📚 GCSE WJEC Physics: Medical Physics – Key Points Explained | GCSE WJEC 物理：医疗物理考点精讲
Medical physics applies fundamental physics principles to diagnose and treat diseases. It covers imaging methods like X-rays, ultrasound, and PET scans, alongside therapies such as radiotherapy and the use of optical fibres in endoscopy. This guide highlights the key facts, equations, and safety considerations required for the WJEC GCSE Physics exam.
医疗物理将基本物理原理应用于疾病的诊断与治疗。它涵盖X射线、超声波、PET扫描等成像方法，以及放射治疗和内窥镜光纤等治疗技术。本指南突出WJEC GCSE物理考试所需的关键知识点、公式和安全注意事项。
1. X-rays: Production and Properties | X射线的产生和性质
X-rays are a form of high-energy electromagnetic radiation with wavelengths much shorter than visible light. They are produced when fast-moving electrons are suddenly decelerated by striking a metal target, usually tungsten, inside an X-ray tube. The electrons are emitted from a heated filament by thermionic emission and accelerated by a large potential difference (e.g. 50 kV).
X射线是一种高能电磁辐射，波长比可见光短得多。当高速电子在X射线管内撞击金属靶（通常为钨）突然减速时，便会产生X射线。电子由加热的灯丝通过热离子发射释放，并被高电势差（例如50 kV）加速。
The kinetic energy (KE) gained by each electron is equal to the product of the electron charge (e) and the accelerating voltage (V). This is expressed by the equation:
KE = e × V
每个电子获得的动能（KE）等于电子电荷（e）与加速电压（V）的乘积，用公式表示：
KE = e × V
X-rays are ionising, meaning they can knock electrons out of atoms. They can pass through soft tissues but are heavily absorbed by dense materials like bone. This absorption contrast forms the basis of X-ray imaging.
X射线具有电离性，意味着它们能将原子中的电子击出。它们能穿透软组织，但会被骨骼等致密物质大量吸收。这一吸收差异构成了X射线成像的基础。
2. X-ray Imaging and Safety Precautions | X射线成像与安全措施
In a conventional X-ray image, bones appear white because they absorb most X-rays, while surrounding soft tissues appear in shades of grey. The image is recorded on a photographic film or by a digital detector placed behind the patient. The degree of blackening depends on the amount of radiation reaching the detector.
在传统X射线图像中，由于骨骼吸收了大部分X射线，骨骼呈白色，而周围的软组织则呈现不同深浅的灰色。图像被记录在患者身后的摄影胶片或数字探测器上。黑化程度取决于到达探测器的辐射量。
Because X-rays can damage living cells and increase the risk of cancer, strict safety measures are implemented. These include reducing exposure time, using lead shielding (such as lead aprons and screens), standing at a safe distance from the source, and wearing a film badge dosimeter to monitor accumulated radiation dose.
由于X射线会损伤活细胞并增加患癌风险，必须采取严格的安全措施。这些措施包括缩短暴露时间、使用铅屏蔽（如铅围裙和铅屏风）、与辐射源保持安全距离，以及佩戴胶片徽章剂量计以监测累积辐射剂量。
Lead is particularly effective at absorbing X-rays because of its high density and atomic number. Collimators are also used to narrow the X-ray beam so that only the region of interest is irradiated.
铅因其高密度和高原子序数而特别善于吸收X射线。还会使用准直器来缩小X射线束，使得只有感兴趣区域被照射。
3. Ultrasound Waves | 超声波的性质
Ultrasound refers to sound waves with frequencies above 20,000 Hz, beyond the range of human hearing. In medical applications, frequencies between 1 MHz and 15 MHz are commonly used. Unlike X-rays, ultrasound waves are longitudinal mechanical waves and cannot travel through a vacuum; they require a medium.
超声波是指频率超过20,000 Hz的声波，超出了人类的听觉范围。在医学应用中，通常使用1 MHz至15 MHz的频率。与X射线不同，超声波是纵波（机械波），不能在真空中传播，需要介质。
The velocity of ultrasound in a medium is given by the universal wave equation: v = f × λ, where v is the speed in metres per second, f is the frequency in hertz, and λ is the wavelength in metres. Sound waves travel at different speeds in different tissues.
v = f × λ
超声波在介质中的速度由普遍波方程给出：v = f × λ，其中 v 是以米/秒为单位的速率，f 是以赫兹为单位的频率，λ 是以米为单位的波长。声波在不同组织中的传播速度不同。
v = f × λ
When an ultrasound wave encounters a boundary between two tissues of different acoustic impedances, some of the wave is reflected. This partial reflection is the key principle behind ultrasound imaging.
当超声波遇到两种声阻抗不同的组织之间的边界时，部分波会被反射。这种部分反射是超声成像背后的关键原理。
4. Ultrasound Pulse-Echo Technique | 超声波脉冲-回声技术
Medical ultrasound imaging uses the pulse-echo method. A transducer (probe) both emits short pulses of ultrasound and detects the returning echoes. The time delay Δt between emitting a pulse and receiving its echo is measured. The distance to the reflecting surface (depth) is calculated using:
depth = (v × Δt) / 2
医学超声成像采用脉冲-回声法。换能器（探头）发射短促的超声波脉冲，并检测返回的回声。测量从发出脉冲到接收到回声的时间延迟 Δt。到反射面的距离（深度）用以下公式计算：
深度 = (v × Δt) / 2
The factor of 2 accounts for the round trip of the pulse (to the boundary and back). The transducer converts the electrical signals into a real-time image on a screen. Ultrasound is particularly valuable because it is non-ionising, making it extremely safe for prenatal scans and repeated examinations.
