1、Flat Panel X ray Imaging 轉(zhuǎn)Flat Panel X-ray Imaging轉(zhuǎn)載Flat Panel X-ray Imaging平板X射線成像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.盡管70年代進(jìn)行了廣泛和深入

2、的研究，有效地設(shè)計(jì)和生產(chǎn)可靠并經(jīng)濟(jì)的數(shù)字X射線成像系統(tǒng)證實(shí)還是很難做到的。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

3、從80年代中期，射線工作者和醫(yī)學(xué)工作者耐心等到了一種技術(shù)，即在射線照相領(lǐng)域發(fā)明了一種體積小，輕便，便于攜帶的成像器，它淘汰了X射線照相所用的膠片和化學(xué)試劑、計(jì)算機(jī)射線成像(CR)中的成像板(IP板)和射線透視成像中笨重的影像增強(qiáng)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

4、has begun to change X-ray imaging-forever！終于在90年代后期，在硅谷瓦里安的金斯敦技術(shù)中心，第一個商業(yè)可用的非晶硅平板數(shù)字X射線探測器誕生了，開始永遠(yuǎn)改變X射線成像技術(shù)！Why Flat Panel?為什么選擇平板?Flat Panel Detectors(FPDs)emerge as next generation digital X-ray technology.平板探測器(FPDs)是新一代的數(shù)字X射線成像技術(shù)。Digital technology has revolutionized our lives.We are collecting,st

5、oring,analyzing and using more and more information at afaster and faster pace.X-ray imaging is no exception.數(shù)字化技術(shù)徹底改變了我們的生活。我們以越來越快的步伐，收集、保存、分析和使用越來越多的信息。X射線成像也不例外。The forces behind the digital X-ray revolution are much the same as those driving home and office technologies.Digital devices are smal

6、ler and more robust.推動X射線技術(shù)革命的力量和推動家庭和辦公室技術(shù)進(jìn)步的力量那么多。數(shù)字化設(shè)備原來越小越來越健壯。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.

7、當(dāng)圖像是數(shù)字的，它就變得便于攜帶。這個圖像可以很容易的在多方位同時采集。它可以長距離實(shí)時傳輸。數(shù)字圖像使計(jì)算機(jī)輔助診斷成為可能。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 technolo

8、gies have dramatically improved home audio and video fidelity,digital X-ray technology offers significant improvement in image quality and dose utilization.數(shù)字圖像和模擬圖像、膠片相比，更易于存檔，成本更低廉。數(shù)字圖像、視頻序列甚至立體數(shù)據(jù)流很容易鏈接到病人的病例。就像數(shù)字化技術(shù)極大提高了家庭音頻和視頻的保真度，數(shù)字化X射線技術(shù)在圖像質(zhì)量和劑量利用上都有了很大的提高。Many medical modalities,such as CT,PE

9、T,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.很多醫(yī)學(xué)治療設(shè)備，象CT

10、、PET、SPECT、MRI和超聲波是固有的數(shù)字化。然而，標(biāo)準(zhǔn)的X射線照相和透視仍然主要建立在模擬技術(shù)的基礎(chǔ)上，尤其是屏/膠片和影像增強(qiáng)器。平板探測器(FPDs)是下一代的X射線技術(shù)的產(chǎn)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

11、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射線成像器是以固態(tài)集成電路技術(shù)為基礎(chǔ)，在很多方面和可見光數(shù)字照相和攝像芯片相似。X射線成像和可見光成像的主要的差別是探測器的尺寸。在訂單中用于照相和攝像的CCD和CMOS成像器的尺寸通常是一到兩厘米。Since X-rays are

12、 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

13、 computers and their application is broadening as these displays increase in size and quality.由于X射線不容易聚焦，成像器就需要與成像物體的尺寸相當(dāng)，這就需要尺寸很大的集成電路。幸運(yùn)的是，大面積的集成電路以非晶硅薄膜晶體管(TFT)陣列的形式出現(xiàn)了?，F(xiàn)在這些TFT陣列應(yīng)用在數(shù)百萬的筆記本電腦中，隨著顯示器尺寸和質(zhì)量的不斷提高，它們的應(yīng)用范圍正在擴(kuò)大。Figure 1=Flat panel detector signal chain Anumber of detector technologies ha

