📚 Medical Physics in CIE GCSE Physics | CIE GCSE 物理：医疗物理考点精讲
Medical Physics is a fascinating application of fundamental physical principles to diagnose and treat diseases. In the CIE GCSE Physics syllabus, this topic covers ultrasound imaging, X‑rays, radioactive tracers, and radiotherapy. You will learn how sound waves and electromagnetic radiation are used safely to see inside the body and to destroy cancerous cells. Understanding the underlying physics — reflection, refraction, ionisation, and half‑life — is essential for tackling exam questions with confidence.
医疗物理将基础物理原理巧妙地应用于疾病诊断和治疗。在 CIE GCSE 物理大纲中，这一主题涵盖超声波成像、X 射线、放射性示踪剂和放射治疗。你将学习如何安全地利用声波和电磁辐射来观察人体内部并摧毁癌细胞。理解背后的物理原理——反射、折射、电离和半衰期——对于自信应对考试题目至关重要。
1. Properties of Ultrasound | 超声波的特性
Ultrasound refers to sound waves with frequencies above 20 000 Hz, which is beyond the range of human hearing. These longitudinal waves consist of compressions and rarefactions and travel through media by causing particles to vibrate parallel to the direction of wave travel. In medical imaging, typical frequencies range from 2 MHz to 18 MHz.
超声波指频率超过 20 000 Hz 的声波，超出了人耳的听觉范围。这些纵波由压缩和稀疏组成，通过使粒子沿波的传播方向振动而在介质中传播。在医学成像中，典型频率范围为 2 MHz 至 18 MHz。
When ultrasound waves encounter a boundary between two tissues of different acoustic impedance, part of the wave is reflected and part is transmitted. The amount of reflection depends on the difference in impedance; a larger mismatch gives a stronger echo. This principle is exploited to build up images of organs and unborn babies.
当超声波遇到两种声阻抗不同的组织之间的界面时，一部分波被反射，一部分被透射。反射量取决于阻抗差异；差异越大，回声越强。这一原理被用来构建器官和胎儿的图像。
Ultrasound is non‑ionising and therefore considered safer than X‑rays for repeated examinations such as pregnancy scans. It does not damage cells or DNA, making it particularly suitable for monitoring developing foetuses. However, ultrasound cannot penetrate bone or gas‑filled cavities effectively, which limits its use in some areas of the body.
超声波是非电离的，因此对于孕期扫描等重复检查被认为比 X 射线更安全。它不会损伤细胞或 DNA，因此特别适合监测发育中的胎儿。然而，超声波无法有效穿透骨骼或充满气体的空腔，这限制了它在身体某些部位的应用。
2. How Ultrasound Imaging Works | 超声波成像的工作原理
An ultrasound transducer, which contains piezoelectric crystals, emits short pulses of ultrasound into the body. When these pulses are applied to the crystal, it vibrates and generates sound waves; conversely, when returning echoes hit the crystal, it produces an electrical signal. The same transducer acts as both transmitter and receiver.
超声换能器含有压电晶体，向体内发射短脉冲超声波。当脉冲施加到晶体上时，晶体会振动并产生声波；反之，当返回的回声撞击晶体时，会产生电信号。同一个换能器既是发射器又是接收器。
A coupling gel is applied to the skin to eliminate air gaps between the transducer and the body. Because air has a very low acoustic impedance compared to skin, the large impedance mismatch would cause almost total reflection at the skin surface, preventing sound from entering the body. The gel matches the impedances and allows efficient transmission of ultrasound.
在皮肤上涂抹耦合凝胶是为了消除换能器与身体之间的空气间隙。由于空气的声阻抗与皮肤相比非常低，巨大的阻抗失配会导致在皮肤表面几乎全反射，阻止声波进入体内。凝胶匹配了阻抗，使超声波能够有效传输。
the formula: depth = (speed of sound in tissue × time) / 2. The factor of 2 accounts for the round trip. A computer processes the time delays and amplitudes of echoes to generate a real‑time image on a screen.
发射脉冲与接收其回声之间的时间延迟用于计算反射界面的深度。距离通过公式确定：深度 = （声音在组织中的速度 × 时间）/ 2。因子 2 考虑了往返路程。计算机处理回声的时间延迟和振幅，在屏幕上生成实时图像。
The speed of sound in soft tissue is approximately 1540 m/s. Different media have slightly different speeds: air ≈ 330 m/s, bone ≈ 4000 m/s. The large difference means that at tissue–bone or tissue– oundaries, most ultrasound is reflected, making imaging difficult behind these structures.
