Iris outer boundary(Circle) detection using Emgu CV

Question

How can I detect the outer boundary of an iris in an eye image. I tried using HoughCircles() method,

        Gray cannyThreshold = new Gray(150);
        Gray cannyAccumulatorThreshold = new Gray(150);

        CircleF[] circles = grayscaledImg.HoughCircles(
                                     cannyThreshold,
                                     cannyAccumulatorThreshold,
                                     4,
                                     150,
                                     5,         //min radius
                                     0)[0];    //max radius



        foreach(CircleF circle in circles)
        {
            grayscaledImg.Draw(circle, new Gray(), 2);

        }

But the code draws the circle on the wrong places. Need your help guys.

Output image: