Software used on the computer of an Aberdeen surgeon facing child pornography charges makes it difficult for experts to find evidence of illegal activity, a trial has heard.
Douglas Harris, 29, is alleged to have taken, permitted to be taken or made indecent images, as well as being in possession of pictures.
Grampian Police forensic computer analyst Gordon Simon said software installed could clear archives.
Mr Harris denies the charges.
He has lodged special defences of incrimination and alibi at Aberdeen Sheriff Court.
Before they met
Mr Harris claims someone else could have been responsible for downloading the images.
His girlfriend, Maria Carle, previously told the trial she was "certain in her heart" he was not responsible for the crime.
Mr Simon said there was evidence of child pornography in 2004, almost a year before the couple met.
The offences are alleged to have happened between 2004 and 2006.
The trial continues on Wednesday.