FlatPanelXrayImaging转.doc

Flat Panel X ray Imaging 转Flat Panel X-ray Imaging转载Flat Panel X-ray Imaging平板X射线成像Despite extensive and well-funded research stretching well back into the 70s,the effective design and manufacture of areliable and affordable digital X-ray radiographic imaging system has proven elusive.尽管70年代进行了广泛和深入的研究，有效地设计和生产可靠并经济的数字X射线成像系统证实还是很难做到的。Since the mid-80s,radiologists and hospital care-givers have patiently awaited atechnology to eliminate film and chemicals in radiography,image plates in computed radiography,and bulky image-intensifiers in fluoroscopy with asmaller,lighter,more portable imager.自从80年代中期，射线工作者和医学工作者耐心等到了一种技术，即在射线照相领域发明了一种体积小，轻便，便于携带的成像器，它淘汰了X射线照相所用的胶片和化学试剂、计算机射线成像(CR)中的成像板(IP板)和射线透视成像中笨重的影像增强器。Finally,late in the 90s,the first commercially available flat panel digital X-ray detector using amorphous silicon found its way out of Varians Technology Center in Silicon Valley and has begun to change X-ray imaging-forever！终于在90年代后期，在硅谷瓦里安的金斯敦技术中心，第一个商业可用的非晶硅平板数字X射线探测器诞生了，开始永远改变X射线成像技术！Why Flat Panel?为什么选择平板?Flat Panel Detectors(FPDs)emerge as next generation digital X-ray technology.平板探测器(FPDs)是新一代的数字X射线成像技术。Digital technology has revolutionized our lives.We are collecting,storing,analyzing and using more and more information at afaster and faster pace.X-ray imaging is no exception.数字化技术彻底改变了我们的生活。我们以越来越快的步伐，收集、保存、分析和使用越来越多的信息。X射线成像也不例外。The forces behind the digital X-ray revolution are much the same as those driving home and office technologies.Digital devices are smaller and more robust.推动X射线技术革命的力量和推动家庭和办公室技术进步的力量那么多。数字化设备原来越小越来越健壮。Once an image is digital,it becomes portable.The image can easily be made available in multiple locations simultaneously.It can be transmitted over long distances in real-time.Digital images make possible computer-assisted diagnosis.当图像是数字的，它就变得便于携带。这个图像可以很容易的在多方位同时采集。它可以长距离实时传输。数字图像使计算机辅助诊断成为可能。Digital images are far simpler to archive and much less costly than their analog counterpart,film.Digital images,video sequences and even volumetric data sets are easily linked to apatients electronic record.And just as digital technologies have dramatically improved home audio and video fidelity,digital X-ray technology offers significant improvement in image quality and dose utilization.数字图像和模拟图像、胶片相比，更易于存档，成本更低廉。数字图像、视频序列甚至立体数据流很容易链接到病人的病例。就像数字化技术极大提高了家庭音频和视频的保真度，数字化X射线技术在图像质量和剂量利用上都有了很大的提高。Many medical modalities,such as CT,PET,SPECT,MRI and ultrasound are inherently digital.However,standard X-ray radiography and fluoroscopy are still primarily based on analog technologies,specifically,screen/film and the image intensifier.Flat panel detectors(FPDs)have emerged as the next generation digital X-ray technology.很多医学治疗设备，象CT、PET、SPECT、MRI和超声波是固有的数字化。然而，标准的X射线照相和透视仍然主要建立在模拟技术的基础上，尤其是屏/胶片和影像增强器。平板探测器(FPDs)是下一代的X射线技术的产物。Flat panel X-ray imagers are based on solid-state integrated circuit(IC)technology,similar in many ways to the imaging chips used in visible wavelength digital photography and video.The primary difference between X-ray imaging and visible imaging is the size of the detector.CCDs and CMOS imagers found in cameras and video recorders are typically on the order of one to two cm in area.平板X射线成像器是以固态集成电路技术为基础，在很多方面和可见光数字照相和摄像芯片相似。X射线成像和可见光成像的主要的差别是探测器的尺寸。在订单中用于照相和摄像的CCD和CMOS成像器的尺寸通常是一到两厘米。Since X-rays are not easily focused,the imager is necessarily on the scale of the object being imaged,which requires an enormous integrated circuit.Fortunately,a large-area IC technology exists in the form of amorphous-silicon thin-film-transistor(TFT)arrays.These TFT arrays are currently in many millions of laptop computers and their application is broadening as these displays increase in size and quality.