Harmonic motion imaging of human breast masses: an in vivo clinical feasibility

Abstract

Non-invasive diagnosis of breast cancer is still challenging due to the low specificity of the imaging modalities that calls for unnecessary biopsies. The diagnostic accuracy can be improved by assessing the breast tissue mechanical properties associated with pathological changes. Harmonic motion imaging (HMI) is an elasticity imaging technique that uses acoustic radiation force to evaluate the localized mechanical properties of the underlying tissue. Herein, we studied the in vivo feasibility of a clinical HMI system to differentiate breast tumors based on their relative HMI displacements, in human subjects. We performed HMI scans in 10 female subjects with breast masses: five benign and five malignant masses. Results revealed that both benign and malignant masses were stiffer than the surrounding tissues. However, malignant tumors underwent lower mean HMI displacement (1.1 ± 0.5 µm) compared to benign tumors (3.6 ± 1.5 µm) and the adjacent non-cancerous tissue (6.4 ± 2.5 µm), which allowed to differentiate between tumor types. Additionally, the excised breast specimens of the same patients (n = 5) were imaged post-surgically, where there was an excellent agreement between the in vivo and ex vivo findings, confirmed with histology. Higher displacement contrast between cancerous and non-cancerous tissue was found ex vivo, potentially due to the lower nonlinearity in the elastic properties of ex vivo tissue. This preliminary study lays the foundation for the potential complementary application of HMI in clinical practice in conjunction with the B-mode to classify suspicious breast masses.

An analytical model of full-field displacement and strain induced by amplitude-modulated focused ultrasound in harmonic motion imaging

“…Maximum displacements are between 1 and 2 μm, which is in agreement with displacements reported in vivo, in e.g. Saharkhiz et al (2020).…”

“…In a non-homogeneous situation, where the focus is inside a stiff spherical inhomogeneity with a radius R (R=0-∞ mm) and Young's modulus contrast of 2× in a softer surrounding tissue, strain measurements converge to values associated with the stiff inclusion as its volume increases. The measured strains and displacements indicate the local stiffness at the focus with a resolution on the order of 6 mm, which is within the range of 1.2-9 mm that has been reported with HMI in ex vivo and in vivo breast tumor detection (Saharkhiz et al 2020, Hossain et al 2021.…”

“…For the semi-infinite medium test, a FEM mesh with a free surface on the top surface and infinite elements on the other free sides was created, and the displacement and strain fields calculated for various depths D of the HMI focus (D=10, 20, 30, 40, 50 mm and ∞). These depths are in line with the measurement depths of in vivo HMI measurements (Saharkhiz et al 2020). The fields display the maximum value of displacement and strain in every material grid-point.…”

“…For the homogeneity test, a spherical inclusion with a 2× higher stiffness of varying radius R was centered at the HMI focus (R=0, 3, 5, 7, 10, 15, 20, 30, 40 and ∞ mm). Inclusion sizes in this work are in line with investigated tumor sizes in in vivo HMI studies (Saharkhiz et al 2020, Hossain et al 2021. For each value of R, CV was calculated as in equation (10) as compared to the reference case with a homogeneous infinite medium for U z and  zz fields to determine the resolution of a single HMI measurement on a small, stiff structure.…”