PET scan of the human heart showing the effect of a blood clot. Here, an undissolved clot has severely restricted blood flow (see mid-section) so that much of the muscle wall is not metabolizing. Compare with PET scan 4k6890, which shows the same heart after t-PA (a protein called tissue-type plasminogen activator) has been administered so that the clot is dissolved and the heart muscle is metabolizing again.