Towards virtual electrical breast biopsy: space-frequency MUSIC for trans-admittance data.

Breast cancer diagnosis may be improved by electrical immittance measurements. We have developed a novel method, space-frequency MUltiple Signal Classification (MUSIC), to determine three-dimensional positions and electrical parameters of focal lesions from multifrequency trans-admittance data recorded with a planar electrode array. A homogeneous infinite volume conductor containing focal inhomogeneities proved to be a useful patient-independent model for the breast containing focal lesions. Lesions polarized through the externally applied electric field are considered as distributions of aligned dipoles. Independence of the lesions' shape and size is achieved by a multipole expansion of such a dipole distribution. Thus, lesions are described by point-like multipoles. Their admittance contributions are given by a sum over products of multipole-specific source-sensor transfer functions, called lead fields, multiplied by their moments. Lesion localization corresponds to multipole search, and uses orthonormalized lead fields for comparison with a signal subspace from a singular value analysis of a space-frequency data matrix. At the locations found, the moments' frequency behavior is calculated which is assumed to be tissue-specific due to their dependence on conductivities. Results from clinical data show that space-frequency MUSIC successfully localizes lesions. Tissue differentiation might be possible, especially when the frequency range of the measurement system will be increased.