A-Mode Ultrasound Differentiation of Soft-Tissue Masses

Abstract

Roentgenography is currently the principal diagnostic method available for the detection of suspected masses within the thorax, abdomen, and retroperitoneal space. Differentiation of these masses, however, is frequently difficult, despite the roentgen findings and the associated clinical data. A-mode ultrasound is a relatively simple technic that can differentiate cystic from solid masses. By utilizing this information in conjunction with the x-ray studies, the radiologist can offer a more accurate differential diagnosis. A-mode has been used successfully for differentiating breast lesions (1) and localizing liver cysts, which are referred to as “fluid flat segments” (2). The method has also been employed to study mediastinal and abdominal masses (3, 4) and to differentiate echinococcal cysts of the lung from solid tumors (5). In this country, ultrasound research has been directed mainly toward the more complex type of ultrasound, known as B-scan, in which cross-section images of organs and tumors are produced (6, 7), and the simple and established A-mode technic for the evaluation of soft-tissue masses has been neglected. To our knowledge, there have been no reports on its use in conjunction with x-ray studies. This report reflects our experience with 100 patients who have had masses in the mediastinum, abdomen, and retroperitoneal region, all of which were proved either by arteriography, surgery, or autopsy. Technic The mass is localized prior to ultrasound study by palpation or, in instances where this cannot be done, by fluoroscopy. If fluoroscopy is not available the roentgenogram is used as a guide to determine the site of study. At each point of study patterns are obtained with four different gain settings on our instrument. For documentation a lead marker is subsequently placed where a reading is obtained and an x-ray localization film exposed. If the transducer is not directly over the mass, the study is repeated. The ultrasonoscope is so set that there is approximately uniform increasing amplification from the crystal surface to a depth of 15 cm. An attenuator is placed between the transducer and receiver, and the lowest and highest gain settings are defined in terms of the number of decibels of attenuation necessary to reduce to 1 cm in height those echoes produced by coupling the transducer to a Lucite block 6.3 cm wide by 13.9 cm long (thickness can be variable). These echoes are displayed on the oscilloscope at 3.6 and 8 cm depths, and at the low-gain setting 24 decibels of attenuation are necessary to produce the 1 cm echo at 3.6 cm depth while no attenuation is required to obtain the same 1 cm echo at 8 cm depth. At the high-gain settings the near echo (3.6 cm) requires 38 decibels of attenuation and the far echo (8 cm) 14 decibels of attenuation. Large amounts of reject (valley clipping) are used. Damping resistance is 1000 ohms.