1、核醫(yī)學(xué)物理基礎(chǔ)原子和原子結(jié)構(gòu)原子（atom）是構(gòu)成元素的基本單位，不同元素的原子具有不同的性質(zhì)，但是原子的基本結(jié)構(gòu)大致相同。 原子核由質(zhì)子和中子組成，它們統(tǒng)稱為核子。通常采用 表示原子的結(jié)構(gòu)，其中X代表元素符號，Z代表質(zhì)子數(shù)，N代表中子數(shù)，A代表原子的質(zhì)量數(shù)，因?yàn)樵胤柋旧砭痛_定了質(zhì)子數(shù)，N= A Z, 故原子結(jié)構(gòu)亦可簡便地只記元素符號和質(zhì)量數(shù)AX，如131I、18F。質(zhì)子帶一個正電荷，中子呈電中性，核外電子帶一個負(fù)電荷，原子核的正電荷數(shù)目與核外電子數(shù)相等，所以原子本身呈電中性。同位素、同質(zhì)異能素、核素同位素(isotope) 凡原子核具有相同的質(zhì)子數(shù)而中子數(shù)不同的元素互為同位素。 如125

2、I、131I、132I均有53個質(zhì)子，但中子數(shù)不同，在元素周期表中處于同一位置，是同一元素-碘元素。一種元素往往有幾種甚至幾十種同位素。一個元素所有同位素的化學(xué)和生物性質(zhì)幾乎都一樣，但物理性質(zhì)可能有所不同。 同質(zhì)異能素 (isomer) 核內(nèi)中子數(shù)和質(zhì)子數(shù)都相同但能量狀態(tài)不同的核素彼此稱為同質(zhì)異能素。 用m表示激發(fā)態(tài)，放在左上角或右上角。例如99mTc或Tc99m是99Tc的激發(fā)態(tài)，與99Tc互為同質(zhì)異能素。 核素（nuclide) 原子核的質(zhì)子數(shù)、中子數(shù)和原子核所處的能量狀態(tài)均相同的原子屬于同一種核素。例如198Au表示不同的核素。同一種核素化學(xué)性質(zhì)和核性質(zhì)均相同，是某一原子固有的特性征。放

3、射性核衰變原子核中子和質(zhì)子的數(shù)目保持一定的比例才能穩(wěn)定，不會自發(fā)地發(fā)生變化而穩(wěn)定地存在。原子量較小的核素，Z/N=1時原子核是穩(wěn)定的。當(dāng)質(zhì)子數(shù)較多時（一般為Z20），質(zhì)子數(shù)多了，斥力增大，必須有更多的中子使核力增強(qiáng)，才足以克服斥力，保持核穩(wěn)定。原子核中質(zhì)子數(shù)過多或過少，或者中子數(shù)過少或過多，原子核便不穩(wěn)定。放射性核衰變不穩(wěn)定核素的原子核能自發(fā)地放出各種射線變成另一種核素，稱為放射性核素(radionuclide)。原子核能穩(wěn)定地存在，不會自發(fā)地發(fā)生變化的核素稱為穩(wěn)定性核素。放射性核素的原子核自發(fā)地放出射線，同時轉(zhuǎn)變成別的原子核的過程稱為放射性核衰變(radioactive decay)，簡稱核

4、衰變。衰變（alpha decay）Alpha-decay occurs in very heavy elements, They can become more stable by emitting an alpha particle. Ra(Radium，鐳)；Rn(Radon，氡)alpha partical（射線)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. 衰變(negatron decay )Beta decay occurs in very neutron-rich elements.They can become more stable by emitting a beta particle. particle（ 射線）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.+ 衰變 (positron decay)Positron decay occurs in nuclides that are neutron-poor .The minimum transition energy required for positron decay is 1.02 MeV. 湮沒輻射電子俘獲（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. 電子俘獲衰變圖原子核俘獲一個內(nèi)層電子（1）外層電子向內(nèi)層補(bǔ)充（2）兩層軌道之間能量差轉(zhuǎn)換成特征X射線（3）或俄歇電子釋放（4）。同質(zhì)異能躍遷（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. 衰變（射線，electromagnetic radiation） or 內(nèi)轉(zhuǎn)換（internal conversion electron）. 內(nèi)轉(zhuǎn)換電子過程激發(fā)態(tài)的原子核把能量轉(zhuǎn)給一個核外電子（1），這個電子被逐出原子成為內(nèi)轉(zhuǎn)換電子（2），外層電子填補(bǔ)空穴，原子核回復(fù)到基態(tài)（3），能量由特征X射線（4）或俄歇電子 （5） 攜走。penetrating powerAlpha particles are easy to stop, gamma ra

10、ys are hard to stop. Units of Radioactivity 放射性活度及單位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，舊制單位) and the becquerel (Bq，貝可，國際制單位) .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. 三者關(guān)系：Radiation Protection輻射防護(hù)Protection for internal contaminationProtection for external exposureMinimize the amount of time spent near a source of radiation（時間防護(hù)）.Maximize the distance from a source(inverse square law) （距離

17、防護(hù)）.Place shielding between the source and persons to be protected（屏蔽防護(hù)）.核醫(yī)學(xué)檢查的安全性是一種無創(chuàng)性檢查方法。輻射劑量明顯小于大多數(shù)X線檢查。無過敏反應(yīng)。非常安全！美國有1/3以上的就診病人接受核醫(yī)學(xué)檢查。 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