Processing of full digital B-ultrasound signals (2)

Early digitalization of analog signals

The receiving circuit of B-ultrasound is used to detect the ultrasonic Information Reflected by human body tissue. The human body information carried by the traditional analog B-ultrasonic echo receiving circuit is demodulated by amplification, gain compensation, detection, filtering noise and A/D conversion of the Ultrasonic Echo Signal, the most comprehensive digital signal is sent to the computer for imaging, while the full digital B-ultrasound is usedHigh speed A/DDirectly sampling the echo signal after pre-amplification, so that the analog signal is converted into a digital signal in advance, that is, the analog signal is digitalized in the early stage.Programmable Logic DeviceSample the data.Filtering, log amplification, and detectionEffectively maintains signal integrity, reduces distortion, and avoids pseudo images. However, the early digitization of A/D digits improves resolution.In theory, the sampling frequency is more than twice the maximum frequency of the echo signal. However, considering the Phase Quantization Error and other reasons, the sampling frequency must be increased or the sampling rate must be improved using the inner interpolation method;However, there is a conflict between the sampling frequency and the number of sampling digits. This is the only consideration. Based on the performance requirements of the system, you can choose between the parameters and performance of the device.

Ultrasonic signal processing process

 

Dynamic Filtering:

The attenuation of the tissue is not only related to the depth of the tested media, but also to the ultrasonic frequency. As the frequency increases, the attenuation coefficient of the medium for ultrasonic energy increases. Therefore, when the sent ultrasonic wave has a wide frequency band, the frequency component of the received echo must be related to the distance. In the near field, the echo frequency composition is mainly concentrated in the high-end of the frequency band. With the increase of the detection depth, the echo frequency composition gradually shifts to the low-end of the frequency band, because with the increase of depth, the attenuation of high-frequency components is greater than that of low-frequency components. When the depth of the detection is large, the high-frequency components cannot even reach the depth of the media and are completely absorbed.

The dynamic filter circuit is used to automatically select the frequency components with diagnostic value in the echo signal, andFilter out a frequency selector that filters out strong echo and far field interference with low frequency as the primary element.In this way, the near-field resolution and remote signal-to-noise ratio are improved, and the echo image quality is improved.

In full-digital B-ultrasound, dynamic filter has several implementation methods:

1.Dynamic Filter implemented based on the concept of matching filterTo effectively improve the signal-to-noise ratio of the system. In the process of signal processing, the matching filter coefficient dynamically changes with the variation of the receiving depth. The input of the signal processing module is the RF data stream output by the beam synthesizer. After matching and filtering, log amplification and envelope detection are performed.

2.Adaptive Threshold-Based Wavelet DenoisingAlgorithmDynamic FilterIt can be well de-noise throughout the entire game without damaging the useful signals of the midfield. This method overcomes the problem that the Wavelet Noise is not suitable for processing time-varying or air-varying noise unevenly. However, the amount of computing has increased and needs to be improved.

The fir Finite Impulse Response Digital Filter should be selected.

 

Log amplification:

The logarithm amplification circuit is used to compress the dynamic range of the echo signal. It is used to ensure that the image is displayed in gray scale to highlight the diagnostic information.

The dynamic range of Ultrasonic Echo amplitude is large, and the range is usually 100 ~ 110db. Among them, the dynamic range caused by the differences in the organizational interface is about 20 dB; the Dynamic Range Caused by the sound bundle and interface from different angles is about 30db. The dynamic range of the human body is 1 dB/cm. MHz due to the ultrasonic attenuation, while the general dynamic range of the CRT display device is only 20 dB ~ About 26db. Obviously, if the echo signal is simply zoomed in and sent to the display, only the brightness of the corresponding amplitude cannot be obtained, the "Attention effect" when a strong signal is generated will make the strong signal image blurred, and the weak signal image will be dotted with stars, so that information with diagnostic value cannot be extracted.

The solution is to compress the logarithm of the echo signal and compress the echo map into a grayscale echo map. Although the dynamic range of the grayscale echo map is smaller than the dynamic range of the original image, the original image information is retained. Therefore, the most comprehensive ultrasonic image contains various amplitude information, which enriches the image layers, the expressiveness is greatly improved. Therefore, log compression of Echo information is an important part of ultrasonic image information processing.

The log amplification in Full-digit B-ultrasound can be implemented based on the field-programmable gate array (FPGA) lookup table (LUT). LUT is essentially a ram.After you describe a logical circuit through a schematic or HDL language, the PLD/FPGA development software automatically calculates all possible results of the Logical Circuit and writes the results to ram in advance, each input signal performs a logical operation, which is equivalent to entering an address for table search, finding out the corresponding content of the address, and then outputting it.

Envelope detection:

The envelope detection circuit is used to convert the high-frequency echo signal output by the logarithm amplifier into the video pulse output. Because the ECHO is an ultrasonic oscillation modulated by a rectangular pulse, the mission of the envelope detector is to convert (demodulated) high-frequency echo into video signal output.

The main methods are as follows:

1. Absolute value low-pass filtering method.

2. Hibert conversion.

3. Vertical filtering method.

4. orthogonal digital envelope detection method.

5. First-order maximum value fitting envelope method.

Secondary Sampling:

In actual ultrasonic signal processing, it is often necessary to adjust the sampling rate based on the useful signal, that is, to extract or interpolation the sampled signal.

There are two methods:

1. A very effective method in High-Speed extraction or Interpolation Systems isUsing the cascade integrator comb (CIC) filter proposed by hogenauer"Implement Multi-sampling rate filtering. Using FPGA to implement the CIC filter has great advantages over using a digital signal processor (DSP.

2. Real-Time Compression of uneven data is based on the point compression ratio concept . The sampling data of Ultrasonic Echo is processed and displayed in frames. Each frame has a fixed length, such as 256 points. each point in the frame corresponds to a point compression ratio. That is, each vertex is actually a vertex with the maximum amplitude of several consecutive points. For example, the point compression ratio of a point is 3, which is the maximum point obtained from three consecutive points.