How to improve the display Tif image quality(DPI)

private void RibbonButton_Click(object sender, RoutedEventArgs e)
      {
          Microsoft.Win32.OpenFileDialog dlg = new Microsoft.Win32.OpenFileDialog();
          dlg.InitialDirectory = "c:\\";
          dlg.Filter = "Image files (*.tif)|*.tif|All Files (*.*)|*.*";
          dlg.RestoreDirectory = true;
          bool? result = dlg.ShowDialog();
          {
              string selectedFileName = dlg.FileName;
              FileNameLabel.Content = selectedFileName;
              BitmapImage bitmap = new BitmapImage();
              bitmap.BeginInit();
              bitmap.UriSource = new Uri(selectedFileName);
              bitmap.EndInit();
              ImageViewer1.Source = bitmap;
          }
      }