核医学课件：核物理

1、核医学物理基础原子和原子结构原子（atom）是构成元素的基本单位，不同元素的原子具有不同的性质，但是原子的基本结构大致相同。 原子核由质子和中子组成，它们统称为核子。通常采用 表示原子的结构，其中X代表元素符号，Z代表质子数，N代表中子数，A代表原子的质量数，因为元素符号本身就确定了质子数，N= A Z, 故原子结构亦可简便地只记元素符号和质量数AX，如131I、18F。质子带一个正电荷，中子呈电中性，核外电子带一个负电荷，原子核的正电荷数目与核外电子数相等，所以原子本身呈电中性。同位素、同质异能素、核素同位素(isotope) 凡原子核具有相同的质子数而中子数不同的元素互为同位素。 如125

2、I、131I、132I均有53个质子，但中子数不同，在元素周期表中处于同一位置，是同一元素-碘元素。一种元素往往有几种甚至几十种同位素。一个元素所有同位素的化学和生物性质几乎都一样，但物理性质可能有所不同。 同质异能素 (isomer) 核内中子数和质子数都相同但能量状态不同的核素彼此称为同质异能素。 用m表示激发态，放在左上角或右上角。例如99mTc或Tc99m是99Tc的激发态，与99Tc互为同质异能素。 核素（nuclide) 原子核的质子数、中子数和原子核所处的能量状态均相同的原子属于同一种核素。例如198Au表示不同的核素。同一种核素化学性质和核性质均相同，是某一原子固有的特性征。放

3、射性核衰变原子核中子和质子的数目保持一定的比例才能稳定，不会自发地发生变化而稳定地存在。原子量较小的核素，Z/N=1时原子核是稳定的。当质子数较多时（一般为Z20），质子数多了，斥力增大，必须有更多的中子使核力增强，才足以克服斥力，保持核稳定。原子核中质子数过多或过少，或者中子数过少或过多，原子核便不稳定。放射性核衰变不稳定核素的原子核能自发地放出各种射线变成另一种核素，称为放射性核素(radionuclide)。原子核能稳定地存在，不会自发地发生变化的核素称为稳定性核素。放射性核素的原子核自发地放出射线，同时转变成别的原子核的过程称为放射性核衰变(radioactive decay)，简称核

4、衰变。衰变（alpha decay）Alpha-decay occurs in very heavy elements, They can become more stable by emitting an alpha particle. Ra(Radium，镭)；Rn(Radon，氡)alpha partical（射线)Alpha particles are made of 2 protons and 2 neutrons.Alpha particles have a charge of +2, and a mass of 4 .Alpha particles are relatively

5、slow and heavy. Alpha particles have a low penetrating power - you can stop them with just a sheet of paper.Alpha particles ionize other atoms strongly. 衰变(negatron decay )Beta decay occurs in very neutron-rich elements.They can become more stable by emitting a beta particle. particle（ 射线）Beta parti

6、cles have a charge of minus 1(-1) , and a mass of about 1/2000th of a proton (the same as an electron). Beta particles are fast, and light.Beta particles have a medium penetrating power - they are stopped by a sheet of aluminium or plastics such as perspex. Beta particles cause less ionisation than

7、alphas.+ 衰变 (positron decay)Positron decay occurs in nuclides that are neutron-poor .The minimum transition energy required for positron decay is 1.02 MeV. 湮没辐射电子俘获（electron capture） In unstable nuclei where the maximum available transition energy is less than 1.02 MeV, decay of neutron-poor radionu

8、clides is by electron capture. In electron capture an electron from one of the orbital shells is incorporated into the nucleus, converting a proton into a neutron. 电子俘获衰变图原子核俘获一个内层电子（1）外层电子向内层补充（2）两层轨道之间能量差转换成特征X射线（3）或俄歇电子释放（4）。同质异能跃迁（Isomeric Transition，IT)The transition from the metastable state t

