CT(设备构成).ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Component,s of a CT scannerCT,扫描仪组成,计算机,操作台,监视器,X,发生器,X-RAY TUBE,COLLIMATOR,探测器,DAS,扫描架,BODY,扫描床,3.1,Components of a CT scannerCT,扫描仪组成,Components of a CT scannerCT,扫描仪组成,3.2 X-Ray Tube,球管的发展,球管功能及构成,球管特性,球管新技术,球管的高压发生器,3.2 X-Ray Tube,I.,球管的发展,固定阳极（,stationary anode,）,X,线管（,I,、,II,代,CT,、牙科,CT,等）,旋转阳极,(rotating anode)X,线管,（全身,CT,）,II.X-Ray tube components and functions,球管构成及功能,球管组成,Envelope,外壳,Cathode,阴极,Anode,阳极,Protective housing X,射线管防护罩,Tube window,：,A segment of glass that is,thinner,than the rest of the glass envelope.,Contributes to inherent filtration,：,0.5mm Al equivalency.,The Filament function,灯丝功能,When current(mA)is applied to the coil of wire,electron are ejected.,发射电子,The Filament materials,灯丝材料,usually made of,tungsten,钨材料,higher thermionic emission,高的热电子发射率,high melting point,高熔点,Append,Thorium,（钍）,增加电子发射率和延长灯丝寿命,II.2.3 The Anode Assembly,阳极组件,Function,：,电子在阳极受阻，韧致辐射,产生,X,线,Type,Stationary anode,固定阳极,Rotating anode,旋转阳极,Stationary Anode X-ray Tube,固定阳极,X,线管,Used for dental x-rays,some portable imaging,牙科,X,射线、部分便携式成像设备,Used when high tube current and power are not required because they are not capable of producing high-intensity x-ray beams in a short time,不需在短时间内产生高强度,X,射线，因此用于球管电流和功率要求不高时,Rotating Anode X-ray Tube,旋转阳极,X,线管,Tungsten-rhenium alloy(,铼钨合金,),：弹性、寿命,Molybdenum,（,钼,）：高热容量,Graphite,（,石墨,）：更高热容量,The advantage of the rotating anode,旋转阳极的优点,High Power,功率大,The effective size of Focus decrease,有效焦点变小,The,function,of Protective Housing,：,Useful X-Ray beam,产生有用,X,线,Provides,mechanical support,（支撑）,for the tube and protects from rough handling,Some tubes,contain oil,that serves as an insulator against electric shock and as a thermal cushion-,dissipate heat,Some protective housing has,cooling fan,to air-,cool the tube and oil,When x-rays are produced,they are isotropic emitted,各向同性,发射,X,线,useful or primary beam,leakage radiation,II.3.1,Focal spot,焦点,The,area,of the anodes target where x-rays are emitted,The smaller,the focal spot,the better,the resolution of the resultant image,Dual Focus Arrangements,双焦点,Most diagnostic tubes have two focal spots;large&small,大焦点和小焦点,Focal spot penumbra effect,焦点与半影,焦点尺寸、焦点到扫描物体距离,半影,Sharp image,Blurred image,Penumbra effect,Flying spot size,飞焦点,Philips,：电场偏转,Siemens,：磁场偏转,Flying spot size,飞焦点,控制电子束的位移，在很短的时间内在两个位置间移动发射,X,线，相当于多了一次采集或采集数量的加倍；,采样增加、空间分辨力增加,有效焦点大小与靶面倾角,effective,focal spot,size vs,anode angle,ELECTRONS,anode,Focal spot,EFFECTIVE,FOCAL,SPOT,EYE,cathode,II.3.3,Heat capacity,and,heat dissipation,热容量,和,散热率（冷却速率）,Heat unit is