Three-dimensional complementary breast ultrasound (3D CBUS): Improving 3D spatial resolution uniformity with orthogonal images

Background: With increasing evidence supporting three-dimensional (3D) automated breast (AB) ultrasound (US) for supplemental screening of breast cancer in increased-risk populations, including those with dense breasts and in limited-resource settings, there is an interest in developing more robust, cost-effective, and high-resolution 3DUS imaging techniques. Compared with specialized ABUS systems, our previously developed point-of-care 3D ABUS system addresses these needs and is compatible with any conventional US transducer, which offers a cost-effective solution and improved availability in clinical practice. While conventional US transducers have high in-plane resolution (axial and lateral), their out-of-plane resolution is constrained by the poor intrinsic elevational US resolution. Consequently, any oblique view plane in an acquired 3DUS image will contain high in-plane and poor out-of-plane resolution components, diminishing spatial resolution uniformity and overall diagnostic utility.

Purpose: To develop and validate a novel 3D complementary breast ultrasound (CBUS) approach to improve 3DUS spatial resolution uniformity using a conventional US transducer by acquiring and generating orthogonal 3DUS images.

Methods: We previously developed a cost-effective, portable, dedicated 3D ABUS system consisting of a wearable base, a compression assembly, and a mechanically driven scanner for automated 3DUS image acquisition, compatible with any commercial linear US transducer. For this system, we have proposed 3D CBUS approach which involves acquiring and registering orthogonal 3DUS images ( $V_A$ and $V_B$ ) with an aim of overcoming the poor resolution uniformity in the scanning direction in 3D US images. The voxel intensity values in the 3D CBUS image are computed with a spherical-weighted algorithm from the original orthogonal 3DUS images. Experimental validation was performed with 2DUS frame densities of 2, 4, 6 frames mm^-1 using an agar-based phantom with a speed of sound of 1540 ms^-1 and an embedded nylon bead. Lateral and axial full-width at half-maximum (FWHM_LAT and FWHM_AX) values were calculated from cross-sections taken at polar view planes ranging from 0° to 90° for 3DUS and 3D CBUS images of a bead phantom in focal zone and far field regions. Kendall's Tau-b correlation coefficients were calculated between FWHM measurements and cross-section angle for all frame density settings at a significance level of $\alpha =0.05$ . Volumetric 3D segmentations were performed for 3DUS and 3D CBUS images of an inclusion phantom to confirm volumetric reconstruction accuracy. For statistical analysis, a repeated measures ANOVA with the Greenhouse-Geisser correction was performed at a significance level of $\alpha =0.05$ .

Results: Experimental validation of the orthogonal 3DUS images show complementary trends of increasing and decreasing FWHM_LAT from in-plane to out-of-plane (0° and 90° and vice versa) views. This is exemplified with the scan taken at 4 frames mm^-1 in the focal zone, where FWHM_LAT ranges from 3.51 to 1.10 mm for $V_A$ and 1.02-3.02 mm for $V_B$ , spanning 0°-90°, respectively. When combined in the 3D CBUS image, the FWHM_LAT exhibits greater uniformity across view angles by mitigating poor out-of-plane resolution using its complementary in-plane component, with corresponding FWHM_LAT values of 1.27 and 1.46 mm. While visual enhancements were seen in the 3D CBUS image, no statistically significant differences were found in volumetric measurements of the spherical inclusions in the 3DUS and 3D CBUS images.

Conclusion: The out-of-plane resolution in the orthogonal 3DUS images is improved upon their combination into a single 3D CBUS image. These results demonstrate that the proposed 3D CBUS generation approach can improve 3D spatial resolution uniformity, while employing a commercial US transducer. The proposed 3D CBUS method shows potential utility for improving image resolution uniformity in 3D ABUS images, with the goal of improving point-of-care breast cancer supplemental screening and diagnostic applications, particularly in women with dense breasts and limited resource settings.