stroke imagebrush windows store apps

Question

I am using imagebrush as stroke when i draw polyline.

var bitmapImage = new Windows.UI.Xaml.Media.Imaging.BitmapImage(new Uri("ms-appx:/Assets/br1.png"));
        img = new Windows.UI.Xaml.Media.ImageBrush()
        {
            ImageSource = bitmapImage,
            Stretch = Stretch.None,
        };

And create polyline:

this._polyline = new Polyline() { 
            StrokeThickness = 23,                
              Stroke = img,                  

        };

But image stretch when i draw polyline in canvas. I can't repeat imagebrush, can you help me?