医学影像物理学课件：01-X-ray

1、X - ray,A kind of electromagnetic wave, radiate with higher energy than those of visible light. Their wavelengths are from 0.01 Angstrom to 100 Angstrom.,Discover of X-ray,In 1895, a German physicist Roentgen discovered a unknown rays which have high energy and can not be seen by eyes but can transm

2、it some nontransparent matters. Because people in that time didntt understand the properties of that ray, so named it x-rays, also is Roentgen rays. Before long x-ray photography was developed which is a revolutionary tool in medical diagnosis. He won a 1901 Nobel Prize.,Conditions of Generating X-r

3、ay,Electrons with high speed Object used to block(stop) electrons stream Target,电子（带电粒子）与物质的相互作用： 碰撞损失 电离与激发 辐射损失 散射和轫致辐射,Equipment of Generating X ray,X-ray Tube Low Voltage Power Supply High Voltage Power Supply Control circuit: variable resistor,Tube current Tube voltage,sketch map,Structure of X

4、-ray Tube,1.Glass Shell:It compose a highly vacuum glass envelope used to house the Cathode and the anode. Vacuum: P 10-4 Pa 2.Cathode:Cathode is a source of electrons, actually it is also a filament, which is made from material tungsten (W). 3.Anode: it is also a target, whose surface is bombarded

5、by these electrons from the cathode.,About glass shell and cathode,Why X-ray tube need a condition of high vacuum oxidation, disturb electrons stream Filament of cathode a small coil of tungsten wire when it is heated to incandescence electrons are released from the surface of the filament. the high

6、er the temperature of the filament, the greater the rate of electron emission.,Structure of Cathode,圆焦点型 线焦点型 双焦点型 凹面阴极,电子聚焦阴极与二次电子(阳极过热发出),The Choice of material of Anode,High melting point and high thermal conductivity Relatively High efficiency to emit X-ray X射线的转化效率，同样速度和数目的电子轰击原子序数Z不同的各种物质做成的靶，

7、产生X射线光子的总数和光子总能量与Z2成正比。 Because only 0.2 percent of the kinetic energy of electrons is converted into X-rays ,while the remaining 99.8 percent is changed into heat. For conduct and release the heat, Cu is a suitable material. Molybdenum(Mo); Alloy of tungsten and rhenium (Re),Target: Tungsten, Main

8、body: Copper Cu,Release Heat,Rotational Anode Cool system for larger power X-ray tube,热负荷： X-Tube 的容量,特殊X-ray Tube,Triode X-ray Tube：高速X射线摄影 软X射线管： 钼靶，低管电压，主要用于乳腺等软组织检查,Focus,Actual Focus(Actual focal spot): The area on anode that is struck by electrons is actual focus. Effective Focus: The projecte

9、d area of actual focus is effective focus. Target angle: the angle of the target relative to the electrons stream.,The magnitude of actual focus is relative with the size of filament. The effective focus is decided by the actual focus and target angle. The smaller the effective focal spot, the sharp

10、er the radiographic image,just like the shadow of incandescent lamp. In practice, large focus is used in therapy and small focus is used in Imaging.,焦点对成像的影响,焦点X射线强度不均匀对成像的影像 Main Focus and Side (Subsidiary) Focus 焦点尺寸的影响 半影，减小病人与胶片之间的尺寸,electrical characteristic of X-ray tube,Tube Current Tube Volt

11、age,V,I,灯丝电流,灯丝电流,I,X-ray Tube 的容量,实际焦点 管电压 管电流 连续使用时间 阳极构造与冷却方式,允许X射线管能承受热量的最大负荷量。,X-ray 物理特性,高频电磁波,在电磁波谱中X线是介于紫外线与射线之间的一种电磁波。由于光子能量大，能使物质电离，也属于电离辐射,X线同时具有波动性和微粒性。前者的特征是具有波长和频率（具有干涉、衍射、反射和折射的特性)，后者的特征是具有能量、动量和质量,其二象性在表现时各有侧重。传播时，主要表现波动性，在辐射和吸收时，主要表现为微粒性。,X-ray 物理特性贯穿本领,指X线通过物质时不被吸收的本领。 与X光子能量有关；与吸收

