About Medical Ultrasound Imaging (Part 1)

Ultrasound refers to mechanical waves with frequencies exceeding the range of human hearing. Medical ultrasound imaging technology utilizes the phenomena and mechanisms of ultrasound in biological tissues to conduct research on acoustic imaging. The wavelength of ultrasound is comparable to the size of biological tissue cells, and cells are the basic units that reflect tissue characteristics; therefore, the interaction information between ultrasound and tissue is abundant. Ultrasound can distinguish various lesions in soft tissues, such as displacement, lesions, and hyperplasia. Ultrasound imaging is a non-invasive method, with no radiation to the human body, and is a safe examination method. It is widely used in cardiology, obstetrics, gynecology, abdominal scanning, eye scanning, and blood flow determination, etc.

The structure of an ultrasound imaging system mainly consists of units such as an ultrasound transducer, high-voltage pulse emission, beam forming, overall system control, echo post-processing, image display, and processing.

As shown in the figure:

Schematic diagram of the principle of medical ultrasound imaging

The ultrasound transducer, also known as a transducer, is a component that performs electro-acoustic and acoustic-electric signal conversion, used to transmit and receive ultrasound, and is a key component of the medical ultrasound imaging system.

Human bodyStructureFor ultrasound, it is a complex medium. Various organs and tissues, including pathological tissues, have their specific acoustic impedance and attenuation characteristics. Therefore, there are differences in acoustic impedance and attenuation. When ultrasound enters the body, from the surface to the deep part, it will pass through organs and tissues with different acoustic impedance and attenuation characteristics, thus producing different reflections and attenuation. These different reflections and attenuations form the basis of ultrasound images. The received echoes are based on the echoIntensityis weak, and displayed on the screen as light and dark spots in sequence, a cross-sectional ultrasound image of the human body can be shown.

The reflected wave is converted into an electrical signal by the transducer, usually called an echo signal. Due to reflection from normal or diseased organs, the ultrasound echo can be anechoic, hypoechoic, or strong echo of varying degrees. For example, the surface of human organs has a membrane. The acoustic impedance difference between the membrane and the underlying tissue is large, forming a good interface reflection, and a complete and clear peripheral echo appears on the image, showing the outline of the organ.

The echo signal is synthesized and processed in the beam forming part. The quality of the beam determines the quality of the image, directly affecting the signal-to-noise ratio of the echo signal. Its implementation is also affected by many factors such as delay accuracy, and has always been a hot issue in the field of ultrasound imaging.

By using the arrival time of the reflected ultrasound echo and the propagation speed of the sound wave, the position where the reflected wave is generated can be inferred, which is the depth from the surface of the incident probe. The commonly used B-mode image displays the ultrasound echo information on a cross-sectional plane of the tissue in a two-dimensional form, using light and dark dots to represent the strength of the echo, thus obtaining the anatomical image of the research area.

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