Strangely enough, but the Japanese helped to resolve the issue. The class
Microsoft.Expression.Shapes.Arc makes drawing circles easier.
using Microsoft.Expression.Shapes;
Arc a = new Arc();
a.ArcThicknessUnit = Microsoft.Expression.Media.UnitType.Pixel;
a.ArcThickness = 10;
a.StrokeThickness = 1;
a.Fill = new SolidColorBrush(Colors.Aqua);
a.Stroke = new SolidColorBrush(Colors.DarkMagenta);
a.StartAngle = 0;
a.EndAngle = 0;
a.Width = 200;
a.Height = 200;
a.Stretch = Stretch.None;
stp.Children.Add(a);
DispatcherTimer dt = new DispatcherTimer();
dt.Interval = TimeSpan.FromMilliseconds(10);
dt.Tick += (s, e) => { a.EndAngle = a.EndAngle + 4; };
dt.Start();