Fatty liver is a disease caused by the excess accumulation of fat in the human liver. The early diagnosis of fatty liver is very important, because fatty liver is the major marker linked to metabolic syndrome. We already proposed the ultrasonic velocity change imaging method to diagnose fatty liver by using the fact that the temperature dependence of ultrasonic velocity is different in water and in fat. For the diagonosis of a fatty liver stage, we attempted a feasibility study of the quantitative assessment of the fat content in the human liver using our ultrasonic velocity change imaging method. Experimental results showed that the fat content in the tissue mimic phantom containing lard was determined by its ultrasonic velocity change in the flat temperature region formed by a circular warming ultrasonic transducer with an acoustic lens having an appropriate focal length. By considering the results of our simulation using a thermal diffusion equation, we determined whether this method could be applied to fatty liver assessment under the condition that the tissue had the thermal relaxation effect caused by blood flow.

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The instability of vessel plaque is thought to be associated with the size and distribution of the lipid core. Ultrasonic B-mode imaging (ultrasonic amplitude imaging) can visualise plaques, but it is unable to provide information about their composition. Based on the fact that the temperature dependence of ultrasonic velocity differs considerably between water and fat, the motivation for this reported work was that it was thought that ultrasonic velocity-change imaging could help characterise biological tissues. This method should be able to detect unstable plaque because it is lipid-rich. A blood vessel phantom of agar base material having similar ultrasonic properties to human soft tissue was made. A small piece of fat was inserted into the blood vessel phantom. Water (instead of blood) was passed through the model vessel using a tube pump. Ultrasonic velocity-change images were constructed from echo pulse waveforms obtained before and after warming using an ultrasonic transducer. Lipid-rich areas in the blood vessel phantom were obvious in the ultrasonic velocity-change image, but not in ultrasonic B-mode images. These results indicate that ultrasonic velocity-change imaging is able to characterise carotid artery plaque.Introduction: Detachment of carotid artery plaque can lead to serious problems, such as brain infarction and cardiac infarction [1]. The instability of blood vessel plaque is thought to be related to the size and distribution of the lipid core [2]. If carotid artery plaque is detected at an early stage, lifestyle modification, exercise therapy or drug administration can be used to treat the plaque. However, the main diagnostic procedure for carotid artery plaque is currently ultrasonic diagnosis, but ultrasonic diagnosis has difficulty in detecting the plaque at an early stage. The reason is that ultrasonic morphological information needs to indicate ischemic injury caused by the narrowing of blood vessels. Thus, early detection of carotid artery plaque has many benefits, and a diagnostic approach that can detect plaque at an early stage is required. As a result of advances in ultrasonic echo equipment in recent years, it has become possible to obtain high-resolution ultrasonic images of the carotid artery, such as B-mode images [3,4]. Although an ultrasonic B-mode image displays the existence of plaque, it does not give any information about the composition of the plaque.A method of characterising blood vessels based on their elastic modulus has been investigated for examining carotid artery plaque. Ingenious methods have been employed to determine the thickness of the blood vessel wall with high accuracy in pulsing motion [5,6].To identify lipid-rich plaque, the near-infrared absorption spectra of human aortic specimens have been measured [7]. Information about the chemical composition of the tissue is obtained from the near-infrared absorption spectra. However, optical methods have difficulty in characterising the coronary plaque from outside of the body because the biological tissue is a s...

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