Ultrasonic Diagnostics in Medicine: Physical Foundations

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Wild was able to obtain a fine image of soft tissue structure using reflection method. He reported a range of possibilities for diagnostic ultrasound. However, he failed in clinical applications because of large ultrasound attenuation, which limited the examination of tissue to a depth of 1. After about a year research effort, diagnostic ultrasound has seen remarkable development and become an indispensable tool in clinical medicine.

In , I planned and started my research on diagnostic ultrasound by applying the reflection echo method while studying in the Department of Neurosurgery, Juntendo University School of Medicine, Tokyo, Japan. I conducted it in collaboration with engineers of Japan Radio Co, Tokyo.

This work performed from —56 marked the dawn of diagnostic ultrasound research in Japan. As there were no precedent trials or literature on diagnostic ultrasound research in Japan at that time, I had to begin by proving whether ultrasonic waves really transmitted within the human body; and if confirmed, whether echoes could really be detected from the inside of the human body. Also, whether ultrasonic waves are really safe for the human body. To answer these fundamental questions, I started by performing experiments. At first, I set out to confirm the possibility of detecting echoes from inside a brain specimen using a prototype ultrasonic flow detector with a frequency of 1 to 10 MHz.

I designed a special examination method that was performed in a water vessel Fig. This method has since been used widely as an immersion water pass ultrasonic examination method. I succeeded in detecting various types of echoes from the ventricle wall, brain hemorrhage, and tumor tissue. The ultrasonic flaw detector used in this pilot study was an A mode display type that displayed ultrasonic echoes as changes in amplitude on the time axis of a cathode ray tube CRT.

Since Juntendo University was, as a newly established private school, poor in the period just after the war, I had no suitable laboratory or research funding at my disposal. Therefore, I experienced many difficulties in advancing my research. Immersion ultrasonic examination of brain specimen and A mode echogram of brain tumor, To confirm these results theoretically and calculate a reflection coefficient, I tried to measure the acoustic impedance values of various brain tissues based on their propagation velocity and density.

However, as there were no methods or devices for taking these measurements at that time, I had to design my own measurement method and device. I succeeded in measuring propagation velocity in even smaller brain specimens by using a specially designed acoustic masking method. I measured brain tissue density using a small brain specimen and employing a sulfuric acid copper method, which was usually used for measuring blood density at that time.

The results of these measurements showed that the propagation velocity of brain tissue was almost the same as that of water, that the tissue density was similar to that of human blood. Also, that there was a small difference in acoustic impedance values between normal and pathological brain tissues.

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Through these measurements and calculations, I was able to also theoretically confirm the possibility of detecting echoes from inside brain tissue. Moreover, the slightly different propagation velocities between normal and pathological tissues allow ultrasound to detect cancer and other diseases. It was there that I first came to know of several international pioneers in this field whose names are mentioned above , as it was difficult to get foreign literature in Japan at that time.

Subsequently, I measured the ultrasonic attenuation of brain tissue in relation to the ultrasound frequency, and found a very interesting ultrasonic attenuation response between normal and pathological brain tissues, which was effective in ultrasonic differentiation between brain disease, cancer, and others later. To advance the experiment more quickly, I wanted to use of a phantom.

This phantom was very useful in conducting a variety of experiments Fig. On the other hand, to make the immersion water pass technique more convenient and to drive the experiments more rapidly, I developed a special type of transducer with a water column attachment. At first, variously shaped glass columns were designed and undesirable echoes reflected from the water column wall were investigated. Finally, I developed a new transducer with an ideal water column Fig. Using it made the immersion method much easier and more effective in conducting experiments, even clinical studies, compared to the previous ones performed in a water vessel.

As the above results were all obtained using a formalin fixed brain specimen, experiments using fresh brain tissue were necessary. I used a fresh cow brain just after butchering, and was able to confirm the same results as those obtained with the fixed brain specimen. Special ultrasonic transducer with water column attachment and its application to the examination of brain specimen, The first stage prototype immersion linear scan ultrasonotomography instrument and B mode image of phantom, Based on the results of these fundamental experiments, it seemed possible to detect echoes from inside the brain, including brain tumors, when ultrasonic examination is performed directly on the dura mata during open-skull brain surgery.

However, before conducting such a trial, the safety of ultrasonic pulsed waves on a living human brain had to be first confirmed.

