外文原文-DR和CR技术上的最新进展

European Journal of Radiology 72 (2009) 194201 Contents lists available at ScienceDirect European Journal of Radiology journal homepage: /locate/ejrad DR and CR: Recent advances in technology C.M. Schaefer-Prokopa,b, D.W. De Boob, M. Uffmannc, M. Prokopd aMeander Medical Center Amersfoort, Utrechtseweg 160, 3800 BM Amersfoort, Netherlands bAcademic Medical Center (AMC) Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, Netherlands cUniversity Hospital Vienna (AKH) Vienna, Dep of Radiology, Waehringer Guertel 18-20, 1090 Vienna, Austria dUniversity Medical Center (UMC), Utrecht, Image Science Center, Heidelberglaan 100, 3584 CX Utrecht, Netherlands a r t i c l ei n f o Article history: Received 7 May 2009 Accepted 7 May 2009 Keywords: Digital radiography Detector systems Dose effi ciency Physical performance a b s t r a c t After some initial reluctance, nowadays transition from conventional analogue-to-digital radiographic techniqueisrealizedinthevastmajorityofinstitutions.Theeventualtriumphofdigitaloverconventional technique is related to its undoubted advantages with respect to image quality and improved image handling in the context of a picture archiving and communication system. CR represents the older system, which matured over decades and experienced some important recent improvements with respect to dose effi ciency and work-fl ow effi ciency that strengthened its position. It represents a very versatile, economically attractive system that is equally suited for integrated systems as well as for cassette-based imaging at the bedside. DR systems offer superb image quality and realistic options for dose reduction based on their high dose effi ciency. While for a long time only integrated systems were on the market suited for a large patient throughput, also mobile DR systems became recently available. While for the next years, it is likely that DR and CR systems will coexist, the long term perspective of CR will depend on further innovations with respect to dose effi ciency and signal-to-noise characteristics while for DR economical aspects and broader availability of mobile systems will play a role. 2009 Published by Elsevier Ireland Ltd. 1. Introduction Since its introduction almost 30 years ago, digital radiography hasbecomeastandardtechnologyinmostinstitutions.Whileother techniques, such as computed tomography or magnetic resonance imaging were digital since their introduction, projection radiogra- phy made the transition from analogue-to-digital technique rather late. The eventual triumph of digital over conventional technique is related to its undoubted advantages with respect to image quality and improved image handling in the context of a picture archiving and communication system. Purpose of the following overview is to familiarize the reader withthevariouscurrenttechniques,tosummarizerecentadvances in detector technology and to discuss aspects that are critical for clinical practice. The relationship between dose and image quality, control of radiation dose in digital radiography and advanced image processing will be discussed in separate articles within this issue. Corresponding author at: Academic Medical Center (AMC) Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, Netherlands. E-mail address: S (C.M. Schaefer-Prokop). 2. Detector systems The physical properties of the detector systems translate into various advantages and disadvantages with respect to image qual- ity, equipment handling and cost 1,2. Detector systems differ with respect to the image receptor material, the readout process and the data processing. Advances occur not only with the introduction of new detectors systems but also with further refi nement of existing technologies in terms of dose effi ciency and spatial resolution. It is no longer suffi cient to globally refer to computed radiography, CR, or direct radiography, DR, but one has to take specifi c generation of technol- ogy into account when comparing system performance 3. 