12、物质性质（密度，原子序数）有关,电离作用,电离特性。X线照射物质时，使该物质的核外电子脱落原子轨道，这种作用称为电离作用。,热特性：物质吸收X 线能最终绝大部分转变成热能，使物体温度升高，这就是X 线的热特性,化学与生物效应,（1）感光特性。X 线照射到溴化银上的时候，由于电离而使溴化银起化学变化，出现银粒沉淀。这就是X线的感光特性。 （2）着色特性。某些物质如铂氰化钡、铅玻璃、水晶等，经X 线照射后其结晶体脱水而改变颜色。这就是着色特性。 生物特性包括： X 线对生物组织的细胞、分子键等有破坏、瓦解的作用称为生物特性。,荧光作用,荧光特性。某些物质如磷、铂氰化钡、钨酸钙等受X 线照射后由于电

13、离或激发使原子处于激发状态，在回到基态的过程中，由价电子的能级跃迁而辐射出可见光或紫外光线谱，这种光谱就是荧光，而X线使物质发生荧光作用的这种特征称为荧光特性。,X-ray 谱,Continuous X-ray spectrum Characteristic X-ray spectrum,X-ray produced from X-ray tube are not monochromatic. It contains many wavelengths of electromagnetic wave.,Correspondent wavelength,Photograph from spectr

14、ograph devices,Spectra,Mechanism of X-ray Continuous Spectrum,It is relative with the deceleration of electrons in the coulomb filed of atomic nucleus. Bremsstrahlung When high speed electrons strike the target,under the action of electrical filed of atoms, their velocity including magnitude and dir

15、ection are changed sharply and the energy of them is converted directly to X-radiation. For single electron, Bremsstrahlung can finished in one time of procession or several times.,The wavelength changing of continuous X-ray,The max intensity The shortest wavelength With the increasing of tube volta

16、ge, the curve will move to the direction of shorter wavelength,Short wave limit,Plancks constant The minimum wavelength of continuous X-ray is inversely proportional to the tube voltage.,If the electrons give up all its kinetic energy in a single procession, the X-ray photon will own the maximum ene

17、rgy.,峰值波长约为短波极限的两倍。,Characteristic X-radiation,Spectrum lines whose wavelengths of are only determined by the element made up the target is called characteristic X-ray spectrum lines. Mechanism of X-ray characteristic Spectrum,If the tube voltage is greater than 70 kV, there are characteristic spect

18、rum lines which are superposed on the continuous X-ray spectrum.,Mechanism of X-ray characteristic Spectrum,The spatial position of electrons of an atom Outer electrons of an atom are the ones responsible for the visible light spectra of the elements. The displacement of inner electrons of atom will

19、 give rise to the characteristic spectrum.,The Shell of electrons of an atom involved with characteristic X-ray,K series L series M series N series,X-ray micro area analysis,Make use of the properties of X-ray characteristic spectrum to identify elements in very small objects, such as biological mol

20、ecule.,The Intensity and Hardness of X-ray,The intensity of X-ray means the energy radiated in unit time through an unit area which is normal to the direction of X-ray. Unit: W/m2,决定因素：,管电压,管电流,靶材料,Intensity of X-ray,The intensity of X-ray radiation is directly proportional to the tube current We co

21、ntrol the intensity of X-ray by the adjustment of Tube current.,在一定的管电压情况下：,K, 常数；Z,靶原子序数，I管电流，U管电压，m，约2,另一种强度表述,Because X-ray has cumulative effect, when we consider the radiation of X-ray, we use the product of the tube current (milliamp) and time.,The Intensity and Hardness of X-ray,The hardness

22、of X-ray means the penetrating ability of X-ray. It depends on the wavelength of X-ray Control the hardness of X-ray by tube Voltage,X-ray with Different hardness be used different purpose,X-ray Diffraction,visible light and slim slot(grating), the size have comparability Crystal, made up of identic

23、al atoms in a cubic arrangement, Laue spot.,K is integral 1,2,3, constructive interference,Bragg equation,X-ray spectrograph,By it we can observe the spectrum and obtain monochrome X-ray,X-ray Diffraction and the Structure of biological Molecules,The spatial structure of Hemoglobin The twin-helix st