14、ve 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.許多探測器技

15、術(shù)以非晶硅薄膜晶體管陣列為基礎(chǔ)發(fā)展起來了，可是最成功和廣泛應(yīng)用的探測器被稱為間接探測器。這些探測器以連接到X射線閃爍體的非晶硅TFT/光電二極管為基礎(chǔ)。在圖1中，顯示了基本的X射線轉(zhuǎn)化鏈。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 F

16、PD technology stems from the fact that both the amorphous silicon and scintillator technologies are well understood and have decades of research behind them.平板成像最通用的閃爍體是氧硫化釓和碘化銫，與標(biāo)準(zhǔn)的屏/膠片照相和透視中使用的相同。平板探測器技術(shù)的成功來源于這樣的事實(shí)：非晶硅和閃爍體技術(shù)是經(jīng)過幾十年的研究的成熟技術(shù)(well understand)。With indirect digital X-ray imaging,an X-r

17、ay 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 ala

18、yer 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.間接的數(shù)字化X射線成像過程是：X射線管發(fā)射的X射線光子穿過目標(biāo)物體，沒有被物體吸收的X射線光子撞擊一層閃爍體材料，閃爍體材料將它們轉(zhuǎn)化成可見光光子。接著，這些光子撞擊光電二極管陣列，光電二極管陣列將它們轉(zhuǎn)化成電子轉(zhuǎn)化成電子，這些電子可以

19、激活非晶硅層的像素。這些激活的像素產(chǎn)生電子數(shù)據(jù)，它可以被計(jì)算機(jī)轉(zhuǎn)化成高質(zhì)量的目標(biāo)物體圖像，目標(biāo)物體圖像在計(jì)算機(jī)顯示器顯示。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 transistor

20、s arranged in ahighly ordered array,like the grid on asheet of graph paper.使用間接的方法，平板探測器由一片附有薄層非晶硅的玻璃組成。非晶硅以極小的尺寸刻印成數(shù)百萬晶體管，排列成高度有序的陣列，就像一頁圖表上的小格子一樣。Each of these TFTs is attached to alight-absorbing photodiode making up an individual pixel(picture element).Photons striking the photodiode are convert

21、ed 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).每個薄膜晶體管附著在高吸收性的光電二極管上，從而形成各個像素。光子撞擊光電二極管，轉(zhuǎn)化成電荷的兩個載體，叫做電子空穴對，一個電

22、子空穴對包括一個帶負(fù)電的電子和一個帶正電的空穴(空白的能量空間就像一個帶正電的電子)。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.由于產(chǎn)生的電荷載體會隨著進(jìn)入的光子的強(qiáng)度變化而變化，電子圖像也就產(chǎn)生了，它可以通過計(jì)算機(jī)

23、迅速讀取和解釋產(chǎn)生數(shù)字圖像。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 visibl

24、e light photons that then pass onto the photodiode array.盡管非晶硅有很好的電子性能，但它還仍然不是一種特別好的X射線光子吸收體。因此，X射線首先撞擊由氧硫化釓或銫化碘組成的閃爍體。這個閃爍物可以吸收X射線并將他轉(zhuǎn)化成可見光光子，然后進(jìn)入光電二極管陣列。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

25、convert to charge carriers,the combination of these two materials has the highest-rated Detective Quantum Efficiency(DQE)in use today.由于碘化銫是一種很好的X射線吸收體，并能把它們轉(zhuǎn)化成可見光光子，非晶硅能把這種能量形式最好地轉(zhuǎn)化成電荷載體。碘化銫和非晶硅這兩種材料的結(jié)合產(chǎn)生了當(dāng)今使用的最高的量子檢測效率(DQE)。DQE is the yardstick by which the performance of imagers is measured.A hig

26、h DQE means images can be acquired with either superior quality or the same quality at alower dose.量子檢測效率是衡量成像器性能的尺度。高量子檢測效率意味著同樣劑量時可以獲得更好的圖像質(zhì)量，或者使用較低劑量卻獲得相同質(zhì)量的圖像。Figure 2-Flat panel detector internal architecture Flat Panel Architecture平板結(jié)構(gòu)The construction of FPDs is similar in many ways to flat pa

27、nel 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的構(gòu)造在很多方面類似于平板顯示器，并使用很多相同的技術(shù)。圖2顯示了典型的FPD構(gòu)造，中間是一個非晶硅TFT/光電二極管排列。與這個陣列緊密結(jié)合在一起的是X射線閃爍體。Gen