2.1. Computed radiography CR (storage phosphor radiography) Computed radiography, CR, was the fi rst digital radiographic technique to be introduced. CR systems are based on an imaging plate that is similar to the amplifi er screen of conventional screens from radiography. As opposed to the screens from conventional radiography, this storage phosphor imaging plate retains the infor- mation of the incident photons as a latent image which can later be retrieved by stimulation by a read-out laser. Theimageplateiscoatedononesidewithalayerofphotostimu- lable phosphor material, which consists of a thin layer of phosphor 0720-048X/$ see front matter 2009 Published by Elsevier Ireland Ltd. doi:10.1016/j.ejrad.2009.05.055 C.M. Schaefer-Prokop et al. / European Journal of Radiology 72 (2009) 194201195 crystals embedded in a binder. The commonest phosphor material is BaFX:Eu2+with X representing bromine or iodine atoms. Expo- sureoftheimageplatetoX-rayphotonsstimulateselectronswithin thephosphorlayerandtrapstheminameta-stablestate.Thislatent image of high-energy electrons can be read out by a thinly focused laser beam that releases the trapped electrons from their meta- stable state. When returning to their ground state, the electrons emit light which can be captured and digitised. The amount of light is proportional to the intensity of the X-ray beam that had exposed theplate.Thestimulatedlightemissioniscollectedviaalightguide, converted to electronic current by a photomultiplier, logarithmi- cally amplifi ed, fi ltered and fi nally digitised using a 12-bit or 16-bit analogue-to-digital converter. The fi rst versions of CR systems required more dose than screen fi lm systems for similar clinical performance. However, already these systems allowed for fl exible use of radiation dose with sub- stantial dose reduction in situations where reduced image quality was acceptable. Detector material and read-out technology were continuously improved over the past decades, which substantially improved dose effi ciency and geometric resolution. With the most recent innovations, such as dual read-out technology or needle- crystalline detectors, substantial improvements of dose effi ciency became available. Dose effi ciency is now approaching that of direct radiography systems and is better than that of a screen fi lm combi- nation with a speed class of 400 1. 2.1.1. Needle-crystalline CR/channelled CR Needle-crystalline detectors use a more effi cient X-ray absorp- tion material (CsBr:Eu2+) and instead of small amorphous crystals, the material is structured in needle-like columns. The result is reduction of lateral scattering of the emitted fl uorescent light: the light is effectively tapped and channelled within each needle crystal. Channelling of the light allows for increasing the detec- tor thickness, which in turn increases X-ray absorption and dose effi ciency without loosing geometric resolution 4. In the design of a storage phosphor plate, the thickness of the phosphor determines the balance between sensitivity and sharp- ness. Studies found that the optimal compromise between dose effi ciency and geometric resolution is a thickness of the phosphor layer of 250?m for a general purpose powder phosphor plate. For a needle-crystalline phosphor plate, the optimum thickness can be increased to 600800?m, which leads to a substantial improve- ment off dose effi ciency. Thegeometricpackingofthephosphorcanimprovedtoapprox- imately 90% with needle-crystalline systems. This compares to a packing density of approximately 70% for a standard powder phosphorplate,inwhichtheamorphousphosphorcrystalsaresus- pended in a binder 1. Increased thickness of the phosphor layer and denser packing of phosphor result in a dose effi ciency that is in the same order of magnitude as that of modern direct radiography systems 4,5 (Figs. 1 and 2). 2.1.2. Dual-sided read-out In standard CR, the latent image is read out by a laser that scans the surface of the imaging plate. Collection of the laser-stimulated phosphorescence only from one side, however, results in a loss of about 30% of potential signal: the scanning laser beam will lose power as it penetrates the phosphor layer, which will lead to less activation of trapped electrons in deeper portions of the phosphor layer.Inaddition,thephosphorescencelightwillneedtoescapethe phosphor layer and will be attenuated on the way to the surface. New types of phosphor crystals that are transparent to the laser and phosphorescence light can solve this issue. In addition, if the support material is also transparent to light, the phosphor plate can be read out from both sides, which will further improve read- out effi ciency. Such a dual-sided read-out system has two optical subsystems to guide and collect the emitted light, including two separate light guides coupled to photomultiplier tubes. The result- ing two electric signals are digitised and subsequently combined. The result is an improved signal and signal-to-noise ratio for a givenabsorptionofradiationwithinthedetector6.Thedual-sided read-out does not affect the spatial resolution since the amount of scattering of light remains unaffected. Literature reports that the dual-readoutincreasesthefractionalX-rayabsorptioneffi ciencyof the image plate by 50%. Physical and clinical evaluations confi rmed a substantial increase in image quality 6,7. 