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 a reliable 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 a technology to eliminate film and chemicals in radiography, image plates in computed radiography, and bulky image-intensifiers in fluoroscopy with a smaller, 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 Varian’s 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 a faster 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 a patient’s 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. CCD’s 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

A number of detector technologies have been developed based on amorphous silicon TFT arrays, but the most successful and widely used detectors are called “indirect” detectors. 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 a beam of X-ray photons through a target. X-ray photons not absorbed by the target strike a layer 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 a layer of amorphous silicon. The activated pixels generate electronic data that a computer can convert into a high-quality image of the target, which is then displayed on a computer 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 a sheet of glass covered with a thin layer of silicon that is in an amorphous, or disordered state. At a microscopic scale, the silicon has been imprinted with millions of transistors arranged in a highly ordered array, like the grid on a sheet 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 a light-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 a negatively charged electron and a positively charged hole(a vacant energy space that acts as if it were a positively 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 a computer to produce a digital 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 a particularly 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 a lower 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 2 shows the construction of a typical 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 a separate detachable sheet which is mechanically forced into close contact with the array. However, if a CsI 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 3 shows a comparison 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 a gadolinium screen, which results in more than four times the signal at the photodiode for a given 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 a rare earth screen because when CsI is deposited on the array it grows in a columnar structure. The columns tend to act as light pipes, reducing the amount of light spreading in the scintillator. So, for example, a 600μm CsI layer can have resolution similar to a 300μm 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，600μm碘化铯层具有相当于300μm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 the “ni-p” type. 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 a hole-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 a capacitance 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 diode’s 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 a single 127μm pixel.

 

The TFT/photodiode matrix is normally scanned progressively, one line at a time 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 a large 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 a charge integrating amplifier which converts the charge packet to a voltage.

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 a programmable gain stage and an Analog-to-Digital Converter (ADC), which converts the voltage to a digital 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 a thin 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 a standard crystalline silicon chip by means of a TAB (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 a typical circuit board can handle. The glass side of the TAB package may have 128 channels at a 100μm pitch, while the board side connections of the package are on a pitch of 400μm 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 100μm，有128个通道，而包的板端连接在每400μm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 6 shows a picture of the row driver and readout chips used in Varian Medical Systems’ PaxScan® 4030A, 40x30cm (12”x16”) 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 (12”x16”)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 a 12”x16” active area (20” diagonal) takes up less than 25% of the volume of a 12” IIT and less than 15% of that of a 16” IIT. 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12”x16”有效区域的FPD，不到12” IIT体积的25%，不到16” IIT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 a reduction 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 a 16” IIT on a mobile C-arm is impractical because of its size and weight, but achieving an even larger active area with a flat panel imager on a mobile C-arm is now straightforward.

由于它的体积和重量，在可移动探壁上安装16” IIT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 a result 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 a matter 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 a charge packet, which makes summing the data from more than one pixel a simple matter of combining the charge packets. Reading out a 2x2 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 a fixed 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 - This is experimental R&F equipment used by Hitachi in a clinical evaluation of flat panel technology. Here a 12”x16”(active area) FPD is mounted on the side of a 12” Image Intensifier Tube (IIT). With this system, either the FPD or IIT could be rotated into place, facilitating straightforward comparison images.

 

The flat panel imager shown in Figure 7 typically has three modes: full field, full resolution at 7.5fps used for DSA and radiography; full field, half resolution at 30fps used for fluoroscopy and cine; and ¼ field of view, full resolution at 30fps which is used for fluoroscopic zoom mode. Since these modes are defined only by the method of addressing the array, the panel can switch between modes in less than one second. Flat panels also are more economical than an IIT of comparable size considering that we know IIT image quality deteriorates as a simple consequence of everday X-ray use, and thus IIT’s have a relatively short service life.

图7显示的平板成像器通常有三个模式：全场，7.5fps全分辨率用于DSA和射线照相：全场，30fps一半分辨率用于X射线透视和摄影；¼场30fps全分辨率用于X射线透视检测缩放模式。因为这些模式被定为阵列的寻址方法，平板可以在不到一秒钟内在两种模式之间转换。平板 和相对大小的IIT 相比更加经济，因为日常对X射线的使用会降低IIT图像质量，因此IIT的使用寿命会相对较少。

 

Varian life tests have demonstrated that the service life of a typical flat panel is significantly longer than that of an IIT, assuming the same usage. In the most favorable cases, the flat panel will match the life expectancy of the equipment.

瓦里安使用寿命测试表明：在相同的使用条件下，平板使用寿命要大大高于IIT。在大多数有利条件下，平板完全可以达到设备使用寿命的要求。

 

 Figure 8 - Signal output comparison vs. X-ray dose between a PaxScan 4030A and an IIT of comparable size.

Flat Panel Image Quality

平板成像质量

 

Figure 9 - Comparison between a 12” IIT system and GE’s 20cm Revolution flat panel detector. Clearly the image intensifier has both distortion and brightness non-uniformity which is absent from the flat panel detector. (Image courtesy of GE Medical Systems.)

