📚 Ultrasound Scanning: Principles and Clinical Applications | 超声扫描的工作原理与临床应用
Ultrasound scanning uses high-frequency sound waves to create images of the inside of the human body. It is one of the most widely used diagnostic imaging techniques in modern medicine because it is non-invasive, real-time and does not use ionising radiation.
超声扫描利用高频声波生成人体内部结构的图像。它是现代医学中最常用的诊断成像技术之一，因为其无创、实时，并且不使用电离辐射。
1. The Physical Nature of Ultrasound | 超声波的物理本质
Ultrasound is a longitudinal mechanical wave with a frequency greater than the upper limit of human hearing, which is approximately 20 kHz. Medical diagnostic ultrasound typically uses frequencies in the range of 1 MHz to 15 MHz. These waves require a material medium, such as soft tissue, water or blood, to travel through.
超声波是一种频率高于人耳听觉上限（约 20 kHz）的纵机械波。医学诊断超声通常使用 1 MHz 至 15 MHz 范围内的频率。这些波需要固体、液体或气体等介质才能传播。
The speed of sound in soft tissue is approximately 1540 m s⁻¹, and this value is used by ultrasound machines to estimate the depth of reflecting structures. Since the wavelength is related to frequency by c = fλ, higher frequency probes produce shorter wavelengths, which improves resolution but reduces penetration depth.
声音在软组织中的速度约为 1540 m s⁻¹，超声仪器使用该数值来估算反射结构的深度。由于波长与频率的关系为 c = fλ，较高频率的探头产生较短的波长，从而改善分辨率，但会降低穿透深度。
2. Piezoelectric Effect and the Ultrasound Transducer | 压电效应与超声探头
The key component of an ultrasound probe is a piezoelectric crystal, such as quartz or lead zirconate titanate (PZT). The piezoelectric effect allows the crystal to convert electrical energy into mechanical vibrations, and conversely mechanical vibrations into electrical signals.
超声探头的核心部件是压电晶体，例如石英或锆钛酸铅（PZT）。利用压电效应，晶体可以将电能转换为机械振动，反之也可以将机械振动转换为电信号。
When an alternating electrical voltage is applied to the crystal, it expands and contracts rapidly, producing ultrasonic waves. Conversely, when reflected sound waves strike the crystal, they deform it and generate a small voltage. The same crystal therefore acts as both transmitter and receiver of ultrasound.
当交变电压施加到晶体上时，晶体迅速膨胀和收缩，产生超声波；反之，当反射回来的声波撞击晶体时，会使晶体变形并产生微弱电压。因此，同一晶体既可作为超声的发射器，也可作为接收器。
3. Acoustic Impedance and Reflection at Interfaces | 声阻抗与界面反射
When an ultrasound wave reaches a boundary between two different media, part of the wave is reflected and part is transmitted. The proportion of reflected energy depends on the difference in acoustic impedance Z of the two media, defined as the product of density ρ and speed of sound c:
当超声波到达两种不同介质的边界时，一部分波被反射，一部分被透射。反射能量的比例取决于两种介质的声阻抗 Z 之差；声阻抗定义为介质密度 ρ 与声速 c 的乘积：
Z = ρc
The intensity reflection coefficient R at normal incidence is given by:
在正常入射条件下，强度反射系数 R 的表达式为：
R = ((Z₂ − Z₁) / (Z₂ + Z₁))²
If the impedances of two adjacent tissues are very different, a strong echo is produced. For example, the large mismatch between soft tissue and air or bone causes a nearly complete reflection, which is why a water-based gel is applied to the skin to eliminate trapped air between the probe and the body.
如果两种相邻组织的阻抗相差很大，就会产生强回声。例如，软组织与空气或骨骼之间的阻抗严重不匹配，导致几乎完全反射，因此需要在皮肤上涂抹水基耦合凝胶，以消除探头与体表之间的空气层。
4. Attenuation of Ultrasound in Tissue | 超声波在组织中的衰减
As an ultrasound beam passes through tissue, its intensity decreases due to absorption, scattering and reflection. This overall loss of energy is called attenuation. In soft tissue, the attenuation coefficient is approximately proportional to frequency, typically around 0.5 dB cm⁻¹ MHz⁻¹.
超声束在组织中传播时，由于吸收、散射和反射，其强度逐渐减小。这种总能量损失称为“衰减”。在软组织中，衰减系数约与频率成正比，典型值约为 0.5 dB cm⁻¹ MHz⁻¹。
For a transducer of frequency f (in MHz), the one-way attenuation through a distance x (in cm) is approximately:
对于频率为 f（单位 MHz）的探头，传播距离为 x（单位 cm）时的单程衰减近似为：
衰减 (dB) ≈ 0.5 f x
Because attenuation increases with depth, echoes from deeper structures are weaker. Modern ultrasound systems use time-gain compensation to amplify later-arriving echoes so that images have uniform brightness from near to far field.
