The invention relates to an assessing device (13) for assessing the suitability of a shape of an instrument like a guidewire for a registration of a position and shape determination device (10) like an optical shape sensing device with an imaging device (2). Curvature values being indicative of the curvature at several positions along the instrument are determined, which are used for determining shape feature values. These shape feature values are then used for determining a suitability value being indicative of the suitability of the shape of the instrument for the registration procedure, wherein an output is provided to a user based on the determined suitability value. The user can therefore modify the shape until the output indicates that the shape of the instrument is suitable for the registration, without requiring any image. This can reduce a radiation dose if the image is, for instance, an x-ray image.