Beyond the improved form factor and flexibility of flat panels, there are also enhancements to the image quality. In IIT-based imaging there are many stages in the signal conversion chain, each of which is subject to losses and distortion. For example the electron optics inside the IIT is influenced by the earth’s magnetic field. And the optics of the output phosphor is subject to veiling glare, which is a kind of a long-range crosstalk. The result is geometric distortion and brightness non-uniformity across the diameter of the image. In comparison, flat panels have a very direct, short signal conversion path, with essentially no optics. The result is a very flat, uniform “film-like” image from edge-to-edge. Similar to the distortion and brightness nonuniformity, the IIT/CCD image chain also suffers from contrast loss at the edges of the image, which is why the IIT performance is always specified at the center.

除了提高了平板的形成因子和灵活性以外，图像质量也大大提高。在IIT为基础的成像中，在信号转化链中有很多步，每一步都会有损耗和失真。例如，IIT中的电子镜片会受到地磁场的影响。输出磷光体的镜片容易遭受强光的遮盖，这是一种大范围的色亮度干扰。这会使图像在图像直径方向几何失真和亮度不一致。想必之下，平板具有直接、简短的信号转换路经，没有镜片。结果是平板的图像从边缘到边缘是平整的、一致的类似于胶片的图像。和扭曲和亮度不一致相似，IIT/CCD成像链容易遭受从边缘到边缘的对比度丢失影响，这就是为什么IIT性能总是定在中心位置。

 

Figure 10 - Entrance dose per frame by application.

 

The ability of flat panel detectors to encompass multiple X-ray modalities is also a function of their very large dynamic range. Figure 10 shows the detector entrance dose per frame by application. An FPD designed for R&F applications can cover the entire range, from low-dose fluoroscopy to radiography. Generally, the Analog-to-Digital Converters (ADCs) inside the FPD have a fixed input range. In order to adapt the signal to that range, there is an analog gain stage prior to digitizing. Higher gain is used for smaller signals, but at the expense of the maximum dose per frame. Because of the very different resolution, energy and dose requirements, mammography requires a specialized flat panel detector.

平板探测器比其他X射线有很多优越性，也是因它的大动态范围功能。图10显示了在应用中平板探测器每帧的输入剂量。为R&F应用设计的FPD可以覆盖很多方面，从低剂量的透视到射线照相。通常，FPD内部的模拟数字转换器(ADCs)有一个固定的输入范围。为了将信号调到那个范围，在数字化之前有一个模拟增益过程。高的增益适用于弱信号，但必须是每帧的最大剂量。由于不同的像素、能量、剂量要求，乳腺X射线照相需要有一个特制的平板探测器。

The large exposure latitude in flat panel imaging means that retakes, due to over and under exposure, are virtually eliminated. The large dynamic range also enables computed tomography applications, where the entrance dose per projection can vary from the low dose fluoroscopy level to the equivalent of radiography shots.

平板成像很大的曝光范围意味着照相时曝光过量和不足实际上被消除了。大的动态范围也能使计算机层析照相(CT)应用于从低剂量的透视水平到设备的射线照相。

 

Figure 11 - Signal vs. entrance dose for a 194μm, 40x30 cm FPD.

 

Figure 11 shows the signal response of an angiography flat panel in the full resolution mode, over the dose range of 1μR to 1.2mR. Particularly for the amorphous silicon TFT/photodiode technology, the response to entrance dose of the FPD is extremely linear. The response of the imager deviates from the straight line curve by less than 0.01%. This degree of linearity is necessary to facilitate the real-time offset and gain correction that is normally performed for each pixel. Because each pixel in the array has its own gain value and zero signal level, the image would be very non-uniform without normalization. If the imager response is non-linear, the arithmetic for normalizing the pixel response in real-time becomes intractable.
图11显示了在1uR到1.2mR的剂量范围内，血管照相平板在全分辨率模式下的信号响应。尤其是非晶硅TFT/光电二极管技术，对进入FPD的剂量响应是极其线性的。成像器的反应偏离垂直曲线图小于0.01%。这样的线性程度对帮助每个像素进行实时偏离和增益校正是很必需的。由于阵列中的每个像素有自身的增益值和零信号电平，没有经过标准化处理，图像很难是一致的。如果成像器响应是非线性的，实时校准像素响应的算法就是相当有难度的。

 

Figure 12 - SNR vs. entrance dose for a 194μm, 40x30cm FPD.

 

Of equal importance is the Signal-to-Noise (SNR) behavior versus dose. The FPD contributes electronic noise to the image. However, if the electronic noise is low enough, the statistical noise in the X-ray beam is dominant. The noise in the beam follows Poisson statistics.  In other words, the noise is equal to the square root of the number of incident X-ray photons. As shown in Figure 12, the SNR of an FPD has a square root dependence on dose, i.e. is X-ray quantum limited over a very large range. This is an indication that the detector contributes effectively no noise to the image over this dose range.