how much heat an anode can store before damage occurs,热容量表示阳极损坏之前的储热能力,单位：,HU,kVp,mA,time,rectification constant,Heat dissipation means how fast a tubes anode cools down,散热率表示阳极冷却时间,单位：,HU/min,H,eat capacity,vs.,anode angle,热容量与靶面倾角,For the same effective focal spot size,the heat capacity increases as the anode angle decreases,有效焦点一样时，热容量增大，阳极角度减小；,HEAT,HEAT,EFFECTIVE,FOCAL SPOT SIZE,EFFECTIVE,FOCAL SPOT SIZE,ELECTRONS,ELECTRONS,ANODE,ANODE,45,0,30,0,II.4 New CT x ray tube,（,Straton x-ray tube,）,Medical Physics-September 2004-Volume 31,Issue 9,pp.2699-2706,A novel cooling principle,on one hand,，,a novel beam control system,on the other hand.,全新的冷却原理和射束控制技术,Straton x-ray tube,Provides,direct cooling of the anode,with all bearings located outside the vacuum.,阳极和轴承置于真空之外，对阳极直接冷却；,Eliminates the need for waiting times due to anode cooling in clinical workflow.,（无需等待冷却）,Straton,球管与传统球馆的不同在于阴极和阳极一起固定在绝缘油中,球管工作时，不单是阳极旋转，而是在马达的带动下,整个球管的旋转,；,阳极的背面直接靠在管壁上极易散热,，冷却油通过循环直接将热带走；,偏转线圈,的作用是使电子束达到适当的阳极位置，轰击靶面产生,X,线；通过偏转线圈的信号控制可,改变焦点位置,飞焦点,。,Straton x-ray tube,Tube Failure,球管损坏,Tube arcing,打火,Filament breakage,灯丝击穿,Stator failure,定子损坏,Normal Anode Wear,阳极击穿,IV.1 Generator purpose,高压发生器作用,Th,e generator produces,high voltage,and,transmits it to the x-ray tube,.This high voltage propels the electrons from the x-ray tube filament(,灯丝,)to the anode,发生器提供高压并将其传输到,X,线管，使得电子从球管灯丝到达阳极,Provide the heat current of filament,灯丝加热电流,Time Control,控制,X,线曝光时间,IV.2 Generator form,发生器形状,In the past,the high-voltage,generator,was a bulky piece of equipment located in a corner of the x-ray room,过去高压,发生器,是大体积的设备，安置在,X,线室的一角。,CT scanner now use high-frequency generators,which are small,compact,and more efficient.It is located,inside the CT gantry,如今,CT,扫描仪使用高频发生器，它的体积更小，更紧凑，效率更高。安装在,CT,机架内,In some scanners,the high-frequency generator is mounted on the rotating frame,安放在,旋转,架内,Whereas in others it is located in one corner of the gantry and does not rotate,安放在机架的一角,固定,不动,IV.3,Single kVp,or,variable kVp,单项,kVp,和,可变,kVp,A generator can have a single kVp or variable kVp output,kVp accuracy is more easily achieved in single kVp systems,单,kVp,输出,系统的,kVp,更精确,Variable kVp,systems do allow for an increased capability to discriminate different tissues,Variable kVp,输出系统区别不同组织的能力提高,Generators have different,power,ratings,from,3 to 60 kW,these ratings enable,a wide range of exposure,techniques to be selected,generally 80,100,120,130,140kV,and 30,50,60,100,125,150,175,200mA;,Wh,en using a variable kVp system,requires a very,sophisticated,generator,（,复杂的发生器,）,IV.3 Generator Pulse or Continuous,Continuous