椎名 毅¹, 新田 尚隆², 山川 誠¹, 近藤 健悟¹, 千田 彰一³, 舛形 尚⁴

Tsuyoshi SHIINA¹, Naotaka NITTA², Makoto YAMAKAWA¹, Kengo KONDO¹, Shoichi SENDA³, Hisashi MASUGATA⁴

¹筑波大学大学院システム情報工学研究科, ²独立法人産業技術総合研究所人間福祉医工学研究部門医用計測グループ, ³香川大学医学部附属病院総合診療部, ⁴坂出市立病院循環器内科

¹Graduate School of Systems and Information Engineering, University of Tsukuba, ²Instituute for Human Science and Biomedical Engineering, National Institute of Advanced Industrial Science and Technology, ³Faculty of Medicine, Kagawa University, ⁴Sakaide City Hospital

キーワード : Asynergy, myocardial local contraction, strain tensor, three-dimensional myocardial strain imaging, two-dimensional array probe

心筋梗塞などの心筋虚血性疾患では, 早期に心筋壁運動に異常が現れるため, 心筋収縮能を客観的, 定量的に把握することが, これらの心疾患の病態評価と適切な治療の上で重要となる. このため従来から超音波像により壁運動異常を診断する様々な手法が用いられている. その多くは, 実時間で空間分布が得られる利点から組織ドプラ法や, 心筋ストレイン法のようにドプラ法を基本にしたものであるが, これらは超音波ビーム方向成分のみの動きの検出であるため, ドプラ角が90°に近い領域では推定不能となるなど, 局所的な異常を適切に把握することは難しい. 我々は, 従来手法の問題点を克服して心筋壁運動の客観的, 定量的な診断を可能とすることを目的に, 二次元アレイプローブを用いて, 心筋のような変動範囲が大きい場合についても各部の三次元的変位ベクトルを高速且つ高精度に計測し, ひずみテンソルに基づくパラメータを導入して, 局所収縮率分布を画像化する三次元心筋ストレインイメージング法を提案した. 本論文では, その原理を概説し, また梗塞心筋モデルを用いたシミュレーションにより本手法の有効性を検証した. また, 現状の超音波診断システムにも適用が容易な形として一次元アレイプローブ用に簡便化する方法を示した. さらに, その結果をもとに基礎実験システムを構成し, 心筋ファントムの計測を行った. これらにより提案手法の有効性の検証と, 実用化の方向を示した.

Decreases in myocardial motion caused by changes in tissue stiffness often appear in the early stage of ischemic heart disease. Since the myocardium exhibits complex 3–D motion, 3–D assessment of the stiffness distribution is required for accurate diagnosis. However, conventional tissue Doppler and strain-rate imaging techniques cannot meet the above requirement completely because they are angle-dependent, use in-plane (2–D) processing, and suffer from aliasing in the case of large myocardial motion. In order to overcome these problems, we propose novel methods to track the 3–D motion by using a 2–D phased array with a small aperture and to assess myocardial malfunction based on full strain tensors obtained by 3–D motion analysis. As a new method of 3–Dmyocardial motion tracking, we incorporated our phase-gradient method, which is capable of real-time 3–D displacement vector measurement using a 2–D phased array, and our combined autocorrelation method, which accurately measures large phase shifts at each element without aliasing. As a new method of assessing a myocardial ischemic region, the full strain tensor invariant obtained by the measured 3–D vectors is visualized as a 3–D myocardial strain image. We evaluated the feasibility of the proposed methods by numerically simulating left ventricle short-axis imaging of a 3–D elliptic myocardial model including infarction. RF signals received at each element on the 2–D array probe were faithfully simulated. When the simulated echoes were processed by the proposed methods, the invariant image obtained by the full strain tensor clearly depicted the hard infarction area where conventional imaging could not. The proposed methods were also modified for systems with an ordinary 1–D array probe. Finally, a phantom experiment was conducted by using the basic system based on the 1–D array probe. These results validated the easibility of the proposed methods.