超音波陣列影像資料壓縮方法與效能評估

使用軟體方式來設計超音波系統的波束成像，具有很高的系統彈性，而且現階段大量的計算需求，都能夠在個人電腦即時完成。但是，這樣的系統設計目前遇到的瓶頸在於資料傳輸速率，也就是大量的超音波陣列資料如果不經過壓縮，依照目前的傳輸速率，是無法即時地從超音波探頭端傳輸到個人電腦端。過去的研究測試了 JPEG 的陣列資料壓縮方式，但是這樣的架構並沒有充分的利用相鄰影像間的相關性來減少資料量。這份研究為了增加壓縮率，使用 MPEG 技術來壓縮超音波的陣列射頻資料。從結果看來， MPEG 方式較 JPEG 方式提供了更好的壓縮率。舉例來說，在一個 8 位元的超音波陣列射頻資料，待測物在影格間的移動距離小於 5 微米的情況下，可以達到小於 0.13 的壓縮比。而這樣的壓縮效率符合了即時影像系統所需要的資料傳輸速率，而且解壓縮後的陣列射頻資料可以提供視覺無失真的B-mode 影像重建。此外我們還發現壓縮率與影格間資料的相關性呈現高度相關。不過，RF解壓縮後的射頻資料在相位上有明顯的失真，在這個架構下無法符合都卜勒影像重建的需求。為了解決這個問題，我們在碼率控制方面，採用量化係數 (Quantization Parameter, QP) 碼率控制法。在量化係數碼率控制法下，射頻資料壓縮時將不會在影像間使用內插法，而且量化係數可以自行選擇以符合重建都卜勒影像的需求。模擬的都卜勒原始陣列資料被壓縮後，可以達到小於 0.12 的壓縮比。包含方均根超過2位元的雜訊的8位元都卜勒陣列資料，使用固定碼率壓縮法，壓縮比大於0.65。但是改用量化係數碼率控制法後，壓縮比可以小於0.3，而且可以進一步透過提高量化係數得到更小的壓縮比。然而，現有的晶片技術所能提供之編碼跟解碼速率仍有限，尚未足以實現超音波陣列系統之即時影像需求。

關鍵字

資料壓縮； MPEG ；超音波影像；都卜勒成像

並列摘要

Using software for beam-forming in ultrasound systems provides high flexibility, and the large amount of computations required in a software-based system can be performed in real time on a personal computer. However, there is a bottleneck in the very large data transfer rate required from the ultrasound front-end to the personal computer host for real-time operation, which cannot be achieved without appropriate compression. Previous studies have examined JPEG compression of ultrasound RF channel data, but the schemes do not exploit temporal redundancy between adjacent frames. This study utilized MPEG technology to process the ultrasound RF data in order to increase the compression efficiency. Our results indicate that MPEG compression generally provides a better compression ratio than does JPEG compression. As an example, the compression ratio of MPEG compression in an 8-bit channel A/D data under the 5 μm interframe displacement is smaller than 0.13 and thus allowing the real-time data transfer requirements to be met. Moreover, the compression efficiency for motions in different directions is shown to be highly dependent on the frame-to-frame correlation. For Doppler imaging, on the other hand, phase distortion resulting from data compression may not be ignored. In order to minimize the distortion, the QP (quantization parameter) rate control was used. Under the QP rate control method, the RF channel data can be compressed without inter-frame interpolation and the quantization parameter can be chosen to meet the requirements for Doppler processing. With MPEG lossless compression, a compression ratio smaller than 0.12 was achieved on simulation data. The compression efficiency increased with the noise level. When the noise RMS value was larger than 2 bit over 8 bit data, the compression ratio was higher than 0.65. With proper rate control, the compression ratio of noisy data became smaller than 0.3, and can be even smaller if higher quantization was made. Nonetheless, the encoding/decoding speed of current state-of-the-art electronics technologies is still insufficient to implement real-time ultrasound array imaging.

並列關鍵字

Data compression ； MPEG ； ultrasound imaging ； Doppler processing

參考文獻

[1] K.E. Thomenius, “Evolution of ultrasound beamformers,” Proc. IEEE Ultrason. Symp., Nov. 1996, pp. 1615-1622.

[2] M. Karaman, A. Atalar, and H. Koymen, “VLSI circuits for adaptive digital beamforming in ultrasound imaging,” IEEE Trans. Med. Imag., vol 12, pp. 711-720, Dec. 1993.

[3] S. Sikdar, R. Managuli, L. Gong, V. Shamdasani, T. Mitake, T. Hayashi, and Y. Kim, “A single mediaprocessor-based programmable ultrasound system,” IEEE Trans. Inf. Technol. Biomed., vol.7, no.1, pp. 64-71, Mar. 2003.

[4] T. Fukuoka, F. K. Schneider, Y. M. Yoo, A. Agarwal and Y. Kim, “Ultrasound color Doppler imaging on a fully programmable architecture,” in Proc. IEEE Ultrason. Symp., 2006, pp. 1639-1642.

[5] L. W. Chang, K. H. Hsu, P. C. Li, “Graphics processing unit-based high-frame-rate color Doppler ultrasound Processing,” IEEE Trans. on Ultrason. Ferroelectr. Freq. Control, Vol. 56, pp. 1856-1860, Sep. 2009.

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超音波陣列影像資料壓縮方法與效能評估

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