📚 Applications of Ultrasound in Medicine | 超声波在医学中的应用
Ultrasound has revolutionised modern medical diagnosis and therapy, offering a safe, non-invasive and real-time imaging modality. Unlike X-rays and CT scans, which utilise ionising radiation, medical ultrasound relies on high-frequency mechanical sound waves to probe the human body. This article explores the fundamental physics underpinning ultrasound—from the piezoelectric effect and acoustic impedance to the Doppler effect—and examines how these principles translate into clinical practice, including A-scan, B-scan, M-scan, Doppler imaging, and therapeutic applications.
超声波已彻底改变了现代医学诊断与治疗，提供了一种安全、无创且实时的成像方式。与利用电离辐射的X射线和CT扫描不同，医学超声依赖高频机械声波来探测人体。本文将探讨超声背后的基本物理原理——从压电效应和声阻抗到多普勒效应——并分析这些原理如何转化为临床实践，包括A型扫描、B型扫描、M型扫描、多普勒成像以及治疗应用。
1. What is Ultrasound | 什么是超声波
Ultrasound is a longitudinal mechanical wave with a frequency exceeding 20 kHz, which is above the upper limit of human hearing. In medical diagnostics, frequencies between 1 MHz and 15 MHz are typically used. The relationship between the speed of sound c, frequency f and wavelength λ is given by:
c = fλ
In soft tissue, the speed of sound is approximately 1540 m s⁻¹. At a frequency of 5 MHz, the corresponding wavelength is approximately 0.31 mm. A shorter wavelength produces finer spatial resolution, but higher frequencies are attenuated more rapidly as they travel through tissue. This creates a fundamental trade-off between resolution and penetration depth that clinicians must consider when selecting the appropriate transducer frequency.
超声波是一种频率超过20 kHz的纵机械波，高于人类听觉上限。医学诊断通常使用1 MHz至15 MHz的频率范围。声速c、频率f和波长λ之间的关系为：
c = fλ
在软组织中，声速约为1540 m s⁻¹。在5 MHz频率下，对应波长约为0.31 mm。波长越短，空间分辨率越高，但高频波在组织中传播时衰减更快。这造成了分辨率与穿透深度之间的根本权衡，临床医生在选择换能器频率时必须加以考虑。
2. Generation and Detection: The Piezoelectric Effect | 超声波的产生与检测：压电效应
Ultrasound waves are generated and detected using piezoelectric transducers, which are the heart of every ultrasound machine. Piezoelectric materials—such as quartz, barium titanate, and lead zirconate titanate (PZT)—have an asymmetric crystal lattice structure. When an alternating potential difference is applied across the crystal, the lattice deforms, causing the crystal to expand and contract alternately. This mechanical vibration generates ultrasound waves at the frequency of the applied voltage.
超声波通过压电换能器产生和检测，压电换能器是每台超声设备的核心。压电材料——如石英、钛酸钡和锆钛酸铅（PZT）——具有不对称的晶格结构。当在晶体两端施加交变电势差时，晶格发生形变，导致晶体交替膨胀和收缩。这种机械振动以所施加电压的频率产生超声波。
Conversely, the same crystal can act as a detector. When returning echoes strike the crystal, they cause mechanical deformation, which separates internal charges and generates a measurable voltage across the crystal faces. The transducer therefore alternates rapidly between transmitting short pulses and listening for echoes. The resonant frequency of the transducer is determined by its thickness: a thinner crystal vibrates at a higher natural frequency, enabling higher-frequency imaging.
