Attempt to Predict the Performance of an Image-Forming Ultrasonic Microscope

Abstract

are encountered in producing this image are discussed with emphasis on diffraction effects which cause the boundaries of an ultrasonic image of low contrast to be uncertain in some parts. Suggestions are made to overcome or bypass some of these problems. Practical details and limitations for work with biological materials motivated the choice of an operating frequency at 9 MC. Experimental work at this frequency is described in the testing of an ultrasonic lens system and the diffraction effects are analyzed. A conclusion is drawn that it is possible to produce, with this system, an image of about 635 square mmconsisting of a maximum of 18 X 18 elements, providing there are no steep gradations of intensity in the picture. It is suggested that the contrast can be increased when necessary by use of the principle of phase microscopy. Greater resolution can be obtained by altering the diffraction pattern using various techniques described or by increasing the signal intensity level range through the use of logarithmic amplifiers. HE LITERATURE contains a number of papers showing ultrasonic real images formed by sonic optical systems and transduced into visible form.’-‘ In practically every case the pictures are of high-contrast objects such as metal inserts in plastic, keys, wires, holes in metal, etc. This type of picture permits the adjustment of apparatus in a mode which, by answering only to high contrast suppresses any adventitious radiation of intermediate amplitudes which would otherwise “spoil” the picture. In the experience of the author such unwanted ultrasonic radintion very frequently occurs when an ultrasonic image is formed. It is presumably due to complex diffraction effects from edges, etc., and causes the boundaries of an ultrasonic image to be uncertain in some parts. There appears to be no doubt that the reason pictures bearing good detail have not been published despite the existence of fairly good ultrasonic image transducers lies in the existence of diffraction effects which tend to confuse the surface of any image not of very high contrast. It is the intention of the present work to discuss the possibilities that actually do exist for obtaining ultrasonic images of some resolution.