亀田 徹

Toru KAMEDA

済生会宇都宮病院超音波診断科

Department of Ultrasound Medicine, Saiseikai Utsunomiya Hospital

キーワード : lung ultrasound, B-line, vertical artifact, simple model, acoustic trap

肺超音波検査はアーチファクトであるBラインが広く利用されている．Bラインの物理的成因や人体でのメカニズムについては十分に明らかにされていない．Bラインの形状は超音波診断装置の設定に影響を受けるが，臨床で考慮されないことがあるのは憂慮すべきである．近年はBラインの定義への懸念が示され，長短を問わず超音波ビーム方向に伸びるアーチファクトを「vertical artifact」と呼称することが提案されている．我々は上記課題解決に向けて臨床と基礎研究に取り組んできた．臨床では空間コンパウンドイメージングとフォーカスの位置がBラインの形状に大きく影響を与えることを報告した．また基礎研究では1本のvertical artifactを生じる胸壁・肺のシンプルモデルを考案した．このシンプルモデルを使って一連の実験を行い，発生源の接地部サイズと，発生源の高さ（ボリューム）がvertical artifactの発生，強度や長さの重要な因子であることを明らかにした．またvertical artifact発生源内で減衰がより大きければ，vertical artifactの強度はより低く，vertical artifactはより減衰する（短くなる）ことを明らかにした．これらの実験から，vertical artifactの物理的成因は多重反射であるという仮説を支持する結果を得た．また人体でvertical artifactが生じるメカニズムを推論するための一助になった．さらにvertical artifactの性状や中心周波数との関連を分析すれば，疾患の鑑別に利用できる可能性が示唆された．

B-lines, artifacts in lung ultrasound, have been widely utilized. The physical basis of B-lines and the mechanism in the human body have not yet been fully elucidated. The configuration of B-lines is affected by settings of ultrasound machines; however, the fact that the settings are not considered in some clinical situations is a serious issue. There have been concerns about the current definition of B-lines; therefore, it has been proposed that the artifacts extending toward ultrasound beams be called “vertical artifacts” regardless of the length. Our group has been engaged in clinical and basic research toward solving these issues. We reported that the configuration of B-lines is strongly affected by the spatial compound imaging and the focal point in clinical ultrasound. In basic research, we devised simple models of the chest wall and lung that generate a single vertical artifact. We conducted a series of experiments using the simple models and revealed that the size of contact and height (volume) of the sources are key factors in the generation, echo intensity, and length of vertical artifacts. And we also revealed that the larger the attenuation inside the sources of vertical artifacts is, the lower the intensity of vertical artifacts is and the more vertical artifacts attenuate. The results obtained from these experiments support the hypothesis that the physical basis of vertical artifacts is multiple reverberations. They help us reason the mechanism of the generation of vertical artifacts in the human body. Furthermore, they indicate that analyses of the properties of vertical artifacts and their relation to central frequencies of the ultrasound beam may be useful for differentiating diseases.