声音在软组织中的速度大约为 1540 m/s。不同介质的速度略有不同：空气 ≈ 330 m/s，骨骼 ≈ 4000 m/s。巨大的差异意味着在组织–骨骼或组织–空气界面处，大部分超声波被反射，使这些结构后面的成像变得困难。
3. Advantages and Disadvantages of Ultrasound | 超声波的优点和缺点
Advantages: Ultrasound imaging is non‑invasive, painless, and does not use ionising radiation, so it poses no known risk to the patient or foetus. Equipment is portable and relatively inexpensive compared to MRI or CT scanners. Real‑time imaging allows observation of moving structures such as heart valves and blood flow (Doppler ultrasound).
优点：超声成像无创、无痛，不使用电离辐射，因此对患者或胎儿没有已知风险。与 MRI 或 CT 扫描仪相比，设备便携且相对便宜。实时成像可以观察运动结构，如心脏瓣膜和血流（多普勒超声）。
Disadvantages: Image quality is highly operator‑dependent; it can be difficult to obtain clear images of structures behind bone or gas. The resolution is generally lower than that of CT or MRI. Ultrasound cannot reliably distinguish between malignant and benign lesions without a biopsy. Penetration depth is limited, especially at higher frequencies.
缺点：图像质量高度依赖于操作者；很难获得骨骼或气体后方结构的清晰图像。分辨率通常低于 CT 或 MRI。没有活检，超声波无法可靠地区分恶性和良性病变。穿透深度有限，尤其是在较高频率下。
4. X‑rays in Medicine | 医学中的 X 射线
X‑rays are high‑energy electromagnetic waves with wavelengths shorter than 10 nanometres. They are produced when high‑speed electrons hit a metal target in an X‑ray tube. X‑rays are ionising radiation, meaning they carry enough energy to knock electrons out of atoms, creating ions that can damage biological tissue and DNA.
X 射线是高能电磁波，波长小于 10 纳米。它们由高速电子撞击 X 射线管中的金属靶时产生。X 射线是电离辐射，这意味着它们携带足够能量将电子从原子中击出，产生离子，可能损伤生物组织和 DNA。
In diagnostic imaging, X‑rays pass through soft tissue relatively easily but are absorbed more by dense materials such as bone. This differential absorption forms a shadow image on a detector or photographic film. Bones appear white, soft tissues appear in shades of grey, and air appears black. Contrast media, such as barium or iodine compounds, can be introduced to highlight soft‑tissue organs like the intestines or blood vessels.
在诊断成像中，X 射线相对容易穿过软组织，但会被骨骼等致密材料更多地吸收。这种差异吸收在探测器或照相胶片上形成阴影图像。骨骼呈白色，软组织呈灰色阴影，空气呈黑色。可以引入造影剂（如钡或碘化合物）来突出肠道或血管等软组织器官。
Because X‑rays are ionising, the dose must be kept as low as reasonably achievable (ALARA principle). Lead shielding is used to protect parts of the body not being imaged, and radiographers stand behind protective screens or wear lead aprons. Pregnant women should avoid X‑rays unless absolutely necessary.
由于 X 射线是电离的，剂量必须保持在合理可行的最低水平（ALARA 原则）。铅屏蔽用于保护未被成像的身体部位，放射技师站在防护屏后面或穿戴铅围裙。孕妇除非绝对必要，否则应避免 X 射线。
5. Interaction of X‑rays with Matter | X 射线与物质的相互作用
X‑ he photoelectric effect and Compton scattering. In the photoelectric effect, an incoming X‑ray photon is completely absorbed by an inner‑shell electron, which is ejected from the atom. This process predominates at lower X‑ray energies (below about 100 keV) and in materials with high atomic number Z, such as bone (calcium, Z = 20).
X 射线主要通过光电效应和康普顿散射与物质相互作用。在光电效应中，入射的 X 射线光子被内壳层电子完全吸收，电子从原子中被击出。这一过程在较低 X 射线能量（约 100 keV 以下）和原子序数 Z 高的材料（如骨骼，钙 Z = 20）中占主导地位。
Compton scattering becomes more significant at higher energies. The X‑ray photon collides with a loosely bound outer electron, transferring part of its energy and changing direction. The scattered photon has reduced energy and can contribute to image noise. Understanding these interactions helps explain why bone absorbs more X‑rays than soft tissue and why contrast is achieved in radiographs.