9、o the ground state is isomeric because there is no change in atomic number. 衰变（射线，electromagnetic radiation） or 内转换（internal conversion electron）. 内转换电子过程激发态的原子核把能量转给一个核外电子（1），这个电子被逐出原子成为内转换电子（2），外层电子填补空穴，原子核回复到基态（3），能量由特征X射线（4）或俄歇电子 （5） 携走。penetrating powerAlpha particles are easy to stop, gamma ra

10、ys are hard to stop. Units of Radioactivity 放射性活度及单位Radioactivity is quantitatively measured as the number of atoms that disintegrate per unit time. Two different systems for expressing radioactivity-the curie (Ci，居里，旧制单位) and the becquerel (Bq，贝可，国际制单位) .The curie (Ci) was named in honor of Marie C

11、urie. This unit was based on the disintegration rate of 1 gram of radium and was defined as the amount of radioactivity that decays at a rate of 3.71010 disintegrations per second (dps). curie(Ci) =100 Ci = 3.7 1010 d.p.s millicurie (mCi) =10-3 Ci = 3.7 107 d.p.s microcurie (Ci) =10-6 Ci = 3.7 104 d

12、.p.s nanocurie (nCi) =10-9 Ci = 3.7 101 d.p.s picocurie (pCi) =10-12 Ci = 3.7 10-2 d.p.sAn alternative to the curie in the international system of units ( Systeme Internationale, SI ) is the becquerel (Bq). Henri Becquerel was the first person to identify radioactivity in 1896. One becquerel is 1 d.

13、p.s. becquerel (Bq) = 1 d.p.s (Bq) kilobecquerels (KBq) =103 d.p.s (Bq) megabecquerels (MBq) =106 d.p.s (Bq) gigabecquerels (GBq ) =109 d.p.s (Bq)1Bq = 2.703 10-11Ci 1Ci = 3.7 1010BqThe rapid method ? mCi 37 = ?MBq 10mCi 37 = 370MBq ? MBq 37 = ? mCi 370MBq 37=10mCi半衰期（half-life） The half-life is the

14、 time required for half of a radioactive sample to decay. The half-life is constant, but is different for each isotope. How to use half-life?A radioactive substance（200mCi）has a half-life of 2 hours. How much of the substance will remain after 6 hours have passed?6/2=3（half-life）200 1/23 = 25 mCiOth

15、er terms of half-lifePhysical half-life (T1/2 ，物理半衰期 ):Biological half-life （Tb ，生物半衰期 ）: To describe the biological clearance of the radionuclide from a particular tissue or organ system. Effective half-life (Teff ，有效半衰期 ) : The actual half-life of a radiopharmaceutical in a biological system, whic

16、h is dependent on both the physical half-life and the biological clearance. 三者关系：Radiation Protection辐射防护Protection for internal contaminationProtection for external exposureMinimize the amount of time spent near a source of radiation（时间防护）.Maximize the distance from a source(inverse square law) （距离

17、防护）.Place shielding between the source and persons to be protected（屏蔽防护）.核医学检查的安全性是一种无创性检查方法。辐射剂量明显小于大多数X线检查。无过敏反应。非常安全！美国有1/3以上的就诊病人接受核医学检查。 Radionuclide studiesDiagnostic procedure Typical effective Equivalent period of dose (mSv) natural backgroundLung ventilation (81mKr) 0.1 2.4 weeksLung perfus

18、ion (99mTc) 1 6 monthsKidney (99mTc) 1 6 monthsThyroid (99mTc) 1 6 monthsBone (99mTc) 4 2.3 yearsDynamic cardiac (99mTc) 6 2.7 yearsUK average = 2.2 mSv per year: regional averages range from 1.5 to 7.5 in 1990s.Radiation Dose ComparisonDiagnostic Procedure Typical Effective Dose (mSv) Number of ChestX ray film for Equivalent Effective DoseTime Period for Equiva