超音波參數對組織諧波合頻成像對比度的影響

Abstract

自從非線性聲學的理論被提出後，組織諧波成像就被廣泛地使用在超音波醫療診斷上。當組織病變時，聲學非線性參數會增加，因此產生更多的諧波，而相較於傳統基頻影像，諧波影像更易觀察人體組織的病變情形，所以現在逐漸成為主流，儘管如此，還是有許多可以改進的地方。 是否容易從一張諧波影像中判斷組織病變狀況深受影像對比度、空間解析度、信雜比等因素影響。提高發射脈衝訊號的持續時間可以增加信雜比，但會因此降低縱向解析度，這個問題可以透過合頻方法來解決。組織諧波合頻成像就是發射多個窄頻的脈衝訊號，利用其脈衝持續時間久，利於產生二次諧波這個特性，來獲得高信雜比，再把傳遞後各自產生的窄頻二次諧波訊號相加，合成一個大頻寬訊號來成像，就可以同時提升影像縱向解析度及信雜比 。 然而，影像對比度在組織諧波成像中的重要性甚至比空間解析度及信雜比更高，如果組織病變並沒有清楚地呈現在影像中，也就是影像對比度不夠高，看起來就會如同一張沒有病變的組織諧波影像。要衡量影像的對比度可以用訊號對干擾比例(SIR)，而超音波中的干擾幾乎都是來自於旁瓣，因此可以從旁瓣強度來評估超音波影像的對比度。 合頻成像雖然可以同時提升諧波影像縱向解析度及信雜比，但對於影像對比度的效果為何目前還是未知，因此本研究的目的為探討各種超音波成像參數對二次諧波在焦距處的旁瓣強度的影響，以得知對組織諧波合頻成像對比度的影響，並找出可以最佳化影像對比度時的超音波參數解。

Since the presentation of Nonlinear Acoustics has been proposed, the Tissue Har-monic Imaging has been widely used in clinical ultrasound diagnosis. The acoustic non-linearity parameter is larger in tissue lesion region than in healthy region and it conse-quently generates more harmonic energy. It can find the pathological changes easier in Tissue Harmonic Imaging than in Fundamental Imaging and hence results in the popular-ity of Tissue Harmonic Imaging. Nevertheless, there is still a lot of things that we can do better. The difficulty we find a pathological change in Tissue Harmonic Imaging is deter-mined by its contrast, spatial resolution, SNR, etc. Increasing pulse duration will improve SNR, but worsen the axial resolution. One way to improve this problem is using the Syn-thetic-Spectrum method. Synthetic-Spectrum Tissue Harmonic Imaging is emitting sev-eral narrowband pulses, taking the advantage of its long pulse duration to have abundant harmonic energy, and hence good SNR. After summing the narrow-band second harmonic signals they generate, it finally obtains a broad-band harmonic signal and then does im-aging. However, the contrast in Tissue Harmonic Imaging is even more important than the spatial resolution and SNR. If the tissue lesions did not appear clearly in the image, in other words, the image contrast is not enough, it will look like a Tissue Harmonic Imaging without lesions. It can use signal-to-interference ratio (SIR) to estimate the contrast of the image. Almost all ultrasound interference comes from sidelobe signal, so we can estimate the contrast by sidelobe level. Although Synthetic-Spectrum Imaging would improve the axial resolution and SNR simultaneously, its effect on the image contrast is still unknown. Therefore, the research purpose is to investigate the effect of the ultrasound imaging parameters to the sidelobe level of second harmonic signal at the focal length, and understand its effect on the image contrast. Finally, it will find the ultrasound parameters which optimize the image contrast.