康普顿散射在较高能量下变得更加显著。X 射线光子与束缚较松的外层电子碰撞，转移部分能量并改变方向。散射光子能量降低，可能导致图像噪声。理解这些相互作用有助于解释为什么骨骼比软组织吸收更多 X 射线，以及如何实现 X 光片中的对比度。
6. CT Scanning | 计算机断层扫描（CT）
Computed Tomography (CT) uses X‑rays to produce detailed cross‑sectional images of the body. The patient lies on a table that moves through a rotating gantry containing an X‑ray tube and an array of detectors. As the gantry rotates, a narrow fan‑beam of X‑rays passes through the body from many different angles, and detectors measure the intensity of transmitted radiation.
计算机断层扫描（CT）使用 X 射线生成身体的详细横截面图像。患者躺在检查台上，检查台穿过一个旋转机架，机架内装有 X 射线管和检测器阵列。随着机架旋转，窄扇形 X 射线束从许多不同角度穿过身体，检测器测量透射辐射的强度。
A computer processes the data using mathematical algorithms (filtered back projection) to reconstruct a 2D slice image, where each pixel represents the X‑ray attenuation coefficient of a small volume element (voxel). The attenuation values are displayed as Hounsfield Units (HU), with water defined as 0 HU. Bone typically ranges from +400 HU to +1000 HU, while air is about −1000 HU.
计算机使用数学算法（滤波反投影）处理数据，重建二维切片图像，其中每个像素代表一个小体积元素（体素）的 X 射线衰减系数。衰减值以亨氏单位（HU）显示，水定义为 0 HU。骨骼通常在 +400 HU 到 +1000 HU 之间，而空气约为 −1000 HU。
Compared to conventional X‑ray radiography, CT provides much higher soft‑tissue contrast and eliminates overlapping structures. However, CT scans deliver a significantly higher radiation dose — typically equivalent to several hundred chest X‑rays — so the clinical benefit must outweigh the risk.
与传统的 X 射线摄影相比，CT 提供了更高的软组织对比度并消除了结构重叠。然而，CT 扫描的辐射剂量显著更高——通常相当于数百次胸部 X 光检查——因此临床益处必须大于风险。
Radioactive isotopes (radionuclides) are used as tracers to investigate organ function. A small amount of a radiopharmaceutical — a compound labelled with a gamma‑emitting isotope — is injected, swallowed, or inhaled. The tracer concentrates in the organ of interest, and its distribution is detected by a gamma camera.
放射性同位素（放射性核素）被用作示踪剂来研究器官功能。少量放射性药物——用发射伽马射线的同位素标记的化合物——被注射、吞咽或吸入。示踪剂在目标器官中聚集，其分布由伽马相机检测。
Technetium‑99m is the most commonly used tracer in nuclear medicine. It has a half‑life of 6 hours, which is long enough for the examination yet short enough to minimise patient radiation exposure. It emits gamma rays of energy 140 keV, which can easily escape the body and be detected. Tc‑99m is produced on‑site from a molybdenum‑99 generator.
锝‑99m 是核医学中最常用的示踪剂。它的半衰期为 6 小时，足够长以完成检查，又足够短以尽量减少患者的辐射暴露。它发射能量为 140 keV 的伽马射线，容易逸出体外并被检测到。Tc‑99m 通过钼‑99 发生器现场制备。
The gamma camera detects gamma rays emitted from the patient and constructs an image showing the concentration of the tracer. Unlike X‑ray imaging, which mainly shows anatomy, nuclear medicine provides functional information — for example, showing how well the kidneys filter blood or how much blood flows through the heart muscle.
伽马相机检测患者发射的伽马射线，并构建显示示踪剂浓度的图像。与主要显示解剖结构的 X 射线成像不同，核医学提供功能信息——例如，显示肾脏过滤血液的效率或流经心肌的血流量。
8. Positron Emission Tomography (PET) | 正电子发射断层扫描（PET）
PET is a more advanced nuclear imaging technique that uses positron‑emitting radionuclides such as fluorine‑18 (¹⁸F). When a positron encounters an electron in the body, they annihilate, producing two gamma photons of 511 keV travelling in almost exactly opposite directions.
