📚 AS Physics: Medical Physics Essentials | AS 物理：医疗物理 考点精讲
Medical physics applies the principles of physics to the diagnosis and treatment of diseases. In AS Physics, you will explore how X-rays, ultrasound, and radioactive isotopes are used in medical imaging and therapy. Understanding the underlying physics helps in interpreting images and minimising risks.
医疗物理学将物理学原理应用于疾病的诊断和治疗。在AS物理中，你将探索X射线、超声波和放射性同位素如何用于医学成像和治疗。理解其背后的物理原理有助于解读图像并将风险降至最低。
1. Medical Imaging Overview | 医学成像概述
Medical imaging techniques rely on different physical phenomena, including ionising radiation (X-rays, gamma rays), sound waves (ultrasound), and nuclear processes (PET). Each modality has unique advantages, limitations, and safety considerations.
医学成像技术依赖于不同的物理现象，包括电离辐射（X射线、伽马射线）、声波（超声）以及核过程（PET）。每种模式都有独特的优势、局限性和安全考量。
Non-ionising techniques like ultrasound are safer for soft tissue examination, whereas ionising radiation such as X-rays can penetrate deeper but require careful dose management.
超声等非电离技术对于软组织检查更安全，而X射线等电离辐射能够穿透更深，但需要仔细的剂量管理。
2. Production of X-rays | X射线的产生
X-rays are produced in an X-ray tube. Electrons are emitted from a heated cathode filament via thermionic emission and are accelerated towards a rotating anode (usually tungsten) by a high potential difference, typically 50–150 kV.
X射线在X射线管中产生。电子由加热的阴极灯丝通过热电子发射产生，并在高电位差（通常为50–150 kV）加速下飞向旋转阳极（通常为钨靶）。
When the fast electrons strike the anode, their kinetic energy is converted into heat (about 99%) and X-ray photons. Two types of X-rays are produced: bremsstrahlung (braking radiation) from electron deceleration and characteristic X-rays from electron transitions in the target atoms.
当高速电子撞击阳极时，其动能转化为热量（约99%）和X射线光子。产生两种类型的X射线：电子减速产生的轫致辐射和靶原子中电子跃迁产生的特征X射线。
The X-ray spectrum shows a continuous bremsstrahlung background with sharp peaks at energies corresponding to electron shell differences. The maximum photon energy is determined by the tube voltage (Emax = e × V).
X射线光谱显示连续的轫致辐射背景，并在对应电子壳层能量差处出现尖锐的峰值。最大光子能量由管电压决定（Emax = e × V）。
To control beam quality and patient dose, filters (often aluminium) are used to absorb low-energy photons that would not contribute to imaging but would increase patient exposure.
为了控制射束质量和患者剂量，使用滤线器（通常为铝）吸收低能光子，这些光子对成像无益反而会增加患者的照射。
3. X-ray Attenuation and Imaging | X射线衰减与成像
As an X-ray beam passes through matter, its intensity decreases exponentially according to the attenuation law:
当X射线束穿过物质时，其强度根据衰减定律呈指数下降：
I = I0 e-μx
where I0 is the initial intensity, μ is the linear attenuation coefficient (depends on material and photon energy), and x is the thickness of the material.
其中 I0 为初始强度，μ 为线性衰减系数（取决于材料与光子能量），x 为材料厚度。
Different tissues (bone, muscle, fat) have different attenuation coefficients. This contrast forms the basis of radiographic imaging: bone appears white because it absorbs more X-rays, whereas air-filled lungs appear darker.
不同组织（骨骼、肌肉、脂肪）具有不同的衰减系数。这种对比构成了射线成像的基础：骨骼因吸收较多X射线而呈现白色，而充满空气的肺则较暗。
To reduce scattered radiation and improve image quality, grids are positioned between the patient and the detector. Contrast media such as barium or iodine can be used to enhance contrast of soft tissues like the gastrointestinal tract.
为减少散射线并提高图像质量，在患者与探测器之间放置滤线栅。钡或碘等造影剂可用于增强胃肠道等软组织的对比度。
4. Computed Tomography (CT) Scanning | 计算机断层扫描（CT）
CT scanning produces cross-sectional images (slices) of the body by rotating an X-ray source and detectors around the patient. Multiple projections are taken from different angles and reconstructed by a computer into a 3D image, providing detailed structural information.
CT扫描通过围绕患者旋转X射线源和探测器，产生身体的横截面图像（切片）。从不同角度获取多个投影，并由计算机重建成三维图像，提供详细的结构信息。
The key advantage over conventional X-ray is the ability to distinguish between overlapping structures and measure attenuation more precisely, expressed in Hounsfield Units (HU). Air is defined as -1000 HU, water as 0 HU, and dense bone up to +1000 HU.
与传统X射线相比，主要优势在于能够区分重叠结构并更精确地测量衰减，以亨氏单位（HU）表示。空气定义为-1000 HU，水为0 HU，致密骨骼可达+1000 HU。
However, CT scans involve higher radiation doses than a single X-ray image, so the benefits must be weighed against the risks, particularly for children and repeated scans.
然而，CT扫描比单次X射线成像涉及更高的辐射剂量，因此必须权衡利弊，特别是对于儿童和重复扫描。
5. Ultrasound Principles | 超声原理
Ultrasound uses high-frequency sound waves (typically 1–15 MHz) above the human hearing range. A transducer (piezoelectric crystal) emits pulses of ultrasound and detects echoes reflected from boundaries between tissues of different acoustic impedances.
超声使用高于人类听觉范围的高频声波（通常为1–15 MHz）。换能器（压电晶体）发射超声脉冲，并检测从不同声阻抗组织界面反射的回声。
The acoustic impedance Z of a material is given by Z = ρ c, where ρ is density and c is the speed of sound in the medium. The fraction of reflected intensity at a boundary depends on the impedance mismatch: R = ((Z2 – Z1)/(Z2 + Z1))².
材料的声阻抗 Z 由 Z = ρc 给出，其中 ρ 为密度，c 为介质中的声速。界面上反射强度的比例取决于阻抗差异：R =
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