ااین کد ها رو زدم، جواب گرفتم:
int aaa = (int)circle.Center.X;
int bbb = (int)circle.Center.Y;
Point start = new Point(aaa - (int)circle.Radius, bbb - (int)circle.Radius);
Point second = new Point(aaa - (int)circle.Radius, bbb + (int)circle.Radius);
LineSegment2D line5 = new LineSegment2D(start, second);
CvInvoke.Line(circleImage, start, second, new Bgr(Color.Green).MCvScalar, 2);