Up-to-date Doppler techniques for breast tumor vascularity: superb microvascular imaging and contrast-enhanced ultrasound

Abstract

Ultrasonographic Doppler techniques have improved greatly over the years, allowing more sophisticated evaluation of breast tumor vascularity. Superb microvascular imaging (SMI) and contrast-enhanced ultrasound (CEUS) with second-generation contrast agents are two representative up-to-date techniques. SMI is a sensitive Doppler technique that adopts an intelligent filter system to separate low-flow signals from artifacts. With the development of second-generation contrast agents, CEUS has also emerged as a useful Doppler technique for evaluating tumor microcirculation. Both techniques can improve the diagnostic performance of gray-scale ultrasonography by providing vascular information useful not only for the morphologic assessment of microvessels, but also for the quantitative analysis of perfusion. In this review, we explain the imaging principles and previous research underlying these two vascular techniques, and describe our clinical experiences.

Keywords: Breast; Contrast media; Microvessels; Ultrasonography, Doppler.

Fig. 1.. Imaging principles of superb microvascular imaging (SMI).

The graph in the left column shows the overlap of a tissue motion artifact (clutter) and low-speed flow signals. The top right graph shows that conventional Doppler techniques apply a single-dimensional wall filter (blue dotted line) to remove clutter, resulting in the loss of the slow flow signals. The bottom right graph shows that the SMI technique applies a multidimensional filter (red dotted line) to remove only the clutter, preserving the slow flow signals.

Fig. 2.. Vascular findings of color Doppler ultrasonography, power Doppler ultrasonography, and superb microvascular imaging (SMI) of a 65-year-old woman with invasive ductal carcinoma.

A. Gray-scale ultrasonography shows an irregular indistinct or microlobulated hypoechoic mass in the left breast, assessed as, Breast Imaging Reporting and Data System category 4C. B. SMI shows multiple branching and penetrating vessels within the mass. C, D. A color Doppler ultrasonography (C) and power Doppler ultrasonography (D) show a few dot-like vessels within the mass.

Fig. 3.. Gray-scale ultrasonography and superb microvascular imaging (SMI) of a 53-year-old woman with invasive ductal carcinoma.

A. Gray-scale ultrasonography shows an oval microlobulated hypoechoic mass in the right breast, assessed as Breast Imaging Reporting and Data System category 4A. B. SMI shows a penetrating vessel (arrow) and both peripheral and central vascularity.

Fig. 4.. Measurement of the vascular index for the superb microvascular imaging (SMI) of a 40-year-old woman with fibroadenoma.

When a Doppler image or video file in Digital Imaging and Communications in Medicine format is opened in the dedicated software and the region of interest (ROI) is set as the entire target lesion (yellow line), the vascular index (red box) is automatically calculated. The vascular index is the ratio between the pixels for the Doppler signal and those for the total lesion. This SMI shows a Breast Imaging Reporting and Data System category 4A mass with a few branching or penetrating vessels and a vascular index of 13.9%. Biopsy revealed fibroadenoma.

Fig. 5.. Contrast-enhanced ultrasonography (CEUS) of a 52-yearold woman with invasive ductal carcinoma.

During CEUS, the contrast-specific mode (left) and the gray-scale mode (right) are displayed side-by-side. The contrast agent in the vasculature enhances the backscatter of ultrasound waves and amplifies the flow signals (Aplio 500 was used as the US equipment and SonoVue as the contrast agent).

Fig. 6.. Time-intensity curve analysis for a contrast-enhanced ultrasonography (CEUS) examination of a 38-year-old woman with invasive ductal carcinoma.

When the region of interest (ROI) (pink circle) was set in the area of the strongest enhancement, the following quantitative parameters were automatically calculated: peak intensity (arrow), time to peak (bracket), mean transit time (brace), slope (dotted diagonal line), and area under the curve. This CEUS image was captured 34 seconds after injection of the contrast agent, when the intensity had reached its peak.

Fig. 7.. A 74-year-old woman with invasive ductal carcinoma.

A. Gray-scale ultrasonography shows an irregular angular hypoechoic mass in the right breast, assessed as Breast Imaging Reporting and Data System category 4C. B. Superb microvascular imaging shows penetrating (arrows) and branching vessels within the mass. When the region of interest (ROI) is set as the entire target lesion (yellow line), a vascular index of the mass is 19.6% (red box). C. Contrast-enhanced ultrasonography shows hyperenhancement, penetrating vessels (arrows), and perfusion defect (asterisk) of the lesion. When the ROI (pink circle) is set in the area of the strongest enhancement for time-intensity curve analysis, the mass shows fast and strong enhancement (higher peak intensity, slope, and area under the curve) (white box).

Fig. 8.. A 36-year-old woman with fibroadenoma.

A. Gray-scale ultrasonography shows an oval microlobulated hypoechoic mass in the left breast, assessed as Breast Imaging Reporting and Data System category 4A. B. Superb microvascular imaging shows linear or dot like vessels within the mass. When the region of interest (ROI) is set as the entire target lesion (yellow line), a vascular index of the mass is 5.5% (red box). C. Contrast-enhanced ultrasonography shows isoenhancement of the lesion (arrows). When the ROI (pink circle) is set in the area of the strongest enhancement for time-intensity curve analysis, the mass shows relatively slow and mild enhancement (lower peak intensity, slope, and area under the curve) (white box).