📚 Edexcel Physics: Medical Physics Exam Essentials | Edexcel 物理：医疗物理 考点精讲
Medical physics is a fascinating application of fundamental physical principles to diagnose and treat disease. In the Edexcel A Level Physics specification, you are expected to understand how X‑rays, ultrasound, nuclear medicine, and MRI work, as well as the safety considerations involved. This guide brings together all the essential points you need for the exam, presented in a clear bilingual format.
医疗物理是将基础物理原理应用于疾病诊断和治疗的一门迷人学科。在 Edexcel A Level 物理大纲中，你需要理解 X 射线、超声波、核医学和 MRI 的工作原理以及相关的安全因素。本文以清晰的双语形式汇总了你备考所需的所有重点。
1. Production of X‑rays | X 射线的产生
X‑rays are produced in a vacuum tube when high‑speed electrons are suddenly decelerated by a metal target (usually tungsten). The kinetic energy of the electrons is converted into electromagnetic radiation through two main processes: bremsstrahlung (braking radiation) and characteristic radiation.
X 射线在真空管中产生，高速电子被金属靶（通常是钨）突然减速。电子的动能通过两个主要过程转化为电磁辐射：轫致辐射和特征辐射。
Thermionic emission from a heated filament provides the electrons. A large potential difference (typically 50–150 kV) accelerates them towards the anode. Less than 1 % of the electron kinetic energy is converted into X‑rays; the rest heats the target, so the anode is often rotated and cooled.
加热灯丝的热电子发射提供电子。一个高电压（通常 50–150 kV）使它们朝阳极加速。电子动能中不到 1% 转化成 X 射线；其余的能量加热靶，因此阳极通常旋转并加以冷却。
The X‑ray spectrum consists of a continuous background (bremsstrahlung) whose minimum wavelength λmin is given by eV = hc/λmin, and sharp peaks (characteristic lines) that correspond to electron transitions between inner atomic shells of the target material.
X 射线谱由一个连续本底（轫致辐射）和锐利尖峰（特征谱线）组成。连续谱的最短波长 λmin 由 eV = hc/λmin 给出，特征线则对应靶材料内部原子壳层间的电子跃迁。
Key exam points: beam intensity ∝ tube current (filament current); penetrating power ∝ tube voltage. Hard X‑rays (high energy) are better for imaging because they suffer less absorption in soft tissue.
关键考点：射线强度 ∝ 管电流（灯丝电流）；穿透能力 ∝ 管电压。硬 X 射线（高能量）更适合成像，因为它们在软组织中吸收较少。
2. X‑ray Attenuation and Half‑Value Thickness | X 射线衰减与半值厚度
When a monochromatic X‑ray beam passes through a material, its intensity I decreases exponentially with thickness x: I = I₀e⁻ᵠμx, where μ is the linear attenuation coefficient of the material. This coefficient depends on the photon energy and the atomic number Z of the absorber.
当一束单色 X 射线穿过物质时，其强度 I 随厚度 x 呈指数衰减：I = I₀e⁻μx，其中 μ 是材料的线性衰减系数。该系数取决于光子能量和吸收体的原子序数 Z。
The half‑value thickness (HVT or x½) is the thickness needed to reduce the intensity to half its original value. From the exponential law, x½ = ln 2 / μ. A material with a larger μ has a smaller half‑value thickness, meaning it is a better absorber.
半值厚度（HVT 或 x½）是将射线强度降至初始值一半所需的厚度。由指数规律可得 x½ = ln 2 / μ。μ 越大的材料其半值厚度越小，意味着吸收能力越强。
For diagnostic purposes, bone (containing calcium, higher Z) attenuates X‑rays much more than soft tissue, producing contrast on the image. Barium and iodine compounds are used as contrast media because of their high atomic numbers, enhancing the visibility of the digestive tract or blood vessels.
诊断中，骨骼（含钙，Z 值较高）对 X 射线的衰减远大于软组织，在图像上形成对比。钡和碘化合物因原子序数高而被用作造影剂，能增强消化道或血管的可见度。
In the exam you may be asked to calculate μ or half‑value thickness from intensity data, or to explain why a rotating anode helps to dissipate heat.
考试中可能要求你根据强度数据计算 μ 或半值厚度，或者解释旋转阳极为何有助于散热。
3. CT Scanning (Computed Tomography) | CT 扫描（计算机断层成像）
A CT scanner uses a thin, fan‑shaped X‑ray beam that rotates around the patient, with a bank of detectors on the opposite side. The X‑ray tube and detectors are mounted on a rotating gantry. As the patient moves slowly through the scanner, many ‘slices’ are recorded.
CT 扫描仪使用一束薄的扇形 X 射线绕患者旋转，对侧装有一排探测器。X 射线管和探测器安装在一个旋转机架上。当患者缓慢通过扫描仪时，记录下许多‘切片’。
Each detector measures the attenuation along a specific path. A computer reconstructs a 3D image from the attenuation data using a mathematical technique called back‑projection. The resulting image can discriminate between tissues with very similar attenuation properties, an advantage over conventional radiography.
