Radioactive Tracers

放射性示踪剂

As a part of the medical subspecialty of Nuclear Medicine, various diagnostic procedures make use of a small amount of a radioactive isotope, usually injected into the patients bloodstream for the purpose of imaging some part of the body. The useful radiation from such isotopes is usually gamma rays, which can be detected outside the body. These gamma rays can be used to image an internal organs or structures. An example is the imaging of blood flow in the heart muscle in myocardial infusion imaging. The radioisotopes used have short half-lives so that they are quickly eliminated from the body. While appropriate cautions must be observed with any use of ionizing radiation, the radiation doses in such procedures are very low and the risks have been carefully examined.

作为核医学的一个分支，各种诊断程序利用少量放射性同位素，通常将它们注入患者的血液中，以便成像身体的某些部分。这类同位素发出的有用辐射通常是伽马射线，这些射线可以在体外检测到。这些伽马射线可用于成像内部器官或结构。一个例子是心肌灌注成像，用于成像心脏肌肉中的血流。用于此类程序的放射性同位素具有较短的半衰期，因此它们会被迅速从体内排出。虽然在使用电离辐射时必须采取适当的谨慎措施，但这些程序中的辐射剂量非常低，风险已被仔细评估。

Radioactivity is Unaffected by Physical or Chemical State

放射性不受物理或化学状态的影响

The radioactive half-life for a given radioisotope is a measure of the tendency of the nucleus to "decay" or "disintegrate" and as such is based purely upon that probability. The tiny nuclear size compared to the atom and the enormity of the forces which act within it make it almost totally impervious to the outside world. The half-life is independent of the physical state (solid, liquid, gas), temperature, pressure, the chemical compound in which the nucleus finds itself, and essentially any other outside influence. It is independent of the chemistry of the atomic surface, and independent of the ordinary physical factors of the outside world. The only thing which can alter the half-life is direct nuclear interaction with a particle from outside, e.g., a high energy collision in an accelerator. Even in that case, the nature of the nucleus is changed, as in the bombardment of nitrogen nuclei in the atmosphere which is the basis for carbon dating. As long as the nucleus of the isotope remains intact, the half-life of a radioactive tracer can be depended upon.

某种放射性同位素的半衰期是衡量其核“衰变”或“分解”倾向的量度，因此它纯粹基于这种概率。由于核的尺寸远小于原子，而核内作用力极其强大，因此核几乎完全对外界隔绝。半衰期与物质的物理状态（固态、液态、气态）、温度、压力、核所处的化学化合物以及本质上任何其他外界因素均无关。它也与原子表面的化学性质无关，也与外界的普通物理因素无关。唯一能改变半衰期的是核与外界粒子的直接相互作用，例如在加速器中发生的高能碰撞。即使在这种情况下，核的性质也会发生变化，例如大气中氮核的轰击，这是碳测年法的基础。只要同位素的核保持完整，放射性示踪剂的半衰期就可以被依赖。