由于X射线不容易聚焦，成像器就需要与成像物体的尺寸相当，这就需要尺寸很大的集成电路。幸运的是，大面积的集成电路以非晶硅薄膜晶体管(TFT)阵列的形式出现了。现在这些TFT阵列应用在数百万的笔记本电脑中，随着显示器尺寸和质量的不断提高，它们的应用范围正在扩大。Figure 1=Flat panel detector signal chain Anumber of detector technologies have been developed based on amorphous silicon TFT arrays,but the most successful and widely used detectors are calledindirectdetectors.These detectors are based on amorphous silicon TFT/photodiode arrays coupled to X-ray scintillators.The fundamental X-ray conversion chain is shown in Figure 1.许多探测器技术以非晶硅薄膜晶体管阵列为基础发展起来了，可是最成功和广泛应用的探测器被称为间接探测器。这些探测器以连接到X射线闪烁体的非晶硅TFT/光电二极管为基础。在图1中，显示了基本的X射线转化链。The most common scintillators used in flat panel imaging are the same ones used in standard screen/film radiography and fluoroscopy,gadolinium oxysulfide and cesium-iodide(CsI).The success of the indirect FPD technology stems from the fact that both the amorphous silicon and scintillator technologies are well understood and have decades of research behind them.平板成像最通用的闪烁体是氧硫化钆和碘化铯，与标准的屏/胶片照相和透视中使用的相同。平板探测器技术的成功来源于这样的事实：非晶硅和闪烁体技术是经过几十年的研究的成熟技术(well understand)。With indirect digital X-ray imaging,an X-ray tube sends abeam of X-ray photons through atarget.X-ray photons not absorbed by the target strike alayer of scintillating material that converts them into visible light photons.These photons then strike an array of photodiodes which converts them into electrons that can activate the pixels in alayer of amorphous silicon.The activated pixels generate electronic data that acomputer can convert into ahigh-quality image of the target,which is then displayed on acomputer monitor.间接的数字化X射线成像过程是：X射线管发射的X射线光子穿过目标物体，没有被物体吸收的X射线光子撞击一层闪烁体材料，闪烁体材料将它们转化成可见光光子。接着，这些光子撞击光电二极管阵列，光电二极管阵列将它们转化成电子转化成电子，这些电子可以激活非晶硅层的像素。这些激活的像素产生电子数据，它可以被计算机转化成高质量的目标物体图像，目标物体图像在计算机显示器显示。With the indirect approach,the flat panel detector consists of asheet of glass covered with athin layer of silicon that is in an amorphous,or disordered state.At amicroscopic scale,the silicon has been imprinted with millions of transistors arranged in ahighly ordered array,like the grid on asheet of graph paper.使用间接的方法，平板探测器由一片附有薄层非晶硅的玻璃组成。非晶硅以极小的尺寸刻印成数百万晶体管，排列成高度有序的阵列，就像一页图表上的小格子一样。Each of these TFTs is attached to alight-absorbing photodiode making up an individual pixel(picture element).Photons striking the photodiode are converted into two carriers of electrical charge,called electron-hole pairs.An electron-hole pair consists of anegatively charged electron and apositively charged hole(a vacant energy space that acts as if it were apositively charged electron).每个薄膜晶体管附着在高吸收性的光电二极管上，从而形成各个像素。光子撞击光电二极管，转化成电荷的两个载体，叫做电子空穴对，一个电子空穴对包括一个带负电的电子和一个带正电的空穴(空白的能量空间就像一个带正电的电子)。Since the number of charge carriers produced will vary with the intensity of incoming light photons,an electrical pattern is created that can be swiftly read and interpreted by acomputer to produce adigital image.由于产生的电荷载体会随着进入的光子的强度变化而变化，电子图像也就产生了，它可以通过计算机迅速读取和解释产生数字图像。Although silicon has outstanding electronic properties,it is not aparticularly good absorber of X-ray photons.For this reason,X-rays first impinge upon scintillators made from either gadolinium oxysulfide or cesium-iodide.The scintillators absorb the X-rays and convert them into visible light photons that then pass onto the photodiode array.尽管非晶硅有很好的电子性能，但它还仍然不是一种特别好的X射线光子吸收体。因此，X射线首先撞击由氧硫化钆或铯化碘组成的闪烁体。这个闪烁物可以吸收X射线并将他转化成可见光光子，然后进入光电二极管阵列。