除以2是因为脉冲的往返路径（到边界再返回）。换能器将电信号转换为屏幕上的实时图像。超声波特别宝贵之处在于它是非电离的，因此对产前扫描和重复检查极其安全。
Doppler ultrasound uses the change in frequency of the reflected wave (the Doppler shift) to measure the speed of blood flow. When the sound is reflected from moving red blood cells, the frequency shift is detected and used to calculate velocity.
多普勒超声利用反射波频率的变化（多普勒频移）来测量血流速度。当声波从运动的红细胞上反射时，检测到的频移即可用于计算速度。
5. Endoscopy and Optical Fibres | 内窥镜与光纤
An endoscope is a medical instrument that allows doctors to see inside the body without invasive surgery. It uses a bundle of fine optical fibres to transmit light. Optical fibres rely on the principle of total internal reflection: when light travels from a denser to a less dense medium and strikes the boundary at an angle greater than the critical angle, it is completely reflected back into the denser medium.
内窥镜是一种医疗仪器，使医生无需进行侵入性手术即可观察身体内部。它利用一束细小的光纤来传输光线。光纤依赖于全内反射原理：当光从光密介质传播到光疏介质，并以大于临界角的角度照射到边界时，会被完全反射回光密介质内。
The central core of an optical fibre has a higher refractive index than the surrounding cladding. This ensures that light signals travel along the fibre with minimal loss, even around bends. Endoscopes typically have separate fibre bundles for illumination, image transmission, and channels for tools or air.
光纤的纤芯折射率高于周围的包层。这确保了光信号以最小的损耗沿光纤传播，即使绕过弯曲处也是如此。内窥镜通常有用于照明的独立光纤束、用于图像传输的光纤束，以及用于工具或空气的通道。
Because visible light is non-ionising, endoscopy is a safe imaging method. It is widely used to examine the digestive tract, joints, and other internal structures.
由于可见光是非电离的，内窥镜检查是一种安全的成像方法。它被广泛用于检查消化道、关节和其他内部结构。
6. Radioactive Tracers for Diagnosis | 用于诊断的放射性示踪剂
A radioactive tracer is a radioisotope that emits gamma radiation, which can be detected outside the body. One commonly used tracer is technetium-99m (Tc-99m). It is injected, swallowed, or inhaled, and its spread through the body is monitored with a gamma camera.
放射性示踪剂是一种能发射伽马辐射的放射性同位素，可在体外被检测到。常用的示踪剂之一是锝-99m (Tc-99m)。它通过注射、吞服或吸入进入人体，其分布情况由伽马照相机监测。
Gamma rays are chosen because they are penetrating enough to leave the body and reach the detector. To minimise harm to the patient, the tracer should have a short physical half-life (T₁/₂) and emit low-energy gamma rays. The half-life is the time for the activity of a radioactive source to fall to half its original value.
选择伽马射线是因为它们具有足够的穿透力，能够离开人体并到达探测器。为减少对患者的伤害，示踪剂应具有较短的物理半衰期（T₁/₂）并发射低能伽马射线。半衰期是指放射源的活度降至其初始值一半所需的时间。
Radioactive tracers allow doctors to study how organs are functioning, such as checking blood flow through the heart or kidney filtration rates, rather than just showing anatomical structure.
放射性示踪剂使医生能够研究器官的工作状况，例如检查通过心脏的血流或肾过滤率，而不仅仅是显示解剖结构。
7. Positron Emission Tomography (PET) | 正电子发射断层扫描（PET）
PET scanners use a radiotracer that emits positrons, such as fluorine-18 bonded to glucose (FDG). A positron is the antiparticle of an electron; when it meets an electron inside the body, the two annihilate, producing two gamma photons that travel in almost exactly opposite directions. Ring-shaped detectors record the simultaneous arrival of these photon pairs to pinpoint the location of the tracer.
PET扫描仪使用发射正电子的放射性示踪剂，例如与葡萄糖结合的氟-18 (FDG)。正电子是电子的反粒子；当它在体内遇到电子时，两者会湮灭，产生两个几乎沿相反方向飞行的伽马光子。环形探测器记录这些光子对同时到达的事件，以精准定位示踪剂的位置。
Because cancerous cells often have higher metabolic rates and absorb more glucose, PET images can highlight tumours. PET is functional imaging: it produces a 3D map of metabolic activity, which helps detect cancer, brain disorders, and heart disease. The radiation dose is a concern, but the clinical benefits are often significant.
由于癌细胞通常具有更高的代谢率并吸收更多葡萄糖，PET图像可以突显肿瘤。PET是功能成像：它生成代谢活动的三维图谱，有助于检测癌症、脑部疾病和心脏病。辐射剂量是一个关注点，但临床获益通常十分显著。
8. Radiotherapy | 放射治疗
Radiotherapy uses ionising radiation to destroy cancer cells or stop them from multiplying. High-energy X-rays (produced by a linear accelerator) or gamma rays from a radioactive source such as cobalt-60 are directed precisely at the tumour. The goal is to maximise the dose to the cancerous region while sparing healthy tissue.
放射治疗利用电离辐射摧毁癌细胞或阻止其增殖。由直线加速器产生的高能X射线，或来自钴-60等放射源的伽马射线，被精确地引向肿瘤。目标是在保护健康组织的同时，最大限度地向癌变区域给予剂量。
To achieve this, the beam may be shaped by multileaf collimators and delivered from several angles, intersecting at the tumour. This concentrates the absorbed dose at the target. However, some damage to nearby normal cells is unavoidable, leading to side effects such as fatigue or skin reactions.
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