Ultrasonic Diagnostics in Medicine: Physical Foundations

As there were no reports on the biological and pathological effects of ultrasonic pulsed wave on the brain, I had to resolve the problem by depending on the results of properly conducted biological experiments. First, I confirmed that ultrasonic pulsed waves did not cause a harmful effect on the human blood based on the results of a hemorrhytic experiment and subsequent animal experiments that I conducted. A living rabbit brain was radiated directly by an ultrasonic pulsed wave through a small hole in its skull using a special transducer with a metal attachment developed for this experiment Fig.

During ten hours of continuous ultrasonic pulsed wave radiation directly into the living rabbit brain, physiological changes such as blood pressure and breathing were continuously monitored. I, then, investigated histological changes in the radiated brain in laps of time until three month after the ultrasonic radiation, but detected no changes in the brain tissue.

Based on these experimental results on the safety of radiating ultrasonic pulsed waves into a living brain, I tried applying this technique clinically during brain surgery and succeeded in detecting echoes from a brain tumor on the dura mata and in diagnosing its size and localization. However, the ideal clinical application was to detect echoes of brain diseases through the skull before surgery.

After various investigations on the characteristics of the ceramic transducer, I succeeded in detecting intracranial echoes inside the brain, overcoming the barrier of the skull Fig. At the same time, I discovered that the midline echo originating from the third ventricle wall was detected at the temporal part of the head, which shift was a very important sign for diagnosing brain diseases. Another interesting phenomenon I discovered was that the midline echo showed a pulsating movement.

Concurrently, I applied the A mode technique to the analysis and diagnosis of soft tissue structures such as breast, stomach and lung cancers, gallstones, and others. Detection of brain tumor echo through skull by high-sensitive ceramic transducer and A mode echogram, The above-mentioned results were all obtained by applying a one-dimensional A mode display technique. However, this technique had various limitations such as poor reproducibility and difficulty in confirming the shape of the reflective target, among others. I took up this challenge by developing a two-dimensional display of echoes by applying an ultrasonic scanning technique B mode to overcome the shortcomings of the A mode technique.

With the B mode technique, ultrasonic echoes were displayed as changes in amplitude on the CRT time axis as bright spots brightness modulation. This technique was usually used to display cross-sectional images by applying a scanning technique with an ultrasonic beam. At first, many engineers did not agree with my idea that a B mode technique could be used to scan the human body because it consisted of numerous echoes.

Based, however, on the results of various fundamental experiments carried out using manual and other scanning techniques, I was able to develop the first immersion electrically automatic linear scanning instrument for displaying a cross-sectional image. I did this in by applying the same principles as used in radar and sonar Fig.

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Following these successful results in displaying a cross-sectional image of a phantom, I attempted to display a cross-sectional image of a human head and brain. For this purpose, the head was inserted into a water tank, covered by a rubber membrane, and bound tightly with a rubber cord to make it water tight. It was interesting that the same techniques were attempted by several researchers in other countries approximately 15 years after these trials.

The first-stage ultrasonotomography instrument Fig. I also applied this scanning instrument to the analysis of soft tissue structures such as the breast, abdominal organs and extremities based on their characteristic ultrasound properties. I developed a water cup method for the breast Fig. Although I succeeded in displaying various cross-sectional images of normal and pathological soft tissue organs, these results were still a long way from allowing the technique to be applied to clinical applications because of its poor image quality.

I also attempted to continuously record moving echoes such as the heart valve and bowl peristalsis by applying a time-position-indication technique TM mode.

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Ultrasono-tomography of the breast by immersion water cup method and ultrasonic image of breast tumor, These results obtained through research carried out at Juntendo University School of Medicine from —56 marked the dawn of diagnostic ultrasound research in Japan. Although the reporting of such results at meetings of various medical societies was not practiced at that time in Japan, I was able to continuously report all of my findings mainly at meetings of the Japan Acoustical Society, which specializes in scientific and engineering systems.

Under such stringent circumstances, I was very lucky to have been given a good opportunity to present the results of my early research carried out from —56 at the 2 nd International Congress on Acoustics ICA held at MIT, Cambridge, Mass. USA in June My invitation came from R. Bolt, who was the Congress president and one of the pioneers of diagnostic ultrasound research. The above-description is of the progress I made in research carried out mainly at its early stage from — Following that, I engaged in research to develop diagnostic ultrasound for application to clinical use in cooperation with Japan Radio Co.

I took up this challenge despite the fact that many overseas pioneers had abandoned the idea. I will briefly outline chronologically my research carried out after With regard to immersion automatic scanning instruments, I developed a second-stage ultrasono-tomography instrument Fig. Subsequently, I developed a third-stage instrument aimed at clinical application Fig. Improving these techniques, I was able to make these instruments suitable for clinical use in diagnosing breast and liver diseases Fig.