由于衰减随深度增加，来自深部结构的回声较弱。现代超声系统使用“时间增益补偿”放大较晚到达的回声，使图像从近场到远场具有均匀亮度。
5. Echo Detection and Distance Measurement | 回声探测与距离测量
Ultrasound imaging works on the pulse-echo principle. A short pulse of ultrasound is transmitted into the body, and the time delay t before an echo returns is used to calculate the depth d of the reflecting structure:
超声成像基于脉冲-回波原理。将一束短超声脉冲射入人体，利用回声返回的时间延迟 t 来计算反射结构的深度 d：
d = (c t) / 2
The factor of 2 accounts for the two-way journey of the sound wave. In soft tissue, using c = 1540 m s⁻¹, every 13 μs of round-trip time corresponds approximately to 1 cm of depth. The transducer sends pulses at a pulse-repetition frequency, and between pulses it listens for echoes.
因子 2 表示声波往返传播。在软组织中，取 c = 1540 m s⁻¹，则每 13 μs 的往返时间约对应 1 cm 深度。探头以脉冲重复频率发射脉冲，并在两次发射之间接收回声。
6. A-Mode, B-Mode and M-Mode Scanning | A型、B型与M型扫描模式
Ultrasound imaging can be presented in several modes. In A-mode (amplitude mode), a single beam is directed along one line and the amplitudes of returning echoes are displayed as deflections on a screen. It is useful for measuring distances, such as the depth of the eye.
超声成像有不同显示模式。A型（幅度调制型）使用单一波束沿一条线发射，返回回声的振幅以屏幕上的偏转显示，常用于测量距离，例如眼轴长度。
In B-mode (brightness mode), each echo is converted into a bright dot whose intensity represents the echo amplitude. A two-dimensional cross-sectional image is formed by scanning the beam across the body. B-mode is the standard mode used in most clinical ultrasound examinations.
B型（亮度调制型）将每个回声转换为一个亮点，亮度表示回声强度。通过使波束在体内扫描，形成二维断面图像。B型是大多数临床超声检查使用的标准模式。
In M-mode (motion mode), a single B-mode scan line is displayed over time, showing the movement of structures such as heart valves. M-mode provides accurate timing measurements of cardiac motion.
M型（运动型）将单条B型扫描线随时间展开，显示心脏瓣膜等结构的运动轨迹。M型用于对心脏运动进行精确的时间测量。
7. Doppler Ultrasound and Blood Flow Measurement | 多普勒超声与血流测量
The Doppler effect is used to measure the velocity of moving structures, especially red blood cells. When ultrasound is reflected from a moving scatterer, the frequency of the returning wave is shifted. The frequency shift Δf depends on the angle θ between the ultrasound beam and the direction of motion:
多普勒效应用于测量运动结构（尤其是红细胞）的速度。当超声波被运动中的散射体反射时，返回波的频率发生偏移。频移量 Δf 与超声束和运动方向之间的夹角 θ 有关：
Δf = (2 f v cos θ) / c
Here, f is the transmitted frequency, v is the speed of the moving object and c is the speed of sound in tissue. The factor 2 arises because the Doppler shift occurs twice: once for the transmitted wave meeting the moving red cells and once for the reflected wave returning to the transducer.
其中 f 是发射频率，v 是物体运动速度，c 是组织中声速。因子 2 的出现是因为多普勒频移发生了两次：一次是发射波遇到运动的红细胞，另一次是反射波返回探头时。
If the beam is perpendicular to the blood flow, θ = 90°, then cos θ = 0 and no frequency shift is detected. Therefore, the transducer must be angled carefully to obtain accurate velocity measurements.
当波束与血流方向垂直时，θ = 90°，此时 cos θ = 0，检测不到频移。因此，探头需要调整到合适的角度才能获得准确的速度测量值。
8. Resolution and Choice of Frequency | 分辨率和频率选择
Axial resolution is the ability to distinguish two structures lying close together along the direction of the beam. It is approximately half the pulse length. If the pulse contains only a few cycles at wavelength λ, the axial resolution is roughly λ/2 = c / (2f). Higher frequencies therefore give better axial resolution.
轴向分辨率是指沿波束方向区分相距很近的两个结构的能力，约为脉冲长度的一半。如果脉冲只包含几个波长为 λ 的周期，则轴向分辨率约为 λ/2 = c / (2f)。因此，频率越高，轴向分辨率越好。
Lateral resolution depends on the width of the ultrasound beam; a narrower beam gives better resolution in the direction perpendicular to the beam. Beam focusing is used to improve lateral resolution at a specific depth.
横向分辨率取决于超声束的宽度；波束越窄，垂直于波束方向的分辨率越好。常用聚焦技术来改善特定深度处的横向分辨率。
In clinical practice, a higher frequency probe, such as 7–15 MHz, is used for superficial structures like the thyroid or vessels. A lower frequency probe, such as 2–5 MHz, is used for deep structures like the liver or fetus, because lower frequencies attenuate less and can penetrate deeper.
