📚 IGCSE Edexcel Physics: Medical Physics Key Points | IGCSE Edexcel 物理：医疗物理考点精讲
Medical Physics brings together many of the core principles of waves, radioactivity, and electricity to understand how doctors see inside the human body without surgery. In the IGCSE Edexcel specification, this topic is assessed in Paper 2 and often links calculations, safety precautions, and comparisons between imaging methods. This article breaks down every key concept you need to master, with clear explanations and exam-relevant details.
医疗物理综合了波动、放射性和电学等核心原理，帮助医生在不进行手术的情况下观察人体内部。在IGCSE Edexcel 考试大纲中，本专题在 Paper 2 中考查，经常将计算、安全预防措施以及成像方法对比联系起来。本文拆解你需要掌握的每一个关键概念，并提供清晰的解释和贴合考点的细节。
1. Sound Waves and the Nature of Ultrasound | 声波与超声波的特性
Sound waves are longitudinal mechanical waves that require a medium to travel. The human ear can detect frequencies between approximately 20 Hz and 20 000 Hz. Ultrasound refers to sound with a frequency above 20 000 Hz, typically in the megahertz range for medical use. Because the wavelength is very short, ultrasound can produce detailed images of internal organs.
声波是纵波，属于机械波，需要介质传播。人耳可以探测大约 20 Hz 到 20 000 Hz 的频率。超声波指频率高于 20 000 Hz 的声波，医疗中通常达到兆赫级。由于波长非常短，超声波可以对体内器官生成精细的图像。
In ultrasound scanning, a transducer containing a piezoelectric crystal emits short pulses of ultrasound into the body. When the pulse meets a boundary between different tissues, part of the wave is reflected back to the transducer. The same crystal detects the returning echoes and converts them into electrical signals, which a computer uses to construct a real‑time image.
在超声扫描中，装有压电晶体的换能器向体内发射短脉冲超声波。当脉冲遇到不同组织间的界面时，一部分波被反射回换能器。同一块晶体检测返回的回声并将其转换为电信号，计算机利用这些信号构建实时图像。
2. Ultrasound Calculations: Distance, Speed and Time | 超声计算：距离、速度与时间
A standard exam question asks you to calculate the depth of a reflecting surface under the skin. The total distance travelled by the ultrasound pulse is twice the depth because it goes to the boundary and back. The formula linking speed, distance and time is:
标准考题会让你计算皮肤下反射面的深度。超声波脉冲传播的总距离是深度的两倍，因为它往返于界面。速度、距离和时间的关系式为：
v = 2d / t
Where v is the speed of sound in the medium (typically given as ~1540 m/s in soft tissue), d is the depth of the reflecting surface, and t is the time interval between emission and reception of the echo. Always rearrange carefully and remember the factor of 2.
其中 v 是介质中声速（软组织中通常约为 1540 m/s），d 是反射面的深度，t 是发射与接收回波之间的时间间隔。务必仔细变形并牢记因数 2。
= 0.0308 m ≈ 3.1 cm. This type of question often requires unit conversion, so be comfortable with milliseconds (ms) and microseconds (μs).
计算示例：一个超声脉冲从肝脏界面返回需要 4.0 × 10⁻⁵ s。使用 v = 1540 m/s，深度 d = (v × t) / 2 = (1540 × 4.0×10⁻⁵) / 2 = 0.0308 m ≈ 3.1 cm。这类题目经常需要单位转换，因此要熟练掌握毫秒 (ms) 和微秒 (μs)。
3. Ultrasound in Medicine: Uses and Advantages | 超声的医学应用与优点
Ultrasound is widely used for prenatal scans to monitor foetal development, as well as for examining soft tissues such as the liver, kidneys and heart. Doppler ultrasound can even measure blood flow by detecting frequency changes in reflected waves from moving red blood cells.
超声波广泛用于产前扫描以监测胎儿发育，也用于检查肝、肾和心脏等软组织。多普勒超声甚至可以通过检测来自运动红细胞的反射波频率变化来测量血液流速。
One major advantage of ultrasound is that it is non‑ionising, so it does not carry the same cancer risk as X‑rays. It is also relatively inexpensive and portable, and produces real‑time images without the need for invasive procedures. Because ultrasound reflects strongly at boundaries between materials with very different acoustic impedance, it can clearly distinguish between fluid‑filled cysts and solid tumours.
