●ABSTRACT
Tetsuya Kawagishi
Ultrasound Systems Development Dept., Medical Systems
Research & Development Center, Medical Systems Company,
Toshiba Corporation, Tochigi, Japan
Technical Description of 1.5-Harmonic Imaging, an Effective
Technique for Contrast-Enhanced Ultrasound Diagnosis
When a high-power ultrasound pulse is applied to an ultrasound
contrast agent consisting of air-filled microbubbles,
the microbubbles are destroyed, resulting in the generation
of non-linear echoes with a uniform distribution along
the carrier frequency axis. In the body, the application
of a high-power ultrasound pulse produces a fundamental
echo and harmonic echoes that arise from the nonlinear
characteristics of tissues. Unlike the harmonics generated
by a contrast agent, the harmonics generated by the nonlinear
propagation of ultrasound signals in tissues consist of
only frequency components that are integral multiples
of the fundamental.
The 1.5-harmonic imaging technique extracts and visualizes
a frequency band whose center frequency is higher than
the fundamental by a factor of 3/2. This frequency band
is intermediate between the fundamental and the second
harmonics and includes only bubble echoes and no tissue
echoes. This imaging technique reduces tissue echoes without
causing motion artifacts such as those that are seen in
the pseudo-Doppler imaging technique. Contrast between
tissues and bubbles is improved by 20 dB or more compared
with the second-harmonic imaging technique. The waveform
control feature of the Aplio system suppresses leakage
of the fundamental, and digital signal processing ensures
high performance in the extraction of bubble echoes from
the intermediate frequency band.
●はじめに
静脈投与型の造影剤が開発され，コントラストエコーが研究から臨床に広まりつつある。コントラストエコーの造影剤は気泡であり，肺を通って全身に行き渡るため，静脈投与された造影剤のうち染影に寄与する気泡のサイズは数ミクロン以下である（マイクロバブル）。超音波造影剤（以下，バブル）は，生体組織に比較し強い非線形性を示し，高音圧（〜MPa,
MI値〜1.0）の超音波パルス照射下では破壊されながら強い非線形エコーを返すことが知られている。
経胸壁の循環器のコントラストエコーでは，これまで図1に示したような問題点が議論されてきた。肋骨や肺の影響，心臓の動きなどに起因するものである。「1.5ハーモニックイメージング」1)（以下，1.5
HI）は，このような問題を背景として開発されたものであり，以下に原理と特徴を説明する。
References
1) 川岸哲也，嶺喜隆，神山直久，日本音響学会講演論文集(2002.3),
p1165
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