The main purpose of this work is to present a simple quantitative approach for assessing artifacts in passive instruments when used in an MRI environment. It is mainly based on a quantitative indicator related with the amount of distortion produced by the instrument on a 3D surface obtained from the 2D MR image. After a preprocessing stage for attenuation of the 3D surface low frequency components, an indicator related with the volume of the background distortion is computed for each MRI slice. Then a monotonically decreasing curve is calculated for assessing artifact level using all the slices of a sequence. Results show that an indicator can be computed automatically from all the slices of a given MRI sequence and through this indicator it is possible to perform comparative studies between artifacts produced by different catheters and needles.