Classification and working principle of ultrasonic probe

Ultrasonic probe is a device that converts electrical energy into ultrasonic waves and converts echo signals into electrical signals. Its core component is piezoelectric crystal, which can generate high-frequency mechanical vibrations under the action of alternating current, and then emit ultrasonic waves. When ultrasonic waves encounter the interface of tissues and organs in the body, they will be reflected, and the reflected waves will be received by the probe again and converted into images.
 

1. Working principle of ultrasonic probe

Ultrasonic diagnostic instrument generates incident ultrasonic waves (transmitted waves) and receives reflected ultrasonic waves (echoes) through the probe, which is an important part of diagnostic equipment. The task of ultrasonic probe is to convert electrical signals into ultrasonic signals or vice versa. The probe can transmit and receive ultrasound, perform electroacoustic and signal conversion, and can convert the electrical signals sent by the host into high-frequency oscillating ultrasonic signals, and can also convert the ultrasonic signals reflected from tissues and organs into electrical signals and display them on the display of the host. Ultrasonic probe is made using this working principle. When the power is on, the chip in the probe can produce elastic deformation, thereby generating ultrasonic sound waves; in the opposite case, when the ultrasonic sound wave passes through the chip, it can cause it to produce elastic deformation, which in turn causes a change in voltage. Finally, the signal processing board processes the corresponding electrical signal changes to complete the image detection of the detected object. This processing process is called piezoelectric effect (positive and negative piezoelectric effect).

2. Probe composition

Piezoelectric crystal-core component

Function: Convert electrical energy into ultrasonic waves, and convert the received echo into electrical signals.
Common materials: PZT (lead zirconate titanate), piezoelectric composite materials, etc.
Working principle: When the current is turned on, the crystal deforms and vibrates to emit ultrasonic waves; when the echo is received, the crystal reversely deforms to generate electrical signals.
Each probe may contain dozens to hundreds of piezoelectric crystal arrays.

Matching layer

Function: Improve the acoustic impedance matching between piezoelectric crystals and human tissues, reduce acoustic energy reflection, and improve the penetration efficiency of ultrasonic waves.
Material: Usually resin or composite material, the acoustic impedance is between crystals and human tissues.
Generally, it is a 1~2-layer structure: multi-layer matching layer has better effect.

Acoustic lens or focusing element

Function: Control the propagation direction of the ultrasonic beam, achieve focusing effect, and improve image resolution.
Type: fixed focus or electronic focus.
Action site: Installed at the front end of the probe (the side that contacts the patient).

Backing layer

Function:
Absorb the sound waves emitted from the back of the crystal to prevent echo interference;
Shorten the ultrasonic pulse time and improve the axial resolution.
Material: Usually epoxy resin containing metal particles.

Acoustic window/protective film

Function: Protect the crystal from external damage while allowing ultrasonic waves to propagate smoothly.
Material: Polyurethane film or other biocompatible materials.

Wire and cable system

Function: Transmit the electrical pulses emitted by the host to the piezoelectric crystal, and transmit the echo signal received by the crystal back to the host.
Structure: High-density, multi-channel coaxial cable to ensure signal integrity.
Precautions: Avoid pulling and folding the probe cable to prevent signal loss caused by wire damage.

Shell and handle

Function: protect the internal precision structure and provide operating feel.
Material: medical plastic, silicone, etc., with waterproof, dustproof and corrosion-resistant properties.
Design: ergonomic structure, convenient for doctors to operate for a long time.

Guide device (only for special type probes)

Function: install puncture needle guide to guide biopsy needle or puncture device.
Application: ultrasound-guided puncture (such as thyroid nodule biopsy, liver puncture, etc.)

3. Classification of ultrasound probes

Classification by diagnostic site

There are ophthalmic probes, cardiac probes, abdominal probes, cranial probes, intracavitary probes and pediatric probes, etc. There are characteristic intracavitary probes
Classification by the number of transducers used in the probe, there are unit probes and multi-element probes
Classification by beam control method
There are line scanning probes, phased array probes, mechanical fan scanning probes and square array probes, etc.

Classification by probe geometry

There are rectangular probes, various probes for columnar sections, arc-shaped probes (also known as convex), circular probes, etc.
Classification by the number of piezoelectric chips in the probe
It can be divided into single crystal probes, dual crystal probes and polycrystalline probes. Among them, single crystal probes are expensive in material cost and complex in process. Compared with traditional probes, more uniform, less attenuated and wider bandwidth images can be obtained. Yichao will fully apply this technology to the abdomen, obstetrics and gynecology, heart and other parts

Other special application probes

In recent years, ultrasound intervention has flourished, and puncture probes, dual-plane probes, transesophageal probes, intraoperative probes, laparoscopic probes, etc. have emerged.

4. Probe selection

The emission frequency of the probe is one of the most important characteristic parameters of the probe. In ultrasound diagnosis, different probes are often selected according to different objects and parts of the examination, such as 2 MHz, 2.5 MHz, 5 MHz, 10 MHz, etc. The emission frequency of the probe is determined by the thickness of the crystal. The shape of the chip determines important characteristics such as the shape of the sound beam and the distribution of the sound field.

Mechanical sector scanning probe

The full name is the mechanical sector scanning probe, which was commonly used for abdominal and cardiac ultrasound examinations in the early days, and is now almost only used for ophthalmic A/B ultrasound

Plane linear array

Before the emergence of convex arrays, it was the main force for abdominal examinations, and the frequency was mostly 3.5MHz; after the emergence of convex arrays and becoming the main force for abdominal examinations, it was mainly used for small organs and superficial tissue examinations, and the frequency was generally 5MHz~7.5MHz (even 9MHz).

Convex array

The large R (radius of curvature of the chip) of the convex array is usually above 30mm, which is used for abdominal examination; the small R (10~20mm, doctors often call it micro-convex) is used for heart examination.

Phased array

It is used for cardiovascular color blood flow imaging in color Doppler ultrasound. Because the image is inlaid (superimposed) on the grayscale image of the anatomical structure, the black and white, color images and Doppler spectrum are obtained using different working modes of the same probe.

5. Scanning of the sound beam and the sonograms formed by different probes

This is a brief introduction. I believe that with the continuous development of ultrasound technology, more probes with excellent images and new functions will appear to assist ultrasound diagnosis.

As the core component of the medical ultrasound system, ultrasound probes are not only of various types and functions, but also play an irreplaceable role in the diagnosis process. Understanding the characteristics and working principles of various probes can help doctors make more accurate and rapid judgments in clinical applications, and also help equipment purchasers to make reasonable selections.