临床实践中，高频探头（如 7–15 MHz）用于甲状腺或血管等浅表结构；低频探头（如 2–5 MHz）用于肝脏或胎儿等深部结构，因为低频衰减较小，可穿透更深。
9. Obstetric and Gynaecological Applications | 产科与妇科应用
Ultrasound is most famous for imaging the fetus during pregnancy. It can confirm pregnancy, estimate gestational age, check fetal growth, determine the position of the placenta and detect multiple pregnancies or structural anomalies.
超声最著名的应用是孕期胎儿成像。它可以确认妊娠、估算胎龄、检查胎儿生长情况、确定胎盘位置，并发现多胎妊娠或结构异常。
During the first trimester, transvaginal ultrasound with a high-frequency probe is often used because it provides clearer images of the early embryo. Serial scans are also used to monitor conditions such as placenta praevia, where the placenta covers the cervix.
在妊娠早期，通常使用高频探头的经阴道超声来更清晰地显示早期胚胎。连续多次扫描还可监测前置胎盘（胎盘覆盖宫颈口）等情况。
In gynaecology, ultrasound helps to identify ovarian cysts, uterine fibroids and ectopic pregnancies. It also assists in guiding procedures such as egg collection for in vitro fertilisation (IVF).
在妇科中，超声有助于识别卵巢囊肿、子宫肌瘤和异位妊娠，还可辅助引导取卵等体外受精流程。
10. Cardiac and Vascular Applications | 心脏与血管应用
Echocardiography is the use of ultrasound to image the heart. B-mode echo shows the structure of the four chambers, valves and the thickness of the heart wall. M-mode provides precise timing of valve opening and closing, while Doppler ultrasound measures the speed of blood flow across valves, helping to detect stenosis or regurgitation.
超声心动图是使用超声对心脏成像。B型回声显示四个腔室、瓣膜结构和室壁厚度；M型对瓣膜开启和关闭进行精确定时；多普勒超声测量血流经过瓣膜的速度，有助于检测瓣膜狭窄或反流。
In vascular medicine, Doppler ultrasound is used to assess blood flow in the carotid arteries, peripheral veins and arteries. It can detect deep vein thrombosis (DVT), carotid artery stenosis and chronic venous insufficiency. Colour Doppler adds colour-coded velocity information to a B-mode image, making it easier to visualise blood flow direction and turbulence.
在血管医学中，多普勒超声用于评估颈动脉、外周动静脉中的血流。它可以检测深静脉血栓、颈动脉狭窄和慢性静脉功能不全。彩色多普勒将速度信息以彩色编码叠加在B型图像上，便于观察血流方向和湍流。
11. Safety of Diagnostic Ultrasound | 超声诊断的安全性
Diagnostic ultrasound is generally considered very safe, as it does not use ionising radiation. However, ultrasound waves can cause biological effects through two main mechanisms: thermal effects, where tissue is heated by absorption of acoustic energy; and mechanical effects, such as cavitation, where tiny gas bubbles oscillate or collapse under the alternating pressure of the wave.
诊断超声通常被认为是安全的，因为它不使用电离辐射。然而，超声波可通过两种机制引起生物效应：一是热效应，即组织通过吸收声能而升温；二是机械效应，例如空化现象，即微小气泡在声压交替作用下振荡或破裂。
Ultrasound machines display indices such as the thermal index (TI) and mechanical index (MI) to estimate the potential for heating and cavitation. The ALARA principle (as low as reasonably achievable) is followed clinically, meaning that exposure time and output power are kept to the minimum necessary for a diagnostic image.
超声仪器会显示热指数（TI）和机械指数（MI），以评估致热和空化的风险。临床遵循 ALARA 原则（合理可行尽量低的照射量），即把暴露时间和输出功率控制在获得诊断图像所需的最低水平。
12. Advantages and Limitations of Ultrasound | 超声扫描的优点与局限性
The main advantages of ultrasound include low cost, portability, real-time imaging, absence of ionising radiation and the ability to image blood flow with Doppler techniques. It is also particularly suited for soft tissues that are not obscured by bone or gas.
超声的主要优点是成本低、便携、实时成像、无电离辐射，并能利用多普勒技术观察血流。它还特别适合显示未被骨骼或气体遮挡的软组织。
However, ultrasound also has limitations. It cannot penetrate bone or gas-filled structures such as the lungs or bowel, so it is poor at imaging these regions. The image quality depends heavily on the skill of the operator, and lateral resolution and penetration depth are limited compared with CT or MRI. In addition, ultrasound imaging has a limited field of view for very large patients.
但超声也有局限性：它不能穿透骨骼或充满气体的结构（如肺和肠），因此对这些区域成像能力较差；图像质量严重依赖操作者技术；与CT或MRI相比，其横向分辨率和穿透深度有限；此外，对体型很大的患者，超声视野也受到限制。
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