Using Ultrasound in Medicine | 医学超声的应用

📚 Using Ultrasound in Medicine | 医学超声的应用

Ultrasound is high-frequency sound above the human hearing range, normally above 20 kHz. In medicine, frequencies from about 1 MHz to 15 MHz are used for both diagnosis and therapy. Because ultrasound can travel through soft tissue and reflect from internal boundaries, it produces images without using ionising radiation.

超声是高于人类听觉范围的高频声波，通常指频率超过 20 kHz 的声波。在医学中，常使用约 1 MHz 到 15 MHz 的超声波进行诊断和治疗。由于超声波能够穿过软组织并在内部界面发生反射，它可以在不使用电离辐射的情况下生成图像。

1. Basic Properties of Ultrasound | 超声波的基本性质

Ultrasound is a longitudinal mechanical wave, so it requires a medium and cannot travel through a vacuum. In medical imaging, pulses of ultrasound are sent into the body and echoes are detected from boundaries between different tissues.

超声是一种纵波机械波，因此它需要介质传播，不能在真空中传播。在医学成像中，超声波脉冲被送入人体，并由不同组织之间的边界反射产生回波，这些回波被检测出来。

In soft tissue, the speed of sound is approximately 1540 m s⁻¹. The wavelength λ is related to frequency f and speed v by the wave equation:

在软组织中，声速约为 1540 m s⁻¹。波长 λ 与频率 f 和速度 v 之间满足波动方程：

v = f × λ

For a typical diagnostic frequency of 1 MHz, λ = v ÷ f = 1540 ÷ 1.0 × 10⁶ ≈ 1.54 mm. Higher frequencies give shorter wavelengths, which can resolve smaller structures.

对于典型的诊断频率 1 MHz，λ = v ÷ f = 1540 ÷ 1.0 × 10⁶ ≈ 1.54 mm。频率越高，波长越短，能够分辨更小的结构。

2. Generating Ultrasound: The Piezoelectric Effect | 超声波的产生：压电效应

Ultrasound transducers use the piezoelectric effect. A piezoelectric crystal such as lead zirconate titanate (PZT) changes shape when a potential difference is applied across it. An alternating voltage makes the crystal vibrate and emit ultrasound at the same frequency.

超声换能器利用压电效应。诸如锆钛酸铅（PZT）之类的压电晶体在两端施加电势差时会发生形变。施加交变电压会使晶体振动，并以相同频率发射超声波。

The same crystal also acts as a receiver: when reflected ultrasound echoes strike it, the changing pressure induces an alternating potential difference across the crystal. This signal is then processed to build an image.

同一块晶体也可用作接收器：当反射的超声回波撞击晶体时，变化的压力会在晶体两端感应出交变电势差。该信号随后经处理生成图像。

In A-level work, you should recall that the emitted frequency equals the driving frequency of the alternating voltage, and that the crystal is cut to a suitable thickness to resonate at the desired frequency.

在 A-level 阶段，需要记住发射频率等于交变电压的驱动频率，并且晶体被切割成适当厚度，以便在所需频率下共振。

3. Frequency, Wavelength and Resolution | 频率、波长和分辨率

Axial resolution is limited by wavelength, so higher frequencies give better detail. For example, at 10 MHz the wavelength in soft tissue is about 0.154 mm, allowing structures smaller than a millimetre to be distinguished.

轴向分辨率受波长限制，因此频率越高，细节越好。例如，在 10 MHz 下，软组织中的波长约为 0.154 mm，能够分辨小于一毫米的结构。

However, higher frequency ultrasound is attenuated more rapidly. This means there is a trade-off between resolution and penetration depth. Obstetric scans often use 2-5 MHz to reach deep tissues, while scans of the eye or skin may use 15 MHz or more because these structures are close to the surface.

然而，频率越高的超声波衰减越快。这意味着分辨率和穿透深度之间存在权衡。产科扫描通常使用 2-5 MHz 以到达深部组织，而眼睛或皮肤扫描可能使用 15 MHz 或更高频率，因为这些结构靠近体表。

4. Acoustic Impedance and Intensity Reflection Coefficient | 声阻抗和强度反射系数

When ultrasound meets a boundary between two media, the amount of reflection depends on the difference in acoustic impedance Z. Acoustic impedance is defined as the product of density ρ and speed c of sound in the medium:

当超声波遇到两种介质的界面时，反射量取决于声阻抗 Z 的差异。声阻抗定义为介质密度 ρ 与声速 c 的乘积：

Z = ρ × c

The SI unit of acoustic impedance is kg m⁻² s⁻¹, sometimes written as rayl. For normal incidence, the intensity reflection coefficient R is given by:

声阻抗的国际单位是 kg m⁻² s⁻¹，有时写作 rayl。对于垂直入射，强度反射系数 R 由下式给出：

R = [(Z₂ − Z₁) ÷ (Z₂ + Z₁)]²

Here Z₁ and Z₂ are the acoustic impedances of the two media. If the impedances are equal, R = 0 and there is total transmission; if they are very different, R is close to 1 and almost all ultrasound is reflected.

