 74 kV (RQA5 beam quality) with 21 mm of lower-purity aluminum (99.0% purity, type-1100 alloy) filtration. This finding was ultimately attributed to the larger grain size (approximately 1-2 mm) of high purity aluminum metal, which is a well-known characteristic, particularly in thicknesses greater than 1 mm. The impact of this low frequency mottle is to significantly overestimate the noise power spectrum (NPS) at spatial frequencies < or = 0.2 mm(-1), which in turn would cause an underestimation of the DQE in this range. A subsequent evaluation of ultrahigh purity aluminum, purchased from a second source, suggests, that reduced grain size can be achieved by the process of annealing. Images acquired with this sample demonstrated vertical striated nonuniformities that are attributed to the manufacturing method and which do not appear to appreciably impact the NPS at spatial frequencies > or = 0.5 mm(-1), but do result in an asymmetry in the x- and y-NPS at spatial frequencies < or = 0.2 mm(-1). Our observations of markedly visible nonuniformities in images acquired with high purity aluminum filtration suggest that the uniformity of filter materials should be carefully evaluated and taken into consideration when measuring the DQE.     

Signal-to-noise properties of mammographic film-screen systems

The signal-to-noise ratio (SNR) and the detective quantum efficiency (DQE) have been experimentally determined as a function of spatial frequency for several mammographic film-screen systems. These two parameters were determined from our measurements of noise power spectra and sensitometric properties of each system along with modulation transfer function (MTF) data for the screens which were obtained from others. From the noise power spectra, it was found that film noise contributes significantly to the total noise of mammographic film-screen systems, comprising 30%-50% of the total noise at 1 cycle/mm and as much as 75% at 5 cycles/mm. All systems had approximately the same SNR below 1.5 cycles/mm, but differed at higher frequencies due to differences in screen MTF and in the gradient of the film's sensitometric curve. The DQE curves varied between systems at all frequencies, however, due to differences in system speed, MTF, and gradient. Generally, the DQE of mammographic film-screen systems is between 10%-30% at frequencies below 1 cycle/mm and decreases to about 1% between 8 and 12 cycles/mm. Compared to film-screen systems used in general radiography, mammographic systems have similar DQE values at low frequencies, but are superior at higher frequencies.     

Intercomparison of methods for image quality characterization. I. Modulation transfer function

The modulation transfer function (MTF) and the noise power spectrum (NPS) are widely recognized as the most relevant metrics of resolution and noise performance in radiographic imaging. These quantities have commonly been measured using various techniques, the specifics of which can have a bearing on the accuracy of the results. As a part of a study aimed at comparing the relative performance of different techniques, in this paper we report on a comparison of two established MTF measurement techniques: one using a slit test device [Dobbins et al., Med. Phys. 22, 1581-1593 (1995)] and another using a translucent edge test device [Samei et al., Med. Phys. 25, 102-113 (1998)], with one another and with a third technique using an opaque edge test device recommended by a new international standard (IEC 62220-1, 2003). The study further aimed to substantiate the influence of various acquisition and processing parameters on the estimated MTF. The slit test device was made of 2 mm thick Pb slabs with a 12.5 microm opening. The translucent edge test device was made of a laminated and polished Pt(0.9)Ir(0.1). alloy foil of 0.1 mm thickness. The opaque edge test device was made of a 2 mm thick W slab. All test devices were imaged on a representative indirect flat-panel digital radiographic system using three published beam qualities: 70 kV with 0.5 mm Cu filtration, 70 kV with 19 mm Al filtration, and 74 kV with 21 mm Al filtration (IEC-RQA5). The latter technique was also evaluated in conjunction with two external beam-limiting apertures (per IEC 62220-1), and with the tube collimator limiting the beam to the same area achieved with the apertures. The presampled MTFs were deduced from the acquired images by Fourier analysis techniques, and the results analyzed for relative values and the influence of impacting parameters. The findings indicated that the measurement technique has a notable impact on the resulting MTF estimate, with estimates from the overall IEC method 4.0% +/- 0.2% lower than that of Dobbins et al. and 0.7% +/- 0.4% higher than that of Samei et al. averaged over the zero to cutoff frequency range. Over the same frequency range, keeping beam quality and limitation constant, the average MTF estimate obtained with the edge techniques differed by up to 5.2% +/- 0.2% from that of the slit, with the opaque edge providing lower MTF estimates at lower frequencies than those obtained with the translucent edge or slit. The beam quality impacted the average estimated MTF by as much as 3.7% +/- 0.9% while the use of beam limiting devices alone increased the average estimated MTF by as much as 7.0% +/- 0.9%. While the slit method is inherently very sensitive to misalignment, both edge techniques were found to tolerate misalignments by as much as 6 cm. The results suggest the use of the opaque edge test device and the tube internal collimator for beam limitation in order to achieve an MTF result most reflective of the overall performance of the imaging system and least susceptible to misalignment and scattered radiation. Careful attention to influencing factors is warranted to achieve accurate results.     

