a birthday tribute to dr r.w.b. stephens
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A birthday tribute to Dr R.W.B. Stephens
The thirteenth of October saw the 75th birthday of a man whose outstanding contribution to Ultrasonics is known throughout the world-Raymond Stephens.
He was born in Tunbridge Wells, Kent and later attended Emanuel School. Then in 1921 he went to Imperial College, London where, after two years of study, he qualified as an Associate of the Royal College of Science. A year later he received his BSc in Physics.
His relationship with Imperial College was long and successful, and it was there that his interest in acoustics grew and provided, with his Acoustics Research Group, the encouragement for so many others to enter the field. It is not only his academic skills which have enabled him to foster so much enthusiasm for his subject, but also his warm personality and the personal interest shown to his students.
Dr Stephens achievements have not gone unrecognized, and among the many honours he has received throughout the world are the Rayleigh Gold Medal from the (then) British Acoustical Society, the Gold Medal of the Acoustical Society of America, the Silver Medal of the Groupement des Acousticiens de Langue Fransaise, France, and honorary membership of many acoustical societies.
He was one of the advisory editors of Ultrasonics at its inception in 1963 and remains an invaluable member of the editorial board to this day.
Ultrasonics would like to thank Dr Stephens for his great help in the running of the journal and offer him its warmest congratulations, and looks forward to many more years of fruitful co-operation.
In a recent paper in your Journal the authors have expressed opinions relating to the use of compression amplifiers in ultrasonic diagnostic equipment. They quote that . . . . the inclusion of a compression amplifier in the receiver may give rise to considerable distortion of the effective directivity characteristic of the transducer and hence its lateral resolution. One presumes that their intention is to emphasise that displaying echo signal strength on a logarithmic scale (this being an idealised definition of compression) tends visually to overemphasise the spatial distribution of the stronger echoes. This is surely not a distortion of the directivity characteristic but merely its representation in other than a linear scale. To suggest that this alters the lateral resolution of the transducer is not correct. If one defines resolution by a fixed criterion, say the full width at half maximum, then altering the mode of display will change the appearance but not the parametric measurement of resolution. Although possibly unintentional, their representation of two iso-echo amplitude plots, with a linear and compression amplifier alternatively incorporated in their system, is confusing since the decibel representations are specific to the output from each system and cannot be directly compared.
There seems to be a repeated ambiguity in the paper especially where they discuss beneficial effects of compression. They claim that . . . . the fall off in echo amplitude with distance into the far-field is less than in the linear case, thus the quality of the image will be less critically dependent on the precise value of the time-gain compensation. Experimentally the fall off in echo amplitude doesnt change; only its mode of display determines its appearance. It should also be considered that the criterion, for correct time-gain-compensation, employed by most sonographers is that the low-level grey-scale echoes, such as seen in normal liver parenchyma, should appear uniform in intensity throughout the scan. As mentioned by the uuthors, these small echoes are usually within the linear regions of typical compression curves (see Fig. 1) and as such are noticably effected by changes in time-gain-compensation.
Finally, one must query the statement that . . . . the displayed echo emplitude distribution in the near-field is substantially less non-uniform than in the linear case, which may partly explain why clinical interpretation of images is possible nearer the transducer than the linear beam plot would suggest. This infers that compression may permit better near-field diagnosis. It should be clarified that the
ULTRASONICS. NOVEMBER 1977 265