Generator,连续式发生器,A“continuous on”generator requires less maximum power and permits more photons to power and permits more photons to be used in the imaging process,连续方式对发生器最大功率要求较低，允许更多的光子被激发，同时也就使得用于图象处理中的光子数更多,由于,螺旋,CT,尤其是,MSCT,的需求，,现均采用连续式,Pulsed Generator,脉冲式发生器,要求较高的瞬时功率，并可在脉冲间歇时间内自动地进行每个测量通道的零点校准,投影数与被检测物体的要求相匹配,脉冲方式可以利用适当的发生器来切换从脉冲到脉冲的,X,射线管电压，并可以在测量系统旋转一周时摄制出两幅不同能量的图像,(,连续工作方式却做不到这一点,),。这是有效地应用双谱线法的重要先决条件。,第三代,广泛应用,目前不再采用,3.3,Filters,滤过器,tube output,detector,I,o,I,m,I=I,0,e,-,m,d,I,0,I,d,m,1,m,2,m,3,m,n,I,0,I,I=I,0,e,-d,Sm,i,i=1,n,d,I=I,0,e,-,m,x,dx,m,x,I,0,I,0,D,x,D,Tube output energy,object,Idea,Mono,energetic,Un,iform,Real,Poly,energetic,Non,uniform,I.The Purpose of Filters,To,absorb low-energy x-rays,(soft X-Ray)that do not contribute to the acquisition of CT image,吸收对,CT,成像没有贡献的软射线。,By doing this,patient dose is reduced,and the beam quality increases(,beam “harder”,),降低患者剂量并提高射线质量（射线变硬）,To shape the energy distribution across the radiation beam to produce uniform beam hardening when x-rays pass through the filter and the object,使出射射束能量分布定型为均匀一致的硬射束,2.,Two types of filters,Flat filter,平板滤过器,copper or aluminum,铜或铝制成,to modify the x-ray spectrum uniformly across the entire field of view,在整个,FOV,范围内射线能量均匀,Bow-tie filter,蝴蝶结滤过器或楔形滤过器,To reduce the dynamic range,（减少动态范围）,of the data acquisition system,根据大人、小孩要做更换,Avoiding beam-hardening artifact,Adequate beam filtration,flat filter,“bowtie”filter,3.4,Collimators,准直器,3.4,Collimators,准直器,Purpose:,目的,used for collimating the x-ray beam to a desired width,用于校准,X,线束到,期望宽度,Types:,类型,the tube collimator(,Pre-patient collimator),球管侧准直器（前准直器）,the detector collimator(,Post-patient collimator,),探测器侧准直器（后准直器）,准直器,X-RAY TUBE,1 MM Cu FILTER,BEAM LIMITTING,EARS,POST PATIENT,COLLIMATOR,DETECTOR,SLICE THICKNESS,PLATES,BEAM SHAPER,(NOT IN TOMO M/EG),单层,CT,的准直器,7 mm,5 mm,3 mm,2 mm,10 mm,(COLLIMATOR),Pre-patient collimator,前准直器,Shape the beams,射束塑型,Scan thickness,扫描层厚,（单层面,CT,）,The,umbra,：,uniformity,本影区均匀,The,penumbrae,：,non-uniformity,半影区不均匀,Slice Profile,Post-patient collimator,后准直器,Shapes beam also,射束塑型,helps to remove scatter,消除散射,ensure constant beam width at detector,确保到探测器的射束宽度,一致,可不用，如有前后准直器必须精确对准，否则产生条形伪影,准直器的原理,辐射衰减物质,，用以限制到达探测器组件的放射线角度分布,空间定位,，即仅局限于某一空间单元的射线进入探测器，而其他部分的射线则被屏蔽而不能进入探测器,准直器的材料选择,对,X,射线吸收大，易于加工,经济,一般采用铅或含有少量的锑、铋的铅合金等,探测器的发展,钨酸镉晶探测器,(CdWO4)-70,年 代,高压氙气探测器,(Xenon)-80,年 代,闪烁晶体探测器,(GOS)-80,年 代,稀土陶瓷探测器,(ceramic)-80,年 代 末,“,红宝石,”,探测器,=,石榴石晶体,+,磷化合物,（,LefightSpeed CT750 HD,）,Aquilion,detector,3.5 Detectors,I.,Purpose,Gather information by measuring the x-ray transmission through the