Because CsI is such an excellent absorber of X-rays,and coverts them to visible light photons at energies that amorphous silicon is best able to convert to charge carriers,the combination of these two materials has the highest-rated Detective Quantum Efficiency(DQE)in use today.由于碘化铯是一种很好的X射线吸收体，并能把它们转化成可见光光子，非晶硅能把这种能量形式最好地转化成电荷载体。碘化铯和非晶硅这两种材料的结合产生了当今使用的最高的量子检测效率(DQE)。DQE is the yardstick by which the performance of imagers is measured.A high DQE means images can be acquired with either superior quality or the same quality at alower dose.量子检测效率是衡量成像器性能的尺度。高量子检测效率意味着同样剂量时可以获得更好的图像质量，或者使用较低剂量却获得相同质量的图像。Figure 2-Flat panel detector internal architecture Flat Panel Architecture平板结构The construction of FPDs is similar in many ways to flat panel displays,and uses many of the same technologies.Figure 2shows the construction of atypical FPD.At the core is an amorphous-silicon TFT/photodiode array.Closely coupled to the array is the X-ray scintillator.FPD的构造在很多方面类似于平板显示器，并使用很多相同的技术。图2显示了典型的FPD构造，中间是一个非晶硅TFT/光电二极管排列。与这个阵列紧密结合在一起的是X射线闪烁体。Generally,the X-ray conversion screen,rare earth screens such as gadolinium oxysulfide,can be aseparate detachable sheet which is mechanically forced into close contact with the array.However,if aCsI screen is used,this is often directly deposited on the array,to give the best optical coupling efficiency.CsI is used in applications like low-dose fluoroscopy,where the photon flux is very low.通常X射线转换屏，非植入屏，比如硫氧化钆，是一个独立可拆卸的薄片，通过机械压力与阵列紧密结合在一起。然而，如果将碘化铯屏安装在阵列上就以获得最好的光学耦合功效。碘化铯可以应用于光子通量很低的低剂量X射线透视法。Figure 3-Absorption efficiency of the primary scintillators used in FPDs.(Image courtesy of Thales Electron Devices.)Figure 3shows acomparison between the absorption efficiency of CsI and gadolinium oxysulfide.In addition to its much higher absorption efficiency,CsI also produces roughly twice the light output of agadolinium screen,which results in more than four times the signal at the photodiode for agiven dose.图3对碘化铯和硫氧化钆吸收性能作了对比，在一定剂量下，碘化铯的光输出大致是钆屏的两倍，在光电二极管中产生的信号大约是钆屏4倍。Furthermore,the thickness of the CsI can be greater than that of arare earth screen because when CsI is deposited on the array it grows in acolumnar structure.The columns tend to act as light pipes,reducing the amount of light spreading in the scintillator.So,for example,a 600m CsI layer can have resolution similar to a300m thick rare earth screen.These screens such as gadolinium oxysulfide have the advantage of much lower cost and greater flexibility in that the screen can easily be changed to match the resolution requirements of the application.此外，因为碘化铯附着在阵列上形成柱形结构，碘化铯比非植入屏要厚很多。圆柱可以做光管，减少在闪烁物上的光散射，比如，600m碘化铯层具有相当于300m厚度的非植入屏。像硫氧化钆这样的屏幕具有价格低廉、使用灵活的优势，因为这样的闪烁屏可以很容易的更换以符合应用的要求。Figure 4-The relative light spreading of columnar CsI versus an amorphous phosphor screen.(Photograph courtesy of Hamamatsu Photonics.)The light generated by the scintillator is absorbed by the photodiodes in the array,creating electrons which are stored on the capacitance of the photodiode itself.The peak light absorption efficiency of the photodiodes is in the green spectrum,at 550nm wavelength.Both gadolinium oxysulfide and thallium doped cesium iodide,CsI(Tl),produce their peak light output at this frequency.闪烁体产生的光被阵列中光电二极管吸收，产生电子，电子储存在光电二极管自身的电容中。光电二极管对550nm波长的绿光谱光吸收性能最大。硫氧化钆和铊都掺杂着碘化铯，在这个频率产生最高的光输出。The amorphous-silicon photodiodes are typically theni-ptype.In other words,the layers in the photodiode consist of an electron-rich layer at the bottom,an intrinsic or undoped layer in the middle,and ahole-rich(positively charged)layer at the top.非晶硅光电二极管是典型的ni-p型半导体。