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Figure 2 Simulated maximum frequency waveform.

The superimposed maximum frequency waveform also confirms that MP achieves a good denoising performance.

From Figure 5(d), it is obvious that the Doppler random noise and speckles are effectively suppressed; meanwhile the superimposed maximum frequency waveform indicates that the AD-PCNN obtains a better noise and speckle reduction performance than MPWD. Figure 5 The spectrogram and the maximum frequency waveform, which is superimposed on the spectrogram with a solid curve, of a clinical signal.

The objective effect and subjective indices of the spectrogram and their maximum frequency waveform are used to compare the performance of the AD-PCNN and the MPWD noise and speckle suppression algorithms.

The maximum frequency waveform used in the simulation is shown in Figure 2, and the details of this simulation model can be found in the work by Mo and cobbold [19].

The diagnostic indices which are necessary for a clinical judgment are all extracted from the maximum frequency waveform of the Doppler spectrogram, calculated by using short-time Fourier transform (STFT), and are very useful in diagnosing arterial stenosis or other vascular disease by evaluating the vascular resistance [2].

The maximum frequency waveforms are extracted from the spectrograms.

The maximum frequency waveforms extracted from these estimated spectrograms include considerable distortion and illegibility, which implies the difficulty in finding correct indices used for quantification of vascular diseases' severity from the spectrograms.

Table 1 The mean and standard deviation of the RRMS errors of the maximum frequency waveforms extracted from the signals with different SNR levels based on the STFT, the MPWD methods, and the AD-PCNN methods (×10-3).

From Table 1 it can be concluded that the RRMS errors of the maximum frequency waveforms extracted from the PCNN spectrograms have decreased much more than those before noise and speckle reduction.

For example, the RRMS errors of the maximum frequency waveforms of the original STFT are 2.92 and 3.84 times higher than those denoised by using MPWD and AD-PCNN on Doppler signals with SNR = 10 dB, respectively.