PET 是一种更先进的核成像技术，使用发射正电子的放射性核素，例如氟‑18（¹⁸F）。当正电子与体内的电子相遇时，它们湮灭，产生两个 511 keV 的伽马光子，几乎沿完全相反的方向运动。
Detectors arranged in a ring around the patient register these back‑to‑back gamma rays in coincidence. A line of response is drawn between the two detectors that recorded simultaneous events, and a computer reconstructs a 3D map of the tracer distribution. PET is often combined with CT (PET‑CT) to provide both functional and anatomical information.
围绕患者排列成环状的探测器以符合方式记录这些背对背的伽马射线。在两个同时记录到事件的探测器之间绘制响应线，计算机重建示踪剂分布的三维图谱。PET 通常与 CT 结合（PET‑CT），同时提供功能和解剖信息。
¹⁸F‑FDG is a glucose analogue that accumulates in cells with high metabolic rates, such as cancer cells. Therefore, PET is widely used in oncology for tumour detection, staging, and monitoring treatment response. The short half‑life of ¹⁸F (110 minutes) requires an on‑site cyclotron for production.
¹⁸F‑FDG 是一种葡萄糖类似物，在代谢率高的细胞（如癌细胞）中积聚。因此，PET 广泛用于肿瘤检测、分期和监测治疗反应。¹⁸F 较短的半衰期（110 分钟）需要现场回旋加速器进行生产。
9. Radiotherapy | 放射治疗
Radiotherapy uses high‑energy ionising radiation to destroy cancer cells. The radiation damages the DNA of rapidly dividing cells, causing them to die or stop proliferating. Both normal and cancerous cells are affected, but normal cells are usually better at repairing themselves, so fractionated doses are given over several weeks to maximise tumour kill while allowing healthy tissue to recover.
放射治疗使用高能电离辐射摧毁癌细胞。辐射损伤快速分裂细胞的 DNA，导致它们死亡或停止增殖。正常细胞和癌细胞都会受到影响，但正常细胞通常更善于自我修复，因此在几周内进行分次照射，以最大限度地杀伤肿瘤，同时让健康组织得以恢复。
External beam radiotherapy uses a linear accelerator (linac) to direct a beam of X‑rays or electrons at the tumour from outside the body. The beam is shaped by a multi‑leaf collimator to conform to the tumour’s shape, sparing surrounding healthy tissue. Treatment angles are carefully planned using CT or MRI images.
体外射束放射治疗使用直线加速器从体外将 X 射线或电子束导向肿瘤。射束通过多叶光栅塑形，以符合肿瘤的形状，保护周围的健康组织。治疗角度通过 CT 或 MRI 图像仔细规划。
Brachytherapy involves placing sealed radioactive sources (e.g., iridium‑192 or iodine‑125) directly inside or next to the tumour. This delivers a very high radiation dose to the cancer while greatly reducing the dose to surrounding organs. It is commonly used for prostate, cervical, and breast cancers.
近距离放射治疗涉及将密封放射源（例如铱‑192 或碘‑125）直接放置在肿瘤内部或旁边。这向癌症提供非常高的辐射剂量，同时大大减少对周围器官的剂量。常用于前列腺癌、宫颈癌和乳腺癌。
10. Choosing a Radioisotope for Diagnosis and Therapy | 诊断和治疗放射性同位素的选择
The choice of radioisotope depends on the purpose. For diagnostic tracers, a short half‑life (hours), pure gamma emission, and suitable gamma energy (detectable but not too highly absorbed by body tissue) are key. Technetium‑99m fulfils all these criteria.
放射性同位素的选择取决于目的。对于诊断示踪剂，关键因素是短半衰期（几小时）、纯伽马发射以及合适的伽马能量（可检测但不会被身体组织过度吸收）。锝‑99m 满足所有这些标准。
For therapeutic use, isotopes that emit alpha or beta particles are preferred because these particles deposit their energy over a very short range within the tumour, minimising damage to distant healthy tissue. For example, iodine‑131 emits beta particles and is used to treat thyroid cancer; yttrium‑90 is a pure beta emitter used in radioembolisation of liver tumours.
