Experts not included on any of the lists mentioned in article 157 shall, each time they are appointed, take the oath provided for by the loi n° 71-498 du 29 juin 1971 relative aux experts judiciaires before the examining magistrate or the magistrate appointed by the court. The swearing-in report is signed by the competent magistrate, the expert and the court clerk. In the event of an impediment, the reasons for which must be specified, the oath may be received in writing and the letter of oath shall be attached to the record of the proceedings.