28、erally,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 efficien

29、cy.CsI is used in applications like low-dose fluoroscopy,where the photon flux is very low.通常X射線轉(zhuǎn)換屏，非植入屏，比如硫氧化釓，是一個獨(dú)立可拆卸的薄片，通過機(jī)械壓力與陣列緊密結(jié)合在一起。然而，如果將碘化銫屏安裝在陣列上就以獲得最好的光學(xué)耦合功效。碘化銫可以應(yīng)用于光子通量很低的低劑量X射線透視法。Figure 3-Absorption efficiency of the primary scintillators used in FPDs.(Image courtesy of Thales Elect

30、ron 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

31、 agiven dose.圖3對碘化銫和硫氧化釓吸收性能作了對比，在一定劑量下，碘化銫的光輸出大致是釓屏的兩倍，在光電二極管中產(chǎn)生的信號大約是釓屏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 lig

32、ht 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 req

33、uirements of the application.此外，因?yàn)榈饣C附著在陣列上形成柱形結(jié)構(gòu)，碘化銫比非植入屏要厚很多。圓柱可以做光管，減少在閃爍物上的光散射，比如，600m碘化銫層具有相當(dāng)于300m厚度的非植入屏。像硫氧化釓這樣的屏幕具有價格低廉、使用靈活的優(yōu)勢，因?yàn)檫@樣的閃爍屏可以很容易的更換以符合應(yīng)用的要求。Figure 4-The relative light spreading of columnar CsI versus an amorphous phosphor screen.(Photograph courtesy of Hamamatsu Photonics.)The

34、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

35、 doped cesium iodide,CsI(Tl),produce their peak light output at this frequency.閃爍體產(chǎn)生的光被陣列中光電二極管吸收，產(chǎn)生電子，電子儲存在光電二極管自身的電容中。光電二極管對550nm波長的綠光譜光吸收性能最大。硫氧化釓和鉈都摻雜著碘化銫，在這個頻率產(chǎn)生最高的光輸出。The amorphous-silicon photodiodes are typically theni-ptype.In other words,the layers in the photodiode consist of an electron-

36、rich layer at the bottom,an intrinsic or undoped layer in the middle,and ahole-rich(positively charged)layer at the top.非晶硅光電二極管是典型的ni-p型半導(dǎo)體。換句話說，光電二極管層由多層構(gòu)成，底部是電子層(負(fù)極)，中部是固有的或者不摻雜層，頂部是空穴層(正極)。This type of amorphous-silicon photodiode has the advantages of low dark current and acapacitance that is i

37、ndependent of the accumulated signal.這種非晶硅光電二極管有很多優(yōu)勢，如低暗電流和能獨(dú)立積累信號的電容。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 s

38、ignal information.光電二極管固有的熱產(chǎn)生的暗電流不斷為二極管充電。如果光電二極管有大量的暗電流，大部分二極管信號電容會在沒有信號情況下被充滿。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

39、 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成像器的晶體硅光電二極管相比，非晶硅光電二極管的暗流大大減少，因此在室溫下，非晶硅平板陣列可以超過十秒積分時間是很容易的事。事實(shí)上，二極管電容獨(dú)立于信號有助于檢測系統(tǒng)的線性特征。As is discussed later,the lineari

40、ty 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.在隨后的討論中，檢測系統(tǒng)的線性特征對有效糾正陣列和電子器件中的所有非一致性來源至關(guān)重要。Figure 5-TFT/Photodiode array schematic and view of asingle 127m pixel.The TFT/photodiode matrix is normally

41、scanned progressively,one line at atime from top to bottom.TFT/光電二極管矩陣一般是逐行掃描，從上到下每次一行，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

42、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實(shí)質(zhì)上是個開關(guān)，當(dāng)一個大正極電壓作用在一個閘門線上時，選擇行的(TFTs)開關(guān)關(guān)閉以使他們導(dǎo)電，TFTs導(dǎo)電后，選擇行的像素向數(shù)據(jù)線輸出儲存的信號電子。每個數(shù)據(jù)線末端是一個電荷積分放大器，將電荷信號轉(zhuǎn)化成電壓。At

