Title

運用K-space方法於超音波掃掠式系統之流速向量估測

Translated Titles

Ultrasonic Vector Velocity Estimation in Swept-scan Using a K-space Approach

DOI

10.6342/NTU.2005.02506

Authors

鄭耿璽

Key Words

超音波影像 ; 都卜勒超音波 ; 高頻超音波 ; 血流估計 ; 流速向量估計 ; 掃掠式掃描 ; k-space ; 空間頻率 ; Ultrasonic imaging ; Doppler ultrasound ; high frequency ultrasound ; blood velocity estimation ; velocity vector estimation ; swept scan ; k-space ; spatial frequency

PublicationName

臺灣大學電信工程學研究所學位論文

Volume or Term/Year and Month of Publication

2005年

Academic Degree Category

博士

Advisor

李學智

Content Language

英文

Chinese Abstract

頻率大於20MHz的高頻超音波影像系統能夠觀察細微的組織以及量測微弱血流速度,然而由於高頻陣列探頭製作技術的困難,目前高頻系統大都採用單一探頭、機械掃瞄的方式來取得影像資訊,掃瞄的方法主要採用離散步進的方式(step scan),這種方法相當耗時,無法提供即時的血流資訊。另一種稱作掃掠式掃瞄的技術(swept scan)則是讓探頭連續不間斷地移動,因此能大幅縮短成像的時間。雖然掃掠式掃瞄已應用於目前的高頻系統,然而探頭的連續移動對於流速的估計有著不可忽略的影響。為此,本論文主要目的則是從理論面以及實驗探討掃掠式掃瞄對於流速估計的效應,並針對掃掠式高頻系統提出一個新的定量流速估計方法。 本文首先利用二維空間頻譜的概念(稱為k-space)來量化掃掠式掃瞄對於二維流速估測的影響。我們證明了移動物體的空間頻譜等效於其時間頻譜 (亦即,由都卜勒頻率以及RF頻率所構成的二維頻譜)。另外,相較於離散式掃瞄,掃掠式掃瞄則會造成都卜勒頻寬的變動,此頻寬變動導致流速估計的偏差及變異。為了校正此一速度偏差並提高估計的精確度,我們提出一個基於k-space的流速向量估計方法。我們利用模擬以及體外流體實驗來驗證所提出的新方法,此外,我們亦利用45MHz的高頻系統來測量老鼠尾巴內的靜脈流速,實驗結果顯示所提出的流速向量估計方法適用於掃掠式高頻系統,並能有效地降低流速以及血管角度估計的誤差。 本研究之主要貢獻,在於以k-space之方式建構超音波血流分析之理論架構,並完整分析掃掠式掃描對於流速計算之影響,提升超音波小動物影像中定量血流分析之能力。

English Abstract

The rapid developments in high-frequency ultrasound systems (operating at higher than 20 MHz) have allowed visualization of fine tissue structures and assessment of small vessels with slow flows. Due to the lack of high-frequency arrays, however, most current high-frequency systems use mechanically scanned, single-element transducers that are moved through a series of discrete positions. This scan technique, called the step-scan, is relatively time consuming and cannot provide flow information in real-time. An alternative technique, called the swept-scan, involves continuous scanning a transducer and is capable of improving the data acquisition time. Although the swept-scan technique is currently employed in high-frequency ultrasound systems, the continuous transducer movement may have nonnegligible effects on accuracy of velocity estimation. It is therefore the purpose of this thesis to thoroughly investigate such effects, and to further develop a new quantitative flow estimation method. In this thesis, a spatial frequency domain (i.e., k-space) approach is employed to quantify the effects of swept scanning on the spectral-broadening-based vector velocity estimation method. It is shown that the k-space representation of a 2-D moving object is equivalent to a Doppler-RF frequency domain representation, and that transducer movement in the swept-scan technique results in a change in Doppler bandwidth. The spectral broadening caused by swept scan introduces velocity estimation bias and variance that are not present in the step-scan technique. In order to correct such effects and improve velocity estimation accuracy, a robust vector velocity estimation method is developed based on the proposed k-space approach. Both simulations and in vitro experiments were performed to evaluate performance of the proposed vector velocity estimator. Furthermore, in vivo measurements of mouse tail vessels were also conducted using a 45-MHz transducer. The results demonstrate that the proposed vector velocity estimator is feasible in swept scan and can effectively reduce the velocity and angle estimation errors. The main contributions of the thesis include development of a theoretical framework for ultrasonic flow analysis using a k-space approach. Based on this framework, effects of the swept scan on flow estimation were thoroughly investigated, thus making quantitative flow analysis in ultrasonic small animal imaging more feasible.

Topic Category 電機資訊學院 > 電信工程學研究所
工程學 > 電機工程
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