DependencyProperty does not trigger LayoutProcess

DependencyProperty does not trigger LayoutProcess

I am writing my own Panel (WPF) to draw a model. I have a
Model-DependencyProperty and I want that any changes to my Model will
affect a LayoutProcess.
ModelProperty = DependencyProperty.Register("Model", typeof(Model),
typeof(ModelPanel),
new FrameworkPropertyMetadata(null,
FrameworkPropertyMetadataOptions.AffectsMeasure));
How should I implement my Model-Class so that any changes will affect a
LayoutProcess? I've tried INotifyPropertyChanged. But it didn't work.