换句话说，光电二极管层由多层构成，底部是电子层(负极)，中部是固有的或者不掺杂层，顶部是空穴层(正极)。This type of amorphous-silicon photodiode has the advantages of low dark current and acapacitance that is independent of the accumulated signal.这种非晶硅光电二极管有很多优势，如低暗电流和能独立积累信号的电容。The thermally generated dark current intrinsic to the photodiodes is always working to charge the diode.If the photodiodes have large amounts of dark current,much of the diodes signal capacity will be filled up by charge with no signal information.光电二极管固有的热产生的暗电流不断为二极管充电。如果光电二极管有大量的暗电流，大部分二极管信号电容会在没有信号情况下被充满。Compared to crystalline silicon photodiodes like those used in CMOS imagers,the dark current in amorphous silicon photodiodes is orders of magnitude less.So it is not unusual for amorphous silicon flat panel arrays to be capable of more than ten second integration times at room temperature.The fact that the diode capacitance is independent of signal helps make the detection system linear.和用于CMOS成像器的晶体硅光电二极管相比，非晶硅光电二极管的暗流大大减少，因此在室温下，非晶硅平板阵列可以超过十秒积分时间是很容易的事。事实上，二极管电容独立于信号有助于检测系统的线性特征。As is discussed later,the linearity of the detection system is critical to being able to efficiently correct for all the sources of non-uniformity in the array and the electronics.在随后的讨论中，检测系统的线性特征对有效纠正阵列和电子器件中的所有非一致性来源至关重要。Figure 5-TFT/Photodiode array schematic and view of asingle 127m pixel.The TFT/photodiode matrix is normally scanned progressively,one line at atime from top to bottom.TFT/光电二极管矩阵一般是逐行扫描，从上到下每次一行，The TFTs are essentially switches.When alarge positive voltage is applied to one of the gate lines,the switches(TFTs)in the selected row are closed,causing them to conduct electricity.With the TFTs energized,each pixel in the selected row discharges the stored signal electrons onto the dataline.At the end of each dataline is acharge integrating amplifier which converts the charge packet to avoltage.TFT实质上是个开关，当一个大正极电压作用在一个闸门线上时，选择行的(TFTs)开关关闭以使他们导电，TFTs导电后，选择行的像素向数据线输出储存的信号电子。每个数据线末端是一个电荷积分放大器，将电荷信号转化成电压。At this point the electronics vary by manufacturer,but generally there is aprogrammable gain stage and an Analog-to-Digital Converter(ADC),which converts the voltage to adigital number.One important aspect of the scanning is that the FPD is continuously collecting the entire incident signal；none is lost even during the discharge of the pixel.The FPD is an integrating detector and the integration time for each pixel is equal to the frame time.在这点上，因厂商各不同电子线路不同，但大体上由一个可编程的增益阶段和一个将电压转换成数字信号的模数转换器。扫查的一个重要方面是，即使在输出像素的过程中，FPD不断收集整个入射信号，信号没有一个会丢失。FPD是一个积分探测器，每个像素的积分时间等于帧时间。The electronics can be mounted to the side of the array,out of the beam,as is done in higher energy(MeV)applications to protect against radiation damage.But for diagnostic and interventional procedures,to maintain the best view of the patient,the electronics can also be mounted behind the array and protected by athin layer of lead.电子仪器可以被安装于阵列的侧面、光束以外，就象在高能(MeV)应用中那样以防辐射损坏。但是在诊断和介入过程中，为了能更清楚的看到病人，阵列后面可以安装电子仪器，再用一个薄层的铅保护起来。While amorphous silicon has properties sufficient for the detection electronics,it is not suited to the subsequent signal processing.For this reason,every column and row of the array is brought to the edge of the glass,where it is connected to astandard crystalline silicon chip by means of aTAB(tape automated bonding)package.当非晶硅足以用作检测电子仪器时，它就不适宜做随后的信号处理工作。因此，阵列的每个行和列都在玻璃边缘，这样就可以通过TAB包和标准晶体硅芯片来连接。The TAB package bridges the disparity between the connection density on the glass and what atypical circuit board can handle.The glass side of the TAB package may have 128 channels at a100m pitch,while the board side connections of the package are on apitch of 400m with roughly forty or fewer connections.TAB包消除了玻璃上的连接密度和和典型电流板可处理的密度的不一致性，TAB包的玻璃面，在每100m，有128个通道，而包的板端连接在每400m有大约40或者更少的连接。