24、ructure of DNA,schematic diagram,collimators,X-ray 辐射场,角分布 高能电子的能量与靶的厚度 薄靶与厚靶,X-ray 辐射场的角分布,厚靶X-ray空间分布,靶内部的X-ray辐射 阳极效应：靠近阳极的一端X射线辐射强度下降大,X 线与物质的相互作用 X 线与物质相互作用的过程 X 线通过物质后，其强度减弱的现象称为吸收。由于X 线是一种电离辐射，入射光子作用于物质时，能产生电离电子和散射光子。电离和激发的过程中，物质吸收了能量，必然使入射的X 线强度减弱。,Interaction of X-ray with Matter,Photoelect

25、ric Effect Scattering Effect Pair Production Secondary Photons and Particle Rays,光子与物质的相互作用。 电离辐射，电离（间接）和激发 散射,作用截面,截面：一个入射粒子与单位面积上一个靶粒子发生相互作用的概率。 相互作用的截面：入射粒子以某种作用方式与靶粒子发生作用的概率。,n，靶粒子数；N，入射光子数,线性吸收系数：,能量转移系数与能量吸收系数,能量转移系数 质能吸收系数,能量转移系数：,能量吸收系数：,光子能量的归宿： 1 散射 2 转化为电子的动能,Etr转移为电子的动能,g 次级电子的动能辐射损失,光电效

26、应/Photoelectric Effect,When X-rays irradiate matters, just like visual light or ultraviolet irradiate to metal, the photoelectric effect will be occurred. Inside Shell and Outside Shell 内光电效应,w is the magnitude of the energy with which the electron was bound in this atom. It depends on which shell t

27、he electron was in.,光电效应的发生几率,1 Rule: 作用系数与几率 2 吸收系数的突变：When the energy of X-phton is equal to or just exceeds the binding energy of a electron, the probability which the photoelectric effect happen is maximum. 3 角分布 4 意义：主要的暴光和对比度贡献者。但低能光子的光电效应有损伤,散射效应/Scattering Effect,Compton scattering,发生的位置：外层。

28、 If the electrons binding energy is much less than the energy of the photon, the electron can be regard free, then the interaction is Compton scattering.,散射光子与反冲电子：When a photon of energy hv collides with a electron at rest, the electron recoils with an energy E, the scattered photon has energy hv,

29、of cause v must be less than v. 符合能量守恒与动量守恒：The procession obey the law of conversation of energy and conservation of momentum,康普顿散射,1 Rule: 作用系数与几率 注意与光电效应3次方的对比 与吸收物质原子序数无关。 2 散射光子与反冲电子的角分布 3 意义：散射与灰雾。,电子对生成/Pair Production,Electron pair,An incident photon with energy above 1.02 MeV can produce a

30、particle-antiparticle pair: a negative electron and a positive electron or positron.,The mass and charges of positron are the same as that of electron, only the sign of charge is opposite.,Pair Production,Condition: hv1.02MeV (2me C) 发生几率： 当 hv1.02MeV 当 hv1.02MeV,在医用X射线领域不太可能发生。,Interference Scatter

31、ing(瑞利散射),入射光子与束缚牢固的原子内层电子碰撞时，不能使电子脱离原子，该电子完全吸收了光子的能量后，从低能级跃迁到高能级，使原子处于激发态，原子回到基态的过程中就辐射出一个与入射光子能量相同的散射光子 比例很小,Secondary Photons and Particle Rays,次级粒子与射线,When X-ray is interacting with matter, several secondary rays and secondary particles may occur with these processes. It includes the secondary X

32、-rays (Characteristic X-rays), recoil electrons in Compton scattering and electron pairs. The energies of these secondary photons are often smaller than original photons.,光子与物质作用的三种形式与光子的能量和物质的原子序数Z有关。从图中可见能量低的光子和原子序数高的物质，以光电效应为主；中等能量的射线以康普顿散射为主；电子对生成主要发生在高能光子和高原子序数的物质中，但在能量极高光子作用下，较低原子序数物质中，电子对生成也不可忽视。,几种作用的几率,作用几率决定于X光子的能量与吸收物质的原子序数和电子密度 在水（软组织）中，对于低能X线，光电效应占主导地位（约占50%），对高能X线，则康普顿效应是主要的,瑞利散射相比是次要的。 高原子序数物质，无论光子能量如何变化，都