object;,测试穿过物体的,X,线,Detector element,探测器单元,X-ray beam X,线束,Electrical signal proportional to x-ray beam intensity,与,X,线强度成正比的,电信号,II.Characteristic,Efficiency,效率,Stability,稳定性,Detector response,响应,1.Efficiency,效率,The ability to capture,absorb,and convert x-ray photons to electrical signal;,捕捉、吸收、转换,X,线光子形成电信号的能力；,The total detector efficiency,referred to as the dose efficiency,is the product of the capture efficiency,absorption efficiency,and conversion efficiency;,总探测器效率（剂量效率）指探测效率、吸收效率和转换效率的乘积。,2.Stability,稳定性,the steadiness of the detector response.,探测器响应的稳定性,If the system is not stable,frequent calibrations are required to render(,补偿,)the signals useful;,如果系统不稳定，就需要频繁的校准来补偿信号的作用。,3.Response time,响应时间,Th,e speed with which the detector can detect an x-ray event and recover to detect another event;,探测器探测一个,X,线信号到为下一个信号作好准备的时间,Afterglow,余辉问题,4.Accuracy,准确性,Low electronic Noise,低电子噪声,Linear,线性,Uniformity,探测器均匀一致性,Instantaneous stability,瞬时稳定性,5.Dynamic range,动态范围,the“ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated”,测量最大信号和分辨最小信号精度的范围,6.Radiation Damage,辐射损伤,探测器受辐照增益变化,III.Detector types,探测器类型,Solid state detectors,固体探测器,Early detectors,scintillators,e.g.NaI,早期固体探测器,Modern,ceramic scintillators,稀土陶瓷探测器,Xenon gas detectors,气体探测器,Scintillation detectors,闪烁体探测器,或,固体探测器,Scintillation,闪烁体,Photon capture,电子俘获,Light,光,Photo-diode,光电二极管,Signal,信号,利用光电倍增管将闪烁转换为电信号，再用电子线路和器件将它放大和存储下来。把发光物质即,闪烁晶体,和,光电倍增管,组合起来，便构成了闪烁计数探测装置。,探测效率高、分辨时间短,固体探测器得到了广泛应用,固体探测器结,构,2.UFC detector,The UFC detector consists of individual elements consisting of ceramic(,陶瓷,)scintillation crystals affixed to the surface of light-sensitive diodes which are mounted on a circuit board;,UFC,探测器由一组黏附在电路板上的光敏二极管表面的陶瓷闪烁晶体单元组成,The separate device controls the temperature and humidity of the detector elements.,有单独的设备控制探测器元件的温度和湿度,Pros-highly efficient;,优点,高效,Cons-expensive;,缺点,贵,Pressurised xenon gas,高压氙气,Ionization,电离,Signal,信号,3.Xenon detectors,气体探测器,X-ray,As radiation enters the detector,it ionizes the xenon gas between electrodes;,射线进入探测器，使电极间的氙气电离,During the isolation process,the positive ions are attracted to the negatively charged plate,the negative ions are attracted to the positively charged plate;,在绝缘过程中，正离子吸引至负收集板，负离子吸引至正收集板,The signal current varies directly with the number of x-ray photons absorbed;,信号电流直接随,X,线光子吸收改变而变化,。,Structure,The xenon gas detector consists of pressure chamber separated internally into many compartments;,气体探测器由分散在很多隔间内部的压力电离室组成。