This is a very, very subtle finding. Identifying a lesion of this size in a posterior location within a large breast requires a highly targeted approach, optimisation of scan parameters, operator skill, and a bit of luck.
Ensure the depth and focus settings are appropriate. Use of harmonics is often of value. Consider changing to a lower frequency probe if the lesion is expected at a greater depth, particularly in a dense breast.
Even so, this lesion remains better seen on mammogram than on ultrasound - and this should ultimately guide the choice of biopsy method, provided stereotactic biopsy equipment is available. Regardless of modality, a clip marker should be placed post biopsy.