对于治疗用途，发射 α 或 β 粒子的同位素更受青睐，因为这些粒子在肿瘤内的极短范围内沉积能量，最大限度地减少对远处健康组织的损伤。例如，碘‑131 发射 β 粒子，用于治疗甲状腺癌；钇‑90 是纯 β 发射体，用于肝肿瘤的放射栓塞。
The half‑life should match the treatment duration: too short and the tumour may not receive sufficient dose; too long and the patient remains radioactive after treatment, posing a hazard to others. Iodine‑131 has a half‑life of 8 days, which works well for thyroid therapy. After such treatments, patients may need to stay in hospital until their radiation level drops to a safe threshold.
半衰期应与治疗持续时间匹配：太短则肿瘤可能得不到足够的剂量；太长则治疗后患者仍具有放射性，对他人构成危险。碘‑131 的半衰期为 8 天，非常适合甲状腺治疗。在此类治疗后，患者可能需要住院直到辐射水平降至安全阈值。
11. Safety Precautions and Risk Comparison | 安全预防措施与风险比较
All medical radiation applications follow the ALARA principle (as low as reasonably achievable). For X‑ray and CT, this means using the minimum exposure time, maximum distance from the source, and shielding (lead aprons, lead glass screens). Staff wear dosimeter badges to monitor cumulative dose. Pregnant patients and children are given extra protection.
所有医疗辐射应用都遵循 ALARA 原则（合理可行的最低水平）。对于 X 射线和 CT，这意味着使用最短曝光时间、尽可能远离辐射源以及屏蔽（铅围裙、铅玻璃屏）。工作人员佩戴剂量计徽章以监测累积剂量。孕妇和儿童得到额外保护。
In nuclear medicine, radioactive materials are handled with gloves, tongs, and syringe shields. Patients are advised to drink plenty of fluids to excrete the tracer quickly and to avoid close contact with pregnant women and infants for a certain period. Radioactive waste is stored in shielded containers until it decays to a safe level.
在核医学中，放射性物质通过手套、镊子和注射器防护罩进行处理。建议患者大量饮水以快速排出示踪剂，并在一定时间内避免与孕妇和婴儿密切接触。放射性废物储存在屏蔽容器中，直到衰变至安全水平。
When comparing risks from different imaging modalities, it is important to consider both the radiation dose and the diagnostic benefit. A typical chest X‑ray delivers about 0.02 mSv, equivalent to a few days of natural background radiation. A CT abdomen‑pelvis scan can deliver about 10 mSv, roughly 3 years of background radiation. The increased lifetime cancer risk from a single CT is small but not negligible, especially in children. Ultrasound and MRI do not use ionising radiation and thus carry no such stochastic risk.
在比较不同成像方式的风险时，必须同时考虑辐射剂量和诊断益处。一次典型的胸部 X 光检查剂量约为 0.02 mSv，相当于几天的天然本底辐射。一次腹部–盆腔 CT 扫描剂量可达约 10 mSv，约相当于 3 年的本底辐射。单次 CT 增加的终生癌症风险虽小但不可忽略，尤其是对儿童。超声波和 MRI 不使用电离辐射，因此不存在此类随机性风险。
12. Ultrasonic vs X‑ray Imaging: A Comparison | 超声成像与 X 射线成像的比较
A common exam question asks you to compare ultrasound and X‑ray imaging. The table below summarises the key differences.
考试中常见的一道题要求比较超声成像和 X 射线成像。下表总结了关键区别。
| Aspect | Ultrasound | X‑ray |
|---|---|---|
| Wave type | Longitudinal sound wave | Transverse EM wave |
| Ionising? | No | Yes |
| Typical frequency / energy | 2–18 MHz | ~10¹⁷–10¹⁹ Hz; energies 10–150 keV |
| Penetration | Poor through bone and air | Excellent through soft tissue; absorbed by bone |
| Image type | Real‑time cross‑sectional | Shadowgram (2D projection); CT provides 3D slices |
| Main use | Foetal imaging, abdomen, heart | Bone fractures, chest, mammography |
| Safety | Very safe; no cumulative dose | Radiation dose must be justified and minimised |
Remember that the choice between ultrasound and X‑ray depends on the clinical question and the body part being examined. For a suspected bone fracture, X‑ray is the first choice because it shows bone clearly. For checking the health of an unborn baby, ultrasound is preferred because it avoids ionising radiation.
请记住，选择超声波还是 X 射线取决于临床问题和检查的身体部位。对于疑似骨折，X 射线是首选，因为它能清晰显示骨骼。对于检查胎儿健康状况，超声波是首选，因为它避免电离辐射。
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