Impedance osteography: a technique to study the electrical characteristics of fracture healing

The occurrence of a fracture in a limb is accompanied by large changes in the electrical characteristics of the region. Resistivity of bone is more than a hundred times that of bone. The relative permittivities of the two are also different. The fracture site is also known to be electronegative. Electrical Impedance Imaging is capable of reconstructing a two dimensional (2D) representation of the average distribution of amplitude and phase in a three dimensional (3D) region. The spatial resolution in impedance imaging is low, yet the technique is sensitive to properties that are rapidly changing in growing tissues. Though the reconstructed image is an average of a thick slice, this may be of some advantage in impedance osteography precluding the need for imaging multiple planes to cover the extent of the fracture. We used the Aberdeen Impedance Imaging System which uses the 4 electrode technique in a split array, and a current of 1 milliampere on a carrier of 10 kHz to collect a number of data sets. The averaged data sets were used to reconstruct images by the Equipotential backprojection method. In this pilot study we concentrated on the real part of the complex impedance to produce images of log resistivity. The a-priori assumption of bilateral symmetry of human limbs allowed us to reconstruct equivalent regions of the normal and fractured limbs by inverting the electrode array. We obtained static images of the distribution of log resistivity when we used a homogeneous distribution as a reference, and a differential image comparing the fractured to the normal limb. The three patients in our study were at different clinical stages of fracture healing as confirmed by radiography. Our results show the feasibility of using the technique to study and monitor fractures during the healing process.