,电离室的,上下夹面由陶瓷拼成,；每个,电离室两侧用薄钨片构成,；电离室内,充满氙气,；各个中心收集电极引线连至相应的前置放大器，,气体探测器特性,The deeper the gas chamber,the higher the pressure,the more efficient the detector;,压力室越深，,压力越大，探测器效率越高,Tolerance to temperature and humidity;,耐温度、湿度,Infrequent calibration requirements;,校准需求低,Constant channel to channel response;,响应一致,High X-ray geometric collection efficiency;,几何效率高,Cheap;,便宜,Long term stability;,长时间稳定,No afterglow;,没有余辉,BUT,！,Low efficiency,效率低,Require,High,mAs,When pressure decrease the detect efficiency decreased remarkably,压力降低，探测效率显著下降,Xenon vs Solid State,气体和固体的对比,固体探测器,气体探测器,温度特性,关系大、需调温,关系不大,噪声、颤噪效应,无,有,饱和效应,无,有,散射线准直,需要,一般不需，准直效果差,剂量利用率,无盲层,有盲层,变形,易变形,无变形,Detector Arrangement,探测器排列,Detectors in 3rd generation scanners arranged in an arc around the patient,；第三代扫描仪的探测器绕着患者排列成,弧状,。,600-900 elements in detector bank-to allow good spatial information,；探测器中有,600 900,个元件,允许好的空间信息,Tube and detectors rotate around patient,球管和探测器都围绕患者旋转。,探测器组件,Reference Detectors,参考探测器,X,射线经前准直器后被参考探测器接收。,测得的信号对应于入射组织的,X,线初始强度。,3.6 DAS,数据采集系统,DAS Data acquisition system,Function,将输出信号放大、积分后，多路混合成一路用,A/D,转换器变为数字信号送入计算机处理；,The components of DAS,1.Pre-amplifier,前置放大器,2.Integrator,积分器,3.Multiplexer,多路转换器,4.A/D,转换器,5.D/A,转换器,由探测器接收信号，经对数压缩使后续电路工作在一个较窄的范围；,积分器接在放大器之后测量每个角度下的光子总和；,各路积分器输出信号经多路混合变成一路使用共同的,A/D,转换器变为数字信号；,将连续时域模拟信号转变为离散的数字化信号；,CT,一般要求大于,16 bit,将数字图像变为显示终端的模拟信号，一般用,68bit,3.7 Gantry,机架,and,Patient table,诊视床,Gantry,机架,床,Gantry(,机架,),俗称：,龙门架,作用：,完成特定扫描方式的扫描，以获得病人扫描层面的原始数据，供计算机系统进行图像处理,放置,X-CT,部件,扫描架性能,扫描架孔径应足够大，绝大多数的机架孔径为,70cm,旋转,倾斜性能,好，大多数可以倾斜,30,。,冷却,：开机后,X,线球管产生热量，且机架内含有众多组件，某些组件直接影响到机架周围的温度，因此,机架的制冷,要考虑,机械设计,：,CT,的机械设计很严格,旋转：,机架旋转时产生的离心力,高要求,旋转部分组件的重量基本上都在,400,1000kg,范围,加速到每秒,2,圈的旋转速度，,集成到旋转轴承内的线性驱动,能够保证足够的匀速运动。,Gantry(,机架,),Patient table(,诊视床,couch),Convenient control/and position readout;,便于控制和读数,Strong and rigid to support the weight of the patient;,足够的,强度,和,刚性,来承载体很重的患者,Accurate;,精确性好,Easy loading;,方便装载,Adjustable;,可以调节,Table down and up,诊视床的上升和下降,The vertical movement should provide a range of heights from the floor to make it easy for patients to get on and off the table;,诊视床应该具备在一定范围内的上升和下降，便于患者上下；,It is especially useful when examining geriatric,trauma,and pediatric patients;,对于老年患者，外伤和儿童患者尤其有用；,Table function,诊视床功能,Horizontal or longitudinal couch movements should enable the patient to be scanned from head to thighs without the need repositioning;,可以,水平和垂直移动,，便于扫描全身，无须重新摆位；,Couch top and pedestal,诊视床面和底座,Couch top rests on a pedestal,（托架）,.Is usually made of,carbon fiber,composites,（碳纤维成分）,because they have low absorption and excellent vibration damping features;,吸收射线少、抗震动性好,；,The pedestal houses the mechanical and electrical components that facilitate couch movement;,诊视床的底座有机械和电子元件使床运动,End,