相反，同一晶体可以作为检测器使用。当返回的回波撞击晶体时，会引起机械形变，使内部电荷分离，从而在晶体两端产生可测量的电压。因此，换能器在发射短脉冲和接收回波之间快速交替。换能器的谐振频率由其厚度决定：更薄的晶体以更高的固有频率振动，从而实现更高频率的成像。
3. Acoustic Impedance and Reflection | 声阻抗与反射
When an ultrasound wave travels through tissue and encounters a boundary between two media with different acoustic impedances, a portion of the wave is reflected while the remainder is transmitted. The acoustic impedance Z of a medium is defined as the product of its density ρ and the speed of sound c in that medium:
Z = ρc
The unit of acoustic impedance is kg m⁻² s⁻¹, also known as the rayl. The intensity reflection coefficient R at an interface between two media with impedances Z₁ and Z₂ is given by:
R = ((Z₂ − Z₁)/(Z₂ + Z₁))²
The greater the difference in acoustic impedance between two media, the stronger the reflection. This is why ultrasound imaging works so well at boundaries between soft tissue and fluid-filled structures, yet fails to visualise regions behind bone or gas-filled organs, where almost all of the incident energy is reflected.
当超声波在组织中传播并遇到两种不同声阻抗介质的界面时，一部分波被反射，其余部分被透射。介质的声阻抗Z定义为该介质的密度ρ与其中声速c的乘积：
Z = ρc
声阻抗的单位是kg m⁻² s⁻¹，也称为瑞利。在阻抗分别为Z₁和Z₂的两种介质界面处的强度反射系数R为：
R = ((Z₂ − Z₁)/(Z₂ + Z₁))²
两种介质之间的声阻抗差异越大，反射越强。这就是为什么超声成像在软组织和含液结构之间的边界处效果良好，却无法清晰显示骨骼或含气器官后方的区域——因为几乎所有入射能量都在这些界面被反射了。
| Medium (介质) | Density ρ / kg m⁻³ (密度) | Speed c / m s⁻¹ (声速) | Impedance Z / kg m⁻² s⁻¹ (声阻抗) |
| Air (空气) | 1.29 | 330 | 4.3 × 10² |
| Water (水) | 1000 | 1480 | 1.48 × 10⁶ |
| Soft tissue (软组织) | 1060 | 1540 | 1.63 × 10⁶ |
| Bone (骨骼) | 1900 | 4080 | 7.8 × 10⁶ |
A coupling gel is applied between the transducer and the patient’s skin to eliminate the thin layer of air trapped between them. Without the gel, the enormous impedance mismatch between air (4.3 × 10² kg m⁻² s⁻¹) and skin (≈1.6 × 10⁶ kg m⁻² s⁻¹) would reflect over 99% of the ultrasound energy, making imaging impossible.
在换能器与患者皮肤之间需要涂敷耦合凝胶，以消除两者之间夹带的薄层空气。如果没有凝胶，空气（4.3 × 10² kg m⁻² s⁻¹）与皮肤（约1.6 × 10⁶ kg m⁻² s⁻¹）之间的巨大阻抗失配将反射超过99%的超声能量，使成像无法进行。
4. A-Scan, B-Scan and M-Scan | A型、B型和M型扫描
Ultrasound systems display the reflected echoes in several distinct modes, each suited to a particular clinical application. The simplest is the A-scan (amplitude mode), in which a single transducer emits a pulse along one line of sight and the returning echoes are displayed as spikes on an oscilloscope screen. The horizontal axis represents time, which is directly proportional to depth, while the vertical axis represents echo amplitude. A-scans are used in ophthalmology to measure the axial length of the eyeball before cataract surgery.
超声系统以多种不同的模式显示回波，每种模式适用于特定的临床应用。最简单的是A型扫描（振幅模式），其中单个换能器沿一条视线发射脉冲，返回的回波在示波器屏幕上显示为尖峰。横轴代表时间，与深度成正比，纵轴代表回波振幅。A型扫描用于眼科测量白内障手术前的眼球轴长。
In a B-scan (brightness mode), the transducer is swept across the body, and each echo is represented as a point whose brightness is proportional to the echo amplitude. The resulting two-dimensional image maps the anatomical structure of the tissue. B-scans are the standard mode for abdominal imaging, obstetrics, and cardiac assessment, providing real-time cross-sectional views of organs.