Performance evaluation of a "dual-side read" dedicated mammography computed radiography system

The image quality of a dedicated mammography computed radiography (CR) system was characterized. A unique feature of this system is that it collects image signals from both sides of the storage phosphor. Measurements of the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) were made. This work included improvements in our measurement methods to specifically account for the detrimental effects of system glare on the MTF and to accurately characterize the low-frequency NPS components. Image quality measurements were performed using a 25 kVp beam filtered with 2 mm Al and an exposure range of 1 to 100 mR (87 to 870 microGy). The DQE was found to decrease with increasing exposure due to an increased contribution of storage phosphor structure noise. The DQE of this system was compared to similar measurements made using a standard CR system. The dual-side read system demonstrated superior DQE compared to the standard system. The decrease in DQE with increasing exposure was more severe for the standard system than the dual-side read system. This finding suggests that the CR system noise was reduced for the dual-side read system compared to the standard system.     

System performance of a prototype flat-panel imager operated under mammographic conditions

The results of an empirical and theoretical investigation of the performance of a high-resolution, active matrix flat-panel imager performed under mammographic conditions are reported. The imager is based upon a prototype, indirect detection active matrix array incorporating a discrete photodiode in each pixel and a pixel-to-pixel pitch of 97 microm. The investigation involved three imager configurations corresponding to the use of three different x-ray converters with the array. The converters were a conventional Gd2O2S-based mammographic phosphor screen (Min-R) and two structured CsI:Tl scintillators: one optimized for high spatial resolution (FOS-HR) and the other for high light output (FOS-HL). Detective quantum efficiency for mammographic exposures ranging from approximately 2 to approximately 40 mR at 26 kVp were determined for each imager configuration through measurements of x-ray sensitivity, modulation transfer function (MTF), and noise power spectrum (NPS). All configurations were found to provide significant presampling MTF at frequencies beyond the Nyquist frequency of the array, approximately 5.2 mm(-1) , consistent with the high spatial resolution of the converters. In addition, the effect of additive electronic noise on the NPS was found to be significantly larger for the configuration with lower system gain (FOS-HR) than for the configurations with higher gain (Min-R, FOS-HL). The maximum DQE values obtained with the CsI:Tl scintillators were considerably greater than those obtained with the Min-R screen due to the significantly lower Swank noise of the scintillators. Moreover, DQE performance was found to degrade with decreasing exposure, although this exposure-dependence was considerably reduced for the higher gain configurations. Theoretical calculations based on the cascaded systems model were found to be in generally good agreement with these empirically determined NPS and DQE values. In this study, we provide an example of how cascaded systems modeling can be used to identify factors limiting system performance and to examine trade-offs between factors toward the goal of maximizing performance.     

Determination of the two-dimensional detective quantum efficiency of a computed radiography system

Based on a recently described method for determining the two-dimensional presampling modulation transfer function (MTF), the aperture mask method, a method for determining the two-dimensional detective quantum efficiency (DQE) of a digital radiographic system was developed. The method was applied to a new computed radiography (CR) system and comparisons with one-dimensional determinations of the presampling MTF and the DQE were performed. The aperture mask method was shown to agree with the conventional tilted slit method for determining the presampling MTF along the axes. For the particular CR system studied, the mean of one-dimensional determinations of the DQE in orthogonal directions led to a representative measure of the average DQE behavior of the system up to the Nyquist frequency along the axes, but a deviation was observed above this frequency. In conclusion, the method developed for determining the two-dimensional DQE can be used to determine the imaging properties of a digital radiographic detector system over almost the entire frequency domain, the exception being the lowest frequencies (< or = 0.1 mm(-1)) at which the validity and the reliability of the method are low.