The chips that need to be directly connected to the array,the readout chip and the driver chip,are mounted in these TAB packages.Figure 6shows apicture of the row driver and readout chips used in Varian Medical SystemsPaxScan 4030A,40x30cm(12x16)angiography panel.需要直接连到阵列的芯片，读取芯片和驱动芯片，都装在TAB包里。图6显示了用于瓦里安医学系统PaxScan 4030A,40x30cm(12x16)血管照相平板的行驱动芯片和读取芯片。Figure 6-Board-side view of TAB packaged row driver and custom ASIC readout chips.Flat Panel Operational Advantages平板应用优势The most obvious advantages of flat panel imagers are size and weight.The Image Intensifiers Tubes(IIT)are large and bulky.平板成像器最明显的优势是尺寸和重量。图像增强器管大而笨。An FPD with a12x16active area(20diagonal)takes up less than 25%of the volume of a12IIT and less than 15%of that of a16IIT.In addition,the FPD takes the place of not only the IIT,but also the attached image recording devices,including the CCD camera,35mm Cine camera,and the spot film device.The result is vastly improved access to the patient in interventional procedures.一个12x16有效区域的FPD，不到12IIT体积的25%，不到16IIT体积的15%。另外，FPD不仅代替了图像增强器，而且还包括附带的影像记录装置，包括CCD摄像机、35mm电影摄像机和现场电影记录设备。结果是，在介入治疗的过程，更容易接触病人。In addition to the reduction in size,the weight of the flat panel imager is 60%less than that of the IIT-based imaging chain.Traditionally,the IIT side of the mechanical structure is the heaviest.With flat panels,the heavy side shifts to the X-ray tube,which offers the possibility of areduction in the bulk and cost of the mechanical superstructure.除了尺寸缩小以外，平板成像器的重量不到IIT成像系列的60%。传统情况下，机械结构的IIT那边是最重的。而平板，重的一面转到了X射线管，从而减轻了所用机械结构的体积，降低了成本。Mounting a16IIT on amobile C-arm is impractical because of its size and weight,but achieving an even larger active area with aflat panel imager on amobile C-arm is now straightforward.由于它的体积和重量，在可移动探壁上安装16IIT是不切实际的,但是，将平板成像器安装在移动探壁上就可以获得更大的动态(活动)范围。The fact that the recording devices attached to an IIT are not required with FPDs is aresult of the multi-mode capability inherent in flat panel technology.From an electrical point of view,the array architecture and readout are very similar to those used for dynamic random access memory(DRAM).Accessing sections or regions-of-interest(ROIs)in the array is only amatter of addressing the proper columns and rows.FPDs不需要在IIT上安装刻录装置是因为平板技术固有的多模式容量。从电子学角度来看，阵列结构和读出与动态随机存取内存(DRAM)很相似。阵列中的存取部分或兴趣区仅仅是对正确的行和列的寻址问题。As with DRAM,the signal is stored as acharge packet,which makes summing the data from more than one pixel asimple matter of combining the charge packets.Reading out a2x2 neighborhood of pixels as one super-pixel is easily done by combining the signals from neighboring pixels at the front-end charge integrating amplifiers.和DRAM相同，信号被储存为一个电荷包，累加多于一个像素的数据是将电荷包简单的结合起来。将邻近像素前置电荷积分放大器的信号结合起来，就可以很轻松的将2x2像素邻域读做一个超像素。Pixel binning offers two important opportunities for trade-offs.The first trade-off is sensitivity,since the super pixel will see more X-ray photons and so have higher signal-to-noise ratio(SNR).Very often the maximum digital data conversion rate of the panel is limited to afixed value.Binning also can reduce the overall matrix size,thus allowing higher frame rates.For example,a 1024x1024 imager capable of 30fps can also be read out as 512x512 super pixels at 60fps or higher.像素绑定提供两个有利条件，第一个换位是灵敏度，因为超像素可以看到更多的x射线光子就有更高的信噪比(SNR)。通常平板最大的数字数据转换率被限定为一个固定值。绑定也可以减少总矩阵大小，因此可以获得更高的帧率。例如，一个30fps 1024x1024成像器也可以读取60fps或更高的512x512超像素。Figure 7-T