信噪比(SNR)活动和计量的比值也同样重要。FPD对图像带来电子噪声。然而，如果电子噪声足够低，在X射线束中，统计噪声是占大部分。。射线束中的噪声带来有害的统计规律。换句话说，噪声相当于X射线光子数量的平方根。如图12所示，FPDSNR的平方根根据剂量而定，也就是说，X射线量子在很大的范围受到限制。这表明在这个剂量范围，探测器可以有效地去除对图像的噪声。

A more accurate way to look at the sensitivity of an imaging system is to evaluate the Detective Quantum Efficiency (DQE) as a function of spatial frequency. Particularly for digital X-ray technologies, this single measure is a common reference point that accounts for noise, signal-loss mechanisms, sensitivity and resolution. Essentially the DQE is equal to the square of the ratio of the imager’s output SNR to its input SNR.

研究成像系统灵敏度的一个更精确的办法是精确一个考虑成像系统是将量子检测效率(DQE)当作空间频率的函数来计算。特别是对于数字X射线技术，这个单一的测量是计算噪声、信号丢失机制、灵敏度和分辨率常用的参考办法。DQE本质上是等于成像器输出信噪比的平方和输入信噪比SNR的平方的比值。如下所示：

 

 

DQE is typically calculated using the formula:

DQE通常是用这个公式计算的：

 

　

Where, for a given input dose, d is the average output signal produced, MTF is the modulation transfer function (a measure of resolution), q is the number of incident X-ray quanta and NPS is the noise power spectrum produced by the imager. Because the X-ray beam statistics are Poisson distributed, the input SNR 2 is in fact equal to the total number of X-ray photons, q. The rest of the equation defines the output SNR 2 as a function of signal, resolution and noise.

一定输入剂量，d是产生的产成的平均输出信号，MTF是调制转换函数(一种分辨率的尺度)，q是入射X射线量子的数量，NPS是成像器产生的噪声能量光谱。因为X射线束统计表是泊松分布，输入信噪比的平方事实上等于X射线光子的总数，q。剩下的方程式将输入信噪比的平 方定义为信号、分辨率和噪声的函数。

 

 

The DQE as a function of entrance dose (fluoro and cine range) for an angiographic flat panel imager is shown in Figure 13. This is typical of the indirect TFT/photodiode technology with a CsI scintillator. Because of the low-loss, high-absorption detection path, the DQE for indirect CsI-based flat panels is the highest available and is more than double that of computed radiography, film screen and CCD based technologies. Higher DQE translates directly into better imager quality for a given dose.

图13表明DQE是血管照相平板成像器输入剂量(透视和摄像范围)的函数。这是典型的具有碘化铯闪烁体的间接薄膜晶体管(TFT)/光电二极管技术。由于低损耗、高吸收探测途径，碘化铯间接平板的DQE是目前最高级的，是计算机射线照相(CR)、胶片屏和CCD为基础技术的两倍多。在一定的剂量下，更高的DQE可以直接转化成更好的图像质量。

So with high DQE detection systems, it is possible to get the same image quality as a low DQE system like screen film at a fraction of the dose.

因此，具有更高量子检测效率(DQE)的检测系统，可以在较小的剂量下，得到和较低检测效率系统如屏幕胶片一样的图像质量

 

Digital X-ray Radiography

数字X射线成像

 

Figure 13 - DQE vs. spatial frequency over the fluoroscopic and cine dose range, for a 40x30cm angiographic flat panel.

 

The numerous advantages of flat panel detectors over X-ray film, computed radiography image plates, and image-intensifier tubes is clear. This is evident to the thousands of leading radiologists and cardiologists performing routine X-ray examinations, test engineers inspecting for cracks in aircraft structures, and security forces screening for and disarming explosives. Digital X-ray imaging available instantly just like that of modern digital cameras and camrecorders is the future. Please contact your local Varian sales office for the latest on our PaxScan family of flat panel X-ray detectors.

和X射线胶片、计算机射线照相(CR)成像板(IP)和图像增强器相比较，平板探测器的很多优势是显而易见的。这一点被很多人认识到了，包括著名的放射工作者、进行日常X射线检查的心脏病专家、检测航行器结构裂纹的检测工程师和防爆安全部队。不久的将来，即时数字X射线成像会象现在的数码相机和摄像机一样普及。请联系你们当地的瓦里安销售办公室，获取关于我们PaxScan系列平板X射线探测器的

 

X-Ray Products

X射线产品

Headquartered in Salt Lake City our flat panel team is committed to meeting the diverse and growing needs of Digital Radiography in many markets: Medical, industrial/NDT, security, and scientific. World renown for our X-ray tubes, flat panel X-ray detectors are an important addition to our growing X-Ray business.

总部设在盐湖城，我们的平板队伍在努力地满足很多市场对数字射线成像不断增长的多样化需求：医学、工业/NDT，安全检查和科学研究。闻名世界的X射线管和平板X射线探测器是我们不断增长的X射线业务的重要组成部分。

 

http://www.huaketest.com/华科平版介绍.htm

图片本自文件原处.