在B型扫描（亮度模式）中，换能器在身体表面扫掠，每个回波被表示为一个点，其亮度与回波振幅成正比。由此产生的二维图像描绘了组织的解剖结构。B型扫描是腹部成像、产科和心脏评估的标准模式，能够提供器官的实时横截面视图。
In an M-scan (motion mode), a stationary transducer repeatedly emits pulses along a single line, and the reflected echoes are displayed against a time axis on the screen. This produces a strip-chart-like display that reveals the movement of structures over time. M-scans are particularly valuable in echocardiography for measuring valve motion and in foetal monitoring for observing the heart rate and rhythm.
在M型扫描（运动模式）中，固定换能器沿一条线重复发射脉冲，返回的回波在屏幕上沿时间轴显示。这产生了类似条带图的显示，揭示结构随时间的运动。M型扫描在超声心动图中特别有价值，用于测量瓣膜运动，也用于胎儿监护中观察心率与节律。
5. Doppler Ultrasound | 多普勒超声
The Doppler effect is exploited in ultrasound to measure the velocity of moving structures, most notably blood flow. When an ultrasound wave strikes moving red blood cells, the frequency of the reflected wave is shifted. The Doppler frequency shift Δf is given by:
Δf = (2fv cos θ)/c
where f is the transmitted frequency, v is the speed of the blood, θ is the angle between the ultrasound beam and the direction of blood flow, and c is the speed of sound in tissue. The factor of 2 arises because the wave undergoes a Doppler shift on the outward journey and an additional shift on the reflected return journey.
多普勒效应被应用于超声中，以测量运动结构的速度，最典型的是血流速度。当超声波撞击运动的红细胞时，反射波的频率发生偏移。多普勒频移Δf由以下公式给出：
Δf = (2fv cos θ)/c
其中f是发射频率，v是血流速度，θ是超声束与血流方向之间的夹角，c是组织中的声速。因子2的出现是因为波在向外传播过程中经历一次多普勒频移，在反射返回途中又经历一次频移。
In practice, the angle θ must be known to calculate the absolute velocity. When θ = 0° (beam parallel to flow), the Doppler shift is maximised; when θ = 90° (beam perpendicular to flow), the shift is zero. Clinicians therefore adjust the transducer orientation to keep θ small. Colour Doppler imaging assigns colours to the direction and speed of flow—typically red for flow towards the transducer and blue for flow away from it—enabling rapid visual assessment of vascular conditions such as stenosis, thrombosis, and valvular regurgitation.
在实际应用中，必须知道角度θ才能计算绝对速度。当θ = 0°（声束与血流平行）时，多普勒频移最大；当θ = 90°（声束与血流垂直）时，频移为零。因此临床医生会调整换能器的方向，使θ保持较小。彩色多普勒成像将颜色分配给血流的方向和速度——通常红色表示流向换能器，蓝色表示远离换能器——从而快速直观地评估血管状况，如狭窄、血栓和瓣膜反流。
6. Applications in Diagnostic Imaging | 诊断成像中的应用
Ultrasound imaging has become the first-line diagnostic tool in an extraordinarily wide range of clinical scenarios. In obstetrics, B-scan ultrasound is used to confirm pregnancy, determine gestational age, assess foetal growth and development, and detect congenital abnormalities. Because ultrasound does not use ionising radiation, it is the imaging modality of choice during pregnancy, and serial scans can be performed safely throughout all three trimesters.
超声成像已成为极为广泛的临床场景中的一线诊断工具。在产科中，B型超声用于确认妊娠、确定孕龄、评估胎儿生长发育以及检测先天性异常。由于超声不使用电离辐射，它是怀孕期间首选的成像方式，在整个孕期的三个阶段的各个时期都可以安全地进行多次扫描。
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Cardiology (心脏病学): Echocardiography uses B-scan and M-scan modes to evaluate heart chamber size, valve motion, ventricular wall thickness, and ejection fraction. Doppler echocardiography measures blood flow velocities across valves to detect stenosis and regurgitation.