其中 Z₁ 和 Z₂ 是两种介质的声阻抗。如果阻抗相等，R = 0，超声完全透射；如果阻抗差异很大，R 接近 1，几乎全部超声波被反射。

For example, at an air-soft tissue boundary, Z₁ ≈ 4.3 × 10² kg m⁻² s⁻¹ and Z₂ ≈ 1.63 × 10⁶ kg m⁻² s⁻¹. The reflection coefficient is approximately ((1.63 × 10⁶ − 430) ÷ (1.63 × 10⁶ + 430))² ≈ 0.999, so almost all ultrasound is reflected. That is why a coupling gel is essential.

例如，在空气-软组织界面，Z₁ ≈ 4.3 × 10² kg m⁻² s⁻¹，Z₂ ≈ 1.63 × 10⁶ kg m⁻² s⁻¹。反射系数约为 ((1.63 × 10⁶ − 430) ÷ (1.63 × 10⁶ + 430))² ≈ 0.999，因此几乎全部超声波被反射。这就是必须使用耦合剂的原因。

5. Impedance Matching with Coupling Gel | 使用耦合剂进行阻抗匹配

Air has a very low acoustic impedance compared with skin, so an air gap between the transducer and the body would cause strong reflection. A water-based coupling gel is placed between the transducer and the skin. Its impedance is close to that of skin and soft tissue, so R is small and most ultrasound enters the body.

空气的声阻抗与皮肤相比非常低，因此探头与身体之间的空气间隙会引起强烈反射。水性耦合剂被涂在探头和皮肤之间。其阻抗接近皮肤和软组织，因此 R 很小，大部分超声波能够进入体内。

This is an example of impedance matching. Without the gel, the reflected intensity would be so high that very little signal would penetrate or return, making imaging impossible.

这是阻抗匹配的一个例子。没有耦合剂时，反射强度会非常高，几乎很少有信号穿透或返回，使成像无法进行。

Typical values of acoustic impedance are shown below.

下表列出了一些典型声阻抗值。

Material	Density ρ / kg m⁻³	Speed c / m s⁻¹	Acoustic impedance Z / kg m⁻² s⁻¹
Air	1.3	330	≈ 4.3 × 10²
Water	1000	1500	1.5 × 10⁶
Soft tissue (average)	≈ 1060	≈ 1540	≈ 1.63 × 10⁶
Bone	≈ 1900	≈ 4080	≈ 7.8 × 10⁶

6. Attenuation in Tissue | 组织中的衰减

As ultrasound travels through tissue, its intensity decreases because energy is absorbed and scattered. This reduction in intensity is called attenuation. Attenuation increases with distance and with frequency, roughly exponentially in homogeneous tissue.

当超声波穿过组织时，其强度因能量被吸收和散射而降低。这种强度下降称为衰减。衰减随距离和频率增加，在均匀组织中大致呈指数规律。

A higher frequency gives better resolution but also attenuates more quickly, so it can only image shallow structures. Lower frequencies are chosen when deep penetration is needed, such as in abdominal or obstetric imaging.

频率越高，分辨率越好，但衰减也越快，因此只能对浅表结构成像。当需要深部穿透时，例如腹部或产科成像，会选择较低频率。

Attenuation is often specified in decibels per centimetre per megahertz. For soft tissue, a useful approximation is about 1 dB cm⁻¹ MHz⁻¹. This means a 5 MHz pulse loses intensity about five times as quickly per centimetre as a 1 MHz pulse.

衰减通常以每厘米每兆赫的分贝数来表示。对于软组织，一个有用的近似值约为 1 dB cm⁻¹ MHz⁻¹。这意味着 5 MHz 脉冲每厘米的强度损失大约是 1 MHz 脉冲的五倍。

7. A-scan and B-scan Imaging | A 型扫描和 B 型扫描成像

In an A-scan, the transducer sends a short pulse into the body and records the amplitude of echoes as a function of time. The time delay gives the depth of each reflecting boundary because the pulse travels at a known speed. A-scans are used to measure distances, for example the length of the eye before cataract surgery.

在 A 型扫描中，探头向体内发射短脉冲，并记录回波振幅随时间的变化。由于脉冲以已知速度传播，时间延迟给出了每个反射界面的深度。A 型扫描用于测量距离，例如白内障手术前测量眼球的长度。

In a B-scan, the transducer is moved or an array of transducers is used. The echo amplitudes are converted into brightness levels on a two-dimensional image. B-scan is the standard real-time ultrasound imaging mode used for observing a fetus, the heart, the liver and other organs.

在 B 型扫描中，探头移动或使用换能器阵列。回波振幅被转换为二维图像上的亮度等级。B 型扫描

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Using Ultrasound in Medicine | 医学超声的应用