每个探测器测量沿特定路径的衰减。计算机利用一种称作背投影的数学技巧，从衰减数据中重建出三维图像。所得图像能分辨衰减特性非常接近的组织，这是优于常规 X 光摄影的地方。
CT scans deliver a higher radiation dose than a single X‑ray, so the medical benefit must always be weighed against the risk. Modern scanners use iterative reconstruction algorithms to reduce dose while preserving image quality.
CT 扫描比单次 X 射线的辐射剂量更高，因此必须始终权衡医疗收益与风险。现代扫描仪使用迭代重建算法在保持图像质量的同时降低剂量。
4. Ultrasound Imaging: Principle and Pulse‑Echo | 超声波成像：原理与脉冲回波
Ultrasound uses high‑frequency sound waves (typically 1–15 MHz) generated by a piezoelectric transducer. A short pulse of ultrasound is sent into the body, and the transducer then acts as a receiver to detect echoes reflected from boundaries between tissues of different acoustic impedance.
超声波使用由压电换能器产生的高频声波（通常 1–15 MHz）。一短脉冲超声射入体内，然后换能器充当接收器，探测从不同声阻抗组织的边界反射回来的回声。
The time delay Δt between transmitting the pulse and receiving an echo is used to calculate the depth d of the reflecting surface: d = (c × Δt)/2, where c is the speed of sound in the medium (≈1540 m s⁻¹ in soft tissue). The factor of ½ arises because the sound travels to the boundary and back.
发射脉冲到接收回声的时间延迟 Δt 用于计算反射界面的深度 d：d = (c × Δt)/2，其中 c 是介质中的声速（软组织中约 1540 m s⁻¹）。系数 ½ 源于声波往返于边界。
There are two common display modes. A‑scan (amplitude scan) shows the echo amplitude on a timebase, giving one‑dimensional depth information. B‑scan (brightness scan) converts echo amplitudes into grey‑scale dots, building up a 2D image by moving the transducer or using an array of elements.
有两种常见显示模式。A 型扫描（振幅扫描）在时基上显示回波振幅，提供一维深度信息。B 型扫描（亮度扫描）将回波振幅转换成灰度点，通过移动换能器或使用阵列探头建立二维图像。
Ultrasound is particularly useful for real‑time imaging of soft tissues, prenatal scanning, and blood flow measurement (Doppler effect). It does not use ionising radiation, so it is safe for repeated use.
超声波特别适用于软组织实时成像、产前扫描和血流测量（多普勒效应）。它不使用电离辐射，因此可安全地重复使用。
5. Acoustic Impedance and Reflection | 声阻抗与反射
The acoustic impedance Z of a material is defined as Z = ρc, where ρ is its density and c is the speed of sound in it. The unit is kg m⁻² s⁻¹ or rayl. Tissues with very different Z produce strong echoes, which is the basis of image contrast.
材料的声阻抗 Z 定义为 Z = ρc，其中 ρ 是其密度，c 是其中的声速。单位是 kg m⁻² s⁻¹ 或 rayl。声阻抗差异很大的组织会产生强烈回声，这是图像对比的基础。
When ultrasound strikes a boundary at normal incidence, the fraction of intensity reflected is given by (Z₂ − Z₁)²/(Z₂ + Z₁)². For example, at a soft‑tissue–bone boundary almost all the sound is reflected, making deeper structures invisible. A coupling gel is used between the transducer and the skin to eliminate air pockets; air has a very low impedance, causing almost 100% reflection.
当超声波垂直入射到边界时，被反射的强度比例由 (Z₂ − Z₁)²/(Z₂ + Z₁)² 给出。例如，在软组织‑骨骼边界，几乎所有声音都被反射，使得更深的结构无法显示。在换能器和皮肤之间使用耦合凝胶是为了消除空气间隙；空气的阻抗非常低，会引起几乎 100% 的反射。
In the exam you need to be comfortable using this reflection coefficient to explain image features or to calculate the required thickness of a matching layer to minimise reflection.
考试中你要能熟练运用该反射系数解释图像特征，或计算使反射最小的匹配层厚度。
6. Positron Emission Tomography (PET) | 正电子发射断层扫描（PET）
PET is a nuclear imaging technique that detects gamma rays emitted indirectly by a positron‑emitting radionuclide such as fluorine‑18 (¹⁸F). The radiotracer is attached to a biologically active molecule (e.g. glucose) and injected into the patient. It accumulates in tissues with high metabolic activity, such as tumours.
PET 是一种核成像技术，它探测由发射正电子的放射性核素（如氟‑18）间接放出的伽马射线。放射性示踪剂附着在生物活性分子（如葡萄糖）上，注射到患者体内。它聚集在代谢活跃的组织（如肿瘤）中。
A positron emitted by the tracer travels a short distance (≈1 mm) before annihilating with an electron, producing two identical 0.511 MeV gamma photons that travel in opposite directions. This back‑to‑back emission is detected by a ring of scintillation detectors. Only coincident events (photons arriving within a few nanoseconds) are recorded, which accurately locates the annihilation site along the line of response.
