Main performance parameters of ultrasound diagnostic instrument

(1) Dead Zone

The dead zone refers to the depth of the nearest echo target that can be identified by the ultrasound equipment. A smaller dead zone is beneficial for detecting lesions close to the body surface. This performance mainly depends on the characteristics of the amplifier and the performance of the variable aperture technology. In addition, reducing the amplitude of the emitted pulse entering the amplifier and adjusting the time constant of the amplifier will also affect the size of the dead zone. However, for transducers with added water bags, the dead zone is meaningless.

(2) Detection Depth

Under the conditions of normal image display, maximum allowable sensitivity, and brightness, the maximum depth of the echo target observed by the ultrasound equipment is called the detection depth. The larger this value, the wider the range of examination within the biological body. Several factors influence this performance:
1. Transducer Sensitivity
During the process of transmitting and receiving ultrasonic waves, the transducer achieves electro-acoustic and acoustic-electric conversion efficiency. The higher the sensitivity, the greater the detection depth. Sensitivity mainly depends on the electromechanical properties of the crystal and the matching status of the acoustic and electrical matching layers of the transducer.
2. Transmission Power
Increasing the acoustic power radiated by the transducer can increase the detection depth. However, increasing the acoustic power requires increasing the transmission voltage of the circuit. This not only brings difficulties to the overall machine design but also requires limiting the acoustic power within the safety dose threshold. The technical index of this safety dose threshold is usually expressed by acoustic intensity, that is, the acoustic intensity should not exceed 10mW/cm².
3. Receiving Amplifier Gain
Increasing the gain of the receiving amplifier can increase the detection depth. However, increasing the amplifier gain amplifies the weak echo signal while also amplifying the system noise signal, causing the useful signal to be drowned out by the noise. Therefore, the gain should be moderate.
4. Operating Frequency
The acoustic attenuation coefficient of biological tissues has a linear relationship with frequency. The lower the frequency, the longer the wavelength, the smaller the amplitude attenuation, and the greater the detection depth, but the resolution is worse. Conversely, the higher the frequency, the smaller the detection depth, but the better the resolution. To improve the overall performance of the machine, dynamic frequency scanning and dynamic tracking filtering technology are generally adopted to achieve a balance between high resolution and detection depth. Despite this, to meet clinical needs, transducers of different frequencies are still needed to diagnose different parts of the organism.

(3) Axial Resolution (Longitudinal Resolution)

Refers to the minimum distance at which two echo targets can be distinguished in the B-ultrasound image display along the direction of the acoustic beam axis. The smaller this value, the clearer the layering of the longitudinal interface on the sonogram. For continuous ultrasonic waves, the theoretical resolution that can be achieved is equal to half the wavelength. Therefore, the higher the frequency, the better the resolution. Since biological tissue interfaces are not completely identical targets, it is impossible to achieve the theoretical resolution value in practice, but rather a value equivalent to 2-3 wavelengths. In an ultrasonic pulse echo system, axial resolution is related to the effective pulse width (duration) of the ultrasound pulse. The narrower the pulse, the better the axial resolution. To improve this characteristic, multi-layer optimal impedance matching technology is commonly used in current transducers. At the same time, to ensure a steep pulse leading edge, the best dynamic tracking filter is used in the receiving amplifier by various manufacturers to improve this characteristic.

(4) Lateral Resolution (Transverse Resolution)

Refers to the minimum distance at which two echo targets can be distinguished in the scanning plane of the ultrasonic beam, perpendicular to the direction of the acoustic beam axis. The smaller this value, the clearer the layering of the transverse interface on the sonogram. The influencing factors include:
1. Acoustic Beam Width
The narrower the acoustic beam, the better the lateral resolution. The acoustic beam width is related to the crystal diameter and the operating frequency. However, the transducer size cannot be made very large, and the frequency cannot be infinitely high. Therefore, designers have adopted lens and variable aperture technology, and applied segmented dynamic focusing and continuous dynamic focusing in the design, thereby improving lateral resolution.
2. System Dynamic Range
Within the directional sound field generated by the transducer, the sound pressure (or sound intensity) is not uniformly distributed. Generally, there is such a rule: as the gain increases and decreases, the acoustic beam width correspondingly widens and narrows, and the lateral size of the target echo image correspondingly lengthens and shortens.
3. Display Brightness and Medium Attenuation Coefficient
Display brightness and medium attenuation coefficient will affect lateral resolution. Therefore, when measuring lateral resolution, the gain and brightness of the equipment must be adjusted to the optimal state.