2.1.3. Line-scan CR reader Standard single-sided read-out and dual-sided read-out employ a point-scan mechanism using a laser that sequentially addresses each pixel location on the imaging plate sequentially. To increase read-out speed and read-out effi ciency a line- mechanism was introduced in which a whole row of pixels is read outsimultaneously:itusesascanheadthatconsistsofalineararray of solid-state laser diodes for stimulation of phosphorescence that is combined with an array of solid-state photodiodes for capturing the emitted light8. The light collection and detection system con- sists of specially designed optics and multiple linear, asymmetric charge coupled devices (CCDs). In a dedicated line-scan CR reader, the readout time can be reduced to less than 10s, resulting in a throughput of 240plates/h comparedto5080plates/hwithconventionalsinglecassetteread- Fig. 1. Postero-anterior chest radiographs in a patient after bone marrow transplantation: images were obtained with powder CR (a) and with needle-crystalline CR, the latter with 50% dose reduction: note the at least equivalent display of mediastinal structures with dose reduction in (b). 196C.M. Schaefer-Prokop et al. / European Journal of Radiology 72 (2009) 194201 Fig.2. Antero-posteriorradiographofapatientwithgonarthrosisobtainedwithapowderCR(acquisitiondoseequivalenttospeed400)andneedle-crystallineCR(b)obtained with the same dose: note the superior signal-to-noise ratio in (b). ers or 100150plates/h with conventional readers with cassette stacking facilities 1. It has to be noted that this scan-head design is available in various confi gurations and optimised for various clinical applica- tions. Also the needle-crystalline detector is read out using this line-scanning approach. 2.1.4. CR system confi gurations The fi rst CR systems used to be a cassette-based and required specifi c readout units. These readout units have continuously decreased in size and have become increasingly affordable. Cassette-based systems are still in widespread use, for example on intensivecareunits.Whilethesesystemsareveryfl exibleandinex- pensive, wear and tear of the phosphor plates becomes an issue. Needle-crystalline detectors are available for cassette-based sys- tems as well. CR systems have been integrated in dedicated chest stands or Bucky units that offer automatic recount and require no manual interactionwiththecassettes.Dualread-outandneedle-crystalline detectors are also available for these systems. The scan head technology can be integrated in an X-ray cassette that can be used for rapid bedside imaging: the phosphor plate can be read out immediately after exposure, and the whole cassette has a similar size as a mobile direct radiography cassette. 2.2. Selenium drum In the past, a dedicated chest stand was on the market consist- ing of a drum covered with a selenium layer and a charge meter for read-out. It incorporated an air gap and an optional grid for scatter reduction. Thoravision represented a fi xed unit for upright chest radiography and could not be used on an ICU ward. Since the intro- ductionofdirectradiography(DR)systems,seleniumdrumsystems are not produced any more, yet are still in use in some institutions and produce high quality chest radiographs usually at a dose level comparable to a 400 speed conventional fi lm-screen system. 2.3. Direct radiography DR (fl at-panel detector systems) Radiographic fl at-panel detectors for direct radiography are characterizedbyadirectreadout-matrixofelectronicelementsthat are made of thin layers of amorphous silicon thin-fi lm transistors (a Si-TFT elements) that are deposited on a piece of glass. This TFT layer is coupled with an X-ray absorption medium. Depending on the material used, there are two types of DR detec- tors: (a) Detectors using a scintillator (Cesium Iodide=CsI or Gadolinim Oxysulphide=GOS) and light sensitive photodiodes are called indirect conversion TFT detectors or opto-direct systems. Simi- lar to CR technology, the absorbed radiation is