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Electromagnetic induction tomography (EMT) is a non-invasive imaging technique for determining the conductivity and magnetic permeability distributions in a region of interest (ROO, from measurement of inductances at its boundary. This paper describes how this technique may be used for imaging steeI reinforcement bars embedded in a nonconducting and non-magnetic medium, from measurements made on only one side of the ROI. Reconstruction of the ROI is first carried out using the simultaneous increment reconstruction technique (SIRT), which gives a crude estimate of the position and size of the reinforcement bars. These estimates are then refined using a non-linear reconstruction technique based on a parametric representation of the ROI, I INTRODUCTIONElectromagnetic induction tomography (EMT) is a noninvasive imaging technique for determining the conductivity and magnetic permeability distributions in a region of interest (ROI) fiom measurements of inductance made at its boundary. In many situations, it is possible to surround the ROl with a sensor array composed of a number of coils mounted on ferrite core. Each coil may be excited by a time-varying current, which produces a timevarying magnetic field over the ROI. The number of projections ofthe ROI is equal to the number of coils in the sensor array. In each projection, the flux linked with all the other coils may be measured in the fom of an inductance or impedance. By using suitable reconstruction algorithms, it may be possible to reconstruct the material distribution in the ROI from the boundary measurements.An overview on this technique is given in [I].h certain situations, where it is not possible to surround the ROI with a sensor array, a planar array is placed on the only accessible side. A practical example is in the nondestructive evaluation of a steel reinforced concrete structure, where measurements may be made on only one side. In this case, the number of projections is considerably reduced, and reconstruction of the ROI is complicated by the limited data available. Following a preliminary study on the feasibility of planar EMT [2], improvements were proposed by a more efficient modeling of the sensor array [3], and reconstruction fiom measurements made on an experimental sensor array using the simultaneous increment reconstruction technique (SIRT) [4].This paper describes how the qualitative results given by tbe SIRT algorithm may be refined. T h e refmement is based on a parametric representation of the ROI. The set of parameters, which gives a best fit to the boundary data, is found by a non-linear optimization technique. The 0-7803-9 Mauritius charlie@uom.ac. mu algorithm is tested on measurements made on the experimental sensor array, and was found to give a sufficiently close reconstruction within the limits of experimental error. J I THE PLANAR ELECTROMAGNETIC INDUCTIONSENSOR ARRAY Fig. 1 shows an experimental sensor array for planar electromagnetic induction tomography, and its twodimensional representation is shown in Fig. 2. The seusor arra...