High-frequency ultrasound imaging for breast cancer biopsy guidance

Image-guided needle biopsy is the gold standard for cancer diagnosis. These biopsies are generally performed under ultrasound guidance for masses and stereotactic guidance for microcalcifications.1,2 Ultrasound-guided biopsy is the preferred method of choice since it is more comfortable for patients and does not require ionizing radiation. Stereotactic-guided biopsy, however, is chosen over ultrasound-guided biopsy for microcalcifications since current standard ultrasound systems cannot reliably visualize microcalcifications, the presence of which can suggest early breast carcinoma such as ductal carcinoma in situ (DCIS). Thus, there is a clinical need for an ultrasound imaging tool that can be used in conjunction with the external ultrasound system to guide the needle during the biopsy of microcalcifications for accurate tissue sampling. 1.1. Clinical Need Background Clinical ultrasound imaging has been validated as an effective tool in guiding percutaneous needle procedures such as central venous catheterization3 and other needle biopsy procedures including transbronchial4 and pancreatic fine needle aspiration.5 Standard clinical ultrasound imaging relies on transmitting and receiving ultrasound energy. The redistribution of this energy from a transmitted wave occurs through either reflection, when the wavelength is smaller than the object it encounters, or scattering, when the wavelength is greater than or comparable to the dimension of the object. This is particularly relevant in breast imaging as the ability to identify and sample microcalcifications increases the probability of sampling potentially malignant tissue. The wavelengths of current clinical imaging systems (5 to 15 MHz) with respective wavelengths of 300 to 100  μm, limit the ability of receiving strong echo signals from microcalcifications. The size of microcalcifications, usually <100  μm, then entails that “scattering” becomes the dominant mode of energy redistribution obscuring their identification.6 Since the fundamental limitation of wavelength cannot be changed for these clinical imaging systems, attempts to improve image processing of ultrasound echo data to highlight these microcalcifications has been implemented by systems such as the Toshiba Aplipure System.7 The Aplipure system uses compound imaging to improve the contrast of breast images, which helps the user visualize microcalcifications within the tissue.8 Providing enhanced microcalcification visualization enables physicians to better target tissues they hope to sample with the core biopsy needle. As sampling error is the primary cause for false negative results, improving the resolution of specific structures (i.e., microcalcifications) that may represent DCIS is a promising solution that will improve the sensitivity of breast cancer biopsy procedures under ultrasound guidance. In summary, radiologists need an ultrasound system that can provide high-resolution images of tissue and structures such as microcalcifications in real-time to guide the biopsy needle, improve diagnostic accuracy, and reduce sampling error. We have evaluated external and internal imaging arrays to address this clinical need. Given the small size of microcalcifications and the fact that lesions within the breast can be several centimeters below the skin’s surface, imaging systems require axial and lateral resolutions that correspond to center frequencies above 50 MHz to resolve these individual structures. An external imaging probe is not a viable solution here since acoustic energy attenuation in tissue would be prohibitively large. Ultrasound probes that image the lesion from within the tissue, through an interventional tool such as a core biopsy needle, becomes a more viable solution. Under these conditions, the requirement for depth of penetration is limited to the tissue sampling distance of the core biopsy needle, which is <4  mm for most existing core biopsy needles. We have built and are currently evaluating this internal interventional imaging array and have designed a clinical imaging study to validate that high-frequency ultrasound imaging is useful for identifying tissue features such as microcalcifications to guide the tissue biopsy.