transferred into light signals in the CsI layer. However, based on the needle-type structure there is only minimal scattering of the light along the silicon elements. CsI-TFT systems are widely applied for chest and skeletal radiography and are also amenable to real-time display. More recently also mobile DR units became available, suited to be used at the bedside: they use fl exible substrates in place of glass to make the detector more robust and Gadolinim Oxysuplhide=GOS as scintillator. (b) In direct conversion systems, the detector elements consist of condensator elements made up of amorphous selenium (or other semiconducting material) that is deposited on the TFT array. Absorbed X-ray energy is directly converted into charge, obviating the intermediate step of a scintillator to provide con- version to visible light. These systems are not amenable to real-time imaging due to the tendency to produce persistent latent images. They are mostly applied in mammography units because they provide a higher dose effi ciency for the high fre- quency ranges needed in mammography. DR systems have marked advantages over standard CR. As com- pared to most modern CR units, however, differences with respect to image quality and work-fl ow organization become much less prominent 1,2,9,10. C.M. Schaefer-Prokop et al. / European Journal of Radiology 72 (2009) 194201197 Fig. 3. Four regions of interests with a simulated nodule and simulated micro-nodular interstitial markings obtained with CR-single read out (a), CR dual read-out (b), DR (a-Si-TFT) (c), and DR (50% dose reduction, d). Note the superior quality of (b) over (a) (impact of read-out technology) and the equivalent quality of (b) and (c) (superior dose effi ciency of a-Si TFT). (a) DR has a markedly higher dose effi ciency as compared to stan- dardCR.Ascomparedtodual-readoutCRandneedle-crystalline CR, however, difference in terms of dose requirements become much smaller (Figs. 3 and 4). (b) DR (CsI/TFT) systems are amenable to high frame rates (up to 30/s) making them also suited for fl uoroscopy applications. (c) Byintegratingacquisition,read-outandprocessingintoonesys- tem, throughput and workfl ow can be optimised. Integrated systems are available for both CR and DR. When CR is used as cassette-based system, e.g., at the bedside, these advantages (e.g., immediate availability of pre-read, no cassette handling) ameliorate. 2.4. CCD and CMOS-based detectors Charge coupled devices (CCDs) and complimentary metal oxide semiconductor devices (CMOS) were initially suffering from lower doseeffi ciencythatcouldnotcompetewithDRorCRsystems.They also use scintillating phosphor as an absorption medium; emitted light is directed to multiple CCD or CMOS cameras that form the radiograph. Various optical arrangements including lenses or fi ber- optic tapers are used for the coupling between the phosphor layer and the mostly relatively small cameras 11. Because only a fraction of the light could be captured by the cameras, image formation was less effi cient with respect to signal-to-noise. This limitation was even more obvious for clinical applications that require a large area (e.g., chest) than for imaging of skeletal parts that need only a small area detector. Recent developments markedly improved the coupling effi - ciency by using larger sensors and improved phosphor effi ciency, making these systems also suitable for high quality chest and large area skeletal radiography. CCD and CMOS systems have a wider distribution in the US, but are only scarcely applied in Europe. 2.5. Slot-scanning technology The slot-scan technology provides excellent scatter rejection by irradiating the body by a sliding slit beam instead of irradiating the whole body at once. The increased signal-to-noise yielded by scatter reduction effectively compensates for the 2.5 times lower intrinsic detective quantum effi ciency (DQE) of CCD technology 12. No demagnifi cation is required for slot-scanning CCD tech- nology: a CsI scintillator is coupled to a linear array of CCDs that covers the whole slot that is used to scan the chest. In a comparison study of eight digital chest systems, CCD slot- scantechnologyperformedequivalenttoCsI-DR.Despitea75%dose reductionforCsI-DRanda50%reductionforCCDslotscanning,they both outperformed a standard CR system. Advantages of the CCD technology were especially prominent for the mediastinum 13. Though these systems provide excellent image quality at rea- sonable