1、Digital Radiology with Photon Counting Detectors Uwe ZSCHERPEL 1,David WALTER 1,Bernhard REDMER 1,Uwe EWERT 1,Christer ULLBERG 2,Niclas WEBER 2,Tuomas PANTSAR 3 1 Division 8.3,BAM Federal Institute for Materials Research and Testing,Berlin,Germany Phone:+49 30 8104 3677,Fax:+49 30 8104 1837;E-mail:u
2、we.zscherpelbam.de,david.walterbam.de,bernhard.redmerbam.de,uwe.ewertbam.de 2 XCounter AB,Danderyd,Sweden;E-mail:christer.ullbergxcounter.se,niclas.weberxcounter.se 3 Oy Ajat Ltd,Espoo,Finland;E-mail:tuomas.pantsarajat.fi Abstract The progress in X-ray detector electronics(sensitivity and speed)allo
3、ws meanwhile fast single photon detection by a matrix detector.Combined photon counting and energy discrimination is implemented in the electronic circuit of each detector pixel.The company XCounter developed detectors based on CdTe single crystals,which can be tiled to larger areas and have a pixel
4、 size of 100m.The largest area available in beginning of 2014 is 50 x75 mm.These detectors have very promising properties,which make them very suitable for NDT applications:1.A CdTe attenuation layer of 750 m thickness allows efficient X-ray detection up to ca.300 keV.In counting mode only photon no
5、ise is important;no other detector noise sources need to be considered.There is no offset signal without radiation.2.Each of the detector pixels has two energy thresholds.These can be used for dual energy imaging for materials separation.Also the suppression of scattered radiation by energy threshol
6、ding will improve the image contrast sensitivity.First experiments will be presented which demonstrate the advantages of this new detector technology over the conventional charge integrating detectors.A challenge is the development of a modified detector calibration procedure,which becomes critical
7、at longer exposure times.Keywords:materials characterization,Radiographic Testing(RT),digital detector array,photo counting,image quality,calibration,dual-energy,energy discrimination 1.Introduction Photon counting detectors offer some unique features for x-ray imaging.If designed correctly,photon c
8、ounting detectors have no readout noise and no dark counts.This is an important feature for high detection efficiency and constant detector performance at various ambient temperatures requiring minimum calibration.Additionally,it is also possible to incorporate pulse height discrimination of each ph
9、oton event,thus enabling the recording of images from multiple energy intervals in a single exposure.The work presented here is based on the first tests with the XCounter PDT25-DE and XC-T 7550 detectors.These detectors are based on an upgraded version of the earlier MGC700 platform 1 that now incor
10、porates smaller pixels(100m),dual energy thresholds and faster counting capabilities 2.2.Detection principle and Detector construction The Detector PDT25-DE consists of two XCounter XC225 ASICs pump-bonded to two solid-state CdTe-crystals using ohmic contacts(detection area of 25x25 mm),whereas the
11、XC-T 7550 detector consists of 2x6 tiles of the same modules resulting in a larger detection area of 50 x75 mm.The PDT25-DE uses a USB2 interface for control and image transfer,which limits the frame rate to max.10 frames/s.The XC-T 7550 detector implements a GBit-Ethernet interface,which allows var
12、ying the frame rate from 300 frame/s down to 2 frames/s.Fig.1 provides the construction principle of larger tiled detector areas.The detection crystal is bump-bonded to an underlying ASIC for pixel read-out.The ASIC XC225 has a pixel size of 100 m and implements two counters of 12 bit each,which cou
13、nts the photons passing two 11th European Conference on Non-Destructive Testing(ECNDT 2014),October 6-10,2014,Prague,Czech Republicadjustable thresholds(the first for discrimination of photon counting events against noise and the second for low energy counts against high energy counts).Fig.1:State o
14、f the art of photon counting detectors in medicine and NDT(left side construction principle of the detection area from assembled tiles,right side image of the detector PDT-25 using 2 tiles from XCounter.Various other manufacturers exists too)Fig.2:View of the XC-T7550 detector with the 75x50 mm dete
15、ction area(covered by CFRP,black)In Fig.2 a larger detector version XC-T75550 with 75x50 mm detector area is shown without the radiation shielding case made from Densimet.Also,the temperature stabilization of the detector ASICs using water cooled Peltier elements is not shown here.3.Results and Disc
16、ussion 3.1 Dual energy thresholding for reduced detection of scattered radiation One major advantage of the photon counting principle is the adjustable energy threshold.Only photons having a distinct minimum threshold reaching the detector will be counted.The usage of energy thresholding was investigated first for reduction of the influence of scattered radiation on the radiographic image.In Fig.3 a Fe steel step wedge is shown and two images(low energy up to 80 keV and high energy image above 8
