We used the collagen I-binding peptide EP-3533 and our novel 'T-peptide' targeting matrix metalloproteinases-2-digested collagen IV to image interstitial atrial fibrosis, AF and heart failure (HF) that develop in the double transgenic mouse model dnPI3K-Mst1 (termed 'AF + HF'). The gene discussed is MMP2; the disease is heart failure.