IGCSE CIE Physics: Medical Physics – Key Points | IGCSE CIE 物理：医疗物理 考点精讲

📚 IGCSE CIE Physics: Medical Physics – Key Points | IGCSE CIE 物理：医疗物理 考点精讲

In the CIE IGCSE Physics course, the topic of medical physics brings together concepts from waves, nuclear physics, and optics. You will learn how ultrasound, X-rays, radioactive sources, and optical fibres are used to diagnose and treat medical conditions. Understanding the physics behind these technologies, as well as the associated safety measures, is essential for the examination. This revision guide highlights the key points you need to know.

在 CIE IGCSE 物理课程中，医疗物理将波动、核物理和光学的知识结合起来。你将学习超声波、X射线、放射源和光纤如何用于诊断和治疗疾病。理解这些技术背后的物理原理以及相关的安全措施是考试的关键。这份考点精讲将梳理你需要掌握的重点。

1. Ultrasound and its Medical Applications | 超声波及其医学应用

Ultrasound refers to sound waves with frequencies above 20 kHz, beyond the range of human hearing. In medicine, ultrasound is produced and detected using a piezoelectric transducer, which converts electrical signals into vibrations and vice versa.

超声波指频率超过 20 kHz 的声波，超出人耳可听范围。在医学上，超声波通过压电换能器产生和检测，它将电信号转换为振动，反之亦然。

During an ultrasound scan, short pulses of ultrasound are sent into the body. These pulses reflect off boundaries between tissues of different acoustic impedance (density × speed of sound). A gel is applied to the skin to eliminate air gaps and maximise transmission. The time taken for the echo to return is measured, and the distance to the reflecting surface is calculated using the formula:

超声波扫描时，向体内发射短脉冲。这些脉冲在不同声阻抗（密度×声速）的组织界面处反射。皮肤上涂抹耦合凝胶以消除空气间隙、最大化透射。测量回声返回的时间，利用下列公式可算出反射面的深度：

d = (v × t) / 2

High-intensity focused ultrasound is used to break up kidney stones or gallstones — a procedure called lithotripsy. The ultrasound beam is focused on the stone, causing it to vibrate and shatter into fragments without the need for invasive surgery.

高强度聚焦超声用于击碎肾结石或胆结石，这种操作称为体外碎石。超声波束聚焦在结石上，使其振动并碎裂，无需侵入性手术。

2. X-rays and Radiography | X射线与放射成像

X-rays are a form of high-frequency electromagnetic radiation, with wavelengths around the size of atoms. They are produced when fast-moving electrons hit a metal target in an X-ray tube, converting kinetic energy into X-ray photons. X-rays are ionising and can penetrate soft tissues but are absorbed more by dense materials like bone and metal.

X射线是一种高频电磁辐射，波长接近原子大小。X射线管中高速电子撞击金属靶时产生X射线，将动能转化为X射线光子。X射线具有电离能力，能穿透软组织，但会被骨骼和金属等致密物质更多吸收。

In a conventional radiograph, a patient is placed between an X-ray source and a photographic film or digital detector. Tissues that absorb X-rays appear white, while areas where X-rays pass through appear black. Bones show up clearly, but soft tissues show little contrast unless a contrast medium like barium or iodine is used.

在常规X射线摄影中，患者位于X射线源与照相底片或数字探测器之间。吸收X射线的组织呈白色，而让X射线透过的区域呈黑色。骨骼清晰可见，但软组织对比度低，除非使用钡剂或碘剂等造影剂。

Because X-rays are ionising, they can damage cells and cause cancer. Safety measures include using the minimum exposure time, wearing lead aprons, standing behind protective screens, and keeping a safe distance from the source.

由于X射线具有电离作用，会损伤细胞并可能导致癌症。安全措施包括尽量缩短照射时间、穿戴铅围裙、站在防护屏后面以及远离辐射源。

3. Computed Tomography (CT) Scanning | 计算机断层扫描 (CT)

A CT scanner consists of an X-ray tube that rotates around the patient, along with multiple detectors on the opposite side. As the tube rotates, narrow beams of X-rays pass through the body at different angles, and a computer processes the signals to build a detailed 3D cross-sectional image.

CT扫描仪由围绕患者旋转的X射线管和对侧多个探测器组成。当球管旋转时，窄束X射线从不同角度穿过身体，计算机处理信号，构建精细的三维截面图像。

CT scans provide much more detail than conventional radiographs and allow doctors to visualise soft tissues, blood vessels, and organs with high contrast. However, the radiation dose is higher than a single X-ray exposure, so it is used only when the diagnostic benefit outweighs the risk.

CT扫描比常规X射线片提供更丰富的细节，医生可以高对比度地观察软组织、血管和器官。但辐射剂量高于单次X射线检查，因此仅在诊断获益大于风险时使用。

4. Radioactive Tracers | 放射性示踪剂

A radioactive tracer is a chemical compound labelled with a radioactive isotope. When introduced into the body (by injection, ingestion or inhalation), it accumulates in a target organ. The isotope must emit gamma rays so that radiation can be detected outside the body. It should have a short half-life to minimise the dose, and must not emit alpha or beta particles that would be absorbed internally.

放射性示踪剂是用放射性同位素标记的化合物。进入体内（注射、口服或吸入）后，它积聚在靶器官。同位素必须发射γ射线，以便在体外被探测。它的半衰期应很短以减少剂量，且不得发射会被体内吸收的α或β粒子。

A common example is technetium-99m (Tc-99m), which has a half-life of about 6 hours and emits gamma rays. It is used to image organs such as the thyroid, bones, heart, and kidneys.

常见的例子是锝-99m（Tc-99m），半衰期约6小时，发射γ射线，用于甲状腺、骨骼、心脏和肾脏等器官的成像。

5. Gamma Camera and PET Scans | γ相机与正电子发射断层扫描

A gamma camera detects gamma rays emitted by a tracer inside the body. It contains a collimator (usually lead) with many parallel holes that only allow gamma rays travelling in a straight line to reach a scintillator crystal. The crystal produces flashes of light when struck by gamma photons; these are converted into electrical signals and used to build a 2D image of the tracer distribution.

Published by TutorHao | IGCSE Physics Revision Series | aleveler.com

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