玉城 信治, 土谷 薫, 安井 豊, 泉 並木

Nobuharu TAMAKI, Kaoru TSUCHIYA, Yutaka YASUI, Namiki IZUMI

武蔵野赤十字病院消化器科

Department of Gastroenterology and Hepatology, Musashino Red Cross Hospital

キーワード : acoustic structure quantification (ASQ), liver fibrosis, steatosis

肝実質のスペックルシグナルは内部が均一であると仮定した場合には，エコー信号の確率密度分布（probability density function: PDF）はレイリー分布に近似されることが知られている．肝病態の進展によって線維や脂肪などの構造物が増加するとPDFはレイリー分布から逸脱することとなる．Acoustic structure quantification（ASQ）はこのレイリー分布からの逸脱度を定量化する手法である．すなわちASQは肝実質エコーの粗雑さを定量化する手法であり，肝線維化や肝脂肪化診断における有用性が報告されている．レイリー分布からの逸脱度はROI内の分散の平均とレイリー分布の分散値の比（Cm²）で計算される．肝線維化との関連を検討すると線維化の進行に従いCm²が有意に上昇する相関関係が報告されている．また，肝脂肪化診断において，ASQの手法を用いた肝局所の不均一性を評価するパラメータ（focal disturbance-ratio: FD-ratio）の有用性が報告されており，肝脂肪の増加によりFD-ratioが低下し，脂肪化診断に有用であると報告されている．肝臓の硬さを定量評価するelastographyとは異なり，ASQは肝実質の粗雑さを定量化する手法であり，びまん性肝疾患を定量化する新たな手法として今後さらなる発展を遂げることが期待される．

It is known that if the speckle signals in liver parenchyma are assumed to be uniform, the probability density function (PDF) of echo signals can be approximated using the Rayleigh distribution. The increase of structures such as fibers and fat as liver disease progresses causes the PDF to deviate from the Rayleigh distribution. Acoustic structure quantification (ASQ) is a technique to quantify the degree of deviation from the Rayleigh distribution. In other words, ASQ is a method of quantifying crudeness of liver parenchyma echo scans, and its applications in diagnosing liver fibrosis and liver fatty metamorphosis have been reported. The degree of deviation from the Rayleigh distribution is calculated as the ratio of mean dispersion within the region of interest (ROI) to Rayleigh distribution dispersion values (Cm²). It has been reported that there is a correlation between Cm² values and liver fibrosis, with the Cm² values rising significantly as fibrosis progresses. Furthermore, it has been reported that a parameter, focal disturbance-ratio (FD-ratio), which assesses the non-uniformity of local areas in the liver using ASQ, is useful when diagnosing liver fatty metamorphosis, under the condition of which the FD-ratio decreases as liver fat increases. In contrast to elastography, which is used to quantitatively evaluate the stiffness of the liver, ASQ is a means of quantifying the crudeness of liver parenchyma, and its further development as a new method for quantifying diffuse liver disease is anticipated.