(5) Geometric Position Indication Error

Refers to the accuracy of the actual target size and distance displayed and measured by the ultrasound equipment. In practical applications, mainly longitudinal
Geometric position indication error and transverse geometric position indication error are measured. This technical parameter has a great influence on the measurement of lesion size in the biological body, involving the consistency of diagnosis and treatment. The factors affecting this accuracy are related to the uniformity of the fan-shaped image related to the sound speed setting and scanning pattern, which is slightly less accurate in geometric position than plane linear array scanning.

(6) Acoustic Beam Slice Thickness

Refers to the thickness of linear array, convex array, and phased array transducers in the direction perpendicular to the scanning plane. The thinner the slice, the clearer the image; otherwise, it will lead to image compression and artifacts. The slice thickness depends on the size and inherent frequency of the crystal in the short-axis direction. Solution: A focusing acoustic lens is usually added in front of the crystal, and focusing technology is used in the whole machine.

(7) Contrast Resolution

Refers to the minimum difference in echo amplitude that can be detected on the image. The better the contrast resolution, the stronger the sense of layering of the image, the richer the detail information, and the more delicate and soft the image. The factors influencing this are mainly determined by the bandwidth of the acoustic signal and the grayscale of the display circuit.

(8) Operating Frequency, Bandwidth, Repetition Frequency

1. Operating Frequency
The operating frequency refers to the emission frequency of the ultrasonic probe, which is related to the inherent frequency of the probe crystal and other parameters of the emission oscillation circuit.
2. Bandwidth
In pulsed echo systems, pulsed ultrasound waves are emitted, and these pulse groups have very short durations. According to spectral analysis, a pulse contains many harmonics, i.e., it has a bandwidth; this wideband containing many harmonics is called bandwidth; its maximum energy is concentrated near the center frequency, and it can also be inferred that: when the pulse frequency is 1 MHz, the duration of the pulse group is only 1 μs (microsecond); while the actual pulse (working) frequency of the B-ultrasound diagnostic instrument is greater than 1 MHz, the duration is less than 1 μs (microsecond).
3. Repetition Frequency
The ultrasound emitted by the ultrasound diagnostic instrument is a pulsed signal; the same pulsed signal is repeatedly emitted at a certain time interval, this time interval is called the repetition period T, the number of times the pulse appears per second is called the repetition frequency F, and the pulse duration is called the pulse width W.
The lower the repetition frequency, the deeper the ultrasonic detection depth. The wider the bandwidth (frequency band) of the ultrasonic emitted pulse, the higher its longitudinal resolution.

(9) Transmission Power

Increasing the acoustic power radiated by the transducer can increase the detection depth. However, increasing the acoustic power requires increasing the transmission voltage of the circuit. This not only brings difficulties to the overall machine design, but also requires limiting the acoustic power within the safe dose threshold for design and use, especially for fetuses and children, reducing and avoiding the use of ultrasound for examinations, and using low-power ultrasound diagnostic instruments when necessary; the Technical Index of ultrasound transmission power is expressed by sound intensity, and the sound intensity of the ultrasound diagnostic instrument should not exceed 10 mW/cm2.
The transmission power of the ultrasound diagnostic instrument is directly proportional to the electromechanical conversion coefficient of the transducer chip, the capacitance of the transmission circuit energy storage capacitor, and the repetition frequency of the transmission pulse, and inversely proportional to the area of the transducer chip.

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