Eddy Current Testing (ECT) is used for in-service inspection of tubes in steam generators, heat exchangers and condensers in nuclear or conventional power plants as well as in chemical installations. ECT is particularly attractive because it offers both very high detectability and high scanning speeds. Direct contact with test material or a coupling medium is not necessary and the test is easily automated. For the tube inspection, the probes embedded sensor coils are usually inserted in the tubes. Therefore the detecting capability for an outer defect is low in comparison with an inner one by skin effect.