示踪剂发出的正电子行进很短距离（约 1 mm）后与电子湮灭，产生两个相同的 0.511 MeV 伽马光子，它们沿相反方向飞行。这种背对背发射由一圈闪烁探测器探测。只有符合事件（光子在大约几纳秒内到达）被记录，这就能沿响应线精确定位湮灭位置。
PET provides functional information about metabolism, often combined with CT (PET‑CT) to provide both anatomical and metabolic detail. The main exam points include the conservation of momentum and energy in pair annihilation, and the advantage of coincidence detection.
PET 提供有关代谢的功能信息，常与 CT 结合（PET‑CT）以提供解剖和代谢两方面的细节。主要考点包括正电子湮灭中动量和能量守恒，以及符合探测的优势。
7. Magnetic Resonance Imaging (MRI) | 核磁共振成像（MRI）
MRI exploits the magnetic properties of hydrogen nuclei (protons) in the body. When placed in a strong static magnetic field B₀, a small fraction of the protons align with the field and precess at a precise frequency, called the Larmor frequency: f = γB₀/2π, where γ is the gyromagnetic ratio for protons.
MRI 利用体内氢原子核（质子）的磁性。当置于强静磁场 B₀ 中时，一小部分质子与磁场对齐，并以精确的频率进动，此频率称为拉莫尔频率：f = γB₀/2π，其中 γ 是质子的旋磁比。
A radio‑frequency (RF) pulse at the Larmor frequency is applied, tilting the net magnetisation away from B₀. When the pulse ceases, the protons relax back to the equilibrium state, emitting RF signals that are picked up by receiver coils. Two relaxation times are crucial: T₁ (spin‑lattice relaxation) and T₂ (spin‑spin relaxation). Tissues differ in their T₁ and T₂ values, creating image contrast.
施加一个与拉莫尔频率相同的射频脉冲，使净磁化矢量偏离 B₀。脉冲停止后，质子弛豫回平衡态，发射出被接收线圈捕获的射频信号。两个关键的弛豫时间是 T₁（自旋‑晶格弛豫）和 T₂（自旋‑自旋弛豫）。不同组织的 T₁ 和 T₂ 值各异，从而形成图像对比。
Spatial localisation is achieved by superimposing gradient magnetic fields, which make the Larmor frequency depend on position. MRI gives excellent soft‑tissue contrast without ionising radiation, but it is expensive and cannot be used for patients with certain metallic implants.
空间定位是通过叠加梯度磁场实现的，这使得拉莫尔频率依赖于位置。MRI 能提供极佳的软组织对比且无电离辐射，但设备昂贵，且不适用于某些带金属植入物的患者。
8. Radiotherapy and Radiation Protection | 放射治疗与辐射防护
Radiotherapy uses ionising radiation to destroy malignant cells. High‑energy X‑rays (from a linear accelerator) or gamma rays (from cobalt‑60) are directed at the tumour from multiple angles to maximise dose to the tumour while sparing surrounding healthy tissue. This is known as conformal radiotherapy or intensity‑modulated radiotherapy (IMRT).
放射治疗使用电离辐射摧毁恶性细胞。高能 X 射线（来自直线加速器）或伽马射线（来自钴‑60）从多个角度照射肿瘤，使肿瘤剂量最大化而保护周围健康组织。这称为适形放疗或调强放疗（IMRT）。
Proton therapy is a growing field: protons have a Bragg peak – a sharp maximum in energy deposition just before they stop – allowing even better sparing of tissues beyond the tumour. Brachytherapy involves placing small radioactive sources directly inside or next to the tumour.
质子治疗是一个发展的领域：质子具有布拉格峰——在它们即将停止时能量沉积存在一个尖锐的极大值——从而能更好地保护肿瘤之后的组织。近距离放疗则将小放射源直接放入肿瘤内部或旁边。
Radiation protection follows three cardinal principles: justification (the benefit must outweigh the risk), optimisation (doses must be As Low As Reasonably Achievable – ALARA), and limitation (dose limits must not be exceeded). Equivalent dose in sieverts (Sv) considers the type of radiation and its biological effect: H = Q × D, where D is the absorbed dose in grays and Q is the quality factor.
辐射防护遵循三条基本原则：正当性（收益必须大于风险）、最优化（剂量应达到可合理达到的尽可能低——ALARA）、以及限值（不得超过剂量限值）。用希沃特（Sv）表示的当量剂量考虑了辐射类型及其生物效应：H = Q × D，其中 D 是以戈瑞为单位的吸收剂量，Q 是品质因数。
For patients and staff, shielding (lead aprons, lead‑glass screens), increasing distance, and minimising exposure time are essential practical measures. In exams you may be asked to calculate absorbed dose from activity and exposure time, or to evaluate risks of different imaging modalities.
对于患者和工作人员，屏蔽（铅围裙、铅玻璃屏）、增大距离和缩短暴露时间是基本的实用措施。考试中可能让你根据活度和暴露时间计算吸收剂量，或评估不同成像方式的辐射风险。
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