Nephrosonography: Ultrasound Differentiation of Renal Masses

Abstract

One of the more common yet challenging problems in diagnostic radiology is the differentiation of mass lesions of the kidneys. In order to establish a correct diagnosis, several elaborate, time consuming, costly, and potentially dangerous procedures have been utilized, including nephrotomography (1), retroperitoneal and perirenal pneumography (2), and arteriography (3). During the past two years our department has used ultrasonic changes in conjunction with routine intravenous urography in the study of 100 patients with proved renal masses. Principles When ultrasonic waves traverse tissues a portion of each wave is reflected at each interface depending upon the specific acoustic impedance of the tissues. The differences in the density and elasticity of the tissues at an interface determines the amount of reflected waves or echoes. Generally, the maximum reflection and the strongest echo complex will be found at a fluid-solid interface, which is the situation that occurs in the presence of a renal cyst. In contrast, solid masses (solid-solid interface) will produce a weaker echo complex. If the ultrasound beam is amplified, additional echoes are produced in a solid mass, which contains multiple interfaces, while cysts will remain echo-free because there are no reflecting interfaces within the fluid (Fig. 1). Although much experimental work has been accomplished with complex cross-section technics of ultrasound (4, 5), these have yet to be as widely accepted as the simple A-mode ultrasound procedure. In A-mode (amplitude modulation) ultrasonography the echoes (reflected sound) are displayed on an oscilloscopic screen as vertical deflections, with the height representing the strength of the echo. The horizontal width separating the echoes represents the distance, measured in centimeters, between the reflecting interface and the transducer. Technic Instrumentation: A commercially available ultrasonoscope is set to provide uniform increasing amplification of the echoes from the crystal surface of a 2.25 megacycle transducer to a depth of 15 cm, and the display on the screen is recorded with self-developing film. Use of the proper settings, which have been previously described (6), is of the utmost importance. In principle, low and high amplification settings of the ultrasound beam are used. The low power settings are adjusted so that only the echoes of the front and back walls of the mass are obtained. The power is then increased to a point where the echo patterns of solids will fill in, while fluid-filled structures will remain echo free (Fig. 1). Localization of Masses: Renal masses are usually detected on routine intravenous urography, and the transducer is positioned in accordance with the x-ray localization. Tracings are obtained at low and high power, a lead marker is placed on the skin at the point where the echo pattern is recorded, and a roentgenogram is exposed for the purpose of documentation (Fig. 2).