📚 IGCSE CCEA Physics: Medical Physics Key Points | IGCSE CCEA 物理：医疗物理 考点精讲
Medical physics applies the principles of physics to healthcare, enabling diagnosis and treatment of diseases. In IGCSE CCEA Physics, you need to understand how X-rays, ultrasound, fibre optics, and radioactivity are used safely and effectively to produce images and treat conditions without causing unnecessary harm.
医疗物理将物理学原理应用于医疗保健，实现疾病的诊断和治疗。在 IGCSE CCEA 物理中，你需要理解 X 射线、超声波、光纤及放射性如何被安全有效地用于成像和治疗疾病，同时避免不必要的伤害。
1. X-ray Production and Properties | X 射线的产生与性质
X-rays are produced when high-speed electrons collide with a metal target (often tungsten) inside a vacuum tube. The sudden deceleration of electrons causes the emission of high-energy electromagnetic radiation. X-rays have very short wavelengths (about 10⁻¹⁰ m) and high frequencies, giving them strong penetrating ability.
X 射线由高速电子在真空管内撞击金属靶（通常是钨）产生。电子的突然减速导致高能电磁辐射的释放。X 射线波长极短（约 10⁻¹⁰ m），频率很高，因此具有较强的穿透能力。
Their penetration depends on the material’s density and atomic number. They pass easily through soft tissue but are significantly absorbed by denser materials such as bone and metal. This difference in absorption forms the basis of X-ray imaging.
其穿透能力取决于物质的密度和原子序数。它们容易穿透软组织，但会被骨骼和金属等密度更高的材料大量吸收。这种吸收差异构成了 X 射线成像的基础。
2. X-ray Imaging and Safety | X 射线成像与安全
In a conventional X-ray machine, the beam passes through the patient and strikes a photographic film or digital detector. Dense structures appear white because fewer X-rays reach the detector, while soft tissues appear darker. Contrast can be improved using substances like barium or iodine, which absorb X-rays strongly and outline organs such as the digestive tract.
在传统的 X 光机中，射线穿过患者并照射到胶片或数字探测器上。因为到达探测器的 X 射线较少，密度大的结构呈白色，而软组织较暗。可用钡或碘等对比剂提高对比度，这些物质强烈吸收 X 射线，勾勒出消化道等器官的轮廓。
X-rays are ionising radiation and can damage living cells, increasing the risk of cancer. Safety measures include using the minimum exposure time, standing behind lead shields, wearing lead aprons, and monitoring cumulative dose with film badges. As low as reasonably achievable (ALARA) is the guiding principle.
X 射线是电离辐射，会损伤活细胞，增加癌症风险。安全措施包括使用最短曝光时间、站在铅屏蔽后面、穿戴铅围裙以及用辐射剂量计监测累积剂量。合理可行尽量低（ALARA）是指导原则。
3. Computed Tomography (CT) | 计算机断层扫描 (CT)
A CT scanner rotates an X-ray source and a set of detectors around the patient, capturing numerous 2D projection images from different angles. A computer reconstructs these into cross-sectional slices and finally into a detailed 3D image. The patient lies on a motorised table that moves slowly through the gantry.
CT 扫描仪围绕患者旋转 X 射线源和一组探测器，从不同角度获取大量二维投影图像。计算机将这些图像重建成横截面切片，最终合成精细的三维图像。患者躺在电动床上缓慢通过扫描架。
CT provides much greater detail than a single X-ray, allowing identification of tumours, internal bleeding, and bone fractures. However, a CT scan involves a significantly higher radiation dose, so the clinical benefit must outweigh the risk.
CT 比单次 X 光片提供更丰富的细节，能识别肿瘤、内出血和骨折。但 CT 扫描的辐射剂量明显更高，因此必须确保临床获益大于风险。
4. Ultrasound Waves and Echoes | 超声波与回声
Ultrasound describes sound waves with frequencies above 20,000 Hz, typically 1–10 MHz for medical imaging. A transducer containing piezoelectric crystals produces short pulses of ultrasound and then switches to receive echoes reflected from tissue boundaries. The time delay between transmission and echo reception is measured.
超声波指频率超过 20,000 Hz 的声波，医学成像常用 1–10 MHz。包含压电晶体的换能器发射短脉冲超声，然后切换至接收模式，接收从组织界面反射的回声。测量发射与回声接收之间的时间延迟。
Using the known speed of sound in soft tissue (about 1540 m/s), depth is calculated as:
利用已知的软组织声速（约 1540 m/s），深度计算如下：
depth = (speed × time) / 2
The division by 2 accounts for the pulse travelling to the boundary and back. Higher frequencies give better resolution but penetrate less deeply.
除以 2 是因为脉冲往返于界面。频率越高分辨率越好，但穿透深度越浅。
5. Ultrasound Scanning in Medicine | 医学中的超声扫描
Ultrasound is widely used to monitor foetal development during pregnancy because it does not involve ionising radiation. It also images the heart (echocardiography), liver, kidneys, and blood flow via the Doppler effect. A water-based coupling gel is applied to the skin to eliminate air gaps, ensuring good acoustic coupling.
由于不使用电离辐射，超声波广泛用于孕期胎儿发育监测。它还通过多普勒效应对心脏（超声心动图）、肝脏、肾脏和血流进行成像。皮肤上涂抹水性耦合凝胶以消除气隙，保证良好的声学耦合。
Ultrasound is safe for repeated scans, portable, and relatively low-cost. Its main limitation is that it cannot penetrate bone or air-filled structures effectively, making it less suitable for lungs or mature bone.
