Pergamon Ultrasound in Med. & Biol.. Vol. 21, No. 6, pp. 745-751, 1995 Copyright 0 I995 Elsevier Science Ltd Printed in the USA. All rights reserved 0301.5629/95 $9.50 + .OO 0301~5629( 95)00020-8 *Original Contribution A NOVEL ULTRASONIC TECHNIQUE FOR DIFFERENTIATING CYSTS FROM SOLID LESIONS: PRELIMINARY RESULTS IN THE BREAST KATHRYN R. NIGHTINGALE, + PHYLLIS J. KORNGUTH,~ WILLIAM F. WALKER, + BRUCE A. MCDERMOTT+ * and GREGG E. TRAHEY tS *Department of Biomedical Engineering, Duke University, Durham, NC, USA; $Department of Radiology, Duke University Medical Center, Durham, NC, USA; and *Siemens Medical Systems, Ultrasound Group, Issaquah, WA, USA (Received 17 October 1994; in jnal form 29 December 1994)) Abstract-The feasibility of a new ultrasonic technique to distinguish cysts from solid lesions is explored. High intensity pulses are used to induce acoustic streaming in cyst fluid, and this motion is detected using Doppler techniques. Acoustic streaming cannot be generated in solid lesions, therefore, its detection would indicate a cyst. In six of seven breast cysts motion was clearly generated and detected in viva. Ultrasonic pulses with intensities up to 4.4 W cm-’ (ZSpb in water) were focused on the cysts for 10 s. Lesion diameters ranged from 0.6 to 2.5 cm; induced flow velocities were less than 4.0 cm s-’ . Key Words: Radiation force, Acoustic streaming, Cyst, Breast. INTRODUCTION The primary use of ultrasound in breast imaging is to evaluate mammographically detected or palpable lesions to differentiate between cysts and solid masses. While complex cysts and solid lesions require further investigation, the diagnosis of simple cysts by ultra- sound is clinically significant since it eliminates the need for further diagnostic measures including aspira- tion, biopsy or follow-up (Hilton et al. 1986). Ultra- sonic cyst diagnosis is presently performed by as- sessing several characteristic features of simple cysts, such as round to oval shape, well-circumscribed mar- gins, increased through-transmission of sound and an- echoic interiors (Bassett and Kimme-Smith 1991) . However, simple breast cysts can exhibit internal echoes. These low- to medium-level internal echoes have been attributed to solid material within the cyst fluid such as cholesterol crystals, proteinaceous parti- cles, fat globules or cellular debris (Bassett and Kimme-Smith 199 1; Khaleghian 1993; Stavros and Dennis 1993). As the resolution and dynamic range of ultrasound machines improve, more of the weak echoes from these inhomogeneities will become appar- Address correspondence to: Kathryn R. Nightingale, Depti- ment of Biomedical Engineering, Duke University, P.O. Box 90282, Durham, NC 27708-0282, USA. ent in cysts, causing them to appear hypoechoic rather than anechoic, thus complicating the ultrasonic diagno- sis of simple cysts. In this article, results from a pilot study performed to investigate a new ultrasonic technique for differenti- ating cystic from solid lesions are presented. This tech- nique takes advantage of two phenomena associated with acoustic wave propagation: radiation force and acoustic streaming. The radiation force phenomenon is associated with all forms of wave motion. It is caused by a transfer of momentum from a wave to absorbing and reflecting obstacles in its path (Torr 1984). When a wave propagates through a fluid, this momentum transfer generates a bulk steady motion of the fluid in the direction of wave propagation, called acoustic streaming (Wu and Du 1993). Starritt et al. ( 1989) performed a study of acoustic streaming gener- ated by pulsed diagnostic ultrasound beams in water. They observed velocities up to 14 cm s -I, with average generation times of 500 ms. Dymling et al. ( 1991) presented a method for fluid property measurement that used ultrasound to generate acoustic streaming, and then evaluated variations in streaming velocity to obtain information about the fluid viscosity. They used this technique to show the difference in viscosity be- tween fresh and sour milk. Stavros and Dennis ( 1993) 7457-S Ultrasound in Medicine and Biology Volume 2 1, Number 6, 1995 observed acoustic streaming in B-mode images of breast cysts and attributed the moving echoes to cho- lesterol crystals within the streaming cyst fluid. A new ultrasonic technique is envisioned for non- invasively differentiating fluid-filled and solid lesions. This technique could serve as an adjunct to existing ultrasound examinations that would allow the physi- cian to positively diagnose a cystic lesion, even when the cyst appeared hypoechoic on B-mode examination. In this proposed method, the questionable lesion would be located using conventional B-mode ultrasound. High intensity acoustic pulses would then be focused in a narrow portion of the lesion. If the lesion was fluid filled, the radiation force generated by the high intensity beam would induce acoustic streaming in the cyst fluid. The direction of acoustic streaming would be in the direction of wave propagation, away from the transducer. Doppler processing techniques with their associated wall filters would be used to detect fluid motion. Detection of acoustic streaming would unam- biguously indicate a cystic lesion, as solid lesions can- not exhibit streaming. The hypothesis that acoustic streaming can be generated and detected in vivo using this technique is investigated. Results from a pilot study of seven pa- tients with simple breast cysts are presented. MATERIALS AND METHODS The study was performed using a modified com- mercial SI 1200 phased array ultrasound scanner (Sie- mens Medical Systems, Ultrasound Group, Issaquah, WA). The center frequency of the transmitted pulses was 6.75 MHz. This machine has a fixed transmit focus in both color-B/color-M mode (B/M/C mode) and color M-mode (M/C mode), which were the two Doppler