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1、第9卷增刊 2000 年 9 月 (95- 99)CT理论与应用研究 CTTheor y andApplicationsVo1 .9, Supplement Sept-,2000TheResearchRepor t ontheIndustrialCf O Cr)ICrX0171raoYongjie (BeijingYouLinnSiTeTechnicalCompany,LTD.) (Summar y )1The Selection ofthe technicalplan1.1 Aboutthe勺 peofscan:ICTcartbethe2ndgene r ationCr or the3rdg
2、enerationCTac cordingtoits scanningtype.Fo r the 2ndgeneration,此 ob jectbeing seamed translates at oneconstant anglelocationat f irst Havingcompletedthetranslationtheobjectr otates ace r tainangle(generally 1 2 degrees).Thenanew translationbeginsatthenewanglelocation . Inthiswaythetranslation-r otat
3、 ionr epeats15timestocove r the total 180degrees.Thetranslationdistanc e isthelengthofthechordofthedetector s arrayar e infaceofthe fan-beamdivergence angle the objec t diameter.The nu mbe r of detectors:64,32or 24.The advantages of 2ndgenerationscan二二 】assamountofdetectors,thef i eldofviewisnot lim
4、itedbythefanbeamangleand if ssatiationscopeisbigger,the imagehasnoringartifac t Theshortcomingsofthe2ndgenerationscanare theslowe r scan ning speed and the compl icated mechanical movement.We choose the second generation scanstyletoincr e asetheaccuracyandthescanningscope. 1.2Abouttherec o nstructio
5、noperation:Cr isakindoftechnology tomaptheinter naldensit y distri bution f r om the x-ray integration signalswhich pass through the ob ject via a conversion .We use the convolution-back : pr ojectionmethodtocompletether econstructionope rat ion.Itisthemostpopular way in cu rrent Cr image reconstruc
6、 t ion because of its good reconstr u c t ed image qual it y and less calculat i on requirement Theconvolut ionofthepr oject ionwiththefi lte r ingfr a ct ionmeansspacialf i lteringandremoves theblu r , so better r e constructed image can be obtained af ter back pr ojec t ion . To achieve the attenu
7、ation coef ficientofanarbitrar y pixeltheinterpolationoperationsforthepro jectionlinecoordina吸“二 】 reede比 The reconstruction needs massive triangular funct ion operations andconvolu tion operations, and the operation time isquitelong.We二。 vatc xl away that al l the calculations二 car v ed out intheti
8、me domainandthe wholeoperationsarebasical lyaddit ioncalculations.Theoperationtimedu ration:isr e markabb r educed.Itis suitable to the demand in china 13About the f i lteri ng function: Having investigated into 二f i ltering ， func t ions u sed in the reconstructio n ope ration, wechoosetheButtex wc
9、 rthf i lter Its二features are:bigf requencyregulat ing range,theoperatorcanfreely 呐ustthe f i lte r parameters includingfrequencyparameter Fm, pass-bandFp, stop-bandFs.Theshapeofthedispersio n curveofthefilte r canbechangedbymeansoftheaboveparameter r egulationtoimpr ovetheimagequality.Firstwecalcul
10、atethef i m ctionR(f)inf r equencydomain,thencar) outtheinverseFFTto尹 theconvolutionf i mctionoftheButterwor thflite r intherealspaceandgoonthe backprojectionoperation.25ystem C ompositionThepr oductconsistsofx-ra ysource,sc anm e chanism ,f ront and“collimator,32channelsdetector s ,Ampl if ier circ
11、uit,Samplingcontr olsystems,f r ontcomputer,二 computer andlaserprinter. X-raysource:WeuseaGerman-madeequipment,focusspotsige:0.4mm,energy 225kv , current 3mA, highvoltagestabilization:0.01%. Scan mechanism: It has 3 degrees of f reedom: t ranslation, rotation , up-and-down movement All96CTTheor y an
12、d Applicat i onsVol., movements are driven 切 step motor contr o l 妙computer. T he repeantion accuracy of the mechanism 0刀Imm Frontandrearcollimators:Itisusedtoensurethex-raybeamtobecollimatedtolessthan0.3mminwidth Detecto r s: Sol i dcrystale detecto r s andPMT ar e used Signal conversion speed canr
13、each 10MHZ The del icage detector structure increasesthe S/Nvalue Ampl if i cationandsampl ingcircuits:Weusehighspeedamplif i cationand disc riminationcircuits,422 transmissionlineforsignaltransmission,32channelssampling,high-speedstorage. 32channelshigh-speedsynchronousdataacquisitionareguaranteed
14、Elec trical and mechanical contort: The motor movements are oontr oled 卜 the computer inte r face Since the driving pulses are obtained by the f r equency division of 此 c rystal oscillat ion, the motor movementsare inhighaccuracy. Fr o nt computer.It isinchargeofdata acqu isition,sendingsignaltocont
15、rolthemotors,thecoordination ofal l themovingpartsinCr scan ner,thedata Aeparationanddatacorrect ion,etc. Rear computer:Itisinchargeofthereconst ruct ionoperat ionanddigitalimageproessing. Design parametersselection: Theenerg y select ion:For obfectswith dif f erent densit ies dif ferent source ener
16、gy should be arranged ICTis used tomeasu re the density of an object and the densityof the object dependsonitsabsorpt ioncoef f i cient Thecountselect ion:Countrat e isthemainfactortoaf f ectthehomo-geneityofthedensity.Itsamountwil l causethecountf l uctuation.T hecountrateisconshamted妙thex-rayintensityandtheamplif icationcircuits. Thestatist icsobeytherule(s-error,N-thecountrateofthedetectors). Thespeedse l ection:Thesc anningspeeddirectlydeb a rminesthetotalscantimeofonesl ic e ofimage.Itis