超声波可安全用于重复扫描，便于携带且成本相对较低。其主要局限是无法有效穿透骨骼或充满空气的结构，因此不太适用于肺部或成熟骨骼。
6. Optical Fibres and Endoscopy | 光纤与内窥镜检查
An endoscope contains two bundles of flexible optical fibres. One bundle carries light from an external source into the body to illuminate the area; the other transmits the reflected light back to an eyepiece or camera, forming an image. This allows doctors to view internal cavities without major surgery.
内窥镜包含两束柔性光纤。一束将外部光源的光导入体内照亮区域；另一束将反射光传回目镜或摄像头形成图像。这使医生无需大手术即可观察体腔内部。
The guiding principle is total internal reflection. Light travels through the core of the fibre, which has a higher refractive index than the surrounding cladding. When the light ray hits the core–cladding boundary at an angle greater than the critical angle, it reflects completely and continues along the fibre with negligible loss.
其指导原理是全内反射。光在纤芯中传播，纤芯的折射率高于周围的包层。当光线以大于临界角的角度射到纤芯与包层的界面时，会发生全反射，并沿光纤几乎无损耗地继续传播。
7. Radioactive Tracers | 放射性示踪剂
A radioactive tracer is a radioisotope introduced into the body, usually by injection or ingestion. It follows a specific metabolic pathway or accumulates in a particular organ, emitting gamma rays that are detected externally by a gamma camera. This reveals the function of organs rather than just their structure.
放射性示踪剂是引入体内的放射性同位素，通常通过注射或吞服。它遵循特定的代谢途径或积聚在特定器官中，发射的伽马射线由体外伽马相机探测。这能揭示器官的功能而不仅仅是结构。
Technetium-99m is a common choice because it emits pure gamma rays with an energy suitable for detection, has a half-life of 6 hours, and can be chemically bound to different pharmaceuticals. A short half-life minimises the patient’s radiation exposure while allowing enough time for the scan.
锝-99m 是常用选择，因为它发射纯伽马射线，能量适合探测，半衰期为 6 小时，并能与不同药物化学结合。短半衰期可在允许足够扫描时间的同时，最大限度减少患者的辐射暴露。
8. Positron Emission Tomography (PET) | 正电子发射断层扫描 (PET)
PET uses radiotracers that decay by positron emission, such as fluorine-18 attached to glucose (FDG). Once injected, the tracer concentrates in areas of high metabolic activity, like cancer cells. A positron travels a short distance and annihilates with an electron, producing two gamma photons that fly apart in exactly opposite directions.
PET 使用通过发射正电子而衰变的放射性示踪剂，例如标记在葡萄糖上的氟-18（FDG）。注射后，示踪剂富集在代谢活跃的区域，如癌细胞。正电子穿行短距离后与电子湮灭，产生两束沿严格相反方向飞行的伽马光子。
Detectors arranged in a ring around the patient only record an event when two photons arrive simultaneously (coincidence). This allows the computer to pinpoint the location of the annihilation and build a 3D map of metabolic activity. PET is often combined with CT (PET-CT) to overlay functional and anatomical data.
围绕患者排列成环状的探测器仅在两个光子同时到达（符合）时记录事件。这使计算机能精确定位湮灭位置，构建代谢活动的三维图谱。PET 常与 CT 联合（PET-CT），将功能与解剖数据叠加。
9. Radiation Therapy | 放射治疗
Radiation therapy uses high-energy ionising radiation, such as accelerated X-rays or gamma rays from sources like cobalt-60, to destroy cancerous cells. The radiation damages the DNA of rapidly dividing cells, preventing them from proliferating. Multiple beams are focused on the tumour from different angles to concentrate the dose and spare normal tissue.
放射治疗使用高能电离辐射，如加速 X 射线或钴-60 等放射源产生的伽马射线，来摧毁癌细胞。辐射损伤快速分裂细胞的 DNA，阻止其增殖。多束射线从不同角度聚焦于肿瘤，集中剂量并保护正常组织。
Treatment planning involves precise dose calculations and the use of custom-made shields or multi-leaf collimators to shape the beam. Patients are carefully positioned using lasers and immobilisation devices. Side effects occur because healthy cells near the tumour are also affected, though modern techniques minimise this.
治疗计划包括精确的剂量计算，并使用定制屏蔽或多叶准直器塑造射束。通过激光和固定装置仔细摆位患者。由于肿瘤附近的健康细胞也会受影响，可能出现副作用，但现代技术已将其降至最低。
10. Comparing Medical Imaging Techniques | 医学成像技术比较
Each imaging modality has distinct advantages and limitations. The table below summarises key differences in terms of ionising radiation use, the type of image produced, and potential risks.
每种成像方式都有独特的优势和局限。下表从是否使用电离辐射、图像类型和潜在风险等方面总结了主要区别。
| Technique | Ionising radiation? | Image type | Main risks |
|---|---|---|---|
| X-ray | Yes | 2D projection; bone and dense structures | Cell damage, increased cancer risk |
| CT | Yes | 3D cross-sectional; soft tissue and bone | Higher radiation dose, same as X-ray risks |
| Ultrasound | No | Real-time 2D; soft tissue, blood flow | No known risks; heating effect at very high intensities |
| Gamma camera (tracers) | Yes (gamma) | Functional map of organ activity | Radiation dose from tracer; allergic reaction rare |
| PET | Yes (positrons → gamma) | 3D metabolic activity; often fused with CT | Radiation dose; risk from co-registered CT |
Choosing the appropriate technique depends on the clinical question, the need for soft-tissue contrast or function, and the acceptable radiation risk.
选择合适的技术取决于临床问题、对软组织对比度或功能的需求以及可接受的辐射风险。
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