Overview of evaluation indicators of image quality in B-ultrasound diagnosis

1 blind zone
Blind Zone refers to the nearest echo target depth that can be identified by B-ultrasonic equipment. If the blind zone is small, it is helpful to check the lesions close to the body surface. This performance mainly depends on the characteristics of the amplifier. In addition, reducing the amplitude of the transmit pulse entering the amplifier and adjusting the amplifier time constant will also affect the blind area. However, the blind zone of the transducer with water tank is meaningless.
2. Probe depth
The maximum depth of the ECHO target observed by B-ultrasonic equipment under the conditions of maximum sensitivity and brightness permitted by normal image display is called the detection depth. The larger the value, the larger the number of items in the organism. However, there are several factors that affect this performance:
2.1 transducer sensitivity
In the process of transmitting and receiving ultrasonic waves, the transducer achieves electrical → acoustic → electrical conversion efficiency. The higher the sensitivity, the larger the probe depth. The sensitivity mainly depends on the Mechanical and Electrical Performance of the chip and the matching of the acoustic and electrical matching layers of the transducer.
2.2 transmit power
Increasing the acoustic power of the transducer radiation can increase the detection depth. However, to increase the acoustic power, increase the transmit voltage of the circuit. This not only brings difficulties to the design of the entire machine, but also limits the acoustic power within the safe dose threshold. The technical indicators are usually expressed by the acoustic intensity, that is, the acoustic intensity should not be greater than 10 mW/cm2.
2.3 receiving Amplifier Gain
Increasing the gain of the receiving amplifier can increase the probe depth. However, the gain of the amplifier is increased, while the weak echo signal is amplified, the system noise signal is also amplified, so that the useful signal is drowned in the noise, so the gain is moderate.
2.4 operating frequency
The acoustic attenuation coefficient of biological tissue is linearly related to the frequency. The lower the frequency, the longer the wavelength, and the smaller the amplitude attenuation, the larger the detection depth, but the worse the resolution. On the contrary, the higher the frequency, the smaller the probe depth, but the better the resolution. In order to improve the performance of the entire machine, dynamic frequency scanning and dynamic tracking and filtering are generally adopted, which can be applied with both high resolution and detection depth. However, in order to meet the clinical needs, a different transducer is still needed to diagnose different parts of the organism.
3. axial resolution (longitudinal resolution)
Axial resolution refers to the minimum distance between two ECHO targets in B-ultrasound image display along the beam axis. The smaller the value, the clearer the layer of the portrait interface on the image. For continuous ultrasound, the theoretical resolution is equal to half a wavelength. Therefore, the higher the frequency, the better the resolution. Because the biological tissue interface is not exactly the same target, it is impossible to reach the theoretical resolution value, but equivalent to 2 ~ Three wavelength values. In the ultrasonic pulse echo system, the axial resolution is related to the effective pulse width (duration) of the ultrasonic pulse. The narrower the pulse, the better the axial resolution. In order to improve this feature, the multi-layer optimal impedance matching technology is widely used for the transducer. In addition, to ensure the steep pulse front, various manufacturers have adopted the best dynamic tracking filter in the receiving amplifier.
4. lateral resolution (horizontal resolution)
Lateral resolution refers to the minimum distance between two ECHO targets in the scanning plane of the ultrasonic beam and perpendicular to the beam axis. The smaller the value, the clearer the layer of the image horizontal interface. Its Influencing factors include:
Width of 4.1 Beam
The narrower the sound bundle, the better the lateral resolution. The beam width is related to the wafer diameter and operating frequency. However, the size of the transducer cannot be very large, and the frequency cannot be infinitely high. Therefore, the designer adopts the lens and variable aperture technology to apply the segmented dynamic focus and continuous dynamic focus in the design, thus improving the lateral resolution.
4.2 System Dynamic Range
Sound pressure (or sound intensity) is not evenly distributed in the directional acoustic field generated by the transducer. Generally, with the increase or decrease of the gain, the width of the sound bundle is reduced accordingly, and the horizontal size of the target echo image is shortened accordingly.
4.3 display brightness and media attenuation coefficient
The brightness of the monitor and the attenuation coefficient of the media affect the lateral resolution. Therefore, when measuring the lateral resolution, you must adjust the gain and brightness of the device to the optimal condition.
5 geometric position error
Geometric location indication value error refers to the accuracy of displaying and measuring the actual target size and distance by B-ultrasonic equipment. In practical application, the error of the vertical geometric position and the error of the lateral geometric position are measured. This technical parameter is used to measure the accuracy of the lesion size in the organism, involving the consistency of diagnosis and treatment. The factors that affect the accuracy are the speed setting and Scanning Pattern. the uniformity of the slice image is less accurate than that of the geometric position of the flat line array scan.
6-beam slice thickness
The thickness of the beam slice refers to the thickness of the line array, convex array, and phased array transducer perpendicular to the scanning plane. The thinner the slice, the clearer the image. Otherwise, the image will be compressed to produce a pseudo image. The slice thickness depends on the size and inherent frequency of the chip's short axis. Solution: Generally, the focus sound lens is installed in front of the chip and the focus technology is used in the whole device.
7. Contrast Resolution
Contrast Resolution refers to the minimum difference in the ECHO amplitude that can be detected on the image. The better the Contrast Resolution, the higher the attention of the image, the richer the details, and the more delicate and soft the image. The reason for this is mainly dependent on the bandwidth of the acoustic signal and the gray scale of the display circuit.
According to the development of ultrasonic technology and the status of ultrasonic diagnostic parameters in the world, the Institute of acoustics of the Chinese Emy of Metrology and the Chinese Emy of Sciences re-modified and formulated the JJG639-1998 of the verification procedure for the medical ultrasonic diagnostic instrument ultrasonic source in 1998, it was implemented on July 15, March 1, 1999. In order to achieve comparable performance evaluation of machines, the new procedure divides the tested instruments into four levels: A, B, C, and D, respectively, that is, the color Doppler ultrasound is; high-grade black/white B ultrasound with multi-probe, multiple scanning modes, M-type function and Doppler technology is B-class; digital scanning converter (DSC) portable, freeze and electronic cursor ranging functions, and some B-mode with carts are c files; the simplest B-mode without DSC is D files. Then, the four instruments are distinguished by different scanning methods and operating frequencies, so as to develop the technical indicators of ultrasonic equipment of different grades, it is used to evaluate the image quality and the threshold value of ultrasonic equipment.