Responsive WPF UI

I ahve a WPF application not implementing any mVVM.
I have  button click event, where I would like to show a progress Image, and then start a background thread to get some data.
In the background thread when the data has ben retrieved, I want to hide the Progress image.

  private void Button_Click(object sender, RoutedEventArgs e)
        {          
           
            //imgProgress.Visibility = System.Windows.Visibility.Visible;
            Application.Current.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Normal, new Action(
                delegate()
                {
                    imgProgress.Visibility = System.Windows.Visibility.Visible;
                              }));
         
            StartBackgroundThread();
         
        }

 public void StartBackgroundThread()
        {
            BackgroundWorker bgWorker = new BackgroundWorker();
            bgWorker.WorkerSupportsCancellation = true;
            bgWorker.DoWork += new DoWorkEventHandler(bgWorker_DoWork);
            bgWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bgWorker_RunWorkerCompleted);          
            bgWorker.RunWorkerAsync();
        }

        void bgWorker_DoWork(object sender, DoWorkEventArgs e)
        {
       
            BackgroundWorker bgWork = (BackgroundWorker)sender;
            if ((bgWork.CancellationPending == true)) { e.Cancel = true; }
            else
            {
                GetData(0;
            }
        }

 void bgWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
imgProgress.Visibility = System.Windows.Visibility.Hidden;
}

THE UI freezes when the data retrival takes a long time, so does the Progress image.