超声的一个主要优点是非电离性，因此不具有与 X 射线相同的致癌风险。它还相对廉价、便携，并且无需侵入性操作即可生成实时图像。因为超声波在声阻抗差异较大的材料界面附近反射强烈，所以能清晰区分充满液体的囊肿与实体肿瘤。
4. Production and Properties of X‑rays | X 射线的产生与特性
X‑rays are high‑frequency, short‑wavelength electromagnetic waves. In an X‑ray tube, electrons are emitted from a heated cathode filament and accelerated by a high voltage (typically tens of thousands of volts) towards a metal anode target, often made of tungsten. When the fast electrons strike the target, some of their kinetic energy is converted into X‑ray photons.
X 射线是高频、短波长的电磁波。在 X 射线管中，电子从加热的阴极灯丝发射出来，被高电压（通常数万伏）加速后撞向金属阳极靶（通常由钨制成）。当快速电子轰击靶材时，它们的一部分动能转换为 X 射线光子。
X‑rays are ionising radiation, meaning they can remove electrons from atoms and damage living cells. They travel in straight lines, cannot be focused by lenses, and are absorbed more by dense materials such as bone and metal. The intensity of an X‑ray beam decreases as it passes through matter, and the amount of absorption depends on the thickness and atomic number of the material.
X 射线是电离辐射，意味着它们可以使原子核外电子脱离，损伤活细胞。它们沿直线传播，不能被透镜聚焦，且容易被骨和金属等致密材料吸收。X 射线束通过物质时强度会减小，吸收程度取决于材料的厚度和原子序数。
5. X‑ray Imaging and Safety Precautions | X 射线成像与安全措施
A conventional X‑ray photograph is a shadow image. The patient is placed between the X‑ray tube and a detector (photographic film or a digital sensor). Hard tissues like bone absorb more X‑rays and appear white, while soft tissues allow more radiation through and appear darker. X‑rays are particularly useful for viewing bone fractures, dental problems and chest infections.
常规 X 射线照片是一幅阴影图像。患者位于 X 射线管和探测器（照相胶片或数字传感器）之间。像骨骼这样的硬组织吸收较多 X 射线，在图像上呈现白色；而软组织让更多辐射通过，呈现较暗色调。X 射线特别适用于观察骨折、牙科问题和胸部感染。
Because X‑rays are ionising, strict safety measures are essential. Radiographers leave the room or stand behind a lead screen. Patients are exposed for the shortest possible time and only the required area is irradiated. Lead aprons can shield parts of the body not being imaged. The triad of time, distance and shielding minimises the risk of cell mutation and cancer.
由于 X 射线具有电离能力，严格的安全措施必不可少。射线技师会离开房间或站在铅屏后面。患者接受照射的时间尽可能短，并且仅对所需区域进行辐照。铅围裙可以屏蔽不成像的身体部位。时间、距离和屏蔽三原则可最大程度地减少细胞突变和癌症的风险。
6. CT Scans: Computerised Tomography | CT 扫描：计算机断层成像
A CT scanner takes many X‑ray images from different angles around the body and uses a computer to combine them into a three‑dimensional image. During a scan, the X‑ray tube rotates around the patient while the couch moves slowly through the gantry. This produces cross‑sectional 'slices' that can show the shape and position of organs and tumours with high precision.
CT 扫描仪从身体周围不同角度获取大量 X 射线图像，并由计算机将其组合成三维影像。扫描过程中，X 射线管围绕患者旋转，检查床缓慢通过机架。这样能产生横截面的“切片”，高精度地显示器官和肿瘤的形状与位置。
CT scans provide far more detail than conventional X‑rays and can distinguish between tissues of similar density. However, the total radiation dose is much higher — equivalent to many single X‑ray exposures. Therefore, doctors balance the diagnostic benefit against the increased risk. CT is invaluable in emergency situations, such as detecting internal bleeding or complex fractures.
