Real‐time transvaginal elastosonography of uterine fibroids

Abstract

Uterine fibroids are the most frequently occurring benign tumors originating in the uterus, and are usually round or partially rounded in shape (Figure 1). Although they are composed of the same smooth-muscle fibers as the uterine wall, they are many times more dense than is normal myometrium. This characteristic is frequently responsible for the poor visualization of fibroids on transvaginal ultrasonography, due to strong acoustic shadowing. Conventional B-mode imaging with power Doppler demonstrating two uterine fibroids, measuring 23 mm (D1) and 31.9 mm (D2) in diameter. Peripheral vascularization of the fibroids is evident. Because the distribution of fibroids can be difficult to determine on conventional B-mode ultrasonography, their number and size can be underestimated1. Three-dimensional ultrasound, computed tomography and magnetic resonance imaging have all been proposed for better fibroid visualization. However, a new, easy-to-use ultrasound tool, real-time transvaginal elastography2 has been suggested in two recent in-vitro studies as a potential method for evaluating fibroids3, 4. The principle of the ultrasonographic technique is based on slight external tissue compression on the structures examined, which produces strain (displacement) within the tissue, with subsequent calculation of the strain profile along the axis of compression (using the Hitachi ECAM (extended combined autocorrelation method (Hitachi Medical Corporation, Tokyo, Japan)) algorithm to produce the elastography image). The strain profile is converted into an elastic modulus image, i.e. the tissue elasticity distribution, called an elastogram. The calculated elasticity values are then color-coded and superimposed on the translucent, corresponding B-mode scan image (Figure 2). The stiffness of the tissue is displayed in a range of color from red (components with the greatest strain, i.e. the softest components) to blue (components with no strain, i.e. the hardest components). The components with average strain are displayed as green. Real-time elastosonography images of the two fibroids shown in Figure 1. The strain ratio is evaluated by comparing the mean strain in a region of interest centered on the myoma, with the mean strain in a region of interest in the surrounding myometrium close to the probe. The fibroid corresponding to D1 in Figure 1 has a strain value of 0.70%, and that corresponding to D2 has a strain value of 0.69%. The ratio shows a mean strain about 10 times greater for normal myometrium compared with the fibroids. Elastography images were obtained using a Hitachi EUB-8500HV ultrasound machine equipped with an integrated elastography (HI-RTE) hardware card and a 5.9-MHz EUP-V53W transvaginal probe. Real-time elastosonography parameters were set as follows: frequency, T-Elasto-H; frame rejection, 5–6; noise rejection, 4; dynamic range, 3; smoothing, 3; persistence, 6–7; frame rate, H or M (high or mid)5. This technology has been available from Hitachi for 5 years. We observed 10 patients with uterine fibroids who were referred for infertility investigations, and who subsequently underwent conservative surgical treatment. Our study was approved by the ethics committee of the French Obstetrics and Gynecology Society (CEROG-2009-012), and informed written consent was obtained from patients before the procedure was performed. The diagnosis of uterine fibroids was confirmed by histology retrospectively in each case. The mean strain value was 0.08% for uterine fibroids and 0.77% for the normal surrounding myometrium, giving a myometrium-to-fibroid strain ratio of 11 (P = 0.017, Student's paired t-test). All fibroids were seen easily on the color display in elastography mode, and their extent was easier to define than it was in conventional B-mode. The distance between the fibroid and the endometrial cavity or uterine serosa could also be measured easily in each case (Figure 3). Color mapping of uterine strain allows precise determination of the so-called ‘security wall’ between the fibroids and the endometrium or uterine serosa before surgical resection with laparoscopy or hysteroscopy, respectively. Here an intramural fibroid distance of 4.7 mm to the endometrium and a submucosal fibroid distance of 9.3 mm to the uterine serosa (arrows) are presented. Endovaginal ultrasonography is safe, accessible and inexpensive, and remains the primary imaging method for gynecological evaluation. Real-time elastosonography offers complementary diagnostic and mapping information. It is easy to perform, and the procedure requires only a few seconds of manipulation. With conventional ultrasound, the ultrasound beam is often strongly attenuated by the fibroid. As a result, with low gain, the posterior wall of the uterine myoma is poorly vizualized, but as the gain is increased, noise or artifactual echoes appear inside the mass, obscuring the image of the fibroid1. Real-time elastosonography provides an instantaneous color map that precisely delineates the fibroids, thus overcoming the limitations of conventional ultrasound. Real-time elastosonography encodes the ultrasonographic displacement of tissues under an external pressure constraint, using a temporal parameter which originates from an image sequence analysis and comparison over a short period. This temporal parameter is absent from conventional B-mode sonography, which displays only the instantaneous ability of tissues to reflect or absorb ultrasound waves, and this may explain why attenuation of ultrasound and shadowing artifacts affect elastography less than they do conventional B-mode ultrasound imaging. There are only a few reports in the literature on the in-vivo use of real-time transvaginal elastosonography in the field of gynecology. The usefulness of this technique has been documented for breast cancer and liver fibrosis, and has been of interest in the exploration of malignant tumors in the cervix, prostate and thyroid6-11. In this study, we have demonstrated the feasibility of in-vivo real-time transvaginal elastosonography of fibroids. In conclusion, real-time elastosonography is a promising tool that can provide detailed mapping and characterization of uterine fibroids. This could improve the gynecological ultrasound evaluation of size, volume and delineation of uterine fibroids before surgery or embolization. Future studies should aim to investigate strain contrast differences between fibroids and adenomyomas, and the characterization of uterine or adnexal pathologies.