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Abdominal imaging (腹部成像): Ultrasound visualises the liver, gallbladder, kidneys, spleen, and pancreas. It is particularly sensitive for detecting gallstones, kidney stones, liver tumours, and cysts, and for guiding needle biopsies.
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Emergency medicine (急诊医学): The FAST scan (Focused Assessment with Sonography for Trauma) rapidly detects free fluid—indicative of internal bleeding—in the abdomen and pericardium of trauma patients.
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Vascular imaging (血管成像): Doppler ultrasound measures blood flow in carotid arteries, peripheral veins, and deep veins, detecting deep vein thrombosis (DVT) and carotid artery stenosis.
Ultrasound is also used in guiding interventional procedures such as central venous catheter placement, joint injections, and regional anaesthesia nerve blocks, where real-time visualisation improves accuracy and reduces complications.
超声还用于引导介入操作，如中心静脉导管置入、关节注射和区域麻醉神经阻滞，实时可视化提高了操作准确性并减少了并发症。
7. Therapeutic Applications | 治疗应用
Beyond imaging, ultrasound is a powerful therapeutic tool. The mechanical energy carried by ultrasound waves can produce thermal and non-thermal biological effects that are exploited for treatment purposes.
除了成像以外，超声波也是一种强大的治疗工具。超声波携带的机械能量可产生热效应和非热生物学效应，这些效应被用于治疗目的。
In physiotherapy, low-intensity ultrasound (typically 1–3 MHz) is applied to soft tissues and joints to generate deep heating. This increases blood flow, promotes tissue healing, reduces pain, and improves flexibility. The thermal effect arises because the mechanical vibrations of the tissue dissipate energy as heat through internal friction and viscous losses.
在物理治疗中，低强度超声（通常为1–3 MHz）被应用于软组织和关节以产生深层加热。这增加了血流量，促进组织愈合，减轻疼痛并改善柔韧性。热效应源于组织的机械振动通过内摩擦和粘滞损耗将能量以热的形式耗散。
Extracorporeal shock wave lithotripsy (ESWL) is a dramatic application of focused ultrasound for breaking kidney stones. High-energy ultrasound pulses are generated outside the body and focused precisely onto the calculus using an ellipsoidal reflector. The intense pressure waves fracture the stone into small fragments that can then be passed naturally in urine, eliminating the need for invasive surgery.
体外冲击波碎石术（ESWL）是聚焦超声粉碎肾结石的典型应用。高能超声脉冲在体外产生，并通过椭球面反射器精确聚焦到结石上。强烈的压力波将结石破碎成小碎片，随后可随尿液自然排出，从而无需进行侵入性手术。
High-intensity focused ultrasound (HIFU) is an emerging non-invasive treatment for tumours. A large number of transducer elements focus ultrasound energy onto a small target volume, raising the local temperature above 60 °C and causing coagulative necrosis of cancerous tissue while sparing surrounding healthy structures. HIFU is currently used to treat uterine fibroids, prostate cancer, and certain liver tumours.
高强度聚焦超声（HIFU）是一种新兴的无创肿瘤治疗方法。大量换能器单元将超声能量聚焦到一个小目标体积上，将局部温度升至60 °C以上，导致癌组织凝固性坏死，同时保护周围健康结构。HIFU目前用于治疗子宫肌瘤、前列腺癌和部分肝肿瘤。
8. Advantages over Other Imaging Techniques | 相较于其他成像技术的优势
Each medical imaging modality has strengths and weaknesses, and ultrasound offers several distinct advantages that make it indispensable in clinical practice.
每种医学成像方式都有其优缺点，超声波具有多项显著优势，使其在临床实践中不可或缺。
| Feature (特性) | Ultrasound (超声) | X-ray / CT (X射线/CT) | MRI (磁共振) |
| Ionising radiation (电离辐射) | None (无) | Yes (有) | Published by TutorHao | A-Level Physics Revision Series | aleveler.com
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