CT 扫描提供的细节远比传统 X 射线丰富，能够区分密度相似的组织。但总辐射剂量高得多——相当于多次单次 X 射线照射。因此医生会在诊断收益与增加的辐射风险之间权衡。CT 在检测内出血或复杂骨折等紧急情况中极有价值。
7. Endoscopy and Total Internal Reflection | 内窥镜与全内反射
An endoscope is a flexible tube containing bundles of optical fibres that allows a doctor to see inside the body without major surgery. The key physics principle is total internal reflection. Light travels along the core of the fibre, repeatedly reflecting off the core–cladding boundary because the angle of incidence always exceeds the critical angle. The cladding has a lower refractive index than the core, and it protects the core surface to maintain the high reflectivity.
内窥镜是一种含有光纤束的可弯曲管道，能让医生无需大开刀即可观察体内。关键的物理原理是全内反射。光线在光纤芯中传播，由于入射角始终大于临界角，会在芯与包层界面反复反射。包层的折射率小于芯层，并保护芯层表面以维持高反射率。
One bundle of fibres carries light into the body to illuminate the area, while a second bundle returns an image to the eyepiece or a camera. Endoscopes are used to examine the stomach, colon, bladder and joints. They can also carry small surgical tools for taking biopsies. The procedure is minimally invasive and does not involve ionising radiation.
一束光纤将光线导入体内照亮检查区域，另一束将图像传回目镜或摄像头。内窥镜用于检查胃部、结肠、膀胱和关节。它们还可以携带小型手术工具进行活检。该过程属于微创操作，且不涉及电离辐射。
8. Radioactive Tracers and Gamma Rays | 放射性示踪剂与伽马射线
In nuclear medicine, a small quantity of a radioactive isotope (a tracer) is introduced into the patient's body, often by injection or swallowing. The tracer emits gamma rays, which can be detected from outside the body to follow its path or uptake in organs. Technetium‑99m is a commonly used tracer because it emits gamma rays with an energy suitable for detection and has a half‑life of about 6 hours — long enough for the procedure, but short enough to minimise long‑term radiation dose.
在核医学中，少量放射性同位素（示踪剂）通常通过注射或吞服进入患者体内。示踪剂发射伽马射线，可以在体外探测以追踪其路径或在器官中的摄取情况。锝‑99m 是一种常用的示踪剂，因为它发射的伽马射线能量适合检测，半衰期约 6 小时——足够完成检查，且足够短以尽量减少长期辐射剂量。
A gamma camera detects the emitted gamma photons and builds up a picture of how the tracer is distributed. This functional imaging can show whether an organ such as the thyroid or kidney is working correctly, or highlight cancerous areas that absorb the tracer abnormally. Unlike X‑rays, this method shows physiology rather than just anatomy.
伽马相机探测发射出的伽马光子，生成示踪剂分布的图像。这种功能性成像可以显示甲状腺或肾脏等器官是否运作正常，或突出异常吸收示踪剂的癌变区域。与 X 射线不同，该方法显示的是生理功能，而不仅仅是解剖结构。
9. Positron Emission Tomography (PET) Scans | 正电子发射断层扫描 (PET)
PET scanning is an advanced nuclear medicine technique that uses tracers emitting positrons (β⁺ particles). A common tracer is fluorine‑18 attached to glucose. When a positron meets an electron inside the body, the two particles annihilate and produce two identical gamma photons travelling in exactly opposite directions. Detectors arranged in a ring around the patient register these pairs and pinpoint the location of the annihilation event.
PET 扫描是一种先进的核医学技术，使用发射正电子（β⁺ 粒子）的示踪剂。一种常见的示踪剂是与葡萄糖结合的氟‑18。当正电子在体内遇到电子时，两粒子湮灭，产生两个沿精确相反方向传播的同能伽马光子。环绕患者的环形探测器记录这些光子对，从而定位湮灭事件发生的位置。
PET scans are exceptionally useful for detecting cancer, because rapidly dividing cancer cells take up more glucose than normal cells, leading to a higher concentration of tracer. PET is often combined with CT (PET‑CT) to overlay functional PET information onto detailed anatomical CT images. The tracers used have very short half‑lives and must be produced in a cyclotron close to the hospital.
