脳の超音波吸収についての実験—連続測定による知見

ULTRASONIC ABSORPTION OF THE BRAIN:Observation by the continual measurement of the absorption coefficient

I．はじめに

　生体内に入射した超音波パルスは，生体内の数多くの構造物の雑多な界面における反射によつて，また種々の構造物内での超音波吸収によつてもその勢力は減衰する。この現象は生体組織の状態変化に対応する音響学的性状の変化として，すなわち，超音波吸収値，すなわち超音波吸収係数（音響学上の用語）の変動として把握できる。しかし上述のような超音波吸収係数の変化が果してあるのかどうか，生体について明らかな成績はない。

　超音波の医学への応用研究の当初より生体組織内での超音波の減衰について論ぜられ，研究発表も数多く現われている（和賀井^1），犬上^2）3），丸田^4）5）6），永井^8），岩田^7），岡^{11）〜16）}ら多数）。しかし従来より超音波減衰なる語で表現されている，各研究者の抱く概念が必ずしも明確でないことは否定できない。故にこの現象の背景をなすものに対する考察に誤りなきを期し難い。したがつて実験的に検討すべき時には，その測定方法を音響学的に正確なものにすべきは当然である。そこで私どもは超音波吸収の正確な測定によつて，生体についての一定の実験条件の設定と相まつて，その測定値の音響学的な意味づけを明らかにし，かつ生体組織自体の音響学的性状を明らかにすることを目的とした。

This study intends to measure the exact absorption coefficient of the brain, as well as its fluctuation in the course of head injury and of postural changes. Recent reports may not seem to have been done by the exact ultrasonic method. The measuring device here presented is so correctly arranged that the fluctuation of ultrasonic absorption coefficient of the brain tested represents their minute sonic conditions. The ultrasonic pulses of 970 kc (kilo-cycles) are sent through the brain widely exposed not to disturbed paths of transmitted pulses which are measured with their sonic power.

Results are summarized as follows ;

1. The absorption coefficient increases in several to 25%, i. e., in less than+0.1 db/cm, within about 15 minutes after death, and the increase is not due to the fall of body temperature.

2. Brains after cerebral concussion show the defi-nite decrease of absorption coefficient in about 0.1 db/cm within an hour and show marked fluorescein accumulation in the brain tissue which indicates to be edematous and possibly represents the increase not of blood volume but of water content in the brain.

3. There is no marked change in absorption coeffi-cient during the forced artificial respiration. If there may be some changes of absorption coefficient, they may be due to the displacement or shifting of brains. 4. No distinct fluctuation can be observed in the absorption coefficient during the course of the inha-lation of amyl nitrate, as the fluctuated value observ-ed is less than 0.002 db/cm.

5. Very soon after setting the head downwards the absorption coefficient decreases in less than 5% (within 0.05 db/cm), and the increase of within 2% is observed after setting the head upwards. These changes may be possibly due to the displace-ment or shifting of brains in the cranial cavity.