43、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 los

44、t 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.在這點(diǎn)上，因廠商各不同電子線路不同，但大體上由一個可編程的增益階段和一個將電壓轉(zhuǎn)換成數(shù)字信號的模數(shù)轉(zhuǎn)換器。掃查的一個重要方面是，即使在輸出像素的過程中，F(xiàn)PD不斷收集整個入射信號，信號沒有一個會丟失。FPD是一個積分探測器，每個像素的積分時間等于幀時間。The electronics can be mounted

45、 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 le

46、ad.電子儀器可以被安裝于陣列的側(cè)面、光束以外，就象在高能(MeV)應(yīng)用中那樣以防輻射損壞。但是在診斷和介入過程中，為了能更清楚的看到病人，陣列后面可以安裝電子儀器，再用一個薄層的鉛保護(hù)起來。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 edg

47、e of the glass,where it is connected to astandard crystalline silicon chip by means of aTAB(tape automated bonding)package.當(dāng)非晶硅足以用作檢測電子儀器時，它就不適宜做隨后的信號處理工作。因此，陣列的每個行和列都在玻璃邊緣，這樣就可以通過TAB包和標(biāo)準(zhǔn)晶體硅芯片來連接。The TAB package bridges the disparity between the connection density on the glass and what atypical circ

48、uit 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包消除了玻璃上的連接密度和和典型電流板可處理的密度的不一致性，TAB包的玻璃面，在每100m，有128個通道，而包的板端連接在每400m有大約40或者更少的連接。The chips that

49、 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.需要直接連到陣列的芯片，讀取芯片和驅(qū)動芯片，都裝在TAB包里。圖6顯示了用于瓦里安醫(yī)學(xué)系統(tǒng)PaxSc

50、an 4030A,40x30cm(12x16)血管照相平板的行驅(qū)動芯片和讀取芯片。Figure 6-Board-side view of TAB packaged row driver and custom ASIC readout chips.Flat Panel Operational Advantages平板應(yīng)用優(yōu)勢The most obvious advantages of flat panel imagers are size and weight.The Image Intensifiers Tubes(IIT)are large and bulky.平板成像器最明顯的優(yōu)勢是尺寸和

51、重量。圖像增強(qiáng)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 dev

52、ice.The result is vastly improved access to the patient in interventional procedures.一個12x16有效區(qū)域的FPD，不到12IIT體積的25%，不到16IIT體積的15%。另外，F(xiàn)PD不僅代替了圖像增強(qiáng)器，而且還包括附帶的影像記錄裝置，包括CCD攝像機(jī)、35mm電影攝像機(jī)和現(xiàn)場電影記錄設(shè)備。結(jié)果是，在介入治療的過程，更容易接觸病人。In addition to the reduction in size,the weight of the flat panel imager is 60%less than t

53、hat 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.除了尺寸縮小以外，平板成像器的重量不到IIT成像系列的60%。傳統(tǒng)情況下，機(jī)械結(jié)

54、構(gòu)的IIT那邊是最重的。而平板，重的一面轉(zhuǎn)到了X射線管，從而減輕了所用機(jī)械結(jié)構(gòu)的體積，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.由于它的體積和重量，在可移動探壁上安裝16IIT是不切實(shí)際的,但是，將平板成像器安裝在移動探壁上就可以獲得更大的動態(tài)(活動)范圍。Th

55、e 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).Accessin

56、g 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上安裝刻錄裝置是因?yàn)槠桨寮夹g(shù)固有的多模式容量。從電子學(xué)角度來看，陣列結(jié)構(gòu)和讀出與動態(tài)隨機(jī)存取內(nèi)存(DRAM)很相似。陣列中的存取部分或興趣區(qū)僅僅是對正確的行和列的尋址問題。As with DRAM,the signal is stored as acharge packet,which makes summing the data from more th

57、an 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，信號被儲存為一個電荷包，累加多于一個像素的數(shù)據(jù)是將電荷包簡單的結(jié)合起來。將鄰近像素前置電荷積分放大器的信號結(jié)合起來，就可以很

58、輕松的將2x2像素鄰域讀做一個超像素。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

59、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.像素綁定提供兩個有利條件，第一個換位是靈敏度，因?yàn)槌袼乜梢钥吹礁嗟膞射線光子就有更高的信噪比(SNR)。通常平板最大的數(shù)字?jǐn)?shù)據(jù)轉(zhuǎn)換率被限定為一個固定值。綁定也可以減少總矩陣大小，因此可以獲得更高的幀率。例如，一個30fps 1024x1024成像器也可以讀取60fps或更高的512x512超像素。Figure 7-