A Toronto court heard explicit details on Tuesday of how a doctor allegedly managed to sexually assault more than 20 women while they were being operated on at North York General Hospital.

Dr. George Doodnaught says he is not guilty of the 21 counts of sexual assault on women ranging in age from 25 to 75 over a four-year period from 2006 and 2010.

At one time, he was facing 29 charges

On Tuesday, Crown prosecutor David Wright showed the court staged photographs of the operating rooms in the hospital where Doodnaught, 64, worked. The anesthesiologist had worked at the hospital since 1981.

The photographs were meant to show how Doodnaught might have been able to carry out the alleged assaults while other doctors and nurses were also in the operating room, just steps away.

George Doodnaught has pleaded not guilty to the charges. ((Toronto Police Service) )

Doodnaught was hidden by what is called a 'sterile screen,' said Wright, where he administered drugs that kept the patients in a semi-conscious state.

The screen is a one by 1½-metre curtain that separates the sterile and non-sterile parts of the operating area

In the 2010 case that ultimately led to the police investigation a woman who was undergoing a hysterectomy found herself being sexually assaulted, the prosecutor said.

"He started kissing her and placed his tongue in her mouth," Wright said. "No one saw her head being turned to the left and George Doodnaught putting his penis in her mouth."

The prosecutor said the woman recalls asking Doodnaught about the others in the operating room and he replied, "I know how to be discreet."

The other allegations are similar.

Female patients claim Doodnaught kissed them, fondled their breasts and placed his penis in their mouths and hands.

The women were sedated, but conscious. They were undergoing various surgeries on their lower bodies.

Most of the charges surfaced after news of Doodnaught's arrest in 2010.

The trial is being heard by Ontario Superior Court Justice David McCombs alone.