PET 扫描对于检测癌症极为有用，因为快速分裂的癌细胞比正常细胞摄取更多葡萄糖，导致示踪剂更浓。PET 常与 CT 联合使用 (PET‑CT)，将 PET 的功能性信息叠加在 CT 的详细解剖图像上。所用示踪剂半衰期极短，必须在医院附近的回旋加速器中生产。
10. Comparing Medical Imaging Modalities | 医学成像方式的比较
Each imaging technique has distinct advantages and disadvantages, and the choice depends on the clinical situation.
每种成像技术都有其独特的优缺点，选择取决于临床情况。
| Technique / 技术 | Ionising? / 是否电离 | Main use / 主要用途 | Key limitation / 关键局限 |
|---|---|---|---|
| Ultrasound | No | Soft tissues, prenatal | Poor at penetrating bone or gas |
| X‑ray | Yes | Bone fractures, chest | Radiation dose, 2D only |
| CT scan | Yes (higher dose) | Detailed 3D anatomy | High radiation dose |
| Endoscopy | No | Internal surfaces, biopsy | Invasive (though minimally) |
| Gamma camera | Yes (gamma rays) | Organ function | Lower resolution than CT |
| PET scan | Yes (positron annihilation) | Metabolic activity, cancer | Very short‑lived tracers, cost |
Exam questions may ask you to justify why a particular imaging method is chosen for a given scenario. Always refer to the required detail of the image, whether the tissue of interest contains bone, and the acceptable level of radiation risk.
考题可能要求你说明为什么在特定情况下选择某种成像方法。答题时应始终提及所需的图像细节、感兴趣区域是否包含骨骼，以及可接受的辐射风险水平。
11. Key Equations and Data to Remember | 需要记住的关键方程与数据
Make sure you are confident with the following relationships and typical values:
请确保你对以下关系式和典型数值烂熟于心：
The speed‑time‑distance formula for ultrasound: v = 2d / t. Remember to halve the total distance to find depth.
超声的速度‑时间‑距离公式：v = 2d / t。记住要将总距离减半以求得深度。
Speed of sound in soft tissue ≈ 1540 m/s (given in questions). In air it is ≈ 330–340 m/s, but ultrasound is not transmitted well through air.
声音在软组织中的速度 ≈ 1540 m/s（题目会给出）。在空气中约为 330–340 m/s，但超声在空气中传播不良。
Total internal reflection requires the core refractive index to be greater than the cladding refractive index, and the angle of incidence > critical angle.
全内反射要求纤芯折射率大于包层折射率，且入射角 > 临界角。
Half‑life of technetium‑99m ≈ 6 hours; fluorine‑18 ≈ 110 minutes. Shorter half‑life means less residual radiation but logistical challenges.
锝‑99m 的半衰期约为 6 小时；氟‑18 约为 110 分钟。更短的半衰期意味着残余辐射更少，但带来物流挑战。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Many students lose marks by forgetting to divide by 2 in ultrasound depth calculations, so always highlight the factor 2 in your working. When describing X‑ray safety, be specific: mention 'lead shielding', 'short exposure time' and 'radiographer leaving the room', rather than a vague 'protection'.
许多考生因超声深度计算中忘记除以 2 而丢分，因此务必在解题步骤中突显因数 2。在描述 X 射线安全措施时要具体：提到“铅屏蔽”、“短时间照射”和“放射技师离开房间”，而非含糊的“防护”。
In questions about endoscopes, clearly link total internal reflection to the refractive indices of core and cladding, and state that the cladding also protects the core from scratches. For nuclear medicine, connect the choice of tracer half‑life to both the diagnostic time window and the radiation dose to the patient. If asked to compare techniques, always address image quality, radiation risk, and whether functional or anatomical information is obtained.
在关于内窥镜的题目中，要将全内反射与纤芯和包层的折射率明确关联，并说明包层还能保护纤芯不被刮擦。对于核医学，需将示踪剂半衰期的选择与检查时间窗口和患者所受辐射剂量联系起来。如果要求比较技术，须始终论及图像质量、辐射风险，以及获得的是功能性还是解剖学信息。
Practice rearranging equations and converting time units (seconds, milliseconds, microseconds) carefully. Drawing a simple diagram of the path of ultrasound or X‑rays can help you visualise the problem and avoid arithmetic errors.
认真练习方程变形以及时间单位（秒、毫秒、微秒）的转换。画一个超声或